Wisconsin Glacial Stage

▪ geology

      most recent major division of Pleistocene time and deposits in North America (1,600,000 to 10,000 years ago). It was named for rock deposits studied in the state of Wisconsin. At least the last half, and possibly all, of the Wisconsin Stage corresponds to the Würm Glacial Stage of classical European usage. The Wisconsin Stage follows the Sangamon Interglacial Stage and represents the last time that major continental ice sheets advanced across the North American continent. Its end approximately coincides with the close of the Pleistocene Epoch in North America, a somewhat vague termination because the glacial ice did not recede everywhere at the same time. Similarly, there is no conclusive evidence that the ice will not return.

      The Wisconsin Glacial Stage is the best known of the glaciations that affected North America, and it has been possible to divide the Wisconsin into early, middle, and late episodes. The beginning of the Early Wisconsin has been dated (during the late 1970s) using cores from the seafloor to about 115,000 years ago. In the Great Lakes region, where deposits of Wisconsin age are well represented, five substages, representing successive advances and retreats of glaciers, are recognized.

      The Wisconsin Glacial Stage had a profound effect upon the landscape of North America. The Great Lakes are remnants of glacial lakes that bordered the vast continental ice sheets. Much material was eroded from various areas, only to be deposited elsewhere. The rich soils of the Great Plains, for example, are largely derived from the silt deposited by streams of glacial meltwater. Sequences of pollen grains preserved in Wisconsin sediments indicate the presence of tundra on the fringes of the ice masses; the tundra was occupied by the woolly mammoth, caribou, and muskox. Bison, horses, giant ground sloths, and a host of other vertebrates occupied the Great Plains to the southwest, while an equally rich and diverse fauna that included peccaries, sabre-toothed cats, mastodons, and camels occupied the region to the southeast. During the Late Wisconsin the first clear traces of human presence in North America became evident, though there are some indications that humans may have arrived even earlier.

      A feature of the end of the Wisconsin Glacial Stage, and so of the Pleistocene Epoch as well, was the extinction of numerous species of large mammals in North America, including the horse, mammoth, mastodon, camel, giant armadillo, and sabre-toothed cats. It has been suggested that overhunting by the first humans in the New World was the major factor in this wave of animal extinction. The true answer is probably more complex, involving diverse climatic and environmental factors, in addition to possible human intervention.

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