Mn/Model Final Report Chapter 12: Landscape Suitability Models for Geologically Buried Precontact Cultural Resources

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Chapter 12

Landscape Suitability Models for Geologically Buried Precontact Cultural Resources

By Curtis M. Hudak and Edwin R. Hajic, with contributions by Patricia A. Trocki and Rose A. Kluth

Statewide Survey Impelmentation Model Map

12.10 ST. CROIX RIVER MODEL

12.10.1 Introduction

The Minnesota side of the St. Croix River Valley was mapped from the Minnesota - Wisconsin state line at the northern limit, downstream to its confluence with the Mississippi River at Hastings, Minnesota, on the south end. The following comments, therefore, refer to the mapped Minnesota side of the valley. The St. Croix River has very low sinuosity and numerous islands in the reach north of Stillwater. South of Stillwater, the river turns into a riverine lake, Lake St. Croix, presumably in response to hydraulic damming by the Mississippi River. The St. Croix River, its narrow floodplain, and Lake St. Croix are confined by the most deeply scoured inner channel of late glacial catastrophic flooding. Upstream of North Branch, the valley is wide and characterized by continuous catastrophic flood channels and streamlined erosional forms. Some of the former are filled with post-flood peat. Downstream of North Branch, the Minnesota side of the valley is usually narrow, widening at times where Catastrophic Flood channels lie at the foot of large arcs carved into the valley wall.

12.10.2 Overview of Past Work

The history of the St. Croix Valley as a glacial spillway is poorly to well documented depending on the time interval in question, but chronology is somewhat tenuous for all but the latest glacial lake discharge (Clayton 1983, Hobbs 1983, Drexler et al. 1983, Clayton and Moran 1982, Wright 1990). With advance of the Grantsburg Sublobe of the Des Moines Lobe, the Mississippi River was diverted eastward around the lobe causing flow to traverse down the St. Croix Valley (Wright 1990). During the Cass Phase of Lake Agassiz, about 11,500 B.P., it is argued that lake waters in the eastern arm of Lake Agassiz and Lake Koochiching made it to the St. Croix Valley. For a brief time, the Agassiz waters drained down the Prairie River to Lake Upham-Aitkin, then down the St. Louis River to the edge of the Superior Lobe of glacial ice, and subsequently down the Kettle River to the St. Croix River. The St. Croix River also received lake waters via the Brule and Portage spillways when the waters in the Lake Superior Basin were at various lake levels. The large magnitude pulse of lake water that carved the Brule spillway is still unidentified. The event that carved the Brule may also have been the last large magnitude flood to modify the St. Croix. The most likely times that the Brule would have been functioning were during the Superior Phase B (ca. 11,400-10,800 B.P.) and Phase D (ca. 9,950-9,700 B.P.) (Clayton 1983). The latter interval corresponds with the Marquette advance into the Superior Basin, which would have forced glacial lake waters down the Brule spillway.

Considerable interest has been shown in the paleomagnetic record of sediments in Lake St. Croix as a means to examine local secular variation of the earth's magnetic field during the Holocene. Lund and Banerjee (1985) analyzed two parallel long cores. A series of radiocarbon ages range from about 10,600 to 1,080 B.P from the bottom to the top of one core (Lund and Banerjee 1985:805-806). Lund and Banerjee, however, argue that these radiocarbon ages are about 980 years too old based on independent evidence. The same cores were analyzed for pollen (Eyster-Smith et al. 1991). In the same paper, these authors argue that Lake St. Croix in the lower St. Croix valley formed in response to backflooding from the Mississippi River.

Hudak (unpublished data) obtained three radiocarbon assays from deep cores across the valley at Stillwater. One date, >55,500 B.P. on wood, was interpreted as coming from an exhumed valley fill intersected by the downcutting of the modern St. Croix. The younger dates are from Holocene organic fill and are reported in Table E.2.

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12.10.3 Specific Methods and Data

12.10.3.1 Cores

Twenty-six (26) cores were collected from the St. Croix River valley between October 10 and 28, 1996. All of these cores were taken with the Giddings hydraulic soil probe. Several cores were examined from the Trunk Highway 36 proposed bridge crossing at Stillwater. These cores were collected by MnDOT during some foundations work and turned over to the Mn/Model geomorphology team after their initial use. All cores were logged in a laboratory and are presented in Appendix E.1. The methods discussed in Section 12.2 apply to this field and laboratory work.

12.10.3.2 Cross-Section Profiles

A series of six cross-sections and one long-section were constructed from the soil profiles of the St. Croix River valley. These figures and one drawing are presented in Appendix E.2 and E.3, respectively.

12.10.3.3 Radiocarbon Dates

A variety of organic samples were collected from the various cores. A master list of organics and their identifications are presented in Appendix E.4 as Table E.1. These samples yielded twelve radiocarbon dates from the St. Croix River valley. The radiocarbon dates and their associated data are presented in Appendix E.5 as Table E.2. These radiocarbon dates are also displayed in the above-mentioned cross-sections.

12.10.3.4 Site-Specific Field Methods and Mapping

The nature of the valley, which is gorge-like, prevented much field sampling of the already sparse and geomorphically lower landforms. Nearly all cores were taken in peat that fills a series of broad channels on the highest valley surfaces. The Giddings soil-probe was not equipped with a peat sampler; therefore, the recovery was poor for some cores.

High altitude color infrared aerial photographs from the USGS NAPP program were the primary source of geomorphic information. Tonal contrasts on these air photographs were sufficient to delineate features even finer than those mapped. The Chisago and Washington county NRCS soil surveys were useful in supplying subsurface textural information. Pine County has not been mapped by the NRCS. Mapping was accomplished by thin pencil directly on USGS 7.5 topographic maps. Many of the geomorphic boundaries were transferred to the maps from mylar overlays of NAPP aerial photography, with appropriate adjustments for distortion of the aerial photographs.

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12.10.4 Landform Sediment Assemblages (LfSA)

Landform sediment assemblages of the St. Croix valley are outlined in Table 12.9 and discussed below.

12.10.4.1 Landscapes

The Upland, Catastrophic Flood, Paleo-Valley, Floodplain, and Valley Margin LsSA’s were used in mapping the St. Croix River Valley. The undifferentiated Upland landscape consists of a few upland areas isolated by glaciofluvial deposits. Most of the valley area is mapped as various LfSA’s belonging to the Catastrophic Flood landscape. The Paleovalley LsSA is a minor component and refers to several irregular valleys inset into erosional catastrophic flood landforms. The Floodplain landscape is a minor component in square area and amounts to a series of islands and narrow belts along the river. The Valley Margin suite is also a minor component.

Upland landscape. Several undifferentiated upland areas are completely surrounded by St. Croix Valley LsSA’s. These upland areas are typically underlain by till.

Catastrophic Flood landscape. This landscape consists of erosional and depositional landforms created by, and collectively characteristic of, one or more extremely large magnitude floods, including the one(s) that might have cut the St. Croix valley. Catastrophic Flood landforms are the highest and oldest geomorphic surfaces inset below the surrounding uplands. Basic landforms include bars (CB), terraces (CT), marginal channels (CMC), erosional residuals (CER), paleochannels (CPC), and erosional straths (CST) that dominate the valley landscape. Associated sediment assemblages overlie bedrock or glacial drift. Coarse material deposited during the flood comprises bars and terraces, and mantles to varying degrees marginal channels, erosional residuals and erosional straths. Fine material represents local tributary overbank deposits, local sheetwash deposits, and loess. Silt interpreted as loess is restricted to the southernmost part of the valley where the Mississippi Valley is closest.

Most, if not all, coarse material in this LsSA was deposited during the catastrophic flooding. Nearly all fine material that mantles flood erosional landforms or overlies coarse material post-dates catastrophic flooding. Peat and organic muck found in flood channels postdates catastrophic flooding. The age of the Catastrophic Flood landscape is interpreted to be of the proper time range to contain cultural resources.

Catastrophic Flood bars along the St. Croix are primarily streamlined midchannel bars that occur between marginal flood channels. Bars occur in a swarm up-valley of the mouth of the Kettle River and as relatively smaller forms associated with younger flood channels closer to the modern St. Croix River. They also occur as several relatively large forms near the town of North Branch and are scattered down the remaining length of the valley. The associated sediment assemblage consists of laterally continuous coarse material, greater than two meters thick, that forms the bars and dates to the time of flooding. Bars are differentiated based on the presence (CBFCL) or absence (CBCO) of loess, and on whether an exposed strath terrace underlies the bar (CBFCLK, CBCOK).

Catastrophic Flood terraces are generic surfaces formed by catastrophic flooding and are elevated above the modern floodplain. These surfaces lack forms of other more recognizable Catastrophic Flood landforms (i.e., bars, channels). Also grouped under the terrace LfSA are slip-off slopes and more flat-lying pendant and alcove bars. Sediment assemblages are dominated by sand, with or without gravel, in varying thicknesses that date to the time of flooding. When fine material, peat, or organic muck is present, it overlies coarse material and most likely postdates the catastrophic flooding. The sediment assemblage is greater than 2 m thick and laterally continuous. Catastrophic flood terraces are differentiated based on the presence (CTFCL) or absence (CTCO) of loess; on whether an exposed strath terrace underlies the terrace (CTFCLK, CTCOK); and on the presence of a surficial increment of peat (CTPC).

Erosional residuals are streamlined lemniscate-shaped landforms that form by erosion of the substrate during catastrophic flooding. Swarms of erosional residuals occur within and immediately beyond the mouth of the Kettle River and immediately upstream of the large bars in the vicinity of the town of North Branch, and as a couple of isolated occurrences to the south. The associated sediment assemblage is meager and consists of little or no material overlying the streamlined form. Erosional residuals are differentiated based on the presence (CERL) or absence (CER) of loess.

Strath terraces, as mapped, are formed on bedrock or glacial drift, lack a streamlined form (erosional residuals), and are overlain by little or no overlying material that ranges from a continuous to discontinuous cover of material less than 2 m thick. The primary differentiation of strath terraces is thickness and continuity of sediment on the strath. In most cases, coarse material is discontinuous and very thin (CST). In some cases it is continuous but still less than a meter thick (CSTCO). These two situations are further differentiated if loess is present (CSTL, CSTFCL, respectively). Additional differentiation is based on the presence of younger overbank deposits (CSTFCA), and the presence of a surface lag of boulders (CSTR).

The lowermost reaches of several upland paleochannels are truncated by the Catastrophic Flood valley margin within the lowest reach of the St. Croix River valley. These paleochannels sit high on the landscape and are cut into upland sediment assemblages. They probably were formed during the earliest stages of catastrophic flow. The sediment assemblage is laterally continuous and greater than 2 m thick. The paleochannels are differentiated on whether the material texture is fine (CPF) or coarse (CPC) and whether loess is present (CPCL).

Marginal channels are broad concave to flat sluiceways. They typically occur between valley walls and flood bars or erosional residuals, but as mapped refer to former flood channels regardless of relationship to valley walls. Marginal channels occur along the length of the St. Croix Valley in Minnesota. North of the town of North Branch, where the valley in Minnesota is comparatively wide, they are fairly continuous and join similar sluiceways emanating from the valley of the Kettle River. South of North Branch, where the valley in Minnesota is comparatively narrow, the marginal channels are discontinuous. Marginal channels mapped closer to the St. Croix River tend to be at lower elevations where either flood scour was greater during later stages of a single catastrophic flood, or deeper scour occurred during a younger large magnitude flood. The associated sediment assemblage overlies glacial drift and bedrock. The drift/bedrock surface cut by catastrophic floods, exhibits greater relief than surfaces defined by the sluiceway due to younger infilling of depressional areas following catastrophic flooding. The sediment assemblage is variable, ranging from peat more than 5 m thick to coarse material less than 1 m thick. Peat formed in bogs occupying more poorly drained former channels. These areas are interpreted to be fed by groundwater moving down and laterally from the adjacent uplands. Early and late peat tends to be woody, whereas peat intermediate in peat sequences tends to be fibrous. Biogenic calcite is a very minor constituent, and organic muck is limited to the base of the peat, if present at all. Coarse material in channels primarily was deposited during waning flood currents. Fine material represents local tributary overbank deposits and sheetflood deposits that post-date catastrophic flooding. Some surfaces of lower marginal channels immediately adjacent to the modern St. Croix River have a younger veneer of relatively fine overbank deposits.

Marginal channels are differentiated on the type and thickness of the sediment assemblage. Coarse material may be greater than 2 m thick (CMCCO, CMCCO>), less than 2 m thick (CMCCO<), or absent to discontinuous and thin (CMCCO-). Similarly, fine material may be greater than 2 m thick (CMCF>) or less than 2 m thick (CMCF). Fine material often overlies coarse material. This sediment assemblage with a loess cover is further differentiated if it is greater than (CMCFCL, CMCFC>L) or less than (CMCFC<L) 2 m thick. There are also cases where this sediment assemblage is overlain by type "a" overbank deposits (CMCFCA). One LfSA has fine type "a" overbank deposits overlying peat (CMCFPA). Peat or organic matter may be greater than (CMCP>) or less than (CMCP) 2 m thick. Peat may also overlie fine material (CMCPF) or coarse material (CMCPC). Finally, in a number of marginal channels, surface material is absent or discontinuous, but there is a lag of boulders (CMCR-).

The aforementioned features and their distribution, coupled with the underlying sand and gravel deposits over reddish brown, rhythmically bedded, glaciolacustrine sediments exhibited beneath peat in marginal flood channels, strongly suggests a catastrophic flood origin. A very early Holocene age is proposed for the catastrophic flood responsible for cutting the St. Croix Valley as it exists today, and forming the major geomorphic features of the valley. The flood was triggered by events in the Superior Basin during the Marquette advance, ca. 9,950 – 9,700 B.P. (Clayton 1983). Radiocarbon ages on wood from the base of thick peat near, but not at, the axis of a marginal flood channel are 9,340 ± 60 B.P. (Beta-107069) and 9,360 ± 80 B.P. (Beta-107071) (Figure E-94). From another marginal channel, uncarbonized wood collected from the contact of peat over a thin sediment increment transitional to the underlying sand and gravel yielded an age of 10,120 ± 100 B.P. (Beta-107107) (Figure E-97). These ages suggest that peat began forming immediately after the marginal flood channel was cut. The hypothesis that the entire valley as it exists today was formed by a single catastrophic event at the Pleistocene – Holocene transition is bolstered by the basal radiocarbon age from the previously mentioned Lake St. Croix (Lund and Banerjee 1985:805-806) and the new date of 8,370± 70 (Beta-107078) reported herein (Appendix E). In the broader context of the Mississippi Valley, Hajic and Bettis (1997) identified a marker bed of reddish brown clay (Lake Superior Basin origin) preserved at the base of broad flood channels. The marker bed dates to between 9,900 and 9,700 B.P. And was linked to a catastrophic flood from the Lake Superior Basin that entered the Mississippi Valley via the St. Croix Valley. Some of the marker bed clay could have been derived from the glaciolacustrine deposits cut into by St. Croix Valley catastrophic flood channels.

Paleovalley landscape. Paleo-valleys are valleys that support underfit tributary streams of the St. Croix but were not formed by them. The few examples mapped occur in, and adjacent to, the lowest reach of the St. Croix valley. The example adjacent to the St. Croix valley (YPCN) is undifferentiated in terms of origin. The others (YPC, YPCL) were formed by ice disintegration, which implies that they did not necessarily carry meltwaters, or act as a river valley. The ice disintegration valleys formed indiscrimanently within the Catastrophic Flood LsSA and postdate flooding. Materials vary within these valleys, but are only generally differentiated. Typically they have coarse textured fills of ice-contact material greater than 2 m thick. Because they are in the lower reaches of the valley, loess may mantle or be incorporated in fill within the valley (YPCL).

Valley Terrace landscape. A range of degrees in tributary valley incision is expressed between and within valleys as they traverse scarps separating one Catastrophic Flood feature from another. Although low terraces are undifferentiated from floodplains at this scale, high terraces are often distinct and mappable. These high terraces are distinguished based on whether the tributaries are flowing through primarily bedrock (TVT) or glacial lobe (PVT) geomorphic regions, or flowing within the St. Croix Valley (VT). Tributary valleys and terraces are inset into Catastrophic Flood features and postdate large-magnitude flooding. Multiple terrace levels were probably cut rapidly following formation of the St. Croix Valley, as tributaries adjusted to new regimes and lengths. Later terrace formation may have occurred as courses and gradients changed with the development and growth of peatlands and alluvial fans.

Floodplain landscape. The St. Croix River is a major underfit stream that is very restricted in lateral movement. It occupies the deepest scoured Catastrophic Flood channel in the valley and is unable to effectively erode its inherited channel. Downstream of Lake St. Croix, the deepest Catastrophic Flood channels are occupied by a riverine lake. The floodplain above Lake St. Croix similarly is very narrow and discontinuous. It primarily is characterized by a lack of, or muted, lateral channel migration features, and is therefore classified as type "y" floodplain (FFY). Numerous small islands are similarly mapped as floodplain or island (FI). Only one reach of natural levee is mapped where there is a very slight rise to the landscape paralleling the channel (FNL). Fine textured alluvium overlies coarse textured alluvium with the sequence being greater than 2 m thick and laterally continuous. The alluvium postdates catastrophic flooding, but the thickness of the coarse textured material and nature of the basal contact with flood deposits currently is unknown. In the tributaries, floodplain and low terraces are undifferentiated (FFN). Where tributaries flowed across catastrophic flood features to reach the St. Croix River (FR), larger tributaries support their own meander belts (FMB). Where not deeply incised, belts of overbank deposits flank the meander belts. Tributary textures currently are not differentiated along the St. Croix (FOB).

Valley Margin landscape. Both alluvial fans and colluvial slopes are represented along the St. Croix Valley, having formed after the rapid incision of the valley by catastrophic floods. Fans (MAFC, MAFF) and slopes (MC) were not recognized upstream of the Kettle River valley. Most overlie the Catastrophic Flood LsSA, but a few fans bury and interfinger with the floodplain LsSA. The largest fans tend not to occur at the foot of the valley wall, but rather at the foot of scarps within the Catastrophic Flood LsSA. Streams may produce more than one fan as it drops from one Catastrophic Flood surface to another. Textures, although undifferentiated, are usually coarse, or fine with coarse clast inclusions. Alluvial fans and colluvial slopes postdate catastrophic flooding. Alluvial fans are differentiated on whether they overlie catastrophic flood LfSA’s (MAFC) or floodplain LfSA’s (MAFF).

12.10.4.2 Landform Sediment Assemblage Codes

Table 12.9 provides details on each of the specific LfSA codes used for the St. Croix River valley model.

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12.10.5 Landscape Suitability Rankings

If the origin of the St. Croix Valley was by catastrophic flooding sometime during the first few centuries of the Holocene, then the full range of LsSA’s are temporally suitable for containing cultural deposits. However, the vast majority of the valley is composed of catastrophic flood LfSA’s for which depositional environments would have been unsuitable for preservation, much less habitation. Excluding post-flood deposits, it is not possible (0) for bar, terrace, marginal channel, erosional residual, and strath terrace LfSA’s to have naturally buried cultural deposits (CBCO, CBCOK, CER, CMCCO, CMCCO-, CMCCO<, CMCCO>, CPC, CST, CSTCO, CSTR, CTCO, and CTCOK).

Where loess is incorporated into Catastrophic Flood LfSA’s (CBFCL, CBFCLK, CERL, CMCFC<L, CMCFC>L, CMCFCL, CPCL, CSTFCL, CSTL, CTFCL, and CTFCLK), the 0-1 m interval is ranked high (3). This ranking is warranted because eolian sedimentation of loess is ideal for burial and preservation of cultural deposits, and the loess must postdate about 9,700 B.P. Although not taken into account in the Landscape Suitability Rankings (LSR’s), these LfSA’s are likely to have been subjected to localized sheetwash erosion and sedimentation and biomantle formation processes, given their ages. This applies whether or not loess is present.

The situation is a little different for marginal channels not filled with flood sand and gravel. Where fine sediment is present (CMCF, CMCF>), a ranking of moderate (2) is applied to a depth of 2 m. Where the fine material is greater than 2 m thick, a ranking of low (1) is assigned to the interval greater than 2 m. The origin of the fine material may be related to the latest stages of flood flow; however, in many cases, it is clearly related to St. Croix River overbank sheetflood deposits (CMCFCA, CSTFCA), tributary overbank deposits, localized sheetwash deposits, and upland-derived wash beyond the limits of alluvial fans. All of these environments are conducive to preservation of cultural deposits.

