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• 9 <br />with disturbance. Available geologic evidence, although scanty, suggests <br />that climates of this region during the last 10,000 years have been both <br />wetter and drier than today. The postglacial thermal maxiimun, culmina- <br />ting about 6,000 years ago, was apparently drier than today but ensuing <br />neoglacial times have resulted in colder, and probably wetter periods over <br />the last 4,000 years. <br />During times of glacial maxima, and during times of aridity greater than <br />today, the high plains have been sites of significant wind erosion, trans- <br />port and deposition. Absence of plant cover, coupled with increased frost <br />action and surface soil freezing would be probable causes during the glacial <br />periods. Proximity to great glacial outwash plains and similarly fine- <br />grained proglacial lake beds provided enormous souces of silt for eolian <br />transport and soils throughout the region are noted to contain loess ma- <br />terials, particularly in swales and on lee slopes. During arid times, mo- <br />bilization of soil weathering products and remobilization of wind deposited <br />materials are manifest as blowout depressions and eroded fine-grained soil <br />profiles on exposed sites. <br />Soils of the~oLthern high._plains_in.hbntana and h'yoming are almost every- <br />where very ilmnaturQ I~bst lack textural B-horizons or have only weakly <br />developed thin clay-B zonation where those clays have formed in place. <br />Color zonation in these soils is generally the result of oxidation and tends <br />toward lOYR hues. Calcium carbonate contents generally increase in the <br />C-horizons or at the base of the oxidized zone but fully cemented caliche <br />