SOILS AND GEOMORPHOLOGY OF THE LOWER LITTLE CEDAR RIVER VALLEY, NORTHEAST IOWA

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1 SOILS AND GEOMORPHOLOGY OF THE LOWER LITTLE CEDAR RIVER VALLEY, NORTHEAST IOWA Dale K. Splinter Environmental Science Program School of Geology Oklahoma State University Stillwater, Oklahoma Dennis E. Dahms and David W. May Department of Geography University of Northern Iowa Cedar Falls, Iowa Abstract: Few studies exist of the alluvial stratigraphy and geomorphology of the Iowan Surface landform region. We investigated the alluvial stratigraphy, soils, and geomorphology along a section of the lower Little Cedar River to aid in understanding the processes forming this landform region. We used terrace- and floodplain-surface height above the Little Cedar River, soil development, sedimentological characteristics, and stone lines to distinguish alluvial units and landforms. Three Pleistocene surfaces were mapped in the valley, including two terraces and an erosion footslope. The soils and sedimentary characteristics of these surfaces are extremely variable and are used to interpret geomorphic forces acting on the landscape. Holocene alluvial units identified include three terraces and the modern floodplain. Elsewhere in the Midwest, these units have been described as the DeForest Formation, and include the Camp Creek, Roberts Creek, and Gunder Members. The most strongly developed Holocene soils are found within the Gunder Member. Profiles exhibit either A/Bt/C or A/Bw/C horizonation. Soils in Roberts Creek alluvium are not as developed as those in the Gunder Member. These profiles are predominately A/Bw/ C. Soil development within Camp Creek alluvium is minimal with A/C profiles. [Key words: soil, stone line, alluvium, geomorphology, Iowa.] INTRODUCTION A generalized model of river response to changes in Holocene climates of the upper Midwest has been developed by Knox (1983). More recent alluvial stratigraphic studies and paleoenvironment reconstructions have supplied a significant amount of site-specific information regarding the response of rivers to climatic fluctuations during the Holocene (Chumbley et al., 1990; Baker et al., 1992, 1996, 2002; Denniston et al., 1999; Knox, 1993; Knox, 2000). Studies that have placed emphasis on Holocene alluvial stratigraphy are usually integrated into a generalized alluvial stratigraphic model (Bettis, 1990, 1992; Bettis et al., 1992, 1996). This model is based upon four lithostratigraphic units of the DeForest Formation (Table 1). 52 Physical Geography, 2005, 26, 1, pp Copyright 2005 by V. H. Winston & Son, Inc. All rights reserved.

2 IOWA SOILS AND GEOMORPHOLOGY 53 Table 1. Generalized DeForest Formation Characteristics for Eastern Iowan. Taken from Bettis (1992) DeForest formation Soil characteristics Gunder Member Roberts Creek Camp Creek 10 YR 4/3 5/4 to 2.5 Y 5/2 silt loam, silty clay loam, or loam grading to sand and gravel at depth; usually noneffervescent; lower part may be stratified; detrital organic matter often present in lower, stratified, coarse part of unit, moderately well to somewhat poorly drained Mollisols and Alfisols (A/Bw/C, A/Bt/C, or A/E/Bt/ C profile) developed in upper part; C horizons usually contain strong brown, yellowish red, or dark red (7.5 YR 5/8, 5 YR 5/8, or 10 YR 3/3) mottles; usually comprises low terrace that merges with a sideslope in a smooth concave upward profile; ranges in age from 10,500 to about 3000 B.P. 2.5 Y 3/0 to 10 YR 3/1 3/2 silt loam, silty clay loam and loam grading downward to sand and gravel; usually noneffervescent; thick sections are stratified at depth; detrital organic matter in lower part; relative thick Mollisol (A/C or A/Bw/C profile) developed in upper part; strong brown and yellowish red (7.5 YR 5/8 and 5 YR 5/ 8) mottles may occur throughout unit; found within the present floodplain, usually parallels the modern channel, also found in fan trenches; ranges in age from about 4000 to 500 B.P. 10 YR 3/2 5/4 silt loam to loam (sandy loam if sandy source materials are common) grading to sand and gravel in channel belt; usually noneffervescent; horizontally stratified where greater than 0.25 m in thickness; surface soils are Entisols (A/C profiles); unit often buries pre-settlement surface soil; thickest in and adjacent to modern channel belt, and at the base of steep slopes; ranges in age from 400 B.P. to modern. The general alluvium and soil characteristics of the DeForest Formation Members were originally based upon work conducted in loess blanketed regions of Iowa and Nebraska (Daniels et al., 1963; Daniels and Jordan, 1966). Because of the increasing alluvial stratigraphic studies away from its original loess source area, the model was revised to incorporate soil and alluvium characteristics of the upper Midwest (Bettis, 1990). To our knowledge there has been no attempt to fit the alluvial stratigraphy and soil characteristics of the Iowan Surface landform region in northeastern Iowa into the DeForest Formation. The lack of studies related to the Holocene alluvial stratigraphy of the Iowan Surface provides a unique opportunity to examine the alluvial stratigraphy and soil characteristics along a section of the Little Cedar River and to fit them into the DeForest Formation (Fig. 1). Along with investigating the Holocene alluvial stratigraphy we incorporate the geomorphology, soils, and sedimentary characteristics of Wisconsin age surfaces in this area. Just as there is little known about the Holocene alluvial stratigraphy and geomorphology of the Iowan Surface, studies of the late-pleistocene geomorphic history of this region are also lacking. In particular, we focus our attention on the role of stone lines as stratigraphic indicators that can be used to separate Wisconsin alluvium from post-wisconsin erosion deposits. The goals of this study were to (1) map the alluvial fill sequences and separate Wisconsin alluvial fills and erosion surfaces from Holocene alluvial fills, and (2) see how the DeForest Formation