Where peat is present in marginal channels (CMCP, CMCJP>, CMCPC, CMCPF), cultural deposits as old as late Paleoindian could be buried deeply, particularly on the flanks of the marginal channels. The hydrologic history of the marginal channel basins is unknown, but these paleo-landscape positions would have been adjacent to whatever wetland resource was present in the local basins. As peat continued to grow, these positions would become buried. Figure E-93 indicates a time-transgressive basal age for the peat, as well as earlier peat formation along the valley wall compared to the central part of a marginal channel. The latter further suggests the adjacent upland as an important source for water feeding the wetlands. In a few instances, overbank sediments from the St. Croix River shallowly buries peat near the channel (CMCFPA). A low (1) ranking is extended to the 2 m depth in this later case. Peat growth appears to span the Holocene into the Historic period, although basal ages are likely to vary widely (Figure E-93). Peatland would be a location for only isolated artifacts because of the very poorly drained conditions, but the preservation potential is excellent. Prehistoric cultural deposits associated with the peat are likely to be buried; the landsurface is modern and ranked not possible (0), and the upper 2 m of peat is ranked low (1) because of the poor drainage conditions.

The Paleovalley landscape is somewhat varied in terms of age. but nothing is known about these valleys. The paleo-valleys formed by ice disintegration processes are determined to postdate catastrophic flooding because they are inset into catastrophic flood terraces. The examples adjacent to the St. Croix Valley were probably cut during early stages of the catastrophic flood. All afford moderate to poorly drained conditions. To err on the side of caution, these LfSA’s are ranked low (1) for the 0-1 m depth interval. However, where silt loam is present and interpreted as loess, this interval is ranked high (3), and the underlying meter is ranked low (1).

Terraces in tributary valleys postdate catastrophic flooding because they are inset into Catastrophic Flood LfSA’s, but nothing else is known about their age. The associated sediment assemblage can be greater than 2 m thick, but often is less than 2 m thick. A low (1) ranking is assigned to the surface of the terraces.

The age of the limited floodplain of the St. Croix River is unknown, but it presumably started developing shortly after the valley formed. Sediment assemblages of the Floodplain landscape are greater than 2 m thick, and typically are comprised of coarse sediment, or fine sediment overlying coarse sediment. All floodplain LfSA’s except islands (FI) are ranked moderate (2) to a depth of 2 m and low (1) below 2 m based on drainage conditions and depositional environments. Islands are ranked low (1). Examination of floodplain sediments (FFY) at a few locations indicated little change in depositional conditions below the 2 m depth, so the moderate (2) ranking is extended below 2 m. Overbank deposits, especially post-Euroamerican contact in age, were not distinguished during this study, but it appears that if any are present, they are either thin or discontinuous.

Deposition of alluvial fans and colluvial slopes was initiated shortly after the valley formed. Beyond this, nothing is known about their age or internal structure. Presumably, like other fans elsewhere in Minnesota, sheetflood processes dominate and the potential for burial of cultural debris is great. Alluvial fans and colluvial slopes are ranked high (3) for their entire thickness.

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12.11 ANOKA UPLANDS

12.11.1 Introduction

The mapped quadrangles (Centerville and Linwood) are situated on the Anoka Sand Plain (Wright 1972), a broad, sandy area associated with the Grantsburg Sublobe of the Des Moines Lobe. The Anoka Sand Plain is situated behind the Pine City end moraine. The sand plain is characterized by a sandy surface marked by several indistinct, north-northeast to south-southwest trending, broad to narrow troughs and substantial peatlands. Chains of lakes and accumulations of peat often mark the troughs. The area chosen for mapping encompasses some of the Pine City end moraine to the east and a trough, sand plain, and peatland to the west.

12.11.2 Overview of Past Work

The Des Moines Lobe entered Minnesota and Iowa from Canada and North Dakota, flowing along a pre-existing topographic low. The history of the Des Moines Lobe is one of repeated surging with intervals of stagnation (Kemmis 1991). The Grantsburg Sublobe surged off the northeast flank of the Des Moines Lobe to occupy a somewhat depressed area that had previously been overridden by an advance of the Superior Lobe (Wright 1972). Final retreat and wastage of the Des Moines Lobe and Grantsburg Sublobe was rapid. During stagnation, a karst drainage system developed in the Des Moines Lobe ice that ultimately determined many of the features of the modern landscape - sediment assemblages (LsSA). This ice karst system is not readily apparent on the Grantsburg Sublobe and the till is relatively thin. The troughs occupied by chains of lakes are interpreted to be palimpsest, reflecting former tunnel valleys related to flow beneath Superior Lobe ice (Meyer and Patterson 1997).

The Anoka Sand Plain was interpreted as an outwash plain because of its sandy character (Cooper 1935; Wright 1972), although Stone (1965; 1966) interpreted lacustrine sediments beneath the sand plain. More recently, the plane-bedded sand and finer-texture beds have been interpreted as a former lake plain of Glacial Lake Anoka (Meyer 1993; Meyer and Patterson 1997; Meyer, in prep.). Three levels of the hypothesized glacial lake and associated outlets are recognized. Outlets and lake levels associated with the two lower levels are believed to have been controlled primarily by downwasting of stagnant ice dams, and incremental incision of the St. Croix River (Meyer 1993). An alternative hypothesis essentially renews the outwash plain hypothesis. If the nearby St. Croix Valley was formed rapidly due to catastrophic flooding relatively late in deglacial history, as geomorphic and temporal information suggest (see Section 12.10), then the latest increments of the Anoka Sand Plain could have formed from outwash predating the incision, or early stages of flood flow prior to downcutting, or both. This could explain the washed character of large areas of the Pine City Moraine, as well as the cutting of a scarp presumed to be a shoreline of Glacial Lake Anoka. Presumed lake outlets simply may be flood overflow channels. A focused investigation emphasizing sedimentology of key locations on the Anoka Sand Plain will be required to definitively determine the origin of the sand plain and associated geomorphic features.

Activity of the Des Moines Lobe has been tightly bracketed by radiocarbon ages in Iowa and by radiocarbon ages post-dating retreat and stagnation of glacial ice in Minnesota. This lobe entered Iowa shortly before 15,000 B.P. By 13,800 B.P., It reached its maximum extent at the position of the Bemis Moraine and the city of Des Moines (Bettis et al. 1996). By 13,500 B.P., the lobe had stagnated, re-advanced to the position of the Altamont Moraine, then stagnated again. A final advance in Iowa reached its maximum extent at the position of the Algona Moraine about 12,300 B.P. The glacier again stagnated and rapidly wasted. By 12,000 B.P., glacial ice was no longer active in Iowa.

Meyer (in prep) has reviewed available radiocarbon evidence from the Anoka Sand Plain and related areas. Radiocarbon ages on wood from sand geomorphically below the lowest level of Glacial Lake Anoka are in the 11,700 to 12,000 B.P. range. Two radiocarbon ages, on samples presumably associated with slackwater sediments overlying the "middle" terrace level of the Mississippi Valley in the Twin Cities area, yielded ages of about 11,800 (on wood) and 10,200 (on peat) (Wright and Rubin 1956). If the St. Croix Valley formed around the Pleistocene – Holocene transition, as suggested in Section 12.10, then some of the features on the Anoka Sand Plain could be as young as earliest Holocene in age, as earliest catastrophic flood waters would have flowed through the basin, possibly eroding and/or depositing sediment. Assuming the aforementioned radiocarbon ages are representative of the deposits on the "middle" terrace in the Mississippi Valley, although there is much lacking in contextual information, their antiquity still does not preclude younger flood waters from coursing across the higher Anoka Sand Plain and spilling into the Mississippi Valley.

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12.11.3 Mapping Methods and Data

High altitude color infrared aerial photographs from the USGS NAPP program were the primary source of geomorphic information. Tonal contrasts on these air photographs were sufficient to delineate features even finer than those mapped. The Anoka and Chisago county NRCS soil surveys were useful in supplying subsurface textural information. Mapping was done independent of previously available maps of the area and partly because of the difference in scale of the pre-existing maps and of the mapping being done, and partly because of the desire to map without reference to existing geologic models of the area. Mapping was accomplished by thin pencil directly on USGS 7.5' topographic maps. Many of the geomorphic boundaries were transferred to the maps from mylar overlays of NAPP aerial photography, with appropriate adjustments for distortion of the aerial photographs. No fieldwork was conducted in the mapped area.

12.11.4 Landform Sediment Assemblages (LfSA)

Landform sediment assemblages of the Anoka Sand Plain are outlined in Table 12.10 and discussed below.

12.11.4.1 Landscapes

Active Ice landscape. The active ice landscape is associated with the Pine City Moraine. The Pine City Moraine is an end moraine located on the southeast side of the mapped area. A generally low relief plain (PIP) characterizes this area, with few hummocks (PIHU) that rise above the general plain. There are a few hummocks mapped to the west of the general limit of the moraine. Although surrounded by a glaciofluvial plain that is at least in part collapsed, they are included with the Active Ice landscape. Both landforms are underlain by a coarse textured diamicton representing till associated with the Pine City Moraine. These surfaces are considered susceptible to localized sheetflood erosion and sedimentation, as well as biomantle evolution. Two small disturbed areas were mapped on the moraine (PIDI).

Glaciofluvial landscape. Located west of and parallel to the belt of lakes lies a narrow sandy plain. Smaller sandy glaciofluvial remnants occur within the lake belt and to the west of the narrow but continuous plain. All these remnants are part of the larger Anoka Sand Plain. Beneath the glaciofluvial plain (OP), coarse sandy material is generally greater than 1 m thick, but the sandy surface is susceptible to pedoturbations to a depth of about 1 m. A few depressions exist on the outwash surface (OD). These have a thin veneer of coarse post-glacial fill overlying sandy material. The thickness of the depression fill is less than 1 m. A smattering of outwash occurs on the Pine City Moraine in the south central part of the mapped area (POP). In these cases, outwash is thin and discontinuous. Because of the proximity of some of these units to eolian dunes, this sand may have an eolian origin as well. A few building sites and sand quarries represent disturbed areas mapped on the glaciofluvial plain (ODI).

Peatland landscape. The Peatland landscape is widespread, particularly in the northwestern part of the mapped quadrangles and in the collapsed channel that supports the lake belt. These surfaces are lower in elevation than the Glaciofluvial or Active Ice landscapes. The Peatland landscape forms a plain, without classic bog landforms. The plain has three map units that are differentiated on the basis of peat thickness and the texture of underlying material. Relatively thick peat, greater than 2 m thick (BP), is the most extensive unit. Many smaller isolated areas have thin peat (less than 2 m thick) underlain by glaciofluvial sand (BPSB). A paleosol is often developed on the sand and buried by this thin peat. A rare situation is where relatively thin peat (less than 2 m thick) overlies silt, which in turn overlies sandy loam (BPFC). In this case, the silt could be lacustrine in origin. Smaller patches of peatland developed on the Pine City Moraine in the east central part of the mapped area. Peat can be greater than 2 m thick (PBP), less than 2 m thick and overlie sand (PBPSB), or less than 2 m thick and overlie sandy loam (PBPSLB). A buried soil developed in the top of the coarse material is typical in the latter two cases.

Floodplain landscape. Alluvial valleys on the mapped quadrangles are limited to several low order stream valleys on the Pine City Moraine. The larger valleys have narrow floodplain and channel belts that are indistinguishable. Undifferentiated floodplain (PFF) is mapped in these valleys. Alluvial fill has a loam texture, reflecting the surrounding moraine, and is greater than 2 m thick. The remaining low order drainageways are shallow and intermittent with ill-defined floodplains (PFDA). They have loam to loam diamicton fill that is less than a meter thick overlying till. There is one polygon mapped as an undifferentiated floodplain landform (FF). This FF landform is located within the collapsed channel, and represents a short valley between two lake basins. It is underlain by loam in excess of 2 m thick.

Lacustrine landscape. Many lakes occur in the mapped quadrangles (LLN). They are most frequent and largest in the collapsed channel. Several equally large round to subround lakes occur to the northwest of the glaciofluvial plain. These are likely the result of ice block melting. Also northwest of the glaciofluvial plain, there are a number of smaller, irregular lakes associated with the peatland plain. There are less than ten small lakes on the Pine City Moraine (PLLN). The two largest lakes are in subround to round basins and may have been formed by ice block meltout. Exposed lake beds almost always support marshes (LLBMA) that occur around many of the lakes in the lake belt and on lower flanks of the glaciofluvial plain. In these locations, peat is greater than 2 m thick, and buried soils are likely to be present. On the Pine City Moraine, several of the lake basins have exposed lake beds that support marshes (PLLBMA). These exposed lake beds are less extensive than on the glaciofluvial plain. All of the larger lake basins have continuous to discontinuous shoreline features associated with them (LSH, PLSH). In most cases, these are beaches composed of coarse material greater than 2 m thick. Also in most cases, there are units of exposed lake beds found between the existing lake and the shore features. This geographic position indicates higher lake levels than in the past. It is also possible, and likely, that there were lower lake levels as well.

Eolian landscape. Eolian dunes may occur either singly or in clusters, and are located on both the glaciofluvial plain, and more extensively within the peatland to the west of the plain. In this latter location, the dunes are being buried by peat growth. The glaciofluvial plain upon which the dunes reside in this Peatland landscape is already completely buried. Eolian dunes are composed of sand and are generally greater than 2 m thick (EED). The potential for buried soils is great. There are a limited number of cases where the arrangement of dunes created interdunal depressions. Sand is also greater than 2 m thick in these locations. Dunes are limited on the Pine City Moraine (PED) and are presumed to have blown onto the moraine from the west.

12.11.4.2 Landform Sediment Assemblage Codes

Table 12.10 provides details on each of the specific LfSA codes used for the Anoka Sand Plains model.

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12.11.5 Landscape Suitability Rankings

Till deposited by active ice and incorporated in Active Ice LfSA’s has no suitability for in situ, naturally buried cultural deposits because of both age and depositional environment factors. The cultural deposits may have been deposited on older till plain or hummock surfaces and subsequently shallowly buried by surficial sheetwash sedimentation or biomantle formation. Thus, the surface is ranked possible, and the 0-1 m depth of the PIP and PIHU LfSA’s have low Landscape Suitability Rankings (LSR’s). The likelihood of cultural deposits being affected by these processes increases with the age of the cultural period. If biomantle formation proves to be a significant process in this part of Minnesota, then the resulting buried cultural deposits are not in situ, a fact that will have to be considered when evaluating site significance. The PIDI LfSA has a LSR of not possible (0) for all depth intervals because of its disturbed character.

The outwash plain (OP, POP) LSR’s are moderate (2) for the 0-1 m depth interval because of the strong likelihood surficial sheetwash erosion and sedimentation, eolian modification, and biomantle formation may have buried cultural resources in a moderately to well drained environment. For similar reasons, depressions on the outwash plain (OD) are ranked low (1) for the 0-1 m depth interval because of potential infilling by sheetwash sediments in a generally wet or poorly drained environment. Disturbed areas on outwash (ODI) are ranked not possible (0).

The Peatland LsSA seems an unlikely location for buried cultural deposits because of the poorly drained conditions; however, they are assigned low (1) LSR’s throughout the upper 2 m of peat because these environments have a local history for big game kill sites. Artifacts are expected to be sparse within the peat and have excellent preservation. Because so little is known about the history of water table fluctuations, the LSR’s are conservative. Where the Peatland Plain peat is greater than 2 m thick, (BP, PBP) the increments below 2 m depth are ranked as not possible (0) because of the relatively low landscape position. Where peat is less than 2 m thick and underlain by apparent lacustrine sediments over outwash (BPFC), a similar ranking is applied. Where peat less than 2 m thick is underlain by outwash (BPSB), the 1-2 m depth interval is ranked moderate (2). This ranking is because of the presence of a buried soil starting at the outwash surface that reflects an entire buried landscape of which we know little. Under similar Peatland environments on the Pine City Moraine (PBPSB, PBPSLB), the 1-2 m depth interval is ranked low because of the poor drainage situation and the smaller size of the peatland patches with better drained ground nearby.

The floodplain LfSA’s (FF, PFF) are ranked low (1) to the 2 m depth in low order alluvial valleys with relatively thick fill. The floodplain LfSA (PFDA) is ranked low (1) to the 1 m depth in low order alluvial valleys with relatively thin fill. The low (1) rankings reflect overall poorly drained conditions, but favorable sedimentation processes for burial.

The lake LfSA (LLN, PLLN) and exposed lake bed LfSA’s (LLBMA, PLLBMA) are considered to have some potential for buried cultural deposits. The unknown history of lakes on the Anoka Sand Plain could include substantial lake level fluctuations, particularly rising levels through time, given the thickness of peat accumulation. Former lake shoreline features may be buried. For this reason, the 0-1 m depth interval of lake LfSA’s is ranked low (1), with the deeper underlying deposits ranked as not possible (0). The interval of low (1) rank implies that Historic sedimentation has been less than a meter, which is an untested assumption. The ranking is low (1) because of the overall poor drainage conditions. Similarly, the exposed lake bed LfSA’s have low rankings to a depth of 5 m. The greater depth for possible LSR’s in exposed lake bed LfSA’s compared to that in lakes is due to the exposed lake beds being slightly higher topographically. Lake shore LfSA’s (LSH, PLSH) are ranked moderate (2) for the 0-1 m depth interval and low (1) for the 1-2 m depth interval. The moderate ranking is a balance between the locally better drainage condition afforded by raised shoreline features, and the generally sandy texture that is more easily bioturbated.

Eolian dunes (EED, PED) are ranked high (3) for the 0-1 m depth interval, and moderate (2) down to a depth of 5 m. The high and moderate rankings reflect well drained conditions and the likelihood of preservation of cultural deposits through dune growth and migration (although migration also can cause erosion of cultural deposits). The moderate (2) ranking is taken to the 5 m depth to emphasize that while not all dunes are this thick, they can be. Interdunal depressions (EDP) are given a low (1) ranking for the 0-1 m depth interval only. This is based on the more poorly drained local conditions surrounded by landforms with well drained conditions.

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12.12 BEMIDJI UPLANDS MODEL

12.12.1 Introduction

Two USGS quadrangles—Bemidji East and Bemidji West—were mapped as part of this project area model. The landscape covers a variety of landforms and geologic glacial settings. At the time of mapping little deep geologic coring was accomplished. Most of the subsurface data was derived from some coring previously accomplished as part of the Upper Mississippi River model. Originally, the Bemidji East quadrangle was the most upstream map within the Mississippi River project area model (Section 12.4). Only a small portion of the quadrangle was mapped where the Mississippi River Valley joined up with Lake Bemidji. Since these first river models were developed, the Bemidji East quadrangle has been separated and the rest of the "uplands" were mapped in their entirety.

12.12.2 Overview of Past Work

Wright (1972) indicated that the Itasca end moraine, just south of the Bemidji area, is correlative in time with the St. Croix end moraine and both are approximately 20,000 B.P. in age. Clayton and Moran (1982) indicate that the New York Mills margin is also correlative in time with the Itasca and St. Croix margins, and that these three advances are closer to approximately 15,000 B.P. Most authors are skeptical or less detailed about what is happening in the Bemidji area between 15,000 and about 12,000 B.P. By about 12,000-11,700 B.P., Clayton and Moran (1982) indicate that the ice front had receded north of the Bemidji area.

At 11,300 B.P., Clayton and Moran (1982) show the ice margin just north of the Bemidji area where meltwaters could have deposited the uppermost sand along this large plain. Sackreiter (1975) also suggests that the ice advance from the northwest was responsible for the last outwash sheet when this ice receded. He further suggested that the "meltwater channels" carved into the till uplands were last occupied during this recession. Clayton and Moran (1982) show Lake Koochiching has developed by 11,300 B.P. By 11,100 B.P. The ice margin has receded into Manitoba, and Lake Koochiching has joined with Lake Agassiz. The last major sedimentation within the project area probably occurred between approximately 12,000 and 11,300 B.P.

The DNR (1998) 1:100,000-scale GIS geomorphology map indicates three ice margins are recognized in the Bemidji project area, and that the large relatively flat terrain starting at Bemidji and extending northward is part of the Koochiching lobe Geomorphic Association. The DNR maps some of this plain as having superglacial material. Their three recognized ice margins to the south of this plain include: the Wadena lobe’s Itasca ice margin, mostly along the fringe of the southernmost Bemidji project area; the Koochiching lobe’s Guthrie ice margin, constituting most of the southern and eastern half of the combined Bemidji East and West quadrangles; and the Koochiching lobe’s Erskine ice margin, which starts abruptly just off the western edge of the Bemidji West quadrangle. The DNR (1998) mapped the large esker on the Bemidji West quadrangle (west of Bemidji) as belonging to the Wadena lobe’s Itasca ice margin and called it an "outwash channel." Presumably, they interpret it to be an esker inside an outwash "channel," because this landform has positive relief.

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12.12.3 Mapping Methods and Data

High altitude color infrared photography (1:20,000 USGS NAPP) was used in conjunction with the NRCS soil series maps and aerial photography to map the Bemidji Uplands area. The Beltrami County soil series was complete, whereas the Hubbard County soil series was a set of hand-drawn maps that had not yet been completed nor proofed. The relatively high relief landscape coincided neatly with the aerial photographic signatures. Most of the textural data was obtained from the NRCS soil series, although MnDOT did perform some coring in the area for the Upper Mississippi River valley model (Section 12.4).

