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Wisconsin glaciation
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Wisconsin glaciation
The *Wisconsin* (in North America ), *Devensian*
(in the British Isles ), *Midlandian* (in Ireland
), *Würm* (in the Alps ), and *Weichsel*
(in northern central Europe) *glaciations* 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 .
Vegetation types at time of last glacial maximum.
Enlarge
Vegetation types at time of last glacial maximum
.
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 .
Weichsel glaciation, in Scandinavia
Several glacial cycles, from ice core data
Enlarge
Several glacial cycles, from ice core data
During the glacial maximum in the
Scandinavia, only the western parts of Jutland (a part
of Denmark ) were ice-free, 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 Scandinavian
ice-age animals older than 13,000 BC are found. In the period following
the last interglacial 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
meter in 100 years. This is important for archaeologists
since a village that was coastal in the Nordic
Stone Age now is inland.
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.
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. Little is known about the Tenaya. The
Tioga was the least severe and last of the Wisconsinan group. It began
about 30,000 years ago, reached its greatest advance 20,000 years ago,
and ended about 10,000 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
.
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.
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.
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)
See also
* Glacial history of Minnesota
* Last Glacial Maximum
* Timeline of glaciation
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