3 54 SPLINTER ET AL. Fig. 1. The digital raster graphic (DRG) was acquired from the U.S. Geological Survey at scale of 1: Soil pit locations are noted on the DRG. alluvial units compared with the alluvial units identified in this study on the Iowan Surface. STUDY AREA The geomorphic evolution of the Iowan Surface landform region has long been a subject of debate. During the 1800s and into the early 1900s it was believed that the landform region currently known as the Iowan Surface showed evidence of a glacial advance known as the Iowan (McGee, 1891; Calvin, 1899; Alden and Leighton, 1917; Kay and Apfel, 1929). Studies that followed suggested that the

4 IOWA SOILS AND GEOMORPHOLOGY 55 Iowan glacial advance did not exist and that the surface was actually a separate advance of the Wisconsin (Leighton, 1931; Kay and Graham, 1943). However, this interpretation was challenged when Ruhe (1969) suggested that the Iowan Drift did not exist and that the Iowan Surface landform region was the product of extensive, regional erosion that occurred during the height of the late-wisconsin glaciation. Continued studies suggest that during the late-wisconsin glacial period (22,000 to 16,500 years B.P.) the southern margin of the Laurentide Ice Sheet covered much of the upper Midwest. The Iowan Surface is one region in the upper Midwest (NE Iowa, SE Minnesota, NW Illinois, and SW Wisconsin) that remained unglaciated during this period and was dominated by periglacial conditions. Evidence of icewedge casts and pattern ground features indicates past periglacial environments within the Iowan Surface (Walters, 1994). Work in southeast Minnesota and southwestern Wisconsin (Mason and Knox, 1997) suggests the mass wasting of hillslopes between 22,000 and 16,500 years B.P. This weathered material was probably transported into the valleys by low-order streams, sheetwash of slopes, and turbulent winds that scoured the sides of the valleys (Prior, 1991). Perhaps the most distinguishing feature of this mass wasting and erosion episode is a stone line. This stone line has become a stratigraphic marker that separates older (>16,500 years B.P.) Wisconsin alluvium and pre-illinoian till from latest (<12,000 years B.P.) Wisconsin deposits here. The stone line also represents the maximum depth of the mass wasting that occurred during the erosion period. Site Selection METHODS The study site along the lower Little Cedar River was selected because there were well-preserved terraces along the valley. Preliminary investigations suggested that there were sufficient Holocene alluvial surfaces to encompass all the alluvial fills associated with the DeForest Formation. Also, it appeared that there were surfaces higher than those associated with the DeForest Formation which would aid in explaining the geomorphic evolution of the valley. Surface Heights above the Modern River Terrace surfaces were delineated by walking escarpments and mapping the boundaries of each unit with a Trimble TSC1 Asset Surveyor. The data then were transferred into Pathfinder Office software and the line files were differentially corrected. We then exported the files into ArcView and overlaid the data onto a digital DRG of the study area. Within each delineated surface boundary, a series of points was taken with a global positioning system (GPS) receiver to achieve an estimated elevation of the surface height. However, because of error values commonly associated with GPS elevation data, the point locations were overlaid onto a topographic map and surface elevations were inferred using both GPS point elevation values and elevations from topographic maps.