The Upper Mississippi Valley model originally included some of the Bemidji East quadrangle. Later, as the "Upland" model areas were being selected, the Bemidji region was chosen because of proposed highway and city road work in this archaeologically sensitive vicinity. The Bemidji East quadrangle has subsequently been removed from the Upper Mississippi model, and they have been edge-matched and joined. For now, they are separated for discussion purposes and also because a greater level of mapping sophistication was employed for the Bemidji models in 1998 than the Upper Mississippi models in 1997. For example, the Bemidji Upland models use the USDA texture terminology instead of more general grain-size terms.

12.12.4 Landform Sediment Assemblages (LfSA)

Landform sediment assemblages of the Bemidji Uplands are detailed in Table 12.11 and described below.

12.12.4.1 Landscapes

Stagnant Ice landscape. Much of this modeled landscape could fall into the Active Ice landscape if one interprets the high ground as belonging to an end moraine. The current interpretation is that of a ground moraine with a thin to thick supraglacial deposit of sands and gravels. One large pitted esker (SEKH) is located in the Bemidji West quadrangle. The DNR interprets that this esker must have been deposited as part of the Wadena lobe recession, which means that the subsequent Koochiching lobe that they mapped either overrode it without destroying it or mysteriously avoided it. Depressions with varying thickness of peat or organic muck were mapped on both the esker (SDMAO< and SDMAO>) and the till uplands (SDMAS< and SDMAS>). Stagnant ice plains are also distinguished by having different near-surface textures or boulder/cobble lag deposits at the land surface (SPOU, SPOUS, and SPROU).

Glaciofluvial landscape. This landscape has 11 different LfSA’s. The first two are hillslope landforms with hillslope colluvium or biomantle and that have either a boulder or cobble lag deposit (OHRH), or no lag (OHH). These two LfSA’s are mapped along the outwash channel banks such as those forming the Necktie River Valley. Coarse-grained kame deposits with and without hillslope colluvium are also recognized along the higher till plain on the Bemidji East quadrangle (OKH and OK). A large variety of outwash paleochannels (or meltwater channels) are also recognized along the upland and lowlands of this area. They are further distinguished into LfSA’s by their near-surface textures and the corresponding thickness of these near-surface deposits (OPC-, OPCCF, OPCMA, OPCMA<, and OPCMA>). Many of these paleochannels are thought to be related to the Glaciolacustrine model discussed below and are not well represented at this scale of USGS topographic mapping. These paleochannels are at various elevations on the Bemidji East quadrangle and are best seen in person. The highest channel is at approximately 1385 ft above mean sea level (asl) and is south of the Mississippi River in the northeast corner of the mapping area. Other nearby channels have channel grade elevations of 1365, 1355, and 1345 ft. The latter elevation belongs to the Necktie channel and is the next lowest channel bottom to the current water elevation of the Mississippi River reservoirs (1335 ft). Also, of possible importance is the relatively gentle side-sloped walls of the very highest channels, which may indicate that the these channels were not necessarily cut by Catastrophic floods or surges. The sediments within these higher channels are coarse grained sands and gravels, but are very thin. This would suggest that the waters carving these higher channels were either short-lived, and/or had little sediment load. If these waters were derived from a nearby glacial lake then that could explain the relatively low sediment load (see discussion in the next paragraph). The OPCC LfSA is an ancient albeit small channel within the broad lowlands on the west side of Lake Bemidji. This valley appears to be collapsed on itself and may represent the last traces of a subglacial drainage flowing into a larger remnant of an ancient lake or depression. The OT LfSA is an outwash terrace and is found within the Necktie paleochannel.

Glaciolacustrine landscape. Much of this landscape could have easily been (and has been by other researchers) classified as part of the Glaciofuvial landscape. The somewhat radical idea that a Glaciolacustrine landscape is present here is supported by two main facts. First, the steep escarpments (AHH, AHRH) that may or may not be mantled by a boulder lag; and second, the need for a body of water to be high enough in elevation to cut the outwash channels on the Bemidji East quad discussed in the above paragraph. The current model envisions an ice front somewhere off, but not too far off, to the west that has prevented drainage to the north or west, and thereby forcing meltwater trapped between itself and the steep escarpments (AHH and AHRH) to the east. This eastward flow over the "uplands" is apparently how the higher and relatively gentle side-sloped channels at 1385, 1365, and 1355 ft developed. The primary evidence against this model is the apparent lack of lacustrine sediments within the uppermost several meters of the Glaciolacustrine plain LfSA’s (APCOU, APMAP, APMAPC, APOU) and the interpreted wave-cut platform (AWCOU). The wave-cut LfSA is an untested hypothesis, and it could easily belong with either the Glaciolacustrine or Glaciofluvial plain LfSA, depending on further field investigations. Regardless, the uppermost strata in these Glaciolacustrine LfSA’s are currently interpreted to be of Glaciofluvial origin. The APCOU LfSA appears to be an irregular outwash surface, which has collapsed as a result of the melting of buried ice blocks. This LfSA could be an ancient end moraine that has been buried by subsequent geologic processes, possibly related to the damming of the glacial lake. The APMAP and APMAPC LfSA’s have natural, shallow standing water with peat thickness greater than 1 m or less than 1 m overlying coarse textures, respectively.

Valley Terrace landscape. Three LfSA’s are recognized at this scale of mapping. The first is an undifferentiated Valley Terrace along Grant Creek in the northwest corner of the mapping area. The VVE LfSA is the most common and is made up of "v"-shaped valleys that drain the till uplands. The VVMA LfSA is also a "v"-shaped valley that may have intermittent flow, but mostly contains a marshy peat deposit of undifferentiated thickness.

Floodplain landscape. Seven LfSA’s have been identified at this scale of mapping with the project area. The FFN LfSA (undifferentiated floodplains and terraces) is found within the Mississippi River channel between Lake Bemidji and the eastern edge of the Bemidji uplands mapping area. Floodplains probably dominate this LfSA, therefore it was placed in the Floodplain landscape instead of the Valley Terrace landscape. The FFNMA LfSA dominates the Mississippi River Valley between Lake Irving and the western edge of the Bemidji uplands mapping area. This LfSA is characterized by having intermittent standing water and peat deposits (marshes or bogs) on the undifferentiated floodplains and terraces. The three paleochannel LfSA’s are found within either the Mississippi or Grant Creek floodplains. One paleochannel was recognized at this scale within the delta upstream from Lake Irving. The last two LfSA’s are the active river channels and the riverine lakes, which are connected by the Schoolcraft and the Mississippi rivers.

Lacustrine landscape. Nine Lacustrine LfSA’s are recognized at this scale of mapping. Some of these could have been interchangeable with the Floodplain landscape such as the LDEMA landform, which are deltas located at the upstream ends of Lake Marquette and Lake Irving. These larger riverine lakes also have submerged islands (LIWS) and submerged terraces (LTWS). The submerged terraces were mapped at between 5 and 10 ft depths below the current lake surface. All the larger riverine lakes have this peculiar submerged terrace, which was not tested during the Mn/Model field work. These submerged terraces may result from the supraglacial or englacial sediment shedding off the remnant ice blocks that formed these depressions, and filling in the recently voided areas around the edges of the melting ice block (where ice once occupied). These ice-block kame terraces are similar to other kame terraces except for being ring-shaped and developing around the circumference of stagnant ice blocks. Alternatively, these terraces could also have been formed during lower water levels of these large lakes. The remaining LfSA’s are common to both geographically small and large lakes, such as peninsulas (LPE; which could also be mapped as part of the uplands or surrounding lake landforms), or undifferentiated terraces (LT). The others are distinguished by the underlying strata, such as outwash or till (LLNO, LLNS) or thickness of peat under a terrace (LTMA, LTMA>). The LLNO LfSA’s are associated with the large esker landform on the Bemidji West quadrangle.

12.12.4.2 Landform Sediment Assemblage Codes

Table 12.11 provides details on each of the specific LfSA codes used for the Bemidji Uplands model.

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12.12.5 Landscape Suitability Rankings

The majority of the Bemidji upland area is interpreted to be too old for geologically buried archaeology sites, or at a minimum, at the fringe of a oldest possible age (ca. 12,000 B.P.) as indicated by the Minnesota SHPO archaeologist. Deeply buried sites are interpreted to be any in situ cultural resource that is deeper than the standard 1 m deep shovel test routinely currently performed by the archaeological community.

All eight Stagnant Ice LfSA’s have the possibility to contain in situ cultural resources at the land surface (0 m depth). Four of these (SDMAO, SDMAO<, SDMAS, and SDMAS<) are currently interpreted to have a low potential for suitable landscapes at the 0-1 m depth interval because of relatively recent peat formation. SEKH is interpreted to have a moderate chance for a suitable landscape at the 0-1 m depth interval because of the apparent hillslope colluvium on this relatively steep-sided pitted landform. The SDMAO and SDMAS LfSA’s have a low probability for a suitable landscape at the 1-2 m depth interval because of the thickness of peat.

All eleven Glaciofluvial LfSA’s are interpreted to have the possibility of in situ cultural resources at the land surface (0 m depth). Seven of these (OHH, OHRH, OK, OKH, OPCMA, OPCMA<, and OPCMA>) are currently interpreted to have a low potential for suitable landscapes at the 0-1 m depth interval because of either peat growth or hillslope colluvium. Two of these (OPCMA, OPCMA>) extend their possibilities for suitable landscapes to depths greater than 1 m, once again because of peat thickness; however, they remain low rankings because of the poor drainage conditions.

All seven Glaciolacustrine LfSA’s are interpreted to have the possibility of in situ cultural resources at the land surface (0 m depth). Four of these (AHH, AHRH, APMAP, and APMAPC) are currently interpreted to have a low potential for suitable landscapes at the 0-1 m depth interval because of either peat growth or hillslope colluvium. Only APMAP has any interpreted potential to have suitable landscapes between 1-5 m depth, and this is because of peat growth. The low ranking is assigned because of the poor drainage conditions.

Two of three Valley Terrace LfSA’s (VT and VVMA) are interpreted to have the possibility of in situ cultural resources at the land surface (0 m depth). The VT LfSA has high to low chances for suitable landscapes from 0-1 m to 2-5 m depth intervals. This grading is because of the greater chances for overbank sediments to be nearer to the land surface and higher energy fluvial or glaciofluvial deposits to be at depth. The VVMA LfSA has low potentials for suitable landscapes between 0-2 m depths because of peat formations, which indicates poor drainage conditions. The VVE LfSA is interpreted to be eroded at the land surface; therefore, the chances of finding in situ cultural deposits on the surface are considered impossible. Furthermore, this "v"-shaped valley is cut into older strata, which makes the chances of finding suitable landscapes impossible at depth.

Six of seven Floodplain LfSA’s (all but FR) are interpreted to have the possibility of in situ cultural resources at the land surface (0 m depth). The riverine lake (FRL) LfSA’s "surface" is interpreted to be on top of the first deposited sediment under the water surface. This LfSA is also treated as if it could have had variable beach lines through time, which could have created more terraces than are currently recognized at this scale. The FFN LfSA is ranked similarly to the VT LfSA mentioned above, although the variability is probably greater because of the known but unmapped complex of terraces and floodplains. The active river channel (FR) is the only current Floodplain LfSA that has no interpreted chance for suitable landscapes. The rest of the LfSA’s have low chances for suitable landscapes at varying depths depending upon the depth of infilling in paleochannels or depths of peat. Regardless, these landforms are in poorly drained settings, which explains the low rankings.

Five of nine Lacustrine LfSA’s (LDEMA, LPE, LT, LTMA, and LTMA>) are interpreted to have the possibility of in situ cultural resources at the land or substrate surface (0 m depth). Those that do not have a chance for surficial resources (LIWS, LLNO, LLNS, and LTWS) are interpreted to have wave-modified surfaces, or very recent peat growth, which would create a newer surface. The Lacustrine LfSA’s have the greatest chances for suitable landscapes. A complicated "expert system" ranking involves evaluating the textures, buried soils, thickness, square area, geologic process dynamics, and drainage conditions. Buried soils would raise the LSR’s to high regardless of the sedimentary textures because the soil had to form under relatively low energy environments; however, as in the case of the delta (LDEMA), the paleosols are probably peat, and peat deposits are interpreted as a low LSR because of poor drainage conditions. The peninsula (LPE) LfSA has a great variety of possible LSR’s because it was mapped in this area purely on landform criteria, and not landform sediment assemblage criteria.

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12.13 GLACIAL LAKE AGASSIZ BASIN MODEL

12.13.1 Introduction

The Fargo South, Sabin, Glyndon South, Downer, and part of the Hawley Southeast USGS quadrangles were mapped for the Glacial Lake Agassiz Basin model. The Fargo South quadrangle was also partially mapped as part of the Red River of the North model (Section 12.7). Modifications were made to the code and style of mapping between the time that the Red River was modeled in 1997 and when the Glacial Lake Agassiz basin was modeled in 1998. The overview of past work is similar for both model areas.

12.13.2 Overview of Past Work

The DNR (1998) has mapped the uplands on the eastern edge of this Mn/Model project area as part of the Red River lobe’s Erskine ice margin. Both till plains and supraglacial materials have been mapped by the DNR as part of the Erskine ice margin. This moraine may date to sometime around 11,700 B.P. based upon the discussion and figures presented in Clayton and Moran (1982) and would represent the last surficial ice-contact deposit of the project area.

The Glacial Lake Agassiz basin and its geological history have been investigated by researchers starting with Upham (1895), and progressing with Leverett (1932), Johnston (1946), Harris et al. (1974), Teller and Thorleifson (1983), Clayton (1983), Fenton et al. (1983), Teller (1985), Harris et al. (1995, 1996) and Teller et al. (1996), just to name a few. More of the early references are provided in Bannatyne et al. (1970), and the Glacial Lake Agassiz volume edited by Teller and Clayton (1983).

Clayton and Moran (1982) interpret that the advent of Glacial Lake Agassiz occurred around 11,700 B.P. when the Red River ice lobe began retreating northward from the Big Stone moraine south of the current model area. The paleogeography maps produced by Thorleifson (1996) indicate that ice was still present over the model area at ca. 11,500 B.P., But by ca. 11,200 B.P., The ice had retreated out of the project area and Glacial Lake Agassiz had inundated this terrain. By ca. 10,900 B.P. The Glacial Lake Agassiz shorelines had moved to the north of the project area and remained so until some time between 10,100 B.P. (at Georgetown, Minnesota; Section 12.7 of this report) and ca. 9,900 B.P. (Thorleifson 1996). The Poplar River Formation, which was a fluvial unit deposited when Lake Agassiz was absent from the area, formed between ca. 10,900 and 9,900 B.P. Glacial Lake Agassiz returned as a result of an ice advance blocking northern and eastern glacial lake outlets, and produced the glaciolacustrine Sherack Formation between ca. 10,100 and 9,300 B.P. near the project area.

Reid and Olson (1996, 1994) have described the postglacial Red River alluvium (Oahe Fm.) from cores near the town of Halstad, Minnesota. These authors indicate that the Red River entrenched into the lake plain as early as 9,000 B.P., And then began to aggrade. At 7,500-5,000 B.P. during the Altithermal, a depositional hiatus took place. Michlovic (1985, 1986, 1987) reports radiocarbon dates of approximately 3,400 B.P. from an unpublished buried archaeology site (less than 2.0 m depth) at Halstad, and between 4,000 and 3,000 B.P. From the Canning Site (approximately 1 m depth) near Hendrum, Minnesota. Other dates may exist in the "gray" literature of either cultural resource or geotechnical projects; however a dearth of publications exists for the Red River strata and its geological history.

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12.13.3 Mapping Methods and Data

High altitude color infrared photography (1:20,000 USGS NAPP) was used in conjunction with the NRCS soil series maps and aerial photography to produce the Glacial Lake Agassiz basin model. The Clay County NRCS soil series model was used to assign subsurface data to the landforms. The color infrared photographic signatures coincided neatly with the relatively low relief landscape of the Agassiz basin, and moderately well with the high relief landscape of both the beach ridges and till plain uplands identified along the eastern side of the modeled area. The topographic maps had contour intervals of five and ten feet, and were also of value when identifying the beach ridges and terraces. The Agassiz basin models use a combination of the USDA and a more general textural terminology. This combination effectively displayed the major differences of textural trends within the current Agassiz basin model. The general texture terms were used where a mosaic of different but similar textures would have produced an inordinate amount of additional work for apparently little gain in information. Facies models are not specifically handled by the current Mn/Model code system, but may be readily interpreted from the textural data. Some fieldwork was done in conjunction with the Red River Model; however, no fieldwork or ground truthing was accomplished for this model.

12.13.4 Landform Sediment Assemblages (LfSA)

Landform sediment assemblages of the Glacial Lake Agassiz Basin are detailed in Table 12.12 and discussed below.

12.13.4.1 Landscapes

Active Ice landscape. Parts of this landscape could have been easily interpreted as a Stagnant Ice landscape based upon the proximity to major outwash landforms and some of the smaller landforms. For now, the current interpretation is of an Active Ice landscape although admittedly stagnant features may occur such as Depressions (IDMA). The dominant LfSA’s are variations on the Active Ice plain. Some of these variations occur when the LfSA has not been eroded (IP), has been eroded (IPE), or has a intricate mosaic of both eroded and uneroded surfaces, called an erosion complex (IPEC). A few places were documented by the NRCS as having possibly received some hillslope colluvium (IPH). The remainder of the Active Ice plains are variations on having been wave or current modified such as IPWA, IPWACO, IPWAE, and IPWAEC. These landforms are interpreted to be modified first by a high water stage of Glacial Lake Agassiz, and then in some cases they were modified by subaerial erosional forces acting on these wave modified land surfaces (i.e., IPWAE and IPWAEC). The IPWAEC LfSA also has an erosion complex similar to IPEC mentioned above. The last LfSA is a hillslope (IH), which has relatively steep high relief.

Glaciofluvial landscape. The landforms in this LsSA are all part of a paleo-valley subregion. This paleovalley subregion may be part of the Erskine ice margin advance and eventual stagnation. The paleovalley heads into the till plain to the south-southeast. This entire paleovalley has been wave or current modified and is distinguished by having either sand (OPC, OPCE) or fines (OPC-, OPCE-) present at the land surface, and whether it has been eroded since the wave modifications (OPCE, OPCE-). The OPC and OPCE LfSA’s both have at least a 2 m thickness of sand. The other two LfSA’s are more recently "eroded" down to a diamicton.

Glaciolacustrine landscape. Six different landforms and 20 different LfSA’s are present within this landscape. The six recognized landforms are compaction ridge, depression, escarpment complex, plain, beach ridge (spit, cusp, or shore), and wave-cut platform. The compaction ridge LfSA’s, are distinguished between each other by having either a collapsed surface (ACRCA), or a non-collapsed surface (ACRNA). The collapsed surface could have been labeled a depression, but was distinguished from the other depressions because they may actually be old grown-over sand quarries; but they may also be remnants of ice bergs stranded and partially buried on the west side of this topographically higher landform. Only one depressional LfSA (ADMA) has been recognized and is typified by having marshy deposits. Sometimes this LfSA was assigned to ancient tributary valleys that have "in-filled" with peaty strata. The third landform, an escarpment complex, has two LfSA’s that can be distinguished by having either a recently eroded surface (AECEH), or relatively non-eroded surface (AECH). An escarpment complex may have multiple terraces, platforms, and escarpments that are too small to recognize at this scale of mapping (1:24,000), but overall, the escarpment is the dominant landform. Thirteen LfSA’s make up the glaciolacustrine plain. Some of these 13 LfSA’s are separated based upon their geomorphology such as the APCA being a collapsed landform (which again may be an old grown-over quarry), APDDEC having an abandoned dendritic drainage pattern expressed on the surface, and APIA-, APIAF, APIAL/S and APIASIL-L having linear, reticulated or orbicular surface patterns. These latter three are further distinguished by different textures. The remaining seven glaciolacustrine plain LfSA’s are distinguished by their textures and thicknesses of overlying sediments. APA and APA< are distinguished by having "no distinctions" made for their textures, and having either greater than or less than 1 m thickness of overbank strata, respectively. Both of these LfSA’s are located next to larger streams or rivers and have variable textures. APAF has silt loam to silty clay loam textures and is located next to smaller tributary confluences. APASIL-L is distinguished by its silt loam to loam texture. The two LfSA’s with loam over sand textures are distinguished by their thickness of the overlying "overbank" sediment. APAL/S and APAL/S< have greater than and less than 1 m thicknesses, respectively. APAL-S has loam to sand textures, and is the glaciolacustrine plain LfSA closest to the major glacial lake escarpments. The GIS view of this project area model indicates a textural trend from fine to coarse away from the Red River valley and toward the uplands to the east. This trend can be seen by the "banding" of LfSA’s, which are aligned in a north-south direction. The AWC LfSA is a wave-cut platform that is probably cut into glacial drift including till, outwash, and glaciolacustrine deposits. Two topographically distinct wave-cut platforms were recognized at this scale of mapping. The ASHH LfSA is what most workers have commonly called the beach ridges of Glacial Lake Agassiz. Some of these so-called beach ridges appear to have been sculpted by water more typical of underwater currents, and may represent spits, or near-shore bars.

