A Geoarchaeological Overview of South Dakota and Preliminary

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a rich archaeological record (Labelle et al. 2003). In the Southern Plains, dunes develop on the
downside edge of playas, creating a context where archaeological sites can be buried.
Alluvial Environments
Water flowing in streams provides energy for transporting sediment. The size of sediment particles
varies with discharge. Fast flowing water has more energy and can move larger particles. The current is
swiftest, and therefore sediments are coarser, on the channel bed and on the sand and gravel bars that
flank it. As water rises higher and spreads farther from the channel, its velocity decreases, and coarse
materials drop out, until eventually the water is carrying only fine sands, silts, and clay. Alluvial
environments are therefore divided into high energy environments where sands and gravels accumulate
on channel beds and bars, and low energy environments where finer particles are deposited from
overbank floods. The high energy environments are unlikely to attract occupation or preserve
archaeological deposits, but the low energy environments provide not only habitable surfaces, but the
potential for sites to be buried as the floodplain surface slowly aggrades. Terraces, which are former
floodplains abandoned by downcutting of the stream, have the additional advantage (from the
perspective of site formation and preservation) of being elevated above the active floodplain, and
therefore more protected from floods and erosion.
Low-lying terraces, however, can be occasionally inundated by high magnitude overbank floods,
resulting in the deposition of a veneer of younger alluvium over the original terrace surface. A terrace
veneer is like an eolian mantle in being relatively thin and flat. The soil formed into the terrace fill is
sometimes preserved under the veneer. Such veneers may occur on outwash terraces of rivers in the
glaciated part of South Dakota.
Investigations of Holocene valley sediments have been conducted throughout the state, including the
Big Sioux River (Artz and Riley 2010, Lueck et al. 1988), Highland Creek (Fredlund 1996); the
Cheyenne River (Fosha 1992), the Grand River (Brakenridge and McReady 1988), Bear Creek (Harksen
1974), Sage Creek (Kowal 1997; Kuehn 2003), the White River (Hannus and White 1985), and the
Missouri River (Coogan 1987; McFaul 1985, 1986).
Low order valleys are small tributaries that are geomorphologically different from larger valleys in
being narrower and more steeply graded. Because they are narrow, colluvial transport from the valley
walls often has an important contribution to the valley alluvium, to the extent that alluvial (stream
transported) and colluvial (slope transported) sediments are intermingled or interfingered with one
another in the valley fill. Many low-order valleys have a predominantly erosional history, but others are
broad enough to store sediment in terraces. In general, the more V-shaped a valley, the less buried site
potential it has. Lower order valleys with U-shaped or flat cross sections have sufficient valley floor
space to accommodate the storage of alluvial and colluvial sediment without it being flushed out by
flooding of the stream.
When lower order valleys store sediment, buried site potential is high. Wooded draws in the Missouri
Trench are known to contain buried archaeological sites (Artz and Toom 1985; Toom and Steinacher
1980). At Lighting Spring (Keyser and Davis 1984), the confluence at one locus of three small valleys
resulted in the accumulation of 3 m of sediment in ca. 3500 years, with stratified, Late Prehistoric
through Middle Archaic archaeological deposits throughout that thickness. Lange-Ferguson, a Clovis
mammoth kill in the Badlands (Hannus 1985) is another example of a highly significant archaeological
site buried in small valley alluvium.
Canyons in the Black Hills are different from valleys elsewhere in the state because they are highly
constrained in width and depth by resistant bedrock. Steep, impermeable mountain slopes rapidly
contribute runoff to streams, resulting in high energy stream flow that can transport large and abundantly
available cobbles and boulders, in addition to sands, silts and clays. Relatively thick alluvial fills with
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