Everything about Wisconsin Glacier totally explained
» "Last glacial" redirects here. For the period of maximum glacier extent during this time, see Last Glacial Maximum:
"Last ice age" redirects here. See also ice age (disambiguation).
The
last glacial period was the most recent
glacial period within the
current ice age, occurring in the
Pleistocene epoch. It began about 110,000 years ago and ended between 10,000 and 15,000
BP. During this period there were several changes between glacier advance and retreat. The
maximum extent of
glaciation was approximately 18,000 years ago. While the general pattern of global cooling and glacier advance was similar, local differences in the development of glacier advance and retreat make it difficult to compare the details from continent to continent (see picture of ice core data below for differences).
The last glacial period is sometimes colloquially referred to as the
last ice age, though this use is incorrect because an
ice age is a longer period of cold temperature in which
ice sheets cover large parts of the Earth. Glacials, on the other hand, refer to colder phases within an ice age that separate
interglacials. Thus, the end of the last glacial period isn't the end of the last ice age. The end of the last glacial period was about 12,500 years ago, while the end of the last ice age hasn't yet come: few evidence points to a stop of the glacial-interglacial cycle of the last million years.
The last glacial period is the best-known part of the current ice age, and has been intensively studied in North America, northern Eurasia, the Himalaya and other formerly glaciated regions around the world. The glaciations that occurred during this glacial period covered many areas, mainly on the
Northern Hemisphere and - to a lesser extent - on the
Southern Hemisphere. They have different names, historically developed and depending on their geographic distributions:
Fraser,
Pinedale,
Wisconsin (in
North America),
Devensian (in the
British Isles),
Midlandian (in
Ireland),
Würm (in the
Alps),
Weichsel (or
Vistula, in northern
Central Europe),
Valdai in
Eastern Europe and
Zyryanka in
Siberia,
Llanquihue in
Chile and
Otira in
New Zealand.
Overview
The last glaciation centered on the huge ice sheets of North America and Eurasia. Considerable areas in the Alps, the Himalaya and the Andes were ice-covered, and Antarctica remained glaciated.
Canada was nearly completely covered by ice, as well as the northern part of the USA, both blanketed by the huge
Laurentide ice sheet. Alaska remained mostly ice free due to
arid climate conditions. Local glaciations existed in the
Rocky Mountains, the
Cordilleran ice sheet and as
ice fields and
ice caps in the
Sierra Nevada in northern California. In
Britain, mainland
Europe and northwestern
Asia, the
Scandinavian ice sheet once again reached the northern parts of the British Isles,
Germany,
Poland and
Russia, extending as far east as the
Taimyr Peninsula in western Siberia. Maximum extent of western Siberian glaciation was approximately 18,000 to 17,000 BP and thus later than in Europe (22,000 - 18,000 BP). Northeastern Siberia wasn't covered by ice. The
Arctic Ocean between the huge ice sheets of America and Eurasia wasn't frozen throughout, but like today probably was only covered by relatively thin ice, subject to seasonal changes and riddled with
icebergs calving from the surrounding ice sheets. According to the sediment composition retrieved from deep-sea
cores there must even have been times of seasonally open waters.
Outside the main ice sheets widespread glaciation occurred on the
Alps -
Himalaya mountain chain. In contrast to the earlier glacial stages the Würm glaciation was composed of smaller ice caps and mostly confined to valley glaciers, sending glacial lobes into the Alpine forland. To the east the
Caucasus and the mountains of
Turkey and
Iran were capped by local ice fields or small ice sheets.
,
In the
Himalaya and the
Tibetan Plateau glaciers advanced considerably, particularly between 47,000-27,000 BP and in contrast to the widespread contemporaneous warming elsewhere. The formation of a contiguous ice sheet on the Tibetan Plateau is controversial.