Paleovalley landscape. YFN is distinguished in this model by being an ancient set of floodplains and terraces belonging to the Buffalo River. This LfSA may be a deltaic landform remnant caused by the Buffalo River depositing into Glacial Lake Agassiz while it was at a lower stage. The YDMA are depressions within a paleovalley interpreted to be an outwash valley created by the last glacial ice to have reached this area. This LfSA could have been labeled within the outwash landscape.

Valley Terrace landscape. Three LfSA’s are recognized in this landscape. The VPC includes both abandoned "v"-shaped tributary valleys and paleochannels within the larger tributary valleys. Part of this complication arose at the time these landforms were being mapped. The Lake Agassiz basin was one of the few mapped areas that had these abandoned "v"-shaped valleys. These codes were lumped together at the time of mapping and digitizing. The VFN LfSA includes tributary valleys that have floodplains and possibly terraces that were not mapped at this scale (1:24,000). The VV LfSA is a "v"-shaped valley with actively or intermittently flowing water.

Floodplain landscape. Six LfSA’s were interpreted within the Red River valley, and one of these LfSA’s includes the river channel (FR) for both the Red River and the South Branch of the Buffalo River. The FFXA1 LfSA is a type "x" floodplain at the lowest topographic level. The type "x" floodplains may have the remnants of the channel migration features evident, but the channels themselves may not have been occupied other than when the current active channel reaches overbank stage and floods the other floodplain landforms. The FFYA2 and FFYA1 LfSA’s are both type "y" floodplains, but are at different topographic levels. The FFYA2 could have been interpreted as a valley terrace; however, because of the difficulty in determining if the surface was a "terrace" or a slump block, and compounded with the fact that these higher surfaces are frequently flooded at least once per year, the decision was made to lump them into the floodplain landscape. Clearly, many of these FFYA2 surfaces are related to a higher water stage of the Red River although their ages may not be so clear. Channel migration features are not evident in the FFYA2 surface, but these landforms are commonly inundated during flood stage. The FPCA2 and FPCA1 LfSA’s are paleochannels that are differentiated only by their topographic position in the landscape. The FPCA2’s are located on, or equivalent to the FFYA2 LfSA elevations. Some of these paleochannels are clearly acting as a "chute" or sluiceway during the initial flood stage.

Valley Margin landscape. MAFA is the only LfSA within this landscape. As an alluvial fan, it is extremely rare within the Red River valley given the relatively low relief of the basin. These facts could indicate that this LfSA was incorrectly interpreted, and that it may actually be some sort of slump block. Conversely, modern alluvial fans have developed within Midwest valleys (Hudak 1989) as a result of post-EuroAmerican settlement farming practices, which could have produced the "odd" fan.

Lacustrine landscape Natural lakes (LLN) are the only LfSA interpreted within this landscape. These lakes are located within the boundaries of the glacial lobe (uplands) at the east end of the project area. Most of these lakes appear to be shallow and therefore have had the opportunity for variable shoreline configurations.

12.13.4.2 Landform Sediment Assemblage Codes

Table 12.12 provides details on each of the specific LfSA codes used for the Glacial Lake Agassiz Basin model.

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12.13.5 Landscape Suitability Rankings

Two major environments exist within the modeled area, the till uplands and the glacial lake plains/beaches. The tills are interpreted to be too old to contain geologically buried cultural resources. The Sherack and Poplar River Formations are both interpreted to be young enough; however the depositional environment is generally regarded to be subaquatic. The possibility that shorelines were at any one geographic point at a particular moment in time is low, although the shorelines were certainly at any one point during at least two moments in time. The landscape suitability rankings were therefore interpreted to be low although still possible for the glaciolacustrine sediments.

The Disturbed Areas are mostly quarries and would normally be ranked as impossible to still contain cultural resources after the disturbance; however, because the "cuts" are presumably through the Poplar River Formation or other coarse-grained unit, and may be into the Glacial Lake Agassiz formations, a low possibility still remains for these strata. The LSR’s have, therefore, been assigned a low probability between 0-2 m depth.

The majority of the Active Ice landscape is considered too old and geologically unsuitable for any naturally buried cultural resource. The resources can, however, be found at the land surface if the land surface is not eroded. The eroded LfSA’s such as IPE and IPWAE do not have any chance for in situ resources to remain on the surface. The IPEC and IPWAEC have erosional complexes; therefore some areas may have eroded surfaces and others may not. The LSR’s have been given conservative estimates in these cases. The IPH LfSA has a hillslope colluvium deposit; therefore the LSR is moderate for the first 2 m depth.

The Glaciofluvial LsSA has only two LfSA’s with any possibility for containing in situ cultural resources on the surface or at depth. The OPC and OPC- LfSA’s are apparently not too eroded at the land surface, although the underlying strata are too old and deposited in too high an energy environment to preserve naturally buried cultural resources. The OPCE and OPCE- LfSA’s are both eroded at the surface and once again the underlying strata are too old and deposited in too high an energy environment to preserve naturally buried cultural resources.

The Glaciolacustrine LsSA could be argued that the land surface is too young because of the recently recurring flood deposits. This model uses a more conservative estimate because modern flood limits have not been adequately mapped, although the potential is there for future modification of this LSR. Only the AECEH was interpreted as an impossible LSR at the land surface because it has been modeled by the NRCS staff as eroded. The remaining LfSA’s all have a possible age at depth. Typically, high LSR’s are given to LfSA’s with overlying overbank sediments. The high rankings apply to the surfaces immediately below the overbank deposits. These surfaces may be on older overbank deposits, or the ancient glacial lake strata. The depth of the high rankings depends on the model’s interpretations of overbank thicknesses. Moderate rankings are typically associated with colluvial deposits, or slightly higher energy environments such as near-shore environments. Low LSR’s are reserved for "wet" or higher energy environments. The impossible rankings are given to those that are too high in energy to preserve in situ resources, and mostly refers to sand or coarser textures. Sometimes the model interprets that the shoreline environments could not be within the strata at particular depths, and then assigns a "zero" to the depositional environment ranking.

The YDMA LfSA is a depressional marsh, and therefore is assigned low LSR’s to the 2 m depth, and an impossible ranking below 2 m because of the glacial drift environment and age. The YFN LfSA has moderate (2) rankings to 2 m depth because of the variable ages and textures within the different terraces and floodplains.

The Valley Terrace LsSA assigns high rankings to the VFN LfSA because of the ongoing sedimentation during annual flooding. The paleochannel landform (VPC) has a possible chance for the depositional environment to 2 m depth because of the subsequent in-filling since being abandoned; but is ranked low (1) because of the wet, poorly drained conditions. The VV LfSA is similar to the VPC LfSA for the same reasons, except that the VV surface is still being eroded or modified (hence the impossible surface ranking).

The Floodplain rankings all have possible ages except for the active river channels’ surface. The Floodplain LSR’s are generally higher for the topographically higher or older appearing landforms. The more recently created landforms have a relatively lower ranking because of less chance for flood deposits to have covered or mantled the potential cultural resources, and also because the lower surfaces or channeled surfaces within these floodplains receive higher energy and the chances for more frequent flood scouring. The FR LfSA has no chance (0) because of the depositional environment being primarily high energy, wet, cut-and-fill, or reworked slump block sediment.

The alluvial fan LfSA (MAFA), if indeed an alluvial fan, needs to be tested. This landform may have high (3) potential for buried resources because of its relatively low energy depositional environment.

The natural lake ages are interpreted to be acceptable for containing cultural resources except at the substratum surface, which is interpreted to be deposited annually and therefore too young. The lakes are also interpreted to be shallow and prone to shoreline configuration changes through time, which might mean that some of these basins are very likely to contain buried sites when the lakes were smaller. These sites are then submerged and buried during more wet long-term climates when the lakes grow in size.

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12.14 LAKE BENTON UPLANDS MODEL

12.14.1 Introduction

Lake Benton is located on the southwest flank of the Des Moines Lobe in southwest Minnesota. The two quadrangles selected for mapping—Lake Benton and Tyler—cover a range of glacial landscapes. In the southwest corner of the area lies the dissected southwest flank of the Bemis end moraine, a moraine deposited by active glacial ice. Abutting it to the northeast lies the loess-mantled southwest flank of the Bemis moraine. The crest and the northeast flanks of the Bemis moraine do not have a loess mantle. The northeast flank of the Bemis descends to the Altamont ground moraine characterized by a supraglacial drift complex deposited by stagnant glacial ice. Lake Benton is a large northeast – southwest trending lake that occupies a former englacial or subglacial channel that exited through the Bemis end moraine to the southwest.

12.14.2 Overview of Past Work

The Des Moines Lobe entered Minnesota and Iowa from Canada and North Dakota, flowing along a pre-existing topographic low. Along the southwest flank of glacial ice in Minnesota, where Lake Benton is located, the flow was against the regional slope onto the Coteau des Prairies. The history of the Des Moines Lobe is one of repeated surging with intervals of stagnation (Kemmis, 1991). Final retreat and wastage was rapid. During stagnation, a karst drainage system developed in the ice that ultimately controlled many of the details of the modern landscape sediment assemblages (LsSA).

The Quaternary Geologic Map of Minnesota (Hobbs and Goebel, 1982) maps a relatively wide Bemis end moraine for the project area. To the southwest there is a belt of shale-bearing, loess-mantled, late Wisconsinan drift slightly older than the Bemis Moraine. A belt of Bemis ground moraine is mapped to the northeast of the end moraine that totally encompasses Lake Benton. To the northeast lies a wide belt of Altamont/Algona stagnation moraine.

Patterson (1997) has most recently named the geomorphic regions atop the Coteau des Prairies. Her text is the state-of-the-art discussion for southwestern Minnesota, and earlier publications on past regional work. Patterson (1997) interprets the Lake Benton, Lake Shaokatan, and Lake Hendricks linear troughs as tunnel valleys that breached the Bemis Moraine. She also identified the Verdi till plain of the Des Moines lobe to the southwest of the Bemis Moraine.

The Minnesota Geological Survey (MGS) undoubtedly used Patterson’s (1997) work as the basis for mapping the Lake Benton area at a 1:100,000 scale as part of the Minnesota Department of Natural Resources geomorphology coverage of the state (DNR 1998). MGS recognizes two divisions of the Bemis Moraine similar to those mapped in 1982. The DNR’s (1998) Bemis Moraine is narrower than that mapped by Hobbs and Goebel (1982) and has a till plain sediment association. The Verdi Moraine, also with a till plain sediment association, forms the southwest flank of the Bemis Moraine. It is transected by a few drainages mapped as outwash. Northeast of the Bemis Moraine crest, a belt of undifferentiated Wisconsinan till plain with a few outwash channels is mapped (DNR 1998). An undifferentiated Wisconsinan supraglacial drift complex is mapped covering the north and east part of the Lake Benton area. "Undifferentiated" is probably used because the Altamont – Algona distinction cannot be confidently made. The supraglacial drift area is characterized further with the mapping of some "ice-walled lake plains", Holocene lakes and ponds, and a few Holocene peat deposits.

Activity of the Des Moines Lobe has been tightly bracketed by radiocarbon ages in Iowa and by radiocarbon ages postdating retreat and stagnation of glacial ice in Minnesota. The Des Moines Lobe entered Iowa shortly before 15,000 B.P. By 13,800 B.P. It reached its maximum extent at the position of the Bemis Moraine and the city of Des Moines (Bettis et al. 1996). By 13,500 B.P., The lobe had stagnated, re-advanced to the position of the Altamont Moraine, then stagnated again. The Flandreau valley, downstream of Lake Benton, was one of several valleys that carried proglacial meltwater away from the southwest flank of the lobe in the mapped vicinity. A final re-advance in Iowa reached its maximum extent at the position of the Algona Moraine about 12,300 B.P. This moraine is essentially indistinguishable from the Altamont Moraine in southwest Minnesota. The glacier again stagnated and rapidly wasted. By 12,000 B.P., Glacial ice was no longer active in Iowa.

Radiocarbon ages from deposits postdating the Grantsburg Sublobe in east central Minnesota mostly postdate 12,000 B.P. (Meyer in prep.). Glacial ice had retreated to the position of the Big Stone Moraine by about 11,700 B.P. (Fenton et al. 1983). Radiocarbon ages associated with the Moorhead Phase of Lake Agassiz in west central Minnesota indicate an age of about 11,000 B.P. Thus, active glacial ice apparently abandoned the Lake Benton area sometime between 12,000 and 11,700 B.P.; however, due to the presence of stagnant ice, the landscape continued to evolve, a supraglacial drift complex was deposited, and drainage patterns began to develop after 11,700 B.P.

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12.14.3 Mapping Methods and Data

High altitude color infrared aerial photographs from the USGS NAPP program were the primary source of geomorphic information. Tonal contrasts on these air photos were sufficient to delineate features even finer than those mapped. The Lincoln County NRCS soil survey was useful in supplying subsurface textural information. Mapping was done independent of previously available maps of the area, partly because of the difference in scale of the pre-existing maps and of the mapping being done, and partly because of the desire to map without reference to existing geologic models of the area. Mapping was accomplished by thin pencil directly on USGS 7.5' topographic maps. Many of the geomorphic boundaries were transferred to the maps from mylar overlays of NAPP aerial photography, with appropriate adjustments for distortion of the aerial photographs. No fieldwork was conducted in the mapped area.

12.14.4 Landform Sediment Assemblages (LfSA)

Landform sediment assemblages of the Lake Benton Uplands are detailed in Table 12.13 and discussed below.

12.14.4.1 Landscapes

Active Ice landscape. The active ice landscape is associated with the Bemis Moraine. As mapped here, it includes the Bemis and Verdi Moraines of the DNR mapping; however, the Bemis is differentiated into two major landform sediment assemblages (LfSA’s) that correspond with the two DNR moraines. The differentiation is based on whether loess is present on the active ice plain. The area corresponding with the DNR Verdi Moraine has thin (<1 m thick) loess (IPL) whereas loess is not recognized on the crest of the Bemis Moraine (IP). With or without loess, both areas are recognized as likely to have hillslope colluvium or biomantle present. There are a series of low sand and gravel rises on the plain on the southwestern limit of the Bemis Moraine crest (IHUSG). They probably represent locally preserved outwash remnants, but are included in the active ice landscape. A few closed depressions exist on the Bemis Moraine, and most of these are located at the heads of drainages. These depressions are differentiated on the basis of estimated thickness of postglacial fill (ID, ID<, ID-). There are a couple of terraced surfaces included in the active ice landscape (IPT). One large surface occurs on the side of the moraine adjacent to the western one-fourth of Lake Benton. The other occurs near the head of a short drainageway that exits the moraine just downstream of the Lake Benton outlet.

Stagnant Ice landscape. The stagnant ice landscape is associated with the Altamont (Altamont and Algona) ground moraine, or the supraglacial drift complex of the DNR mapping. This landscape covers most of the Lake Benton mapped area. Numerous hummocks and depressions with a poorly developed drainage system characterize this area. Low order swales and intermittent drainages define undifferentiated hummocks. Hummocks are underlain by a loam diamicton and mantled by a discontinuous thin unit of hillslope colluvium or a biomantle (SHU). There are a number of smaller hummocks, generally occurring on the margins of the larger ones, that have a thin veneer of sand or sand and gravel texture (SHUSG, SHUC). On the southwest edge of the stagnant ice landscape, there are a number of hummocks that were modified by meltwaters trapped between the proximal flank of the Bemis moraine and the Altamont ground moraine (SHUT). Some of these hummocks may be underlain by till related to the Bemis moraine. A number of hummocks are characterized by relatively flat tops, and are underlain by thin, stratified, silty clay loam (SIW). These are the plains of former ice-walled lakes. Some of the other undifferentiated hummocks may also be ice-walled lake plains. A number of narrow, prominent, almost always arcuate ridges are underlain by thin, discontinuous sandy loam over gravel. The ridges rise above ice-walled lake plains with which they are often associated. These ridges are interpreted as beaches and shorelines of ancient ice-walled lakes (SIB).

Depressions are undifferentiated as to origin, but it is likely that at least some are part of a linked depression system resulting from karst development in stagnant ice (Kemmis 1991). Depressions are underlain by a variable loam diamicton and have a variable thickness of undifferentiated sheetwash deposits overlying the loam (SD, SD<, SD-). Depression map units are differentiated on estimated thickness of this postglacial fill, which is generally less than 2 m thick. A buried soil is present in at least one depression LfSA (SDS) where substantial fill of Historic age buries the pre-Euroamerican settlement surface soil. Other depressions likely have sufficient thicknesses of Historic age fill that bury the pre-Euroamerican settlement surface soil.

Outwash landscape. The outwash landscape consists of discontinuous remnants of terraces, strath terraces, paleochannels and one outwash fan. Most mapped outwash is associated with valleys draining from either side of the Bemis end moraine. Some remnants of Bemis outwash occurs on the southwest part of the Altamont moraine, beyond the foot of the Bemis moraine.

Outwash terraces are underlain by sandy loam overlying gravel (OT and OTL) or sand (OTS). Some outwash terrace remnants in valleys draining to the southwest are overlain by a thin (<1 m) increment of loess (OTL). Strath terraces are underlain by thin, discontinuous loam overlying clay loam diamicton (till) (OST). Paleochannels have sandy loam sheetwash deposits overlying outwash gravel (OPC). One exception is underlain by sand (OPCS). Outwash paleochannels are differentiated on the thickness of the surficial sheetwash deposit (OPC>, OPC<, OPC-). They range from >2 m thick (OPC>) to thin (<1 m thick) and discontinuous (OPC-). The lone outwash fan (OOF) is on the proximal flank of the Bemis moraine. It is underlain by thin, discontinuous sandy loam overlying gravel.

Valley Terrace landscape. The valley terrace landscape consists of a number of discontinuous high terrace remnants (VHT). Although high, they tend to be inset below mapped outwash terraces, but this does not exclude the possibility that they are outwash-related. All but one example is associated with valleys draining the Bemis moraine. The exception occurs in the largest drainageway on the Altamont ground moraine in the mapped quadrangles. High valley terraces are underlain by undifferentiated coarse sediment.

Floodplain landscape. The floodplain landscape consists of undifferentiated floodplains in the low- to intermediate- order valleys draining both end and ground moraines. All floodplain units are underlain by fine-textured sediment. They are differentiated on the basis of thickness of alluvium overlying undifferentiated glacial material (FF-, FF<, FF, FF>), ranging from >2 m thick (FF>) to thin (<1 m thick) and discontinuous (FF-). Thicker alluvium tends to occur in the valleys draining the Bemis end moraine to the southwest. This is undoubtedly due to loess being eroded from hillslopes and redeposited in stream valleys. The thinnest alluvium, where it is <1 m thick and discontinuous, tends to occur in first and second order valleys draining the Stagnant Ice landscape.

Valley Margin landscape. The valley margin landscape consists of alluvial fans (MAF), colluvial slopes (MC), and hillslopes (MH). Four alluvial fans are located where low order tributaries enter intermediate order valleys, including the area just downstream of Lake Benton. A fifth is located at the foot of the proximal slope of the Bemis moraine. They are underlain by fine-textured sediment. Colluvial slopes occur at the foot of hillslopes in valleys draining the Bemis moraine. They are particularly well developed immediately southwest of the crest of the Bemis where valleys are deepest, slopes are steepest, and uplands are loess-mantled. Smaller colluvial slopes are present on the Altamont moraine at the foot of longer, steeper slopes of hummocks and ice-walled lake plains. Fine textured colluvium is generally <2 m thick. Mapped hillslopes occur in drainages on either side of the crest of the Bemis moraine, along the southwest end of Lake Benton, and along the Flambeau valley that carries drainage from Lake Benton. A probable thin (<1 m thick) and discontinuous mantle of fine-textured hillslope colluvium or a biomantle overlies till. The fine texture reflects the influence of redeposited loess.

