GREAT LAKES' ORIGINS
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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