5 56 SPLINTER ET AL. Soil Field Descriptions Soil profiles were described from cutbank exposures where possible. When this was not possible, pits were dug by hand on terrace treads. Each soil profile was described at least through the soil solum. Cutbank locations usually allowed sediment characteristics to be described well below the solum. Field descriptions included horizon depth, color, soil structure, clay films, texture, and boundaries (Soil Survey Division Staff, 1993; Birkeland, 1999). Field textures were validated using laboratory procedures (Singer and Janitzky, 1986). Soil Laboratory Techniques Particle-size analysis (PSA) was conducted by the pipette method (Singer and Janitzky, 1986) using the U.S.Department of Agriculture system. Preparation for PSA followed the following procedures: samples were air-dried and peds were gently crushed, weighed, and passed through a #10 sieve (2.0 mm) to separate the gravel from the <2 mm fraction. The >2mm fraction was weighed and subtracted from the initial sample weight and gravel percent was calculated. The <2 mm fraction was mechanically split to obtain statistically random samples. Samples weighing approximately 25 g were used in the analyses. Samples were placed in ml beakers and organics were removed with 30% H 2 O 2 on overnight heat at 60 C. Samples were not treated for carbonates as the soils showed no sign of reaction with 10% HCL. Samples were dispersed using sodium hexametaphosphate and overnight shaking. Sands were separated from silts and clays by wet sieving through a #270 sieve (0.5 mm). The sand separates were dried at 105 C overnight and weighed. The silts and clays were transferred into 1000-ml settling tubes. Silt and clay fractions were sampled by pipette at intervals based on Stoke s Law. Interpretation of the Valley Stratigraphy RESULTS AND DISCUSSION There are six distinct alluvial surfaces and an erosion footslope surface in the study area (Fig. 2). There are three late Wisconsin features that include two terraces (T5 and T4) and an erosion footslope surface (EFS). The remainder of the alluvial surfaces (T3, T2, T1, and FP = floodplain) are interpreted to be of Holocene origin. These surfaces were delineated by using topographic surface height above river at low flow, soil development, alluvium properties, sedimentary characteristics, a single radiocarbon date, and stone lines. The Holocene alluvial units were further characterized and given member status using the properties of the DeForest Formation.

6 IOWA SOILS AND GEOMORPHOLOGY 57 Fig. 2. Generalization of landform positions along the lower Little Cedar River. There were no east to west transects that depicted this stratigraphy in its entirety. The sequence represented depicts the variability and complexity of the area. The younger Roberts Creek unit (T1) is not represented as a separate entity of Roberts Creek in this depiction. Soils and Geomorphology of Late-Wisconsin Surfaces The topographically highest surface is T5 and ranges between 10.7 m to 8.0 m above the river. Soil and subsurface properties of T5 (Table 2) were sampled and investigated at ELCR1, ELCR2, WLCR1, and WLCR2 (Fig. 1). The soils of T5 are dominated on the east side of the valley by sandy loam textures grading to sand, while on the west side of the valley the textures grade from loam to sand. Soil color is dominated by 10 YR hues that generally increase in value and chroma in the down profile direction. In the unoxidized sands of the C horizons of T5 are numerous lamellae (Fig. 3). These lamellae begin at 1.0 m below the surface and continue past a depth of 3.0 m. Generally these lamellae are less than 1.5 cm thick; however, in some locations they exceed 8.0 cm. We interpret these to be pedogenic in origin because of their wavy appearance and the way they curve around the tops of pebbles in the C horizon. Horizon boundaries on the west valley side (WLCR1 and WLCR2) tend to be either gradual or clear to the underlying C horizon. On the east side of the valley (ELCR1 and ELCR2) the lower horizon boundaries become important indicators of past geomorphic processes acting on the formation of the Iowan Surface. At ELCR1, the boundary that separates the Bt2 horizon from the CB horizon is a stone line (Fig. 3). The constituents of the stone-line range from large cobble to predominantly gravel in size. The majority of the largest constituents are very weathered granite that when pulled from the stone line can often be broken by hand. Moving west from ELCR1 to ELCR2 the particle-size constituents of the stone line decrease in size. This was evident by sampling halfway across T5 and noting that large cobbles were not incorporated into the stone line. The general size of materials decreased from large cobble to gravel at ELCR1 to small cobble and gravel