Lacustrine landscape. Many of the depressions on the stagnant ice landscape supported lakes or wetlands during and after ice wastage. Most of these were drained and are now farmed. A few still support lakes or wetlands. Lakes, exposed lake beds, shoreline features, and reservoirs are differentiated in the lacustrine landscape. Lake Benton is the largest natural lake in the area. Although it likely functioned as a glacial stream outlet, it clearly was modified by ice stagnation as evidenced by its arcuate basin segments. The other mapped natural lakes (LLN) are much smaller and are all located on the stagnant ice LsSA. Exposed lake beds are further differentiated on the basis of thickness and material type of lacustrine sediments, as well as whether a marsh is present. Exposed lake beds are underlain by silt loam to silty clay loam lacustrine sediments (LLBF), or peat or organic muck wetland sediments overlying silty clay loam or undifferentiated fine lacustrine sediments (LLB). Wetland and lacustrine sediment is <2 m thick and overlies a clay loam diamicton that is part of the stagnant ice LsSA. Those lake basins now supporting marshes that are recorded on USGS 7.5' topographic maps are further distinguished (LLBMA). Most of these occur in association with the stagnant ice LsSA where some of the larger lake basins occupy former or active drainageways. Others occur in small basins at the heads of drainages on the Bemis Moraine. Shoreline features are associated with Lake Benton and small to large former lake basins associated with the stagnant ice LsSA. More shoreline features exist than were mapped, and some have to be shallowly buried by lake and wetland sediments. Shoreline features are differentiated on the basis of whether sedimentary texture could be stated with some confidence (LSHC) or not (LSH). Where these textures could be determined, they usually consist of sandy loam over gravel. Small reservoirs (LLR) are present where low order drainageways draining the southwest flank of the Bemis Moraine were dammed. An additional, slightly larger reservoir is present on the stagnant ice plain where it was constructed on part of an exposed lake bed.

12.14.4.2 Landform Sediment Assemblage Codes

Table 12.13 provides details on each of the specific LfSA codes used for the Lake Benton Uplands model.

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12.14.5 Landscape Suitability Rankings

Landscape suitability rankings for surface and buried archaeological sites in the Lake Benton area vary widely.

Till (generally loam to clay loam diamicton) in active ice LfSA’s has no suitability for in situ, naturally buried cultural deposits because of both age and depositional environment factors; however, cultural deposits may have been deposited on till plain or hummock surfaces and subsequently shallowly buried by surficial sheetwash sedimentation or biomantle formation. Thus, the IHUSG, IP, IPSG, and IPT LfSA’s have a chance for their surface to have cultural resources, and the 0-1 m depth interval has a low (1) LSR. The greater the age of the cultural period, the greater likelihood that they will be affected by these processes. If biomantle formation proves to be a significant process in this part of Minnesota, then the buried cultural deposits are not in situ, which is a fact that will have to be considered when evaluating site significance.

Where loess mantles the southwest flank of the Bemis end moraine, it is less than 1 m thick. Although the precise age of the loess in southwest Minnesota is unknown, at least part of it is likely to be contemporaneous with Paleoindian and possibly Early Archaic cultural affiliations. Loess may potentially bury or contain in situ cultural deposits of these ages. Sheetwash sedimentation or biomantle formation processes resulting in burial of cultural deposits of all periods may also have affected the near-surface loess. The IPL LfSA is ranked possible for the surface and moderate (2) at the 0-1 m depth increment. The moderate (2) ranking for loess reflects a general tendency towards slightly better drainage when compared to till (of those textures present) for the same landscape position.

Active ice depressions (ID, ID-, ID<) have a low (1) likelihood for having buried or surface cultural deposits in sheetwash deposits that overlie the till. Although sheetwash deposits are of appropriate age to contain cultural deposits, the low (1) ranking is based on generally poor drainage conditions in the depressions.

The Stagnant Ice LsSA has two key unknowns- the duration of stagnant ice conditions and the deposition of the stagnant ice complex. This model conservatively assumes that deposition of the stagnant ice complex continued long enough to overlap with at least the early periods of prehistoric human occupation. LSR’s for the various stagnant ice Hummock LfSA’s (SHU, SHUC, SHUSG, SHUT, SIB, SIW) are judged as possible for the surface and low (1) for the 0-1 m depth increments. This low (1) ranking is assigned because of the overall poorly drained conditions of the Stagnant Ice LsSA, although the hummocks would have been better drained than the depressions. The maximum of 1 m depth is due to the likelihood that the majority of the stagnant ice complex was deposited prior to any substantial human occupation. In addition to processes associated with the stagnant ice complex, such as various types of debris flows, sheetwash sedimentation and biomantle formation could also result in the burial of cultural deposits.

Stagnant ice depressions (SD, SD-, SD<) are similarly assigned a possible LSR for the surface and a low (1) ranking for having buried cultural deposits associated with sheetwash deposits overlying the till. The low (1) ranking is based on the poorly drained conditions of the depressions. Although substantial cultural deposits are not anticipated in these environments, they remain untested. The hydrologic history of this part of the state is unknown, although some regional paleoecological studies indicate substantial changes in water table elevations. Former shorelines likely exist in the subsurface and could have been favored locales for prehistoric cultural activities. One LfSA acknowledges the presence of a buried surface (SDS). Enough information was available to interpret the presence of post-Euroamerican settlement depression fill that buried the former landsurface. In this case, the possible depth of low (1) ranking was extended to 2 m depth, and the surface was assigned a LSR of not possible (0) due to the Historic age of fill.

Outwash LsSA’s generally are judged to have a possible LSR for having surface resources and a low LSR for buried cultural deposits to a depth of 1 m. Outwash terraces (OT, OTS) and fans (OOF) are underlain by either coarse-textured outwash or, in the case of strath terraces, till (OST) that essentially pre-dates human occupation. Where thin loess is present (OTL), the 0-1 m depth increment is ranked moderate (2) because of the slightly better drainage conditions. As with many of the previous surfaces that pre-date cultural occupation, there is a low (1) likelihood that cultural deposits could be buried by sheetwash sedimentation or biomantle formation. Outwash paleochannels are poorly drained, but many received postglacial sediments in the form of overbank or hillslope sheetwash sedimentation. The ranking for these postglacial deposits is low (1), with only the depth of that ranking varying based on the thickness of the postglacial deposits (OPC, OPC-, OPC<, OPC>, OPCS).

The High Terrace LfSA is judged to have a possible LSR for the surface and a low (1) ranking for buried cultural deposits. Drainage conditions and the age are acceptable for human occupation, although it is suspected that postglacial deposits form only a thin veneer on these surfaces.

Floodplains and underlying alluvium are judged to have a low (1) ranking, primarily due to unsuitable drainage conditions; however, where alluvium is greater than 1 m thick (FF, FF>), the 0-1 m depth interval is judged to have a moderate (2) ranking. In these cases, the thicker alluvium is often associated with either slightly better drained conditions compared to where thin alluvium is present, or a slightly higher floodplain level. The extent of post-Euroamerican settlement alluvium is unknown; if present, it would alter the surface LSR to not possible (0).

The Valley Margin landscape has variable LSR’s because of the range of valley margin LfSA’s. Alluvial fans primarily are greater than 3 m thick, and the larger ones are greater than 5 m thick. They are known locales for the preservation of prehistoric and historic cultural resources because of their episodic sedimentation. They also have moderate to well drained conditions. Burial of cultural deposits in fans is well documented in Minnesota. The Alluvial Fan (MAF) LfSA is assigned a high (3) ranking for the 0-1 m interval, and a moderate (2) ranking for depths greater than 1 m. This interpretation requires further testing because some fans may be mantled by post-Euroamerican settlement alluvium of variable thicknesses. Colluvial slopes (MC) are thinner, less than 2 m thick, and are not as well drained. They are ranked possible for the surface, and moderate (2) down to a depth of 2 m. Below this depth, they are ranked not possible (0) because of the thickness of deposits likely to be of age to contain cultural deposits. As with Floodplain LfSA’s, the presence or absence of post-Euroamerican settlement alluvium is unknown. The Hillslope (MH) LfSA is primarily an erosional feature. Where mapped, it is developed on till. The range of prehistoric activities would have been limited due to the slope angles of the mapped unit. The possibility of burial of any cultural deposits by local sheetwash erosion and sedimentation and by biomantle formation is, however, possible, although judged low (1) to a depth of 1 m only.

Exposed Lake Beds (LLB, LLBF) seem an unlikely LfSA for prehistoric occupation; however, because the potential for long-term variations in lake levels accompanied by shoreline feature development and burial remains completely unknown, these LfSA’s are assigned a low (1) ranking to a depth of 2 m only. This tends to be the maximum thickness of lacustrine sediments. One of the authors (CMH) believes LSR should be higher (2), and both authors believe that more testing of these LfSA’s is required. Exposed lake beds that support marshes have similar rankings for similar depth intervals, except the surface LSR is ranked as not possible (0) due to its Historic age. Lakes (LLN) are ranked similar to exposed lake beds, except the surface is judged to have a not possible (0) ranking, largely due to its Historic age. Shoreline features (LSH, LSHC) are ranked possible for their surfaces, and moderate (2) down to a depth of 2 m. In the case of the LSHC LfSA, a low (1) ranking is extended to a depth of 5 m. The possibility of buried or surface cultural deposits of Reservoirs (LLR) is judged not possible (0) due to the age and often disturbed character of the deposits.

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12.15 MOUNTAIN LAKE UPLANDS MODEL

12.15.1 Introduction

The Mountain Lake project area is located on the southwest flank of the Des Moines Lobe in southwest Minnesota and is to the southeast, or down-lobe, of the Lake Benton project area. The Mountain Lake quadrangle is situated on the undifferentiated Altamont and Algona moraine immediately inside of the associated end moraine. The general slope of the landscape is to the northeast. The most striking features of the quadrangle are two valleys, one ring-shaped, that originally had flow to the northwest and southwest and eventually joined the Des Moines Valley. The second valley crosses the southern half of the mapped area from west-southwest to east-northeast. These valleys originated as parts of englacial or subglacial valleys that were part of the glacial meltwater system. Today, drainage through the ring-shaped valley leads to a smaller valley that flows to the northeast.

12.15.2 Overview of Past Work

The Des Moines Lobe entered Minnesota and Iowa from Canada and North Dakota, flowing along a pre-existing topographic low. Along the southwest flank of glacial ice in Minnesota, where Mountain Lake is located, the flow was against the regional slope onto the Prairie Coteau. The history of the Des Moines Lobe is one of repeated surging with intervals of stagnation (Kemmis 1991). Final retreat and wastage was rapid. During stagnation, a karst drainage system developed in the ice that ultimately controlled many of the details of the modern landscape–sediment assemblages (LsSA).

The Quaternary Geologic Map of Minnesota (Hobbs and Goebel 1982) indicates undifferentiated Altamont and Algona ground moraine around Mountain Lake. Immediately to the west of the quadrangle, a belt of undifferentiated Altamont and Algona stagnation moraine is mapped. To the west and northwest lie the headwater areas of the Des Moines River.

The Minnesota Geological Survey (MGS) mapped the Mountain Lake area at a scale of 1:100,000 as part of the Minnesota Department of Natural Resources (DNR 1998) geomorphology coverage of the state. They recognize the area as undifferentiated supraglacial drift complex. The two main valleys are mapped as collapsed valleys belonging to the supraglacial drift complex.

Activity of the Des Moines Lobe has been tightly bracketed by radiocarbon ages in Iowa and by radiocarbon ages postdating retreat and stagnation of glacial ice in Minnesota. The Des Moines Lobe entered Iowa shortly before 15,000 B.P. By 13,800 B.P. It reached its maximum extent at the position of the Bemis Moraine and the City of Des Moines (Ruhe 1969; Bettis et al. 1996). By 13,500 B.P., The lobe had stagnated, re-advanced to the position of the Altamont Moraine, then stagnated again. A final re-advance in Iowa reached its maximum extent at the position of the Algona Moraine about 12,300 B.P. In southwest Minnesota, this moraine is essentially indistinguishable from the Altamont Moraine. The glacier again stagnated and rapidly wasted. By 12,000 B.P., Glacial ice was no longer active in Iowa.

Radiocarbon ages from deposits postdating the Grantsburg Sublobe in east central Minnesota mostly postdate 12,000 B.P. (Meyer et al., In prep.). Glacial ice had retreated to the position of the Big Stone Moraine by about 11,700 B.P. (Fenton et al., 1983). Radiocarbon ages associated with the Moorhead Phase of Lake Agassiz in west central Minnesota indicate an age of about 11,000 B.P. Thus, active glacial ice apparently abandoned the Mountain Lake area sometime between 12,000 and 11,700 B.P. However, due to the presence of stagnant ice, the landscape continued to evolve, a supraglacial drift complex was deposited, and drainage patterns began to develop during and after this time span.

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12.15.3 Mapping Methods and Data

High altitude color infrared aerial photographs from the USGS NAPP program were the primary source of geomorphic information. Tonal contrasts on these air photographs were sufficient to delineate features even finer than those mapped. The Cottonwood County NRCS soil survey was useful in supplying subsurface textural information. Mapping was done independent of previously available maps of the area, partly because of the difference in scale of the pre-existing maps and of the mapping being done, and partly because of the desire to map without reference to existing geologic models of the area. Mapping was accomplished by thin pencil directly on USGS 7.5' topographic maps. Many of the geomorphic boundaries were transferred to the maps from mylar overlays of NAPP aerial photography, with appropriate adjustments for distortion of the aerial photographs. No fieldwork was conducted in the mapped area.

12.15.4 Landform Sediment Assemblages (LfSA)

Landform sediment assemblages of the Mountain Lake Uplands are detailed in Table 12.14 and discussed below.

12.15.4.1 Landscapes

Stagnant Ice landscape. The Stagnant Ice landscape is associated with the Altamont (Altamont and Algona) ground moraine, just inside the end moraine. Overall, the topography is somewhat subdued, forming a plain with a gentle southwest to northeast slope (SP). The plain has very subtle broad ridges that run parallel with the dip of the landscape and very subtle shallow troughs that run parallel with the "strike" of the landscape. This patterning probably was inherited from structures in the stagnating ice. The plain is underlain predominantly by till with a clay loam diamicton texture. Locally, erosion has been moderate to severe, as evidenced by lighter tones on aerial photographs (SPE). These locations often coincide with crests of subtle ridges that run parallel with the overall slope of the landscape. Hummocks and depressions are limited in numbers, especially compared to the Lake Benton supraglacial drift complex. Both are underlain by clay loam diamicton. Locally, the few apparent hummocks (SHU) tend to be aligned with the "dip" of the landscape. Some exhibit evidence of moderate to severe erosion (SHUE). Linked depressions (Kemmis 1991) occur locally (SLD), but are not a major feature of the Mountain Lake landscape. Postglacial fill in the depressions is interpreted to be less than a meter thick. This fill typically consists of clay loam diamicton.

Glaciofluvial landscape. The Glaciofluvial landscape consists of a limited number of landforms on the Mountain Lake quadrangle. Two undifferentiated outwash terrace (OT) LfSA’s occur between the low angle junction of a pair of very subtle shallow troughs, which are parallel with the "strike" of the landscape. The terraces are underlain by thin, discontinuous sand. There are two paleochannels cut by outwash. Both are short abandoned segments associated with the valley draining the ring-shaped paleovalley They are underlain by loam overlying clay loam to loam diamicton. The loam represents post-abandonment fill derived from the sideslopes of the paleochannel and local fluvial reworking of the paleochannel floor.

Paleovalley landscape. The Paleovalley landscape is characterized by a range of LfSA’s that occur in both paleo-valleys. There is a series of hummocks in the paleo-valleys (YHUD) that rise to within several meters of the surrounding uplands. They are underlain by till with a clay loam diamicton texture. There is a second series of rounded hummocks (YHUT) at somewhat lower elevations than the YHU units. These lower hummocks are interpreted as having been modified by flowing water, although they are also underlain by clay loam diamicton till. At still lower elevations, there is a series of flat to gently sloping surfaces interpreted as strath terraces (YST). Multiple levels are represented. They are underlain by loam overlying till with a clay loam to loam diamicton texture. The loam is thin and discontinuous and is interpreted as a glaciofluvial deposit. Several paleochannels are represented within the paleo-valleys. Within the linear paleovalley, these occur in till-cored meanders along the valley wall. Paleochannels exhibit a sediment association similar to that of the strath terraces. Locally, postglacial sheetflood deposits may be present. Within the ring-shaped paleovalley, there are two constructed reservoirs (YDI), probably waste treatment lagoons for the town of Mountain Lake.

Floodplain landscape. The Floodplain landscape consists of undifferentiated floodplains in low order valleys, with several floodplain types represented in higher order valleys. In low order valleys and a series of strike-trending, shallow, outwash drainageways on the stagnant ice plain, floodplains are ill-defined and postglacial alluvial fill is thin and discontinuous (FDA). In only one case were coarse sediments present (FDACO). This occurs in a tributary to the ring-shaped paleovalley There is a series of steeper and shorter low order valleys that drain into the paleo-valleys. They are probably postglacial in age. These valleys are "v"-shaped with little or no floodplain (FV). Undifferentiated floodplains (FF) occur within the higher order stream valleys. In the Mountain Lake quadrangle, they occur mostly within the paleo-valleys. They are underlain by undifferentiated fine texture material in excess of 2 m thick. In two cases, the floodplains are underlain by coarse material (FFCO). One of these Floodplain LfSA’s occurs in a narrow valley oriented with the "dip" of the landscape in the vicinity of the town of Mountain Lake. The other occurs in a tributary to the ring-shaped paleovalley Within the ring-shaped paleovalley, there are several examples of type "y" floodplain (FFY). These areas clearly received overbank floodwaters and sediment, and may have been the locations of active channel belts at some time in the past. There are also several areas of type "z" floodplain within the same paleovalley (FFZ). Both are underlain by greater than 2 m of undifferentiated fine textured alluvium. Also within the same paleovalley, immediately upstream of Mountain Lake, there is a broad delta (FDE) consisting of a greater than 2 m thick fine textured material unit. The size of the delta suggests Mountain Lake may have been at higher levels in the past.

Valley Margin landscape. The valley margin landscape consists of alluvial fans (MAF), colluvial slopes (MC), and eroded hillslopes (MHE). Generally small alluvial fans (MAF) are located where "v"-shaped valleys and other low order valleys join the paleo-valleys. They consist of undifferentiated coarse material that is generally greater than 2 m thick. Its trunk stream incises the single somewhat larger fan, located in the ring-shaped paleovalley There is a narrow, discontinuous belt of colluvium at the foot of the walls of the straight paleovalley (MC). The colluvial slope LfSA is typically less than 2 m thick and consists of a coarse texture diamicton. Both paleo-valleys exhibit relatively steep valley sideslopes developed in till (MHE) that exhibit evidence of erosion on aerial photography. They possibly have a thin (<1 m thick) and discontinuous mantle of fine textured hillslope colluvium or a biomantle developed in till.

Lacustrine landscape. Multiple exposed (drained) lake beds occur within the paleo-valleys (LLB) that are now used for row crops. One small lake bed occurs in a linear depression on the stagnant ice till plain. In the straight paleovalley, the lake beds are comparatively substantial in size. The lake beds are underlain by wetland or lacustrine peat, which overlies lacustrine silty clay loam. These postglacial deposits are less than 2 m thick and overlie till. There is one area, associated with the largest exposed lake bed, where peat is not present (LLBF). Associated with this same relatively large exposed lake bed are shoreline features (LSH). Former shorelines are present in the downwind part of the lake bed. These shorelines are surrounded by exposed lake bed, suggesting they mark a former, relatively low lake stand; that a lagoonal situation existed behind the shoreline; or these "shorelines" are actually off-shore bars. Morphology of another unit is suggestive of a spit. In all cases, silty clay loam less than 2 m thick overlies till. The apparent lack of coarse material, particularly for what appear to be beach ridges is curious. This apparent lack suggests that an alternative origin of these features in the stagnant ice domain cannot be ruled out at this time. Two natural lakes (LLN) are present on the Mountain Lake quadrangle. One occurs in the ring-shaped paleovalley The other, smaller, lake occurs in association with the exposed lake bed on the stagnant ice till plain. Mountain Lake is currently a reservoir (LLR), although a natural lake occurred in the basin prior to being managed.

12.15.4.2 Landform Sediment Assemblage Codes

Table 12.14 provides details on each of the specific LfSA codes used for the Mountain Lake project area.

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12.15.5 Landscape Suitability Rankings

The duration of stagnant ice conditions and deposition of the Stagnant Ice LsSA is unknown. The deposition of the stagnant ice complex may have continued long enough to overlap with early periods of prehistoric human occupation, and this is taken into account in the age ranking for the Landscape Suitability Rankings (LSR). Processes in the stagnant ice landscape would have been dominated by debris flows and runoff of various types. The stagnant ice plain (SP), hummocks (SHU), and linked depressions (SLD) are ranked low (1) for both the surface (0 m) and 0-1 m depth interval. This low ranking reflects somewhat poorly drained conditions. The hummocks are poorly drained because of their flat to slightly concave upward tops. Hummocks would have been better drained than the depressions, but the rankings are considered both low. Major changes in the ground water table after the ice had melted are unlikely because the large valleys were probably present early in the evolution of this landscape. The low ranking is extended to only the 1 m depth due to the likelihood that the majority of the stagnant ice complex was deposited prior to any human occupation; plus the likelihood that there was local sheetflood erosion and sedimentation and/or biomantle formation that could have resulted in the burial of cultural deposits in the SP and SHU LfSA’s. Below the 1 m depth, cultural deposits are considered not possible (0) because of both age and depositional environment rankings. Where the plain and hummocks are moderately to severely eroded (SHUE, SPE), the ranking shifts to reflect the removal of material. Although the surface (0 m) is considered to have a possibility for cultural deposits, the 0-1 m interval is ranked as not possible (0) because of both age and depositional environment rankings. The age of the most recent erosional episode is likely Historic in conjunction with modern agriculture, although there is the possibility of previous episodes of erosion.

