Quaternary Geology along the Eastern Flank of the Coteau Des Prairies, Grant County, South Dakota
1990-07
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Quaternary Geology along the Eastern Flank of the Coteau Des Prairies, Grant County, South Dakota
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1990-07
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The Quaternary sediments and landforms of Grant County in northeastern South Dakota were evaluated in an effort to determine the lithostratigraphy, morphostratigraphy, and Late Cenozoic history of the area. Nine lithologically distinct tills were identified from surface exposures, with five additional units, termed drift complexes, recognized in the subsurface of the Coteau des Prairies. Five of the tills (Whetstone, South Fork, Yellow Bank, Hawk Creek, and Granite Falls) and all of the drift complexes (1 through 5) are pre-Late Wisconsin in age, and some may date to the late Pliocene. Clast lithologies and orientations indicate that most of the pre-Late Wisconsin glaciers advanced into the area from the north and northwest, although the Hawk Creek Till was deposited by a glacier from the northeast. Four Late Wisconsin-aged tills were recognized. The oldest unit, the Toronto Till, is exposed in the western third of the County and was deposited between 30,000 and 20,000 yrbp. The New Ulm Till and the till which comprises the Big Stone Moraine are found over the eastern two-thirds of the County and were deposited by the Des Moines Lobe between 14,000 and 11,700 yrbp. The till of the Dakota Moraine was deposited by the James Lobe about 14,000 to 13,500 yrbp. Six distinct geomorphic zones are found in the area. Each can be associated with a specific phase of Late Wisconsin glacial activity or individual ice lobe. In Grant County, the eastern-most extent of the James Lobe is marked by a push moraine (the Dakota Moraine). The Toronto Till Plain is a well-dissected surface lacking constructional landforms. The Bemis Moraine and Altamont-Gary Moraine Complexes are made up of a variety of moderate- to high-relief ice-contact and ice-stagnation features and mark the eastern edge of the Coteau des Prairies. The Minnesota River Valley is an area of relatively low relief ground moraine that covers the eastern half of the County. The Big Stone Moraine is an arcuate belt of low-relief, hummocky topography in the northeastern part of the County and marks the final advance of Late Wisconsin ice in the area.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Jay Phillip Gilbertson in partial fulfillment of the requirements for the degree of Master of Science, July 1990.
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