The present-day lake basins of Lakes Superior, Michigan, Huron, and Erie were formed when large masses of ice gouged out preglacial river valleys. The varying depths of the lakes are in part attributed to the differences in the thickness of the ice at the time of glaciation. The greatest depth of Lake Superior is 1,333 ft; of Lake Michigan, 925 ft; of Lake Huron, 725 ft; and of Lake Erie, 212 ft.
    The thick accumulation of late Devonian and Mississippian shales on the resistant Silurian dolomites around the western, northern, and eastern margins of the Michigan Basin was very important in determining future topographic features. Because the resistant dolomites form a bedrock high, they almost completely encircle the basin.
    The Pleistocene glacier found the weak spots, evident as gorges cut in the ancient rocks, as for example the downwarped trough where Lake Superior now lies. They were not barriers to its progress, but rather they became diversion channels or preferred pathways for the ice, where it could move more rapidly and erode still further.

    In many places softer and less resistant rocks, such as shale, were deeply carved and gouged by ice erosion. Thus, the pre-glacial stream valleys cut in the softer rocks and the edges of the Michigan Basin were widened and deepened into the Great Lakes. The softer rock masses picked up by the ice were usually ground between the harder stones and pulverized to fine sediment known as glacial flour or glacial till.  Thus, erosion of the softer shales by streams and later by glaciers resulted in the formation of Lake Michigan and Lake Huron. Thus the sites of these two lakes were predestined by events that occurred more than 300 million years ago. The location and shape of Lake Superior was determined by events in Keweenawan time, some 1,000-1,200 million years ago.

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