The Glaciofluvial LfSA’s are ranked similar to the Stagnant Ice LsSA’s. There are no post-depositional processes that would have eliminated the possibility of cultural deposits being present at the land surface; however, if present, they are not necessarily in situ. The 0-1 m depth interval is ranked low (1) for the OT LfSA because of the possibility of burial by either local sheetflood sedimentation and/or biomantle formation. In the case of the OPC LfSA, the low ranking at 0-1 m depth is because of a combination of overall somewhat poor drainage conditions and the possibility of accumulation of sheetflood sedimentation from paleochannel sideslopes. Biomantle evolution is a possible means of burial in this latter LfSA.

The Paleovalley LSR’s are the same as for the Stagnant Ice LsSA. The YHU, YHUT, and YST LfSA’s primarily consist of clay loam to loam diamicton that is interpreted as till or debris flow deposits. These deposits predate cultural occupation. Cultural deposits are possible for the surface with a low (1) ranking for the 0-1 m interval and a not possible (0) ranking below this depth. The 0-1 m depth interval is possible because of the possible burial of cultural deposits by local sheetflood sedimentation and/or biomantle evolution. The YPC LfSA is similarly ranked due to the possibility of burial by sheetflood sedimentation of material derived from paleochannel walls. In situ cultural deposits in disturbed areas of this landform (YDI) are not possible (0).

Floodplains and underlying alluvium of low order "v"-shaped valley streams (FV), shallow drainageways with ill-defined floodplains (FDA, FDACO), and low order valleys with coarse alluvium less than a meter thick (FFCO) are judged to have a low (1) ranking to a depth of 1 m. In most cases, this primarily is due to unsuitable drainage conditions. The FV LfSA energy conditions were too great to preserve any in situ cultural deposits, and landform geometry was not favorable for most activities. In contrast, type "y" (FFY), type "z" (FFZ), and undifferentiated floodplains (FF) of higher order valleys are judged to have at least a low (1) ranking to a depth of greater than 2 m based on the age of deposits and the low energy environment of overbank sheetflood deposition. The ranking is low, however, because of generally unsuitable drainage conditions. The delta entering Mountain Lake (FDE) is ranked as moderate (2) for the 0-1 m depth interval, and low (1) below 1 m. The presence or absence of post-Euroamerican settlement alluvium is untested, but if present, could shift the rankings up with depth.

Alluvial fans (MAF) in the mapping area are moderately well drained and further characterized by sheetflood sedimentation and the rapid burial of older fan surfaces. These combined characteristics result in a ranking of high (3) for the 0-1 m interval, and moderate (2) below 1 m. The higher ranking is assigned the 0-1 m increment because fans in at least the southern half of Minnesota exhibit late Holocene fan sedimentation rates that were less than middle or early Holocene rates. Colluvial slopes (MC) in the Mountain Lake area are less than 2 m thick and only moderately drained. They are ranked moderate (2) to a depth of 2 m. Below this depth, they are ranked not possible (0). As with Floodplain LfSA’s, the presence or absence of post-Euroamerican settlement alluvium is unknown. Eroded valley wall sideslopes (MHE) are developed in till and have a ranking of not possible (0) for all depth intervals. Furthermore, the range of cultural activities on this landscape position would have been limited due to the great slope angles. Cultural deposits may occur on the hillslope surface, but are likely to not be in situ.

Lakes (LLN) are generally thought of as not having any potential for cultural deposits; however, the history of lake level fluctuations in Minnesota is relatively unknown. For this reason, the 0-1 m interval of lake sediments is ranked low (1) and underlying deposits are ranked not possible (0). The low (1) ranking for the 0-1 m depth implies that Historic sedimentation has been less than a meter, which is an untested assumption. The LSR is low (1) because of the poor drainage conditions, even though this interval has extremely low energy depositional environment. For similar reasoning, exposed lake bed LfSA’s (LLB, LLBF) have low rankings to a depth of 2 m. The greater depth for possibility of buried cultural deposits compared to that in lakes is due to the exposed lake beds being slightly higher topographically. Shoreline features (LSH) are ranked moderate (2) for the 0-1 m interval and low (1) below that. The slightly higher ranking for the surface increment reflects slightly better drainage conditions. The possibility of buried or surface cultural deposits of Reservoirs (LLR) is judged not possible (0) due to the age and often disturbed character of the deposits.

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12.16 NICOLLET UPLANDS

12.16.1 Introduction

The Nicollet uplands model is located near the axial part of the Des Moines Lobe in Nicollet County of south-central Minnesota. This model area is located on undifferentiated Altamont and Algona moraine that stagnated. The landscape is an intricate, and sometimes subtle, arrangement of landforms, but the variety of landforms is limited. Although immediately adjacent to and northeast of the Minnesota Valley, the area is poorly drained and has little interior surface drainage.

12.16.2 Overview Of Past Work

The Des Moines Lobe entered Minnesota and Iowa from Canada and North Dakota, flowing along a pre-existing topographic low. In south central Minnesota, where the Nicollet uplands are located, the flow was parallel with the regional slope. The history of the Des Moines Lobe is one of repeated surging with intervals of stagnation (Kemmis 1991). Final retreat and wastage was rapid. During stagnation, a karst drainage system developed in the ice that ultimately controlled many of the details of the modern landscape sediment assemblages.

For the Nicollet County area, the Quaternary Geologic Map of Minnesota (Hobbs and Goebel 1982) maps an undifferentiated Altamont and Algona ground moraine for most of the area and undifferentiated Altamont and Algona stagnation moraine in the northwest part of the county. No further differentiation is recognized.

The Minnesota Geological Survey (MGS) mapped the Nicollet upland area as supraglacial drift complex at a 1:100,000 scale as part of the Minnesota Department of Natural Resources geomorphology coverage of the state. Little additional differentiation was made, although peat is mapped in the larger lake basins, and one short collapsed channel is mapped in the northeast part of the mapped quadrangles.

Activity of the Des Moines Lobe has been tightly bracketed by radiocarbon ages in Iowa and by radiocarbon ages postdating retreat and stagnation of glacial ice in Minnesota. The Des Moines Lobe entered Iowa shortly before 15,000 B.P. By 13,800 B.P. It reached its maximum extent at the position of the Bemis Moraine and the city of Des Moines (Ruhe 1969; Bettis et al. 1996). By 13,500 B.P., The lobe had stagnated, re-advanced to the position of the Altamont Moraine, then stagnated again. The Minnesota Valley, on the southwest part of the mapped area, was in the former axial position of the lobe at this time. A final re-advance in Iowa reached its maximum extent at the position of the Algona Moraine about 12,300 B.P. The Altamont and Algona ground moraines are indistinguishable in south central Minnesota. The glacier again stagnated and rapidly wasted. By 12,000 B.P., Glacial ice was no longer active in Iowa.

Radiocarbon ages from deposits postdating the Grantsburg Sublobe in east central Minnesota mostly postdate 12,000 B.P. (Meyer et al. In prep.). Glacial ice had retreated to the position of the Big Stone Moraine by about 11,700 B.P. (Fenton et al. 1983). Radiocarbon ages associated with the Moorhead Phase of Lake Agassiz in west central Minnesota indicate an age of about 11,000 B.P. Thus, active glacial ice apparently abandoned the Nicollet County area sometime between 12,000 and 11,700 B.P.; However, due to the presence of stagnant ice, the landscape continued to evolve, a supraglacial drift complex was deposited, and drainage patterns began to develop after 11,700 B.P.

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12.16.3 Mapping Methods and Data

High altitude color infrared aerial photographs from the USGS NAPP program were the primary source of geomorphic information. Tonal contrasts on these air photographs were sufficient to delineate features even finer than those mapped. The Nicollet County NRCS soil survey was useful in supplying subsurface textural information. Mapping was done independent of previously available maps of the area, partly because of the difference in scale of the pre-existing maps and of the current mapping, and partly because of the desire to map without reference to existing geologic models of the area. Mapping was accomplished by thin pencil directly on USGS 7.5' topographic maps. Many of the geomorphic boundaries were transferred to the maps from mylar overlays of NAPP aerial photography, with appropriate adjustments for distortion of the aerial photographs. No fieldwork was conducted in the mapped area.

12.16.4 Landform Sediment Assemblages (LfSA)

Landform sediment assemblages of the Nicollet Uplands are detailed in Table 12.15 and discussed below.

12.16.4.1 Landscapes

Stagnant Ice landscape. The Nicollet uplands area is dominated by landforms of a Stagnant Ice landscape. This landscape is characterized by well expressed, rounded to irregular, hummocks; and rounded, often linked, depressions, both of which occur above or below a general stagnation plain. Multiple generations of depressions are evident in most areas at different but close elevations. The hummocks and depressions tend to co-occur in several broad belts both parallel and perpendicular to regional ice flow. All stagnant ice landforms are underlain by clay loam diamicton. For the stagnant ice plain and hummocks, either localized sheetflood erosion and sedimentation and/or biomantle evolution may have modified the surficial deposits.

The stagnant ice plain (SP) has low relief, occurs between hummocks and depressions, and is slightly higher in the southeast corner of the mapped area where no obvious hummocks or depressions were mapped. Local small highs are sometimes eroded due to agricultural practices and show up as lighter tones on aerial photographs. These areas are mapped as SPE. Flat to slightly concave tops and relatively moderate to steep, but short, sideslopes usually characterizes hummocks (SHU). Some may represent ice-walled lake beds, but could not be differentiated based on available information. The shoulder and side slopes of hummocks are often eroded (SHUE), as indicated by lighter tones on aerial photographs. Eroded areas often form a ring-like appearance.

Depressions are linked depressions (SLD) as described by Kemmis (1991). In a few places, alignments of smaller linked depressions led to alignments of larger linked depressions, mimicking a drainage network. A line of linked depressions heads most of the short valleys draining to the Minnesota Valley. The majority of linked depressions are underlain by clay loam diamicton suggesting postglacial sheetflood fills might be limited, particularly for those at slightly higher elevations. The presence of post-Euroamerican settlement sheetflood deposits was interpreted based on lighter tones in small linked depressions (SLDS). Because mapping could not be field checked, it is possible that such sheetflood sedimentation is more widespread.

Valley Terrace landscape. There are three examples of strath terraces (VST). They all occur along the southeast edge of the mapped area in relatively short valleys that now drain to the Minnesota valley. They are underlain by a thin, discontinuous increment of loam overlying clay loam to loam diamicton. The surficial loam is interpreted as alluvium overlying till and debris flows associated with the supraglacial drift complex underlying the Stagnant Ice landscape. The largest of the low order valleys to drain the mapped area has a series of terraces (VT). Similar to the strath terraces, they are underlain by a thin, discontinuous increment of loamy alluvium overlying till or debris flow deposits. They are mapped as terraces rather than strath terraces because most are apparently aligned to the same gradient.

Floodplain landscape. Floodplains are very limited in the mapped area because of the paucity of low order valleys. Only the few largest of the low order valleys draining into the Minnesota Valley exhibit floodplains. They are undifferentiated as to type of floodplain (FF). Floodplains in these valleys are underlain by loam to sandy loam alluvium that is greater than 2 m thick. Channel belts in these valleys are too narrow to distinguish from the floodplains.

Valley Margin landscape. The Valley Margin landscape consists of a continuous belt of relatively steep hillslopes that form the valley walls of the Minnesota Valley and larger low order tributary valleys. Hillslopes (MH) are developed on clay loam to loam diamicton (till or debris flow deposits) and have a discontinuous, thin veneer of loam that may represent sheetflood sediments or a biomantle. Lighter tones on aerial photographs indicate that some areas of hillslope are moderately to severely eroded (MHE). This map unit may be more widespread than represented due to thick tree cover on much of the valley walls.

Lacustrine landscape. Two of the larger and lower groups of linked depressions support the extensive Swan Lake and Middle Lake. Many smaller lakes occupy all or parts of linked depression basins (LLN). Exposed lake beds (LLB) indicating higher lake levels at some time in the past surround the two large lakes, and several other smaller lakes. Exposed lake beds have less than 2 m thickness of postglacial sediments that consist of peat over lacustrine silty clay loam. This package overlies clay loam diamicton (till or debris flow deposits). The presence of peat suggests a strong likelihood that buried soils are present. The exposed lake beds are low relief, and minor fluctuations in water level could have caused great lateral shifts in shoreline location.

12.16.4.2 Landform Sediment Assemblage Codes

Table 12.15 provides details on each of the specific LfSA codes used for the Nicollet uplands model.

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12.16.5 Landscape Suitability Rankings

The duration of stagnant ice conditions and deposition of the Stagnant Ice LsSA is unknown. The deposition by the stagnant ice processes may have continued long enough to overlap with early periods of prehistoric human occupation, and this is accounted for in the LSR’s.

Processes in the stagnant ice landscape would have been dominated by debris flows of various types and localized runoff. The stagnant ice plain (SP), hummocks (SHU), and linked depressions (SLD) are ranked low (1) for the surface and 0-1 m depth interval. This low ranking reflects somewhat poorly to poorly drained conditions. The hummocks are poorly drained because of their flat to slightly concave upward tops. Hummocks would have been better drained than the depressions, but the rankings are both still considered low. The low ranking is extended to only the 1 m depth, even in linked depressions, because the majority of the stagnant ice sediments were likely deposited prior to any substantial human occupation; plus local sheetflood erosion and sedimentation and/or biomantle formation could have resulted in the shallow burial of cultural deposits. Soils information indicates no apparent substantial sheetflood, lacustrine, or wetland sedimentation in the majority of dry and farmed linked depressions. Below the 1 m depth, cultural deposits are considered not possible (0) because of both age and depositional environment rankings. Some depressions have evidence of sheetflood sediments of Historic age (SLDB). In these cases, a buried soil marks the pre-Euroamerican settlement soil, and the low (1) ranking is extended to the 2 m depth interval. Where the plain and hummocks are moderately to severely eroded (SHUE, SPE), the ranking shifts to reflect the removal of material. Although the surface is considered to have a possibility for cultural deposits, the 0-1 m interval is ranked as not possible (0) for in situ material. The most recent erosional episode is likely Historic in age due to modern agriculture; however, there is the possibility of previous episodes of erosion.

Both terraces (VT) and strath terraces (VST) have rankings similar to the stagnant ice LfSA’s. The surface and 0-1 m depth interval are ranked low (1). Greater than 1 m is ranked not possible (0) because of both age and depositional environment rankings. Both types of terraces have only a thin (less than 1 m thick) veneer of alluvium overlying till. The alluvium could incorporate or bury Paleoindian cultural deposits. A major incision of the Minnesota Valley occurred shortly before about 10,400 B.P. Terraces and strath terraces likely formed rapidly in response to this downcutting, in part accounting for the thin increments of alluvium.

Undifferentiated floodplains (FF) are ranked low (1) to a depth of greater than 5 m. The floodplain LfSA is definitely thicker than 2 m. They are poorly drained and characterized by deposition of intermediate to coarse textured overbank deposits. The possibility of buried cultural deposits can not, however, be ruled out.

Minnesota Valley wall sideslopes (MH) are developed in till and have a ranking of low for the 0-1 depth interval and not possible (0) below a meter depth. Despite the relatively steep slopes, local sheetflood sedimentation, slumps, and/or biomantle formation could result in shallow burial. In addition to the age and depositional environment of material the hillslopes formed on, the range of cultural activities in this landscape position would have been limited due to the great slope angles. Cultural deposits may occur on the hillslope surface, but are not likely in situ. More severely eroded segments of valley sideslopes (MHE) are ranked not possible (0) for the 0-1 m depth interval as well as underlying depth intervals. This reflects net sheetflood erosion in this landscape position.

Contrary to conventional thinking, which is reflected in a limited selection of survey techniques, lake (LLN) and exposed lake bed (LLB) LfSA’s are here considered to have some potential for buried cultural deposits. The history of lakes (LLN) in Minnesota, which could include substantial lake level fluctuations in this low energy depositional environment, is unknown. For this reason, the 0-1 m interval of lake sediments is ranked low (1) with underlying deposits ranked as not possible (0). The interval of low (1) rank implies that Historic sedimentation has been less than a meter, which is an untested assumption. The ranking is low (1) because of the poor drainage conditions. For similar reasoning, the exposed lake bed LfSA (LLB) has low rankings to a depth of 2 m. The exposed lake beds (LLB) are slightly higher topographically than the lake LfSA (LLN), and therefore are interpreted to have a greater potential at depth.

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12.17 TESTING THE GEOMORPHIC MODELS

12.17.1 Introduction

A model is defined herein as an abstraction of the real world used to accentuate certain facts or features. These accentuated features or facts are then available to view and be tested. Maps are "models" of the real world and are available to be tested. The Mn/Model geomorphic models have been set up with the intention of refining them through testing. Testing these models must take place to avoid self-fulfilling prophecies.

Most cultural resource-related geomorphic models have used the archaeological sites within or near the study area to help build the model. Mn/Model did not use any archaeological sites to build the initial Landform Sediment Assemblage (LfSA) models. Rather the site location and temporal data were used as a test against both the initial LfSA locations and ages. An assessment and refinement process for both delineation and age assignments is further described in Sections 12.17.2 and 12.17.3, respectively.

Ongoing archaeological and geological field work also provides a more detailed (larger-scaled) test of the 1:24,000 scale Mn/Model geomorphic maps and codes (Section 12.17.4). This close-order work is essential to improving upon the accuracy of the Landscape Suitability Rankings. The Mn/Model geomorphic codes allow for these more detailed investigations by having a code set up for sublandforms.

12.17.2 Archaeological Site Location Test

12.17.2.1 Site Location Test Methods

The geologic ages and polygon delineations of the Mn/Model Landform Sediment Assemblages (LfSA) were independently tested against the available archaeological site centroid locations on file (as of January 1997) at the Minnesota State Historic Preservation Office (SHPO). Archaeological data from seven river valleys and one ancient glacial lake bed were reviewed (Minnesota, Mississippi, Rainy, Red, Rock, Root, St. Croix and Red Lake Bog), as well as from the six upland areas described above (Anoka Sand Plain, Bemidji Uplands, Glacial Lake Agassiz Basin, Lake Benton Uplands, Mountain Lake Uplands, and Nicollet Uplands). Approximately 19,905 polygons were mapped and coded for both the valley/bog and upland study areas. These polygons represent approximately 671 different types of LfSA’s. One thousand sixty-three (1,063) archaeological sites were recorded within 199 different LfSA types currently recognized in the combined study areas.

The LfSA’s were developed by the geological mappers independently of any known locations and ages of cultural deposits. The locations of archaeological sites provided one type of check on the validity of the mapped geomorphic polygons and their Landscape Suitability Rankings. This first test was therefore a rough cross-check against the time span of possible human occupation (between approximately 12,500 and 200 B.P.), And a more detailed check on the post-depositional and depositional environments. A more refined temporal test was established later for those archaeological sites of high confidence levels with regard to both absolute and relative ages, and the geomorphic polygon temporal assignments (see Section 12.17.3 below).

Eight hundred eighty (880) and 183 archaeological sites lie within the valley/bog and upland modeled areas, respectively. The site centroids of the archaeological database were overlayed onto the geomorphic polygons for comparison (point-on-polygon overlay). Age and location contradictions required more close examination for their causes. Stages of analysis were set up to help identify the possible causes for aberrancy. The sites that did not appear to fit with the Mn/Model-generated geomorphological data in terms of age or location were culled out and put into an "aberrant site" database. The model testers realized that the aberrant site was only aberrant relative to the geomorphic polygon and that the polygon or its definition could be in error. Archaeological sites located in floodplain polygons were not necessarily aberrant, but were put into the aberrant site list for further scrutiny in hopes of eliciting further information on the dynamic nature of the floodplains.

From the 880 valley/bog sites, a list of 112 "aberrant" archaeological sites was compiled (12.8 percent of total). No sites were "aberrant" within the Upland study areas and the sites in these areas were not part of any further scrutiny. A third-party review was then conducted at the SHPO and the State Archaeologist's Office (SAO) by a research archaeologist. Site files for the 112 sites were reviewed and summarized. Artifacts (particularly diagnostics, if available), approximate site age, and geomorphic location (i.e., terrace, floodplain, upland) for each site were noted. The level of detail in this data varied between sites depending on the type of survey conducted; type of site located; as well as the author, type, and date of the site form.

