GEOLOGIC HISTORY OF GREAT LAKES BEACHES

Jack L. Hough

Abstract


The locations of the Great Lakes and many details of the lake bottom topography bear a distinct relationship to the bed rock structure. Normal stream erosion in pre-glacial time probably etched out the major topographic relief of the region, forming the major basins and even some of the present bays, in the weak rock belts.
Glacial ice, advancing over the region in several stages, followed the lowlands but reshaped them and probably deepened most of them.
The known lake history, beginning with the last retreat of the ice from the southern rims of the Michigan and Erie basins, involves a number of stages at different levels in each of the basins. These lakes discharged at various places at different times, because of readvancement or retreat of the glacial ice front and because of tilting of the earth's surface. The writer's summary of this history is illustrated by a series of sixteen maps. The practical importance of two extremely low lake stages is pointed out. These have affected foundation conditions in the vicinity of many river mouths. The newly established recency of some of the higher lake stages (Nipissing and Algoma), and the revision of the elevations attained by them, affect estimates of the intensity of beach action and they affect conclusions regarding the time of last discharge of water through the Chicago outlet.

Keywords


geology; Great Lakes; lake levels

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