In western Illinois, many soil profiles developed into upland loess deposits (Peoria Silt) contain Archaic period artifacts greater than 3500 yr B.P. in stone zones below plow level. For artifacts in prairie and prairie-forest transition soils (not forest soils), depth distribution curves suggest they were buried in biomantles by small soil fauna. Artifacts of sizes archaeologists routinely collect generally move down while retaining fine-scale horizontal integrity. The process results in stratigraphic separation of Archaic and Woodland period components that otherwise would commingle at the surface. The characteristic distribution of known upland sites, narrowly rimming stream valley headwaters, reflects incomplete burial of materials on steeper forested slopes. On adjacent gently sloping upper shoulder segments of valleys where grassy cover more strongly influences soil development (and by inference on broad level uplands where true prairie soils occur), Archaic artifacts will be buried in the biomantle and go undetected by surface surveyors. ᭧

Late Quaternary hillslope and valley evolution of the loess‐mantled uplands between the Illinois and Mississippi Rivers has produced a landscape composed of surfaces of different ages. Archaeological remains occur in a variety of geological contexts, including intact surfaces that predate the artifacts upon them, as lag on surfaces formed by subsequent erosion, and buried within alluvium, colluvium, and upland loess soil profiles. Holocene channel belts are largely confined to trenches formed by deep stream incision during the late Pleistocene, and much of the alluvial record is intact, especially in valleys with aggraded floodplains. Modest Holocene hillslope retreat in backslope and shoulder landscape settings and temporary storage of footslope colluvium is documented at some sites. Historic land use practices have resulted in severe hillslope retreat and near complete burial of Holocene valleys with a 0.5‐1‐m‐thick veneer of historic sediment. © 1993 John Wiley & Sons, Inc.

Postglacial geomorphic development of the Buchanan Drainage, a small tributary to the South Skunk River, is reconstructed by documenting relationships among four allostratigraphic units and 17 radiocarbon dates. Formation and headward expansion of the valley was both episodic and time-transgressive. Response to downstream conditions in the South Skunk River largely controlled the early formation of the basin. Downcutting through Pleistocene deposits produced a gravelly lag deposit that was buried by alluvium in the downstream portion of the valley during the early Holocene (10,500–7700 yr B.P.). Lag deposits formed in a similar manner continued to develop in the upper portion of the drainageway into the late Holocene (3000-2000 yr B.P.). Episodes of aggradation during the middle Holocene (7700-6300 yr B.P.) and late Holocene (3000-2000 yr B.P.) were separated by a period of soil formation. Holocene geomorphic events in the drainageway coincide with some vegetational and climatic changes as documented in upland pollen sequences from central Iowa. Analysis of plant macrofossil assemblages recovered from alluvium indicates that during the middle Holocene forest contracted and prairie expanded into the uplands within the basin. Vegetational changes within the basin apparently had only minor influence on rates of hillslope erosion, and the widely accepted relationship between prairie (versus forest) vegetative cover and increased rates of hillslope erosion did not hold. Instead, greater amounts of erosion occurred under forested conditions when local water tables were higher and seepage erosion was more effective.

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Explaining prehistoric mound development requires both anthropological and geoarchaeological perspectives. Illinois Hopewell (Middle Woodland) mounds are remarkable for the range of earthen materials used in their construction. Adding to this variety we document the presence of upturned sod blocks in a mound at the Mound House site. There and at other Illinois sites the sods have dark, 3-10-cm-thick A horizons with minimal or no evidence of B horizon development. They required no more than a few decades to form and did so under a grass cover. Humans probably created the conditions that enabled sods to form, but the sod blocks were not cut from soils adjacent to the mounds (unless from another mound surface nearby) or from soils in habitation areas. In some respects, sod blocks would have been a superior earthen building material, appropriately chosen, for instance, to construct stable, near-vertical walls of above-ground tombs. Their selection and use, however, cannot be explained solely according to principles of sound and efficient mound construction. We argue that sod blocks and other kinds of earth for Illinois Hopewell mounds surely had important symbolic dimensions in addition to their structural properties.