7 58 SPLINTER ET AL. Table 2. Field Descriptions of Soils Associated with Wisconsin Age Surfaces Site a Surface b Depth c Horizonation d Texture e Structure f Color g Clay films h Boundary i E LCR1 T A sandy loam sbk 10 YR 3/1 grd/sm Bt1 sandy loam sbk 10 YR 4/3 2d pf grd/sm Bt2 sandy loam sbk 10 YR 4/4 2d pf stone line CB sand abk 10 YR 5/6 clr/sm 90+ C sand sg 10 YR 6/4 Btb sand sbk 10 YR 4/4 E LCR2 T A sandy loam sbk 10 YR 3/1 grd/sm Bw sandy loam sbk 10 YR 4/4 clr/sm BC sand abk 10 YR 5/6 clr/sm 70+ C sand sg 10 YR 7/6 Btb sand sbk 10 YR 4/4 W LCR1 T A loam gr 10 YR 3/1 abr/sm Bt1 loam abk 10 YR 4/2 2d pf grd/sm Bt2 loam sbk 10 YR 4/3 2d pf clr/sm BC loamy sand sbk 10 YR 4/4 grd/sm CB sand sbk 10 YR 4/6 clr/sm 112+ C sand sg 10 YR 6/4 Btb sand sbk 10 YR 4/4 W LCR2 T A loam gr 10 YR 3/1 abr/sm Bt1 loam sbk 10 YR 4/2 2d pf grd/sm Bt2 loam sbk 10 YR 4/3 clr/sm CB1 sand sbk 10 YR 4/4 grd/sm CB sand sbk 10 YR 4/6 clr/sm

8 IOWA SOILS AND GEOMORPHOLOGY C sand sg 10 YR 6/4 Btb sand sbk 10 YR 4/4 W LCR3 T A sandy loam sbk 10 YR 2/1 clr/sm AB loamy sand sg 10 YR 3/1 clr/wavy Bt sandy loam sbk 10 YR 5/6 2d pf stone line Cox sand sg 7.5 YR 5/8 abr/sm 83+ 3C till firm till 10 YR 6/1 abr/wavy W LCR4 EFS 0 25 A sandy loam gr 10 YR 2/1 clr/sm Bw sandy loam abk 10 YR 3/2 clr/sm CB loamy sand abk 10 YR 5/4 89+ C sand sg 10 YR 5/6 Btb sand 10 YR 4/4 a W = west side of river, E = east side of river, LCR = Little Cedar River, and number is the sample pit. b T = terrace, FP = floodplain, EFS = erosion footslope surface, and number is the surface level. c Depth of profile in centimeters where data is blank. Lamellae are noted sporadically throughout the profile. d Standard horizon nomenclature. e Texture based on laboratory analysis of particle-size. SCL = sandy clay loam. f Sbk (subangular blocky), abk (angular blocky), sg (single grain). g Color is given as moist. h Amount: v1 = very few, 1 = few, 2 = common, 3 = many. Distinctness: f = faint, d = distinct, p = prominent. Location: pf = ped faces, po = interstitial pores. i Grd (gradual), clr (clear), sm (smooth), abr (abrupt).

9 60 SPLINTER ET AL. Fig. 3. Subsurface stratigraphy of T5 is represented by a stone line at 60 cm and lamellae throughout the profile. Depth of profile extends to 3.0 m. Stone line is the stratigraphic indicator that separates Wisconsin alluvium (i.e., >16,500 years B.P.) from younger material deposited after the Iowan Surface Erosion Event. in size midway across T5. At ELCR2 there appeared to be only a very slightly expressed stone line, which appeared to be disrupted by bioturbation. On the west side of the valley (WLCR1 and WLCR2) the soil and underlying sedimentological characteristics are similar. There are no stone lines associated with T5 where sampled. We suggest one possible reason for there being no stone line on the west side of T5 is directly related to distance from the valley wall and late-wisconsin erosion material source. On the east side of the valley T5 is closer to the valley wall

