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Research indicates that the late Wisconsin glaciation began to advance about 25 to 27,000 years before present (BP) (Andrews 1987), and that maximum glaciation occurred at 18,000 years BP. Geologists refer to the 2-million year period of Ice Age as the Pleistocene Epoch. During the late glacial period, the Wisconsin, two large ice caps, the Laurentide glacier in the East and Cordellian Glacier in the West, dominated northern North America. Nearly all of Canada lay under the two massive glaciers, which extended into the northern regions of the United States and into the southern one-third of Alaska (fig. 24.3).
These two massive ice sheets were part of an even larger system of ice that dominated the northern hemisphere. Nearly a quarter of the earth's surface lay under the weight of a mountain of ice. The Laurentide ice sheet is believed to have reached a height of 12,500 feet (Hughes 1987). Ice covered nearly 5 million square miles of North America. As the glaciers grew, they drew more than 50 percent of the Earth's available water, affecting precipitation (Delcourt and Delcourt 1979). The ocean levels dropped, exposing what we call the Continental Shelfs. The expansion of the glaciers dramatically affected the distribution and composition of vegetation.
The leading edge of the glacier in the United States is believed to have been over a mile high (Hughes 1987). Nothing could stand in the way of this massive ice field as it pushed south, grinding over mountains and depressing the land under its massive weight. Over the ice caps, a huge high-pressure system pushed the polar jet stream southward, dominating weather patterns over much of the northern hemisphere (Barry 1983, Kutzbach 1987). The ice sheets influenced temperatures far to the south, and both vegetation and wildlife retreated in its front (Graham and Mead 1987, Jacobson and others 1987). The vegetation map (fig. 24.4) depicts the magnitude of the glaciers and the proportion of Earth's water frozen into the 12,000-foot thick ice sheet and graphically illustrates the extent vegetation was influenced by this glacial system.
By 18,000 years BP, boreal species dominated by jack pine and spruce had been pushed as far south as 34° N. latitude, which is near present-day Atlanta, GA. Temperate deciduous tree species dominated by oaks existed just south of the broad boreal region. Temperate forest species extended south onto the exposed Continental Shelf into the Gulf of Mexico (Delcourt and Delcourt 1984, Watts 1980) (fig. 24.1).
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