Index map
The north-pointing, flatiron-shaped Coteau des Prairies
in eastern South Dakota is arguably the most conspicuous landform
of the U.S. Midcontinent shown 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. Moving south along
pre-glacial stream valleys just to the west and east, respectively,
the James River and Des Moines Lobes of the last
Pleistocene ice sheet probably parted at the stream divide
preexisting the present Coteau ("hill" in French) and further
deepened its flanking lowlands- which then drained meltwater as
the glaciers retreated to the north.
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