The Wisconsin Glacial Episode, also called the Wisconsin glaciation, was the most recent glacial period of the North American ice sheet complex. This advance included the Cordilleran Ice Sheet, which nucleated in the northern

North American Cordillera The North American Cordillera, sometimes also called the Western Cordillera of North America, the Western Cordillera or the Pacific Cordillera, is the North American portion of the American Cordillera, the mountain chain system (cordillera) alon ...

; the Innuitian ice sheet, which extended across the Canadian Arctic Archipelago; the

Greenland ice sheet The Greenland ice sheet ( da, Grønlands indlandsis, kl, Sermersuaq) is a vast body of ice covering , roughly near 80% of the surface of Greenland. It is sometimes referred to as an ice cap, or under the term ''inland ice'', or its Danish equiva ...

; and the massive Laurentide Ice Sheet, which covered the high latitudes of central and eastern North America. This advance was synchronous with global glaciation during the last glacial period, including the North American alpine glacier advance, known as the Pinedale glaciation. The Wisconsin glaciation extended from approximately 75,000 to 11,000 years ago, between the Sangamonian Stage and the current interglacial, the Holocene. The maximum ice extent occurred approximately 25,000–21,000 years ago during the

last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...

, also known as the ''Late Wisconsin'' in North America. This glaciation radically altered the geography north of the

Ohio River The Ohio River is a long river in the United States. It is located at the boundary of the Midwestern and Southern United States, flowing southwesterly from western Pennsylvania to its mouth on the Mississippi River at the southern tip of Illino ...

. 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 Kelleys Island is both a village in Erie County, Ohio, and the island which it fully occupies in Lake Erie. The British originally called it Sandusky Island. Later the United States took it over and officially designated it as Island Number 6 ...

in Lake Erie, northern New Jersey and in New York City's Central Park,SERC Media; Glacial Grooves, Central Park; Image 14884 is a 208 by 173 pixel JPEG; Uploaded: Apr5 09; Wayne Powell, CUNY Brooklyn College; http://serc.carleton.edu/details/images/14884.html the grooves left in rock 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, North America's northernmost point that remained south of the continental ice sheets. During much of the glaciation, sea level was low enough to permit land animals, including humans, to occupy

Beringia Beringia is defined today as the land and maritime area bounded on the west by the Lena River in Russia; on the east by the Mackenzie River in Canada; on the north by 72 degrees north latitude in the Chukchi Sea; and on the south by the tip ...

(the Bering Land Bridge) and move between

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

and Siberia. As the glaciers retreated, glacial lakes were breached in great floods of water such as the

Kankakee Torrent The Kankakee Torrent was a catastrophic flood that occurred about 19,000 calibrated years agoCurry, B.B., Hajic, E.R., Clark, J.A., Befus, K.M., Carrell, J.E. and Brown, S.E., 2014. "The Kankakee Torrent and other large meltwater flooding ev ...

, which reshaped the landscape south of modern Chicago as far as the Ohio and Mississippi Rivers.

Times

Two related movements have been termed Wisconsin: Early Wisconsin and Late Wisconsin.Chapter II. Glacial History of the Huron-Erie Basin; Geological Report on Wayne County; W.H. Sherzer; Michigan Geological and Biological Survey, Publication 12, Geological Series 9; Lansing, Michigan; Wynkoop Hallenbeck Crawford Co., State Printers; 1913 The Early Wisconsin was the bigger of the two and extend farther west and south. It retreated an unknown distance before halting. During this period of quiet, the glacial deposits were eroded and weathered. This first Wisconsin period erased all the

Illinoian The Illinoian Stage is the name used by Quaternary geologists in North America to designate the period c.191,000 to c.130,000 years ago, during the middle Pleistocene, when sediments comprising the Illinoian Glacial Lobe were deposited. It precedes ...

glacial topography that extended over. The Late Wisconsin ice sheet extended more towards the west than the earlier movements. This may have been due to changes in the accumulation center of the ice sheet, topographic changes introduced by the Early phase or by pressure changes in the ice mass in the north.

Continental ice sheets

Ice caps

Labrador Ice Sheet

The Labrador Ice Sheet centered east of Hudson Bay. Expanding towards the southwest, it reached into the eastern edge of Manitoba and across the Great Lakes to the

, upwards of from its source. Its eastern lobes covered New England and reached south to Cape Cod and

Long Island, New York Long Island is a densely populated island in the southeastern region of the U.S. state of New York, part of the New York metropolitan area. With over 8 million people, Long Island is the most populous island in the United States and the 18th ...

.Chamberlin, Thomas C. and Salisbury, Rollin T., Geology, 3 Vols. 1906, Vol III., pp. 330-333.