10 IOWA SOILS AND GEOMORPHOLOGY 61 Fig. 4. Subsurface stratigraphy of the EFS. The stone line occurs at approximately 70 cm and overlies pre-illinoian till. than T5 is on the west side of the valley. We propose that when the hillslope erosion occurred during the late Wisconsin, material was moved downslope via solufluction as suggested by Prior (1991). On the east side of the valley, the larger material stopped at the base of the hillslope. Smaller particles continued across the landscape under the influence of sheet-flow conditions. This would explain why the particlesize distributions across T5 follow the spatial distribution noted in this study. The next topologically lower surfaces are T4 and the EFS. They are found adjacent to one another on the west side of the valley and are at the same elevation (Fig. 1). Both surfaces exist at 6.0 m above the modern river. The EFS (WLCR3) has distinctly different soil and sediments characteristics (Table 2) than T4 (WLCR4). Soil color at WLCR3 is 10 YR hue that increases in value and chroma to a depth of 70 cm. At 70 cm a stone line serves as the boundary between the soil and the underlying glacial till (Fig. 4). Below the stone line is sandy sediment that is strongly oxidized (7.5 YR 5/8). This oxidized material lies above clayey pre-illinoian till. This sequence of oxidized sands over firm glacial till probably represents pre-illinoian ablation till that overlies clayey and mottled unoxidized lodgement till. The soil color at WLCR4 is 10 YR hue that increases in value and chroma throughout the profile. In the unoxidized alluvial sands of T4 lamellae were described. These lamellae are the same color and thickness as those associated with

11 62 SPLINTER ET AL. Table 3. Field Descriptions of Soils Associated with Holocene Age Surfaces a Site Surface Depth Horizonation Texture Structure Color Clay films Boundary E LCR3 T A sandy loam abk 7.5 YR 2.5/1 grd/sm Bw sandy loam abk 10 YR 3/2 clr/sm BC1 loamy sand sbk 7.5 YR 4/6 grd/sm BC2 loamy sand abk 7.5 YR 3/4 grd/sm BC3 loamy sand abk 10 YR 3/3 clr/sm 140+ C sand sg 10 YR 6/3 E LCR4 T A loam sbk 7.5 YR 4/1 grd/sm Bt1 clay loam sbk 10 YR 5/3 1f pf grd/sm Bt2 loam sbk 10 YR 6/3 1f pf grd/sm BC loam sbk 10 YR 6/2 clr/sm 90+ C sand sg 10 YR 6/4 W LCR5 T Ap loam gr 10 YR 3/1 grd/sm A loam sbk 10 YR 3/1 2d pf grd/sm Bw1 SCL sbk 10 YR 5/2 2d pf grd/sm Bw2 sandy loam sbk 10 YR 5/4 clr/sm BC1 loamy sand sbk 10 YR 5/8 clr/sm BC2 sandy loam sg 10 YR 5/8 clr/sm 153+ C sand sg 10 YR 7/2 W LCR6 T A clay loam abk 10 YR 3/1 grd/sm Bw clay loam abk 7.5 YR 3/2 grd/sm Bt1 clay loam sbk 10 YR 5/2 2d pf grd/sm Bt2 loam sbk 10 YR 4/3 2d pf grd/sm Bt3 loam sbk 10 YR 5/3 2d pf grd/sm

12 IOWA SOILS AND GEOMORPHOLOGY C sand sg 10 YR 4/4 W LCR7 T2 0 9 A sandy loam gr 10 YR 4/2 clr/sm 9 25 Bw1 loam sbk 10 YR 4/2 grd/sm Bw2 loam sbk 10 YR 5/3 grd/sm BC sand sbk 10 YR 5/2 grd/sm 80+ C sand sg 10 YR 6/4 W LCR8 T A1 sandy loam gr 10 YR 3/1 clr/sm BW sandy loam sbk 10 YR 3/2 clr/sm 85+ C sand sg 10 YR 7/2 E LCR5 T A sandy loam gr 10 YR 3/1 grd/sm Bw sandy loam sbk 10 YR 3/2 clr/sm CB sand sg 10 YR 6/4 clr/sm 92+ C sand sg 10 YR 7/2 E LCR6 T A loam gr 10 YR 4/1 clr/sm Bw loam abk 10 YR 4/2 abr/sm 78+ C sand sg 10 YR 6/4 W LCR9 FP 0 22 A loam sbk 10 YR 4/2 abr/sm 22+ C sand sg 10 YR 6/3 E LCR7 FP 0 10 A loam sbk 10 YR 4/2 abr/sm 10+ C sand sg 10 YR 6/3 a Refer to Table 2 for descriptions of table headings.