Other areas of the Northern Hemisphere didn't bear extensive ice sheets but local glaciers in high areas. Parts of
Taiwan for example were repeatedly glaciated between 44,250 and 10,680 BP as well as the
Japanese Alps. In both areas maximum glacier advance occurred between 60,000 and 30,000 BP. To a still lesser extent glaciers did exist in Africa, for example in the
High Atlas, the Mountains of
Morocco, the
Mount Atakor massif in southern
Algeria and several mountains in
Ethiopia. Already on the Southern Hemisphere, an ice cap of several hundred square kilometers was present on the east African mountains in the
Kilimandjaro Massif,
Mount Kenya and the
Ruwenzori Mountains, still bearing remnants of glaciers today.
Glaciation of the Southern Hemisphere was less extensive because of current continent configuration.
Ice sheets existed in the Andes (
Patagonian Ice Sheet), where six glacier advances between 33,500 and 13,900 BP in the Chilean Andes have been reported.
Antarctica was entirely glaciated, much like today, but the ice sheet left no uncovered area. In mainland Australia only a very small area in the vicinity of
Mount Kosciuszko was glaciated, whereas
Tasmania glaciation was more widespread.
New Zealand saw a glaciation in the New Zealand Alps, where at least three glacier advances can be distinguished. Local ice caps existed in
Irian Jaya,
Indonesia, where in three ice areas remnants of the Pleistocene glaciers are still preserved today.
Named local glaciations
Pinedale or Fraser glaciation, in the Rocky Mountains
The Pinedale (central Rocky Mountains) or Fraser (Cordilleran ice sheet) 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. This glaciation was somewhat distinct from the main Wisconsin glaciation as it was only loosely related to the giant ice sheets and was instead composed of mountain glaciers, merging into the
Cordilleran Ice Sheet. The Cordilleran ice sheet 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.
Wisconsin glaciation, in North America
The Wisconsin Glacial Episode was the last major advance of
continental glaciers in the North American
Laurentide ice sheet. This
glaciation is made of three glacial maxima (commonly called ice ages) separated by
interglacial periods (such as the one we're 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 Wisconsin Episode. 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 such as 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 Episode 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.
With the assistance of several very large glacial lakes, it carved the
gorge now known as the
Upper Mississippi River, creating the Upper Mississippi River, filling into the
Driftless Area and probably creating an annual ice-dam-burst.
In its retreat, the Wisconsin Episode glaciation left
terminal moraines that form
Long Island,
Nantucket and
Cape Cod, and the
Oak Ridges Moraine in south central Ontario, Canada. In Wisconsin itself, it left the
Kettle Moraine. The
drumlins and
eskers formed at its melting edge are landmarks of the Lower
Connecticut River Valley.
Greenland glaciation
In Northwest Greenland, ice coverage attained a very early maximum in the last glacial period around 114,000. After this early maximum, the ice coverage was similar to today until the end of the last glacial period. Towards the end glaciers readvanced once more before retreating to their present extent. According to ice core data, the Greenland climate was dry during the last glacial period, precipitation reaching perhaps only 20% of today's value.
Devensian glaciation, in the British Isles
The name
Devensian glaciation is used by British
geologists and
archaeologists and refers to what is often popularly meant by the latest
Ice Age.
The effects of the this glaciation can be seen in many geological features of
England,
Scotland and northern
Ireland. 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.
Weichsel glaciation, in Scandinavia and northern Europe
During the
glacial maximum in Scandinavia, only the western parts of
Jutland 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 site that was coastal in the
Nordic Stone Age now is inland.
Würm glaciation, in the Alps
The term
Würm is derived from a river in the Alpine foreland, approximately marking the maximum glacier advance of this particular glacial period. The Alps have been the area where first systematic scientific research on ice ages has been conducted by
Louis Agassiz in the beginning of the 19th century. Here the Würm glaciation of the last glacial period was intensively studied.
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.
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'd profound and lasting influence on
geothermal heat and the patterns of deep groundwater flow.
Antarctica glaciation
During the last glacial period Antarctica was blanketed by a massive ice sheet, much like it's today. The ice covered all land areas and probably extended into the ocean. According to ice modelling, ice over central East Antarctica was generally thinner than today.
Further Information
Get more info on 'Wisconsin Glacier'.
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