The Wisconsin Glacial Episode, also called the Wisconsinan glaciation, was the most recent major advance of the North American ice sheet complex. This advance included the Cordilleran ice sheet, which nucleated in the northern North American Cordillera, the Innuitian ice sheet, which extended across the Canadian Arctic Archipelago, the Greenland ice sheet, and the massive Laurentide ice sheet that covered the high latitudes of central and eastern North America. This advance was synchronous with global glaciation during the last glacial period, including the North American alpine glacier advance, known as the Pinedale glaciation. The Wisconsin glaciation extended from approximately 85,000 to 11,000 years ago, between the Sangamon interglacial (known globally as the Eemian stage) and the current interglacial, the Holocene. The maximum ice extent occurred approximately 25,000–21,000 years ago during the last glacial maximum, also known as the Late Wisconsin in North America.
This glaciation radically altered the geography of North America north of the Ohio River. At the height of the Wisconsin Episode glaciation, the ice sheet covered most of Canada, the Upper Midwest, and New England, as well as parts of Idaho, Montana and Washington. On Kelleys Island in Lake Erie or in New York's Central Park the grooves left by these glaciers can be easily observed. In southwestern Saskatchewan and southeastern Alberta, a suture zone between the Laurentide and Cordilleran ice sheets formed the Cypress Hills, the northernmost point in North America that remained south of the continental ice sheets. During much of the glaciation, sea level was low enough to permit land animals, including humans to occupy Beringia (the Bering Land Bridge) and move between North America and Siberia. As the glaciers retreated, glacial lakes were breached in great floods of water such as the Kankakee Torrent which reshaped the landscape south of modern Chicago as far as the Ohio and Mississippi rivers.
Role in human migration
Prehistoric human migration was greatly influenced by the last glacial period, known in North America as the Wisconsin glaciation. During much of the Wisconsin era, a land bridge across the Bering Strait allowed the first humans to reach North America from Asia (see Settlement of the Americas). Human migration routes opened during interglacial periods in Europe and Asia as well.
Flora and fauna
North American flora and fauna species were distributed quite differently during the Wisconsin era, due to altered temperatures, surface water distribution and, in some cases, coverage of earth surface by glaciers. A number of scientific studies have been conducted to determine species distribution, particularly during the Late Wisconsin and early to mid-Holocene. An example of findings is from the investigation of flora species using pollen-core samples in present day northern Arizona. Here in the Waterman Hills researchers found that Juniperus osteosperma and Pinus monophylla were early to mid-Holocene dominant trees, while Monardella arizonica has been a continuously present understory plant. Celtis reticulata is an example of a plant present in the early Holocene following Wisconsin glacial retreat, which species is no longer present at the Waterman Mountains site.
- Ann G. Harris, Esther Tuttle, Sherwood D. Tuttle. 1997. Geology of National Parks: Fifth Edition, Iowa, Kendall/Hunt Publishing ISBN 0-7872-5353-7
- Harm J. De Blij. 2005. Why geography matters: three challenges facing America, 308 pages, p.69
- C. Michael Hogan, 2009. Elephant Tree: Bursera microphylla, GlobalTwitcher.com, ed. N. Stromberg