13 64 SPLINTER ET AL. T5. The main difference between the sedimentary characteristics of T5 and T4 is the lack of a stone line at the lower boundary of T4. The similarity of elevation between T4 and the EFS is not fully understood. We give two explanations of how the same surface elevation may have occurred. The first one is that when the EFS was eroded and cut, T4 was also being eroded to the same height as the EFS. This appears to be possible if we assume that T4 was eroded from T5. This would explain why T4 was not mapped on the east side of the valley. The lamellae described in both alluvial deposits may support this assumption. The lack of a stone line within either T5 or T4 (west-side) is again supported by distance from the valley wall. The second explanation is that T4 was not eroded, but rather was the floodplain and that the EFS was eroded to that surface height. Additional studies are needed to verify the above possibilities. Holocene Alluvial Units: The DeForest Formation The topographically highest Holocene surfaces (T3) are found between 3.0 m and 2.7 m above the river and are the Gunder Member. We correlated this surface with the Gunder Member by using the topographic position of the surface in the landscape, soil development, and alluvium characteristics. Field samples are based upon descriptions from ELCR3, ELCR4, WLCR5, and WLCR6 (Fig. 1). However, with no radiocarbon ages from this alluvium to place this member into an age context, we use the dates offered by the Midwestern model of the DeForest Formation to estimate the age of the landform. Typical ages for the Gunder Member are between 12,500 and 3500 years B.P. (Bettis, 1992). The large range of age of the Gunder Member is not of significance to us here, as we do not attempt to numerically date the surface to form a chronologic reconstruction. Soils developed in Gunder alluvium are the most weathered and deepest of the soils developed in Holocene alluvium (Table 3). This is directly related to time as the key entity in the development of Gunder soils compared to other Holocene units. Soil variability is most extreme in Gunder alluvium. Some soils are extremely sandy, with slight evidence of clay translocation, while others tend to be more clayey and show obvious evidence of clay translocation (Table 3). Baker et al. (2002) suggested that differences in the morphology of the soil and textural variations of the Gunder Member in southeastern Minnesota can be used to differentiate early from late-gunder alluvial deposits. Baker et al. (2002) reported that early Gunder alluvium can be distinguished by its silt loam texture to light (10 YR 4/2-4/3) loam soil texture, with surface soils exhibiting Bt horizons, whereas late-gunder surface soils lack Bt horizons and consist of loam textures. Along the lower Little Cedar River, Gunder alluvium was represented by a 10 YR 4/3 loam texture at only one of the four sampled locations (WLCR6) and is associated with a Bt2 horizon. Gunder alluvium of the Little Cedar River ranges in variability from sandy loam to clay loam, but shows no evidence of silty textures (Table 3). This suggests that the texture variability along the lower Little Cedar River is much greater than that described by Baker et al. (2002). The reason for the difference in texture is a result of the differing geographic positions of the study locations. The study by Baker et al. (2002) was conducted along

