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Chapter 3: Geomorphology and Stratigraphy

Article excerpt

Geomorphic investigations were conducted at 13HA385 in 1997, 1998, and 1999 (corresponding with the Phase I-III archaeological investigations). The primary goal of these investigations was to place the site's archaeological record in a geologic context. Soils and stratigraphic units were defined and described, and sedimentary environments were reconstructed. The geomorphic and soil-stratigraphic data provided the basis for interpreting site formation processes at 13HA385. In addition, these data, combined with temporal information gleaned from radiocarbon assays, were used to reconstruct the history of human occupation at 13HA385.

PHYSIOGRAPHY AND QUATERNARY GEOLOGY

Holocene deposits in the project area consist of alluvium and colluvium. These deposits are confined to the valley floor of the Iowa River. All deposits of fine-grained Holocene alluvium and colluvium in Iowa have been assigned to a single lithostratigraphic unit: the DeForest Formation (Bettis 1990, 1995; Bettis and Littke 1987). This unit was originally defined by Daniels et al. (1963) for a consistent sequence of alluvial fills in the Loess Hills of western Iowa. Subsequent studies of drainage basins in Iowa and adjacent parts of the Midwest have led to expansion and revision of the formation (Bettis 1990, 1995; Bettis et al. 1992, 1996; Dillon 1992; Fosha and Mandel 1991; Mandel 1994a, 1994b, 1996, 1999; Mandel and Bettis 1992). The DeForest Formation consists of eight formal members (Bettis 1990, 1995; Bettis et al. 1996; Mandel and Bettis 2002); site 13HA385 lies within the Corrington Member alluvium of the DeForest Formation.

Corrington Member deposits underlie alluvial fans and colluvial aprons along valley margins. Alluvial fans are located where small streams (first-- through third-order) enter large valleys. The Corrington Member is the most internally variable unit of the DeForest Formation and consists of very dark brown to yellowish brown oxidized loam and clay loam with interbedded lenses of sand and gravel (Bettis 1990, 1995; Mandel and Bettis 1992). The unit is stratified and often contains multiple buried soils. In the vicinity of 13HA385, surface soils developed into the Corrington Member have thick, well-expressed A-E-Bt profiles. The Corrington Member buries coarse-grained older alluvium, glacial diamicton, loess, or bedrock, and can grade laterally into Gunder Member deposits. The Corrington Member accumulated between ca. 9000 and 3000 B.P. (Bettis 1990, 1995; Mandel and Bettis 1992).

METHODS

Two backhoe trenches were excavated at 13HA385 in 1999 (Figure 1.3). Sediments and soils exposed in the trenches and archaeological excavation units were described using standard procedures and terminology outlined by Soil Survey Staff (1992) and Birkeland (1999). Graphic sediment-soil logs were constructed to visually convey stratigraphic and contextual information.

Standard USDA procedures were used to collect soil and sediment samples for particle-size and organic-carbon analyses (Soil Survey Staff 1992). All of the analyses were performed in the Soil Laboratory at the University of Wisconsin-Milwaukee. The particle-size distribution was determined using a modified pipette method described by Gee and Bauder (1986). The clays were separated by centrifuge precipitation of the silts and sands and decantation of the suspended clay fraction. The silt fraction was subdivided into fine (5mu-2mu), medium (20mu-5mu), and coarse (50mu-20mu) fractions. Organic carbon content was determined by the Walkley-- Black method (see Janitzky 1986).

RESULTS OF INVESTIGATIONS

Site 13HA385 is located on an alluvial fan (Allen Fan) that developed where two intermittent drainages flow out onto the valley floor of the Iowa River. The heads of both drainages are cutting into glacial till and outwash along the eastern edge of the Des Moines Lobe. This sandy and loamy glacial drift is the primary source of the sediment composing the Allen Fan. …