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Geologic Map
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Generalized Geologic Map of the Conterminous United States
North America Shaded Relief


  The North American Tapestry of Time and Terrain

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Coteau des Prairies


Close-up of
 the Coteau des PrairiesThe north-pointing, flatiron-shaped Coteau des Prairies in eastern South Dakota is one of the the most conspicuous landforms of the middle U.S. on this map. Some 200 miles (320 km) long, the low plateau of thick glacial deposits is underlain by a small ridge of resistant Cretaceous shale. The stream divide that was in place on either side of the Coteau (French for "hill") is probably the point at which the last Pleistocene ice sheet divided into lobes on either side of the formation. The James River Lobe on the west and the Des Moines Lobe to the east moved south beside the Coteau in pre-glacial stream valleys. This glacial activity further deepened its flanking lowlands, through which meltwater drained as the glaciers retreated to the north.