Wisconsin glaciation
From Wikipedia, the free encyclopedia
The Wisconsin (in North America), Weichsel (in Scandinavia), Devensian (in the British Isles), Midlandian (in Ireland) and Würm glaciation (in the Alps) are the most recent glaciations of the Pleistocene epoch, which ended around 10,000 BCE. The general glacial advance began about 70,000 BCE, and reached its maximum extent about 18,000 BCE. In Europe, the ice sheet reached northern Germany.
The term ice age can refer to all the periods of glaciation during the late Pliocene and Pleistocene, from 2.5 million years ago to 10,000 BCE. In popular usage, "the Ice Age" usually refers to this last cold phase, due to its shaping of some Northern Hemisphere landscapes and its influence on human prehistory.
Contents |
[edit] Weichsel glaciation, in Scandinavia
During the glacial maximum in Scandinavia, only the western parts of Jutland (a part of Denmark) were ice-free during the glaciation and a large part of what is today the North Sea was dry land connecting Jutland with Britain. It is also in Denmark that the only finds of Scandinavian ice-age animals older than 13,000 BC are found. In the period following the last interglacial period before the current one (Eemian interglacial era) the coast of Norway was also ice-free.
The Baltic Sea, with its unique brackish water, is a result of meltwater from the Weichsel glaciation combining with saltwater from the North Sea when the straits between Sweden and Denmark opened. Initially, when the ice began melting about 10,300 ybp, seawater filled the isostatically depressed area, a temporary marine incursion that geologists dub the Yoldia Sea. Then as post-glacial isostatic rebound lifted the region about 9500 ybp, the deepest basin of the Baltic became a freshwater lake, in palaeological contexts referred to as Ancylus Lake, which is identifiable in the freshwater fauna found in sediment cores. The lake was filled by glacial runoff, but as worldwide sea level continued rising, saltwater again breached the sill about 8000 ybp, forming a marine Littorina Sea which was followed by another freshwater phase before the present brackish marine system was established. "At its present state of development, the marine life of the Baltic Sea is less than about 4000 years old," Drs Thulin and Andrushaitis remarked when reviewing these sequences in 2003.
Overlaying ice had exerted pressure on the earth's surface. As a result of melting ice, the land has continued to rise yearly in Scandinavia, mostly in northern Sweden and Finland where the land is rising at a rate of as much as 8-9 mm per year, or 1 meters in 100 years. This is important for archeologists since a village that was coastal in the Nordic Stone Age now is inland.
[edit] Devensian glaciation
The name Devensian glaciation is used by British geologists and archaeologists and refers to what is often popularly meant by the latest Ice Age.
It was the final glacial phase of the Pleistocene and its deposits have been found overlying material from the preceding Ipswichian interglacial and lying beneath those from the following Flandrian stage of the Holocene.
The latter part of the Devensian includes Pollen zones I-IV, the Allerød and Bølling Oscillations and the Older and Younger Dryas climatic stages.
[edit] Wisconsin glaciation, in North America
The Wisconsin or Wisconsinian was the last major advance of continental glaciers in North America. This glaciation is made of three glacial maximums (commonly called ice ages) separated by interglacial periods (such as the one we are living in). These ice ages are called (from oldest to youngest); Tahoe, Tenaya and Tioga. The Tahoe reached its maximum extent perhaps about 70,000 years ago while little is known about the Tenaya. The Tioga was the least severe and last of the Wisconsinan group and reached its greatest advance 20,000 years ago and ended about 10,000 years before present (it started 30,000 years ago). At the height of glaciation the Bering land bridge permitted migration of mammals and humans to North America from Siberia.
It radically altered the geography of North America north of the Ohio River. At the height of the Wisconsin glaciation, ice covered most of Canada, the Upper Midwest, and New England, as well as parts of 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, which is the northernmost point in North America that remained south of the continental ice sheets.
The Great Lakes are the result of glacial scour and pooling of meltwater at the rim of the receding ice. When the enormous mass of the continental ice sheet retreated, the Great Lakes began gradually moving south due to isostatic rebound of the north shore. Niagara Falls is also a product of the glaciation, as is the course of the Ohio River, which largely supplanted the prior Teays River.
In its retreat, the Wisconsin glaciation left terminal moraines that form Long Island, Nantucket and Cape Cod, and the Oak Ridges Moraine in south central Ontario, Canada. The drumlins and eskers formed at its melting edge are landmarks of the Lower Connecticut River Valley.
[edit] Pinedale glaciation
The Pinedale glaciation was the last of the major ice ages to appear in the Rocky Mountains in the United States. The Pinedale lasted from approximately 30,000 to 10,000 years ago and was at its greatest extent between 23,500 and 21,000 years ago. [1] This glaciation was somewhat distinct from the main Wisconsin glaciation as it was unrelated to the giant ice sheets and was instead composed of mountain glaciers. The Pinedale and the main ice sheets of the Wisconsin produced features such as glacial Lake Missoula, which would break free from its ice dam causing the massive Missoula floods. Geologists estimate that the cycle of flooding and reformation of the lake lasted on average of 55 years and that the floods occurred approximately 40 times over the 2,000 year period between 15,000 and 13,000 years ago. Glacial lake outburst floods such as these are not uncommon today in Iceland and other places.
[edit] Würm glaciation, in the Alps
The term Würm comes from a river in the Alps where the glaciation was first identified. Pollen analysis, the statistical analyses of microfossilized plant pollens found in geological deposits, has chronicled the dramatic changes in the European environment during the Würm glaciation. During the height of Würm glaciation, ca 24,000–10,000 ybp, most of western and central Europe and Eurasia was open steppe-tundra, while the Alps presented solid ice fields and montane glaciers. Scandinavia and much of Britain were under ice.
- Main article Swiss plateau
During the Würm, the Rhône Glacier covered the whole western Swiss plateau, reaching today's regions of Solothurn and Aarau. In the region of Bern it merged with the Aar glacier. The Rhine glacier is currently the subject of the most detailed studies. Glaciers of the Reuss and the Limmat advanced sometimes as far as the Jura. Montane and piedmont glaciers formed the land by grinding away virtually all traces of the older Günz and Mindel glaciation, by depositing base moraines and terminal moraines of different retraction phases and loess deposits, and by the pro-glacial rivers' shifting and redepositing gravels. Beneath the surface, they had profound and lasting influence on geothermal heat and the patterns of deep groundwater flow.
[edit] External link
- University of Zurich: Würmian Glaciation in the Alps: Present state of knowledge
[edit] See also
[edit] References
- Geology of National Parks: Fifth Edition, Ann G. Harris, Esther Tuttle, Sherwood D., Tuttle (Iowa, Kendall/Hunt Publishing; 1997) ISBN 0-7872-5353-7
- E. C. Pielou 1991. After the Ice Age : The Return of Life to Glaciated North America (University Of Chicago Press) ISBN 0-226-66812-6 (paperback 1992)
