The age is a major North American geochronological subdivision of the Pleistocene epoch c 75 000-10 000 bp. It was the final glaciation of North American the fourth and last glacial stage of the Pleistocene. It followed the Sangamon interglacial and is the North American equivalent of the Würm glaciation in the Old World; it is broadly correlated with the Weichselian of northwest Europe and the Devensian of Britain. At certain times during this glaciation enough water was locked up in the form of ice sheets to cause a drop in sea level and the creation of a land bridge between Siberia and Alaska. It was probably during one such period that man colonized America from Asia. As a "stage" it is a chronostratigraphic subdivision of the Pleistocene with deposits in the upper U.S. Midwest and adjacent areas of Canada. Most of the Wisconsin deposits can be dated by radiocarbon. The sequence has been divided into early Wisconsin (c 75 000-53 000 bp) Middle Wisconsin (53 000-23 000 bp) and Late Wisconsin (23 000-10 000 bp). The substages have been defined as: Altonian (c 75 000-25 000 bp) Farmdalian (c 25 000-22 500 bp) Woodfordian (c 22 500-12 500 bp) Twocreekan (c 12 500-11 800 bp) and Greatlakean (c 11 800-10 000 or 7000 bp). The latter replaced the Valderan substage.
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