About two million years ago great ice sheets started to move southward, from various centers of refrigeration and accumulation in Canada, over the northern half of North America as far south as the Missouri and Ohio rivers. This vast sheet of ice slowly, relentlessly, plowed over the Great Lakes region, pushed onward by the ever thickening mass of ice on the Canadian plateau until it was brought to a standstill at its southernmost margin. The ice never advanced farther south than about 1600 miles away from its accumulation zone in Canada, or roughly to the position of the present Ohio River, whose course in part was established along its border.
    In its relentless, crushing advance the huge Labradorean ice sheet rasped, scraped off, and absorbed into itself the residual soil and loose rock masses which had covered the old rock surfaces of the state. The ice froze onto, plucked, tore huge blocks loose from bed rock, clutched them in its icy grip, embedded them in its glacial mass, and used them as tools to erode --- scrape, gouge, rasp, and scratch the surface over which it moved. It used fine sands and clays to sand, smooth, and polish not only the bedrock surface but also the boulders churned within the mill of moving ice.

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