Role of Soils and Soil Stratigraphy In Resolving Floodplain, Terrace, and Terrace Veneer Architecture In Meandering Stream Valleys at An Archaeologically Relevant Scale

Reconstructing geoarchaeological paleo-landsystems often involves addressing fluvial components. Geologic maps (‘Qal'), soil maps (NRCS soil series), and some material descriptors (‘mud') developed for other purposes belie complexities of the valley subsystem critical to prehistoric use of past landscapes, and finding, interpreting and managing archaeological remains that result from such use. Alluvial soils and soil stratigraphy are important components in the differentiation of various floodplain depositional environments and fine grain valley fills. Soils are particularly useful for identifying terrace veneers that serve as a common host to prehistoric cultural deposits. Unrecognized terrace veneer facies, if present, can result in underestimation of the age of fluvial incision that forms a terrace, and incorrect interpretation of a former floodplain environment for habitation (now part of the terrace sediment assemblage) when in fact a terrace environment is represented. Sedimentology of terrace veneers also provides information about relative flooding characteristics since terrace formation, particularly if coarse grain traction current sediment is interstratified in the sequence.

Due to the periodic influxes of sediment delivered by floodwaters, soils developed in some floodplain environments, on terrace veneers, and preserved within terrace sediment assemblages consisting at least in part of former floodplain environments, almost by definition are cumulic. Where multiple soils are represented through a given floodplain sequence, they are often welded atop one another, particularly in more humid environments. Nevertheless, identification of alluvial buried soils is important for defining the geometry of significant subdivisions of fine grain valley deposits, that in turn is a key to differentiating and interpreting various floodplain environments and environmental change at a scale that is relevant to prehistoric use of a valley landscape. Illustrative examples are drawn from the middle Mississippi (IL), White (AK), Sac (MO) and Blue Earth (MN) river valleys.