The surficial material throughout one-third of Kansas is the Peoria silt (loess) member of the Pleistocene Sanborn formation. Samples of this loess from an area 400 miles east-west by more than 200 miles north-south, which have been analyzed for particle size, chemical composition, and mineral constituents, show that major constituents of the silt-size fraction are quartz, feldspars, volcanic-ash shards, carbonates, and micas; quartz constitutes more than half the volume. A large suite of secondary minerals is present. The clay fraction consists of montmorillonite, illite, calcite, quartz, and feldspar, with a trace of a kaolinite mineral. Evidence used to determine source areas includes (1) progressive linear change in texture; (2) geographic variations in thickness; and (3) minor geographic differences in composition. The Platte River Valley is identified as the primary source for the Kansas region, and the Arikaree, Arkansas, and Republican valleys as important supplementary sources. The Missouri is the predominant source in the northeastern corner of Kansas. Other principal valleys, which lacked glacial outwash, were not source areas of Peoria loess in Kansas.
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