14 IOWA SOILS AND GEOMORPHOLOGY 65 reaches of streams that were directly affected by loess deposition from the Mississippi River during the early Holocene. There is no evidence that the region surrounding the lower Little Cedar River was affected by similar amounts of loess deposition during this period. This suggests that soils of loess mantled landscapes should have different textures than soils beyond loess regions. Based on topographic position and soil characteristics, the next youngest Holocene alluvial unit in the valley corresponds to the Roberts Creek Member (T2). Field descriptions of the Roberts Creek Member were obtained at WLCR7, WLCR8, and ELCR5 (Fig. 1). The unit correlated here as the Roberts Creek Member is often covered by a younger alluvial deposit. Terrace tread height varies among separate Roberts Creek alluvial fills, but the average elevation of terrace tread height is between 2.3 m and 1.3 m above the river. Along the lower Little Cedar River, this surface has the most undulating topography of all the DeForest Formation. The surface is identifiable by the presence of abandoned channels. Roberts Creek alluvium tends to be the darkest of all the alluvium in the Little Cedar River Valley. This has been described from other locations where the DeForest Formation has been investigated. Bettis (1992) stated that higher water tables during this period (i.e., 3500 to 380 years B.P.) inhibited oxidation of the organic matter which resulted in the darker colors of this unit. Soils associated with the Roberts Creek alluvium tend to have less variability than the Gunder Member soils (Table 3). Textures of the soils are sandy loam to loam and grade to sand in the C horizon. Soil color is represented by 10 YR hue that increases in value and chroma with depth (Table 3). Depths of soil profiles are between 80 and 92 cm, which are less than the Gunder Member soil profiles (Table 3). The Roberts Creek alluvium in this study fits nicely within the Midwest model of the DeForest Formation. A second Roberts Creek fill has been identified in this study (T1). This younger Roberts Creek alluvium is approximately 1.0 m above the river and was investigated at ELCR6. It differs from Camp Creek (discussed later) alluvium in the lower Little Cedar River Valley in that it is not stratified at depth, has much coarser-size constituents in profile, and has a deeper soil profile than any of the profiles described within Camp Creek alluvium. It differs from T2 Roberts Creek in that it is not comprised of the typical dark colored alluvium that is noted in other Roberts Creek fills (Table 3). The soil profile, where described, has an A/Bw/C horizonation (Table 3). This fill was investigated in detail at the Nashua Sand and Gravel Quarry Pit. Numerous buried trees were revealed during an excavation of this sand and gravel unit. A radiocarbon date was obtained on one of the buried logs. Conventional 14 C analysis indicates that the log was buried between 1410 and 1500 AD ( years B.P. [Beta ]). The radiocarbon date on an elm log (Center for Wood Anatomy Research in Madison, WI) establishes the end of Roberts Creek Member deposition (i.e., 500 years B.P.). The unit may have been deposited when large floods occurred during the transition from the warm medieval interval to the cooler Little Ice Age. Knox (1993) suggested that very large floods occurred in the Upper Midwest between about 1250 and 1450 AD. We suggest this unit corresponds to one or more of these flood events because of the dominance of sand and gravel in the alluvium in which the logs are buried.

15 66 SPLINTER ET AL. The modern floodplain is comprised of Camp Creek alluvium. Floodplain surfaces in this area have weakly developed surface soils because of rapidly aggrading alluvium. These surfaces are less than 1.0 m above the river. Camp Creek alluvium was investigated at WLCR9 and ELCR7 (Fig. 1), where the physical properties of the accompanying soil exhibits an A/C profile (Table 3). These soils, where sampled, are no more than 22 cm deep. These soils lack B horizons for two reasons: (1) the soils are too young for pedogenesis to have developed B horizons, and (2) floodplain surfaces in this area flood almost yearly, which interrupts and limits the process of horizon development. This is represented by numerous flood deposits that occur within the unoxidized flood deposits of the C horizon. In the LCR, Camp Creek fits well into the DeForest Formation of the upper Midwest. CONCLUSIONS The lower Little Cedar River provides a unique investigation into the late- Wisconsin and Holocene geomorphic history of the Iowan Surface. Evidence for erosion during the late-wisconsin is evident in two places. These are definable by stone lines overlying both pre-illinoian till and Wisconsin alluvium. The decrease in the particle-size of the stone line across T5 suggests that larger particles were transported to the bottom of the valley wall and stopped because of their size, while smaller eroded particles continued across T5 under the influence of gravity and solifluctional processes. The Holocene alluvial stratigraphy along the lower Little Cedar River is generally consistent with the model of the DeForest Formation. All three Members of the DeForest Formation were identified in the study area. The Gunder Member exhibits the most variable characteristics of the alluvial units in the study area. In particular, the texture of the alluvium differs from that identified in studies conducted in adjacent loess-mantled landscapes. In addition to the Roberts Creek Member outlined in Bettis (1992) we report finding an additional Roberts Creek Member. This younger Roberts Creek fill is distinguished from the Roberts Creek Member of Bettis (1992) by its color and degree of soil development. Camp Creek alluvium in the Little Cedar River Valley is similar to that in other valleys in the upper Midwest and is easily definable by its low topographic position in the valley and A/C soil profile. Acknowledgments: We would like to thank the families of Joel Berends, Jerry Slessor, and Gary Johanningmeier for allowing us to walk, dig pits, and work on their land. This paper was greatly improved by the comments of two anonymous reviewers. We would also like to thank Mark Steger and James Quinn for volunteering time in the field. Cartography was completed by James Quinn. REFERENCES Alden, W. C. and Leighton, M. M. (1917) The Iowan Drift, a review of the evidences of the Iowan stage of glaciation. Iowa Geological Survey Annual Report, Vol. 26, Baker, R. G., Bettis, E. A., III., Denniston, R. F., Gonzales, L. A., Strickland, L. E., and Krieg, J. R. (2002) Holocene paleoenvironments in southeastern Minnesota

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