Evolution of the Driftless Area and Contiguous Regions of Midwestern USA Through Pleistocene Periglacial Processes

ABSTRACT

In the Driftless Area region and contiguous regions (Paleozoic Plateau and Iowan Erosion Surface) within the U.S.A., there are a variety of northwest to southeast oriented landforms. They are interpreted to have been formed by periglacial nival and cryoplanation erosion processes during the Pleistocene. It is proposed that the oriented landforms are initiated by erosion focused along the edges of northwest-southeast trending snow dunes. Over time, cryopedimentation/cryoplanation widens some of the landforms into broad pediments or planed surfaces. These processes left other landforms standing as narrow, northwest-southeast trending, isolated ridges (paha and rock paha). Oriented valleys were carved in bedrock in some areas and were also carved in loess or loess underlain by till within other areas. An overall regional northeast to southwest trend of increasing amounts of pre-Illinoian till cover is interpreted to form by different degrees of widespread nival erosion. The Pleistocene wind was an important, indirect cofactor in the development of both the till pattern and the previously mentioned landforms. Determining these events not only helps us to formulate an evolutionary model for all three regions as a unit but also justifies speculation that the Driftless Area may have been glaciated during pre-Illinoian times.

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