After summarizing this information, the data were compared to the geomorphic polygons for each valley/bog project area. If the site and geomorphic data were contradictory, then the sites remained in the "aberrant" list for further scrutiny. Recent historic or deeply buried sites, for example, were culled from the list at this point, as were prehistoric sites that fit within the geomorphic polygon age and depositional environment rankings. Forty-five (45) of the 112 sites required further analysis. These 45 sites represent 5.1 percent of the valley/bog site population and 4.2 percent of the total modeled areas’ site population.

The next step was to verify the location of the centroids of the remaining 45 archaeological sites relative to the geomorphology polygons. Three sources were checked to determine in which geomorphic polygon the site centroid was located: 1) the SHPO GIS database printout, 2) the pencil-lined topographic quadrangles on which the geomorphic polygons were originally recorded, and 3) the digitized (GIS) versions of the LfSA maps.

The other site data (such as reports, publications, etc.) were analyzed, and conclusions were made regarding the level of confidence of site cultural age and location. The following "errors" were found in the archaeological site data, and they help explain why many site locations did not match the geomorphic polygon data:

Vague site locational data is caused by oversized site boundaries or an imprecise or unknown site location. There is a high level of confidence that a site exists in a given area, but the precise site location is unknown. Most of these aberrant sites are mound groups originally mapped by Lewis and Brower. Because their maps were less than ideal, we have general but not pinpoint locations for these sites. In addition, generous boundaries were placed around sites by cultural resource managers to be safe or to protect the sites from vandals.
Skeptical site locational data is caused by a site with no integrity (not in situ) or an incorrectly mapped site location. Skeptical site locations differ from vague site locational data in that it is unclear if the site is in situ. The artifacts may have moved from their original location due to water, wind, or human processes. The site also may have been recorded based only upon an informant’s report, without field verification. Another cause for a skeptical site location is when the archaeologists’ mapped site locations do not match their own written site locations.
Scale issues include pencil-width problems on the topographic maps, map size, and digitizing points around the polygons. These actions may enlarge or decrease the size of a polygon. Site centroids near polygon borders may then appear to fall within an adjacent polygon.
Incorrectly calculated UTM’s, even a small error, may place the site centroid in the wrong polygon.
Misinterpreted LfSA age or polygon boundary may cause a site to appear as if it is "aberrant." Simply put, the modeler made a mistake in either assigning an age or drawing the polygon. Both cases are usually caused by insufficient data. Possible "scale" issues could also cause the apparent geologic misinterpretation. For example, smaller portions of larger landforms that are not discernable at the current working scale (1: 24,000) may in fact be suitable for cultural deposits.
Digitizing mistakes occur when the boundaries of the geomorphology polygons were incorrectly digitized or not digitized at all. An example may be when a polygon line was not "closed," resulting in the incorporation of a floodplain LfSA into an older terrace LfSA.
Unknown reasons occur when multiple reasons may exist for the site being aberrant. Field checking the archaeology and the geomorphology would be required to distinguish the cause for being aberrant.

12.17.2.2 Results of Site Location Analyses

Vague site location was the most common problem (n = 12) with the archaeological site data, particularly for many of the Lewis- and Brower-located mounds. The precise location of these sites is unknown, and a large area is indicated for the site boundary, often more than a quarter-section (of one square mile) in size.

Skeptical site locations proved to be problematic for six (6) of the 45 sites analyzed. In this case, the site may have lost its integrity, i.e., the in situ location of a site was not being reported on the site form. In several cases the location of artifacts in a plowed field was reported by a landowner, but the location could never be field verified by an archaeologist. Also, several site locations were mapped incorrectly on the site form based upon the written description of the physical site location (i.e., the site form’s or report’s text would indicate that the site was located "On a terrace 30 ft above the floodplain...," but the site form’s attached topographic quadrangle indicated the site location was on the floodplain).

Eight (8) scale problems were found during the analysis, or approximately 0.9 percent of the valley/bog site population and 0.8 percent of the combined valley/bog and upland population. These scale issues are a function of maps themselves and are neither an archaeological nor a geomorphic error. Possible scale issues include pencil-width problems, general map and contour scales, and numbers of digitizing points used for a polygon.

Two (2) of the 45 aberrant site locations fell into polygons that were digitized incorrectly. These two sites account for 0.2 percent of the river/valley site population and 0.2 percent of the combined valley/bog and upland population. After verifying the error, the original topographic quadrangles with the pencil-line polygons were redigitized.

Seven (7) UTM errors were recognized when the site centroid appeared in the middle of the active river channel on the GIS maps. The site form maps for these same sites indicated that the location of the site was adjacent to, or sometimes up to one mile away from the river channel. Once the correct UTM’s were entered, six of these seven sites fit well within acceptable geomorphic parameters. The seventh site was relocated into another questionable floodplain polygon and remained on the aberrant list for further scrutiny (i.e., the site location had two errors).

Nine (9) sites did not match the geomorphic polygons established for their regions and did not appear to fit the error categories. These nine sites equal 1.0 percent of the valley/bog site population and 0.8 percent of the combined upland and valley/bog population. The geomorphic data were re-analyzed for these sites to determine the level of confidence for the modeling in each particular valley and the physical site location. The geomorphic data and the archaeological data were then compared to determine if the geomorphic polygons for that area needed to be reinterpreted. Most (n = 6) of the geomorphic polygon "errors" were restricted to islands in the Upper Mississippi River channel. The Upper Mississippi "river island" LfSA was originally interpreted to have a younger and thicker overbank mantle; however, this LfSA was never sampled (nor radiocarbon dated) for logistical reasons, so educated guesses were used during the initial mapping. Adjustments were made to the LfSA’s age rankings to allow sites to be found within the uppermost strata of these islands. Three more sites—one from the Minnesota Valley, and two from the Root Valley—were also located in floodplain LfSA’s that were interpreted to be too young to contain cultural deposits at or near the surface. Two corrections to the geomorphic mapping fixed two of the geologic misinterpretations. The third site in the Root Valley was probably mapped correctly; however, the site is near the outer boundary of a floodplain LfSA. This location on the LfSA represents the thinnest increment of young overbank sediment (flood deposits), which makes the site more available to discovery than would have been possible closer to the river channel within the same LfSA. Technically speaking, this "error" could have been classified as a scale issue. The mappers tried to represent the "worst case" scenarios for the end users of these models, and therefore the thickest overbank sediments were modeled.

Finally, for two (2) of the 45 sites, a review of the site reports and a geological re-appraisal could not determine what caused the error. Multiple reasons may exist for these sites being aberrant, and without further field investigation, it was not possible to distinguish the cause for the deviation from expected results. Both sites were located in the Root River Valley (21FL0056, 21FL0064).

The UTM errors mentioned above prompted concerns for both the third-party reviewer of the geomorphology models and the GIS archaeological predictive modelers. A test population of 778 valley/bog site records had their site centroid UTM’s recalculated by a third-party research archaeologist using a standard plastic UTM measuring device. The Mn/Model archaeological predictive models are set up on a 30 by 30 m cell grid. The modelers decided beforehand that any UTM greater than 60 m away from the third-party reviewer’s centroid location would be classified as an "error." Fifty-five (55) sites, or 7.1 percent of the recalculated UTM population, have incorrect UTM measurements representing their locations on the SHPO database. These errors are of concern, but are somewhat mitigated in the archaeological predictive models because those models use Euclidean distance calculations for most point-on-polygon comparisons. Large errors, however, would still have a negative impact on predictive model results.

12.17.2.3 Summary of Site Location Test

This test involved a point-on-polygon overlay of 1,063 archaeology site centroids over 19,905 geomorphic polygons. Eight hundred eighty (880) of the sites are located within the valley/bog model areas. One hundred eighty-three (183) sites are located within the six upland model areas, and all of these were in agreement with the geomorphic models. Forty-five (45) sites were placed in an "aberrant" site list because of incompatible geomorphic and archaeologic data. All but 11 sites were aberrant because of scale issues (8), skeptical site locational data (6), vague site locational data (12), incorrect site locational data (7), polygon digitizing errors (2), and indeterminate reasons (2). Approximately 0.8 percent (9 of 1,063 sites) of the archaeological sites did not fit the geomorphic models because of misinterpreted LfSA polygons. Three changes to the LfSA’s resolved all but one of these misinterpretations. This last unresolved site discrepancy is probably not correctable at the current working scale. Clearly, precision mapping of archaeological sites is of great importance in testing geomorphic models, as well as building models. If site centroid UTM’s were miscalculated even slightly, and 7.1 percent of the 778 recalculated centroids were, then the potential exists for a misinterpretation of the geomorphic polygon data. This test has highlighted the problems found in the archaeological site database, namely those mentioned previously under "errors." These errors must be eliminated in the future so that the data can be used to better understand the location of prehistoric sites.

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12.17.3 Archaeological Temporal Test

12.17.3.1 Temporal Test Methods

The geologic ages of the Mn/Model Landform Sediment Assemblages (LfSA) were independently tested against the ages of archaeological sites with intact cultural deposits. The site data were derived from the SHPO site database and technical reports. Archaeological data from 946 sites in the seven river valley (Minnesota, Mississippi, Rainy, Red, Rock, Root, and St. Croix), one ancient glacial lake bed (Red Lake Bog), and six upland (Anoka Sand Plain, Bemidji, Lake Agassiz, Lake Benton, Mountain Lake, and Nicollet) project areas were reviewed.

The geologic ages of the LfSA’s were assigned independently of the known ages and locations of cultural deposits. In situ archaeological sites with good temporal indicators provided a type of check on the geologic ages of the LfSA’s. The archaeological data set consists of 880 archaeological sites located in the valley/bogs and 183 archaeological sites located in the uplands. The archaeological sites had to be in situ and undisturbed by post-depositional processes to provide a reliable check on the geologic data. Prior to comparing the archaeological data set with the geologic data set, written descriptions for all 1,063 sites were reviewed to determine their temporal placement and to assess their level of integrity.

The criteria employed to cull those archaeological sites lacking integrity emphasized the presence of chronometric dates and diagnostic artifacts and/or structures found in undisturbed deposits. Undisturbed deposits include intact soil profiles as well as cultural features. Obviously problems of potential disturbances still exist even within or under "intact" soil profiles. These soil profiles could have developed after the "disturbance," or the field archaeologist may have misinterpreted the soil profiles from the test pit or shovel test. Sites were placed in "high," "moderate," and "low" confidence categories, based upon the information provided in site records and reports.

The high confidence category includes sites that produced one or more of the following: 1) radiocarbon dated organic material from a reliable/undisturbed context; 2) diagnostic artifacts from an undisturbed deposit; and 3) surface material (lithic quarries and rock art) from an undisturbed context. Additionally, sites were placed in the high confidence category if there was adequate historic documentation to support the archaeological evidence.
A moderate level of confidence is assigned to those sites identified from the subsurface sediments of a Phase I shovel test. The recognized soil profile encapsulating or covering the artifact may be a very recent development; therefore, the artifact may not always be in situ. Right or wrong, these Phase I shovel test sites were required to build a testing population of somewhat meaningful size.
The low confidence category includes sites represented by cultural material recovered from a high energy environment or disturbed context as well as artifacts belonging to a private individual’s collection, which lack adequate provenience data. Although the level of funding and time allocation for individual projects may directly affect the quality and thoroughness of archaeological investigations, these were not used as criteria to assess integrity.

An unexcavated mound is considered a surface find. Although they may be categorized as an intact surface find, visual examination is not effective in placing them in a particular time period. Mounds have a long history of use in Minnesota and may be dated from the Archaic to Historic. Such a time range takes in much of the Holocene and is considered too long to be meaningful for this analysis; therefore, the only mound sites included here are those that were excavated and produced either absolute or relative dates. Only the sites in the high and moderate confidence categories were used in this analysis as they held a certain confidence level for integrity and also had a relatively tight temporal span.

In most cases, the reasons for removing sites from the data set were related to the level of field investigation conducted at the time that the sites were recorded and insufficient data on site forms. A short discussion on each follows:

Level of field investigation: Most of the site forms examined during this analysis generally represent the results of Phase I reconnaissance survey. During Phase I archaeological survey, diagnostic material may be identified, but is it very uncommon to find artifacts in exceptionally good contexts such as features. It is equally unlikely for a Phase I investigation to recover charred organic material from a reliable context for dating purposes. Further, if Phase II evaluations were conducted at a particular site, the site form may not have been updated to reflect the new data. The reports that documented the results of the Phase II evaluations of sites in this analysis were reviewed when recognized and available.
Level of detail recorded: Site forms varied in the level of detail regarding the methods utilized to identify a cultural resource, their provenience information, and the types of artifacts identified or recovered. Frequently surface and subsurface methods were employed on sites with diagnostic material, yet no provenience information was given for the diagnostic material. These sites were automatically placed in the low confidence category. Sites were also removed from the test population if they were described as belonging to a specific historic context and had intact subsurface material but actually lacked a chronometric date and/or diagnostics. Sites were also removed from the working population if they were described as a specific historic context but employed negative evidence to support the assignment to that context. For instance, a site form may list an Archaic context solely based on the absence of pottery. The vague provenience and artifact data documented on many of site forms may be attributed to the year the site was recorded. The earliest state site forms, not more than one page long, prompted the archaeologist to record very minimal information, while the current site form is several pages long and requires more detail about methods, level of site disturbance, and recovery of cultural material. Certainly the year of investigation plays a role in the level of information recorded; however, it is not always the cause of incomplete site descriptions. Some of these "updated" site forms, as well as reports dating to the 1980’s and 1990’s, also lack descriptive detail.

Four stages of analysis were used to identify the valley/bog and upland sites with high to moderate levels of integrity and site ages:

The first stage of the analysis required recording site information such as historic context, method of dating the site, artifact provenience (surface, shovel test, excavation units, trenches), radiocarbon dates, diagnostic artifacts, cultural features, and types of disturbances. This information was gleaned from site records, technical reports, and publications.
The second stage of analysis addressed whether sites were represented by surface finds or by private collections. Surface finds and private collections were deemed low confidence and removed from the test population. Multicomponent sites with surface material remained within the test population and were represented only by their intact components. This stage reduced the total number of testing sites to 350, or 241 valley/bog sites and 109 upland sites.
The third stage of analysis was to determine whether any of the remaining sites were included in the "aberrant" site category for which the cause of the aberrancy could not be determined as mentioned in Section 12.17.2.2. One aberrant site (21FL0064) was still included in at this stage of analysis. Confidence levels were deemed low on two issues at this site; the Archaic materials were recovered by a private collector, and one diagnostic Woodland sherd plus several more nondiagnostic flakes were located in a probable disturbed environment within a floodplain LfSA. The lone sherd and several flakes were also very close to a "higher terrace" with many more diagnostic and nondiagnostic artifacts that temporally fit the geologic framework. Farming practices are thought to have dragged the artifacts down the escarpment.
The fourth stage of analysis was to review the level of integrity of sites with subsurface deposits. If disturbances were noted or no diagnostics were recovered, the sites were removed from the working population. After applying the four stages of analysis, the working population included 107 valley/bog sites and 29 upland sites.

The working population of sites that lack radiometric dates were assigned a temporal range based upon the information provided on the site forms and reports. Both general date ranges that apply to prehistoric cultures across the Midwest and date ranges that apply to archaeological cultures identified in Minnesota have been assigned to sites where appropriate. Prehistoric cultures within the Midwest may be placed in a developmental sequence that is divided into four general periods: Paleoindian (11,000-7,000 B.P.); Archaic (7,000 B.P. - 1,000 B.C.); Woodland (1,000 B.C. - A.D. 1,700); and Late Prehistoric or Mississippian (A.D. 1,200-1,750). The temporal range of each archaeological culture defined in Minnesota is based upon several decades of archaeological research in the state and is reinforced by radiocarbon dates that are documented in the SHPO files. The particular range of dates assigned is dependent upon the general cultural tradition (e.g., Late Archaic, Middle Woodland) or the specific archaeological culture within Minnesota (e.g., Laurel or Blackduck) that was described in the documentation associated with each site.

12.17.3.2 Results of Temporal Test Analyses

The locations of apparently in situ archaeological sites with good temporal indicators are used to test the model. The absolute and relative dates of the sites assigned high and moderate levels of confidence were compared to the geologic age of the LfSA within which each site is located. The LfSA age rankings indicating the potential for cultural material on the surface and at depth were also noted. The results follow.

High and moderate confidence levels were assigned to 107 sites located in the river valleys and bog. Almost half (49 sites; 45.8 percent) of the total sites with either absolute or "good" relative dates are located within the Mississippi River Valley; 22 sites (20.5 percent) are located within the Minnesota River Valley; 20 sites (18.7 percent) lie within the St. Croix River Valley; eight sites (7.5 percent) fall within the Red River Valley; six sites (5.6 percent) are located in the Rainy River Valley; and two sites (1.9 percent) are located in Red Lake Bog. The low percentages of archaeological sites within several of the river valleys and bog do not necessarily reflect a lack of sites, but may reflect a lack of archaeological survey or a lack of sites on or near the surface. In spite of the low numbers, the archaeological data allow a comparison with the geomorphic data set. No discrepancies were recognized between the archaeological and the geomorphic data sets for the valley/bog project areas.

Twenty-nine (29) of 183 upland sites reviewed in this analysis were determined to have good temporal indicators. The Anoka Sand Plain, Bemidji, Lake Agassiz, Lake Benton, and Mountain Lake Uplands are represented within this data set; however, the majority of sites occur within the Anoka Sand Plain (18 sites; 62 percent). No high and moderate confidence sites are represented in the Nicollet Upland study area. One site each occurs within Glacial Lake Agassiz (3.4 percent) and Mountain Lake (3.4 percent) study areas. Two sites occur in the Lake Benton Upland (6.9 percent) study area; whereas seven sites occur in the Bemidji Upland area (24 percent). The low percentages of archaeological sites within the upland study areas do not necessarily reflect a lack of sites, but may reflect a lack of archaeological survey or a lack of sites on or near the surface. No discrepancies were recognized between the archaeological and the geomorphic data sets for the upland study areas.

12.17.3.3 Summary of Temporal Test

Site forms and associated reports for 880 valley sites and 183 upland sites were reviewed in order to assign a level of confidence to each site’s integrity and age, if available. The goals of this analysis are to provide a type of check on the geomorphic data defined in Landform Sediment Assemblages (LfSA). This geomorphic information may be used to predict the presence or absence of other in situ archaeological sites on land surfaces of a particular age. At the time this document was prepared, the site ages have not been incorporated into the LfSA models.

The comparative test indicated that temporal data from 136 high to moderate confidence sites were in agreement with the geomorphic ages of their associated LfSA’s.

Assigning a level of confidence to a site’s age is often difficult, particularly because the majority of sites reviewed for this analysis received only a Phase I survey. The level of detail provided on site forms is often inadequate to assess integrity, which is in large measure an artifact of how archaeological reconnaissance survey is conducted. Reconnaissance survey is conducted primarily to determine the presence of historic properties and whether they are in a disturbed context. At this level of investigation, the location of features, the recovery of datable organic material, and a full understanding of a site are rare.

This analysis has demonstrated the importance of recording archaeological sites with details on location, disturbance, and artifact provenience when they are utilized to test models. Without this information, sites are considered to have little utility for testing landscape evolution models, which in turn impacts the ability to locate cultural deposits.

12.17.4 Ongoing Tests

MnDOT has already started to test these landscape suitability models well before the writing of this report. These models can not become self-fulfilling prophecies and are set up to be easily modified by future workers. Some tests, for example, have recognized that deviations do occur between the models and the real world at this scale of mapping. Hudak (1997) has interpreted that possible post-Euroamerican settlement alluvium or colluvium may mantle some the fans in the downstream reach of the Minnesota River Valley; something that could not be recognized at the Mn/Model scale of mapping, nor was evident in Mn/Model’s more upstream field testing areas. Other ongoing archaeological and geomorphological projects are currently operating and will bring new information to the forefront of predictive modeling. The beauty of having these models established in GIS format is the flexibility and ease that one can adjust the data, and hence the output.

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12.18 DISCUSSION ON RADIOCARBON CHRONOLOGY AND LINKING THE STUDY AREAS

12.18.1 Radiocarbon Dates from the Valleys and Bog

Eighty new radiocarbon dates are presented as part of this report. Every valley and bog project yielded valuable radiocarbon information. The Minnesota River Valley yielded radiocarbon dates on basal alluvial fan strata. These dates bracket the time of the last catastrophic Glacial Lake Agassiz floodwaters to sometime before 10,400 B.P. Alluvial fan soil complexes from the Minnesota Valley have also been dated between approximately 2,000 and 4,500 B.P., And 5,400 and 8,000 B.P. These soil-forming time spans indicate relative surface stability across the fan.

