Little is known of pre-Wisconsinian ice advances in Wisconsin. The map shows ice extending to the outer limits of known pre-Wisconsinian glacial deposits, and also follows the suggestion of Gene Laberge in Geology of the Lake Superior Region (Geoscience Press, 1994) that early Pleistocene ice may have governed the present course of the Mississippi River. However, little is known of the extent and timing of individual ice advances and nothing at all is known in areas covered by late Wisconsinian deposits. The area shown as ice covered was probably never entirely ice-covered at any one time.
How much of the so-called "Driftless Area" was truly never glaciated is controversial. Some early workers claimed to have found erratics beneath loess within the conventionally mapped Driftless Area.
Created 17 July 2003, Last Update 26 March 2013
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