The Wisconsin Glacial Episode was the most recent major advance of the North American Laurentide ice sheet.[1] Globally, this advance is known as the last glacial period. The Wisconsin glaciation extended from approximately 110,000 to 10,000 years ago, between the Eemian interglacial and the current interglacial. The maximum ice extent occurred approximately 21,000 years ago during the last glacial maximum, also known as the Late Wisconsin in North America.
The Wisconsin 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[2] 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. At the height of glaciation the Bering land bridge potentially permitted migration of mammals, including humans, to North America from Siberia.
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Prehistoric human migration was greatly influenced by the Last glacial period, known in North America as the Wisconsin glaciation. In the late Wisconsin era, a land bridge across the Bering Strait allowed the first humans to reach North America from Asia (an alternative theory is migration along the coast; see Settlement of the Americas). Human migration routes opened during interglacial periods in Europe and Asia as well.[3]
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.[4]
In the Sierra Nevada, there are three named stages of glacial maxima (sometimes incorrectly called ice ages) separated by warmer periods. These glacial maxima are called, from oldest to youngest, Tahoe, Tenaya, and Tioga[5]. The Tahoe reached its maximum extent perhaps about 70,000 years ago. Little is known about the Tenaya. The Tioga was the least severe and last of the Wisconsin Episode.