The Mississippi River Valley yielded some early Holocene (9,210 ± 50 B.P., 8,110 ± 50 B.P.) dates from near the Mississippi Headwaters State Park at 3.5-5 m depths. Also, the Necktie channel southeast of Bemidji last carried major sediment down the valley sometime prior to 9,860 ± 60 B.P., which is a date on wood from a basal peat section overlying glacial outwash. This date contradicts the interpretations of Hohmann-Caine and Goltz (1986) regarding the Holocene paleo-flowage history of the Mississippi River down the Necktie valley. Two wood dates of 3,940 ± 70 and 3,600 ± 50 B.P. come from basal channel-fill peats and indicate that the current Mississippi floodplain was developed by this time and likely experienced lateral channel migration during the mid- to late-Holocene.

The Rainy River Valley yielded three radiocarbon dates that indicate that the VT2 terrace had been entrenched before 5,850 B.P as suggested by Hajic (1996a). The VT1 terrace formed between 5,000 and 6,000 B.P., although some younger overbank sediments may mantle this terrace. The VT1 terrace was incised by 5,000 B.P. And the modern floodplain began to develop.

The Red Lake Bog yielded several radiocarbon dates indicating that in at least the area of the transect, peat started to develop during the late Holocene (4,470 ± 50 to 180 ± 40 B.P.). The oldest radiocarbon date in the study (5,890 ± 60 B.P.) came from a depth of 4.5 m in lagoonal sediments related to the Upper Red Lake.

The Red River Valley yielded 10 radiocarbon dates, three of which were from stratigraphically beneath the Sherack Formation. These three dates (10,080 ± 70, 10,140 ± 70, and 10,180 ± 70 B.P.) all came from wood or other subaerial plant parts that were pulled out of what this report defines as a wetland paleosol. These dates indicate that Glacial Lake Agassiz did not readvance across the landscape near Georgetown, Minnesota until at least 10,100 B.P. The maximum incision of the Red River occurred some time before 4,070 ± 80 B.P.; And that when this date is combined with the other valley fill dates, their range extends from 4,070 to 1,260 ± 80 B.P. This range implies that relatively extensive lateral channel migration, which may not be much compared to most of the other Mn/Model valleys, took place during the mid to late Holocene.

The Rock River Valley study yielded two radiocarbon dates from a tributary’s meander belt and one more from the Rock River’s meander belt. These three dates (5,430 ± 60, 2,050 ± 40, and 320 ± 70 B.P.) indicate that lateral channel migration was taking place across the relatively small floodplains of both the Rock River and its tributaries during the mid to late Holocene. One basal alluvial date on seeds (7,940 ± 100 B.P.) recovered from the Rock valley indicates that the modern river was in place by the early Holocene age.

The Root River valley in Olmstead County yielded three radiocarbon dates from a floodplain meander belt (3,360 ± 60, 3,320 ± 50, and 350 ± 40 B.P.). One radiocarbon date (2,890 ± 70 B.P.) came from basal channel deposits in a meander belt belonging to Rush Creek, which was in an unmapped area. Another date (4,960 ± 60 B.P.) came from approximately 5 m deep within vertical accretion deposits belonging to the VT1CO LfSA. Houston County also yielded dates of "modern", 560 ± 40, 2,790 ± 70, and 3,360 ± 50 B.P. From channel fill and lateral accretion deposits. These dates indicate that lateral channel migration was prominent during the mid to late Holocene. A muskox skull was found by a land owner in a quarry excavated out of the VT3 LfSA. The landowner indicated that the skull came out approximately 8 m beneath the surface of the VT3 terrace. Purified collagen extracted from the skull yielded a radiocarbon date of 14,420 ± 70 B.P., Which means that the VT3 terrace is younger than 14,420 B.P. And older than the 4,960 B.P. date from the nearby VT1CO LfSA. This bracket fits with the generally accepted ages of the "Savanna Terrace" of the Mississippi Valley.

The St. Croix River yielded four critical dates for the high catastrophic flood marginal channels. Wood from basal peat (or near basal peat) deposits overlying the glaciofluvial deposits in these channels yielded dates of 10,120 ± 100, 9,640 ± 70, 9,360 ± 80, and 9,340 ± 60 B.P. Bracketing dates from another study (10,600 to 9,620 B.P from Lund and Banerjee 1985), and one date from a MnDOT core (8,370 ± 70 B.P. In Table E.2) reported herein, indicate that the St. Croix Valley was carved out or exhumed by catastrophic waters at or near the Pleistocene-Holocene transition.

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12.18.2 Linking the Valley Project Areas

The Mn/Model geomorphic mappers did not actively pursue thinking about or linking the valleys’ landforms in time between each other or other Midwest valleys; however, from the discussion above in Section 12.18.1, a pattern has emerged with respect to the floodplains and VT1 terraces and their depositional environments. Also, the Root River’s VT3 terrace is certainly linked to the Mississippi River’s "Savanna Terrace."

For now, perhaps the most obvious correlation is within the Floodplain and VT1 LfSA’s of each valley project area. A definite pattern exists within these valleys of lateral channel migration starting sometime during the mid-Holocene (approximately 5,000 B.P.) and lasting up through the present in many cases. (The St. Croix is the lone exception for reasons discussed in Section 12.10.4.1, although its tributaries do display meander belts.) The lateral migration may have started by 5,400 B.P. in the Rock valley. This dynamic landscape certainly makes predicting suitable habitat landscapes more difficult at the 1:24,000 scale and further demonstrates the need for more detailed geomorphic investigations at the Phase I and II levels of investigation. These younger landscapes also commonly have late Holocene to Historic overbank sediments that mantle the older channel migration features, which makes predicting precontact suitable landscapes relatively easier at or near the surface and more difficult at depth. As Mn/Model progresses through time and test, the gaps between valleys will be closed where possible.

12.18.3 Comparing the Valley Project Areas

The mappers did not actively pursue comparing valley, bog, or upland projects areas between each other; however, obvious differences became clear as they jumped from valley to valley and then from upland to upland. The need to adjust and modify the mapping code was almost a weekly if not daily ritual between the mappers. The glaciofluvial and catastrophic flood valleys (Rock, Minnesota, St. Croix, and Mississippi) are typified by coarse grained strata and topographically high glaciofluvial or catastrophic LfSA’s. The valleys/bog on glaciolacustrine landscapes (Red, Rainy, and Bog) often have the finer textures.

All of the valleys had some sort of overbank deposit on the floodplain or valley terraces that, when present, typically thickened in the down valley direction. The Red, St. Croix, and upstream reaches of the Mississippi valleys have very thin mantles of overbank sediment relative to the other valleys. Causes for these differences include relatively small amounts of croplands (i.e., soil erosion) feeding the valleys in the case of the St. Croix and Upper Mississippi reaches, and the nature of relatively flat terrain causing low energy floods in the Red River area (i.e., little sediment eroded and deposited during floods because it is mostly dammed up on gentle gradients). The overbank sediment on the Red, St. Croix, and upper reaches of the Mississippi river floodplains are therefore likely to only bury cultural resources at shallow depths.

The Minnesota, Mississippi, and Root valleys have the most LfSA’s. The Mississippi has many because of being divided into three reaches. The other two have many LfSA’s because of a combination of multiple terrace surfaces, high relief, and probably older ages relative to the other valleys. On the other side, the Rainy, Rock, and Red valleys, as well as the Red Lake Bog have the fewest LfSA’s because of relatively low relief, young age, or short reaches.

The Root and possibly the Rock are the only two valleys that are carved into pre-Wisconsinan aged strata. The Rock, however, was carved by Wisconsinan-aged meltwaters. The oldest of all the fluvial strata (belonging to the current valleys) should be found topographically high in the Root River valley, and the youngest of the oldest fluvial strata between the valleys will likely be found high in the Rainy valley.

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12.19 RESEARCH QUESTIONS FOR THE FUTURE

Future geoarchaeological research issues raised by these initial investigations of valleys and uplands are almost limitless. Several key encompassing issues apply to the upland areas, and others apply to nearly all the valleys. These can be addressed in part by programmatically and systematically incorporating certain observations into all future archaeological investigations.

Minnesota is the "Land of 10,000 Lakes," most in upland areas. While much of what is known about late Quaternary vegetation and paleoclimate in Minnesota has been derived from evidence in lake deposits, relatively little is known about the sedimentology of these deposits. In particular, what is the history of lake level fluctuations in possible response to late Quaternary climate change or regional base level changes? What is the geomorphic and sedimentological record of such fluctuations, and can former shorelines, possibly with associated cultural deposits, be buried by lacustrine or wetland sediments? A related set of questions is tied to whether the oft-held assumption of archaeologists (and that which is supported by Mn/Model’s GIS predictive model) that there is a relationship between the location of archaeological sites and water is accurate. Minnesota has an abundance of water at or near the surface. The relationship between archaeological site location and water is an assumption, not a fact, as is often argued, because groundwater has not yet been incorporated into the analysis.

A second upland issue is whether prehistoric cultural deposits maintain any cultural integrity whatsoever, or whether, following original cultural deposition, they have been redeposited by geomorphic and pedologic processes to the degree that they can in no way be classified as significant for the purposes of cultural resource management. To what degree have biomantle formation, soil development, sheetwash erosion and sedimentation, and erosion surface formation affected cultural deposits? How do these effects vary with components of hillslope, scale of geomorphic features, and regions of the state? A related question is what is the distribution of loess, particularly in areas where it is not formally recognized either because it is too thin or too disturbed to have been formally mapped, or recognized. The results regarding these issues related to a dynamic upland may have profound implications for cultural resource management.

A third upland issue relates to the stagnation style of deglaciation that characterizes large areas of many of the glacial lobes and sublobes in Minnesota. Due to the insulating effects of material melted from stagnant ice, stagnant ice and ice stagnation processes may have lasted into the Holocene. If this was the case, ice stagnation would have impacted Paleoindian and Early Archaic settlement decisions, and it definitely could affect the visibility and preservation potential of cultural deposits of these cultural periods. Also, what were the changes in groundwater conditions from deglacial stagnant ice conditions to responses to postglacial stream valley incision and drainage network growth?

Mn/Model’s geomorphology did not address the lower order stream valleys; therefore, our knowledge is severely lacking regarding site potentials and preservation potentials in these smaller basins. How and when were these smaller valleys behaving relative to the larger valleys?

A major issue concerning most major river valleys in the state is how their respective histories of landscape evolution are interrelated. This is important not only for predicting the possible locations of cultural deposits of differing cultural periods, but for interpreting late deglacial history as well. What, if any, patterns are prevalent throughout the upper Mississippi basin? What are the process response relationships among the Mississippi, Minnesota and St. Croix valleys between the dynamic interval of 12,000 to about 9,500 B.P.?

A number of questions for future research can be posed for the individual mapped areas. The most significant one or two of these are listed for most mapped areas. All have considerable implications for cultural resource management and archaeological investigations in the respective areas.

Anoka Sand Plain. What is the character and landscape suitability of the buried soil developed on sand and what is the basal age of the peat that buries it? What is the sedimentology, age, and origin (lacustrine, outwash, or eolian?) of the youngest strata (excluding peat and eolian dunes) underlying the sand plain? Is there any evidence to contradict the possibility that very early Holocene St. Croix catastrophic flood waters flowed across at least parts of the sand plain and contributed to its morphology? How old are the dunes and when did they last move significantly?

Bemidji Uplands. Are the coarse-grained sediments surrounding the chain of lakes glaciolacustrine or glaciofluvial in origin? Are flats and escarpments on the outer limits of the modern lake beds actually ice block kame terraces, or are they former lake shorelines caused by some other process?

Glacial Lake Agassiz Plain. What is the potential for buried Paleoindian cultural deposits being associated with the histic buried soil, dated at about 10,100 to 10,200 B.P. Near Georgetown, and overlain by the Sherack Formation? Are there significant chances for cultural deposits to be buried within the fine-grained facies of the Poplar River and Sherack formations?

Minnesota River Valley. Were the catastrophic flood features in the valley formed by one or more catastrophic floods from the Lake Agassiz basin? What is the basal age of the floodplain LsSA and how does it vary from upvalley to downvalley? Can the early Holocene age for the VT1 terrace be confirmed?

Mississippi River Valley. Can Upper Mississippi River terraces above St. Cloud be traced to downvalley surfaces? What is the relationship of these terraces to terraces in the Minnesota, St. Croix, and Root valleys? Are coarse silt deposits currently interpreted as loess in the Glacial Lake Aitkin basin actually loess or lacustrine deposits? Could these silt deposits be one sedimentary facies of an ice-block kame terrace? What is the stratigraphic sequence and age of deposits underlying deltaic landforms where the Mississippi River enters the natural lakes of the Headwaters reach? What are the interpreted colluvial landforms near Grand Rapids?

Nicollet County Uplands. What is the thickness range of Holocene-age fill in and surrounding linked depressions, and what are the youngest ages of the linked depressions?

Rainy River Valley. How does the history of valley evolution relate to the shrinking of Lake Agassiz down to the Lake of the Woods water levels?

Red Lake Bog. What is the variability of the basal age of the peat? Are there paleo-topographic highs, now buried by peat, that would have had a high landscape suitability before burial? What is the lake level history of Upper Red Lake and Lower Red Lake and how does that relate to adjacent fluvial and peat deposits?

Red River Valley. What are the ages of abandoned paleo-valleys or chutes cut into the Lake Agassiz plain? What thickness and age trends are recognized for the overbank deposits from Wahpeton to the Canadian border? Can we positively identify alluvial fans and slump blocks within the valley?

Rock River Valley. What are the absolute ages of pediments that grade to outwash terraces? Are there erosion surfaces of Holocene age developed on the pediments? What is the age of the VT1 LfSA?

Root River Valley. What are the refined ages of the VT1 and VT2 terrace sediment assemblages? What do the broad "floodplain" fills of the Root tributaries look like at their valley heads, and how did these valley heads form such an abrupt start? What role did groundwater play in forming these unusually shaped valley heads?

St. Croix River Valley. Can a catastrophic flood origin for the formation of the St. Croix valley and most of the major geomorphic features within it be confirmed? Did early phases of catastrophic flood waters course across at least parts of the adjacent Anoka Sand Plain?

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12.20 GENERAL SUMMARY

The Mn/Model geomorphology project is a Pre-Euroamerican contact model of "suitable habitat landscapes." This model does not predict where to find archaeology sites; this model predicts locations where archaeology sites could exist in geologic time and space. Said differently, this model specifies areas with a geologic context of the correct age and environment to have and preserve cultural deposits. Preserving these deposits means having cultural resources that are in situ, or that have "integrity" as defined by federal mandates (Section 36 CFR Part 800).

Mn/Model’s geomorphic modeling for suitable landscapes was deliberately set up with the hope and expectations that the LfSA’s will be modified through time and test. This report demonstrates that a dearth of information exists for Minnesota’s valleys, and that the potential for deeply buried cultural deposits in both valley and upland environments is poorly understood.

Originally, seven river valleys and one large bog were divided into Landform Sediment Assemblages (LfSA’s) at the 1:24,000 scale based upon their landforms and associated underlying stratigraphic package. Temporal information has also been assigned to these LfSA’s where possible. Field work was tied to these mapping projects to add a certain amount of ground truth and temporal information through relative age indicators and radiocarbon chronology. Later, six "upland" areas were mapped at the same scale to provide a demonstration of possibilities for other environments that may have different kinds of deep site burial processes. Field work was not completed in these upland areas; therefore, these models await ground truth.

The valley, bog, and upland LfSA’s have been digitized and assigned attributes (codes) in a GIS database. Thirty-three codes are assigned to each digitized GIS polygon (LfSA). These codes are divided into three themes: geomorphology, sedimentology, and time. Mn/Model’s suitable landscape model has, as a small example, the ability to sort for different attributes such as buried soils, sandy loams, hummocks, terraces, loess, or LfSA’s older than 500 B.P. The coding system was also set up for eventual mapping in greater detail (larger scale than 1:24,000).

The LfSA is the basic mapping unit; however, different landforms or LfSA’s of similar genetic origin have been grouped into one of 15 "landscapes." The currently recognized Mn/Model landscapes include: Upland, Active Ice, Stagnant Ice, Ice Contact, Pediment, Glaciofluvial, Catastrophic Flood, Glaciolacustrine, Paleovalley, Peatland, Valley Terrace, Floodplain, Valley Margin, Eolian, and Lacustrine. The Upland landscape is an undifferentiated area awaiting future distinctions into one or more of the other 14 landscapes.

A Quality Control program was established for both the field work and office work. Assuming the model is conceptually accurate for each respective project area, the greatest potential for errors were in the digitizing and the transfer of attributes to each LfSA. Proofing occurred at four different points and from different perspectives during the process.

Three hundred fifty-three (353) cores were collected and logged as part of the ground truth program (Appendix E): 59 from the Minnesota, 29 from the Mississippi, 17 from the Rainy, 34 from the Red Lake Bog, 54 from the Red, 45 from the Rock, 89 from the Root, and 26 from the St. Croix. The uplands were mapped without ground truth. Sometimes additional core logs were included from other projects using similar standards as Mn/Model.

Eighty (80) new radiocarbon dates are reported herein as part of the Mn/Model suitable landscape study: 23 from the Minnesota, 8 from the Mississippi, 3 from the Rainy, 6 from the Red Lake Bog, 10 from the Red, 5 from the Rock, 13 from the Root, and 12 from the St. Croix.

Approximately 19,905 LfSA’s were mapped and modeled for the valleys, bog, and upland areas. This number includes: 3,985 for the Minnesota, 2,163 for the Mississippi, 754 for the Rainy, 827 for the Red Lake Bog, 727 for the Red, 432 for the Rock, 3,228 for the Root, and 880 for the St. Croix. The uplands had: 1,190 for the Anoka Sand Plain, 406 for Bemidji, 552 for Lake Agassiz, 1,775 for Lake Benton, 347 for Mountain Lake, and 2,639 for Nicollet.

The geomorphic modeling efforts concentrated on two main areas: developing a meaningful, useful, and flexible code; and delineating, digitizing, and proofing 14 sets of LfSA polygons. The numbers of codes (fields) and the code assignments themselves (attributes) evolved as the 14 project areas were being mapped; hence, so did the sophistication of the modeling. The evolution occurred as the modelers shifted from one project area to the next, and discovered needs for the new area that the old area did not require. Within this evolutionary process, the older models were then revisited to update to the newer codes, when practical.

The age, depositional environment, and surficial post-depositional environments were modeled for every differentiated LfSA. The ages were based upon local archaeologists’ hypothetical time periods (ca. 12,500-200 B.P.) that we might expect precontact peoples to have lived in Minnesota. The depositional environments were based mostly upon the energy levels and drainage conditions represented by the sediments and paleo-landscape. The post-depositional environments of the surface were based upon obvious erosion in the aerial photographs and soil series maps. The Landscape Suitability Ranking (LSR) is a product of the age and depositional or post-depositional environments, and is a valuable planning tool for predicting archaeological site integrity, and deeply buried site potentials.

Third-party tests were conducted on the LfSA’s and their LSR’s. These tests used 1,063 archaeological sites from the uplands (183) and valleys/bog (880). The first test was a site centroid on geomorphic polygon comparison (point-on-polygon overlay) and is a check on predicted environments and, to a small degree, a crude check on time. The second test was a more refined temporal comparison of site ages versus LfSA ages. The first test identified 45 sites that were aberrant relative to the interpreted LfSA’s. Further analysis indicated that most of these sites were aberrant because of inadequate site locational data. Less common reasons include scale issues, digitizing errors, and "unknown" causes. Eight of nine sites appeared "aberrant" because of misinterpreted LfSA assignments, which were corrected. The ninth site did not fit the LfSA assignment, but could not be corrected at the 1:24,000 scale of mapping. The temporal test screened all 1,063 archaeology sites for high and moderate confidence levels of archaeology temporal data. One hundred thirty-six (136) high and moderate confidence site ages were compared to the LfSA temporal data and found to be compatible.

12.21 ACKNOWLEDGEMENTS

The authors in charge (Curt Hudak and Ed Hajic) of the Mn/Model geomorphic data collection and digital mapping would like to thank the following individuals for assisting with - 1) field work and data research: Karen Bobbitt Gran, Eric Moshier, Erik Silvola, Anders Noren, and Vanessa Bodrie; 2) electronic data management and graphics: Phil Paradies, Karen Bobbitt Gran, Julie Melville, and Dan Tilly; 3) model testing: Patti Trocki and Rose Kluth; and 4) report chapter and newsletter production: Karen Lamberg, Dan Green, and Sara Bixby. Many NRCS, Minnesota DNR, academic, research, and local government officials offered data or land access permission to help make this project a success. Of special note were the many geologists, engineers, drillers, and technicians at the MnDOT Foundations Department that helped us to collect data, often times under grueling weather conditions. We truly appreciate the countless citizens and private corporations of Minnesota that offered permission for us to collect data from their land. Without the landowner permission and the available "ground truth," most of the interpretations discussed herein would have been merely speculative, especially the absolute ages of the landform sediment assemblages.

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