Ice lobes

Keewatin Ice Sheet

The Keewatin Ice Sheet began west of Hudson Bay in the Canadian Territory of Keewatin. The ice moved south some into Kansas and Missouri. To the west, it reached to the foothills of the Rocky Mountains.

Cordilleran Ice Sheet

The Cordilleran Ice Sheet has left remnants throughout the Northern Rocky Mountains. Unlike the other two ice sheets, this one is mountain based covering British Columbia and reaching into northern Washington State and Montana. The Cordilleran Ice Sheet has more of an Alpine style of many glaciers merged into a whole. The striations made by the ice field in moving over the bedrock show that it moved principally to the west through the passes of the coast range.

Formation of proglacial and prehistoric lakes

Plate 53 - Glacial Lakes Duluth, Chicago, and Lundy (USGS 1915)

Whenever the ice sheet melted from the north at a

moraine A moraine is any accumulation of unconsolidated debris (regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice shee ...

, water would begin to pond in the divide between a moraine and the ice front. The ice would act as a dam as water could not drain through the ice sheet, which in the Wisconsin period covered most of the proglacial river valleys. Numerous small, isolated water bodies formed between the moraine and the ice front. As the ice sheet would continue to melt and recede northward, these ponds combined into proglacial lakes. In areas without an available outlet, the water levels would either continue to rise until reaching one or more low spots along the rim of a moraine, or the ice sheet would retreat, opening access to a lower portion of the moraine. Multiple outlets could form through low spots too until one would become dominant after erosion lowered both the outlet and lake surface.

Running water

Ice melt and rainfall carried large quantities of clay, sand, and

gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...

from the ice mass. Clays could be moved long distances by moving water, while sand and gravel could not. Thus, sand and gravel

landforms A landform is a natural or anthropogenic land feature on the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Landforms include hills, m ...

developed along the sides and front of the ice sheet; elongated accumulations of this material are known as

kame A kame, or ''knob'', is a glacial landform, an irregularly shaped hill or mound composed of sand, gravel and till that accumulates in a depression on a retreating glacier, and is then deposited on the land surface with further melting of the g ...

s. Mounds along the frontal edge of the ice are called moraines. Wherever a subglacial tunnel began infilling, long winding formations known as

eskers An esker, eskar, eschar, or os, sometimes called an ''asar'', ''osar'', or ''serpent kame'', is a long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North Ameri ...

would form. The sweeping plain of sand and gravel beyond the ice margin and a terminal moraine is called an outwash plain . The materials left under the glacier when it melts back is called the ground moraine or

till plain Till plains are an extensive flat plain of glacial till that forms when a sheet of ice becomes detached from the main body of a glacier and melts in place, depositing the sediments it carried. Ground moraines are formed with melts out of the glacie ...

. Till is highly permeable and creates a large ground reserve for water. This formation is highly desirable for human economic development as a source of water.

Stages of the Wisconsin episode

Role in human migration

Prehistoric human migration was likely greatly influenced by this last glacial period, as during much of the Wisconsin era, the formation of a land bridge known as

across the Bering Strait is believed to have allowed human occupation of this area which provided potential access for some of the first humans to move between North America and Siberia in Asia (see Settlement of the Americas). Other human migration routes also opened during interglacial periods in both Europe and Asia.

Flora and fauna

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 A pollen core is a core sample of a medium containing a stratigraphic sequence of pollen. Analysis of the type and frequency of the pollen in each layer is used to study changes in climate or land use using regional vegetation as a proxy. This a ...

samples in present-day northern Arizona. Here in the

Waterman Hills The Waterman Hills are a low mountain range in the Mojave Desert, just north of Barstow in San Bernardino County, California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million res ...

researchers found that '' Juniperus osteosperma'' and '' Pinus monophylla'' were early to mid-Holocene dominant trees, while ''

Monardella arizonica ''Monardella arizonica'' is a plant species endemic to Arizona in the United States, known by the common name Arizona monardella. Ancient history The species has been confirmed to have been native to northern Arizona since at least the Late Wisc ...

'' has been a continuously present understory plant. '' Celtis reticulata'' is an example of a plant present in the early Holocene following Wisconsin glacial retreat, a species no longer present at the

Waterman Mountains The Waterman Mountains are a low mountainous landform in Pima County, United States. Notable among the tree species is the elephant tree (''Bursera microphylla'') which species exhibits a contorted multi-furcate architecture; most of these froze i ...

site.

References

External links and references

britannica.com reference
{{DEFAULTSORT:Wisconsin Glaciation Glaciology of Canada Glaciology of the United States Ice ages Pleistocene geology Pleistocene United States Quaternary Canada Quaternary Colorado Quaternary Michigan Quaternary Minnesota Quaternary Montana Quaternary Wisconsin Quaternary Wyoming Last Glacial Period