Laurentide Ice Sheet

The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square kilometers, including most of Canada and a large portion of the northern United States, multiple times during the Quaternary glacial epochs, from 2.588 ± 0.005 million years ago to the present.[1]

The last advance covered most of northern North America between c. 95,000 and c. 20,000 years before the present day and, among other geomorphological effects, gouged out the five Great Lakes and the hosts of smaller lakes of the Canadian Shield. These lakes extend from the eastern Northwest Territories, through most of northern Canada, and the upper Midwestern United States (Minnesota, Wisconsin, and Michigan) to the Finger Lakes, through Lake Champlain and Lake George areas of New York, across the northern Appalachians into and through all of New England and Nova Scotia.

At times, the ice sheet's southern margin included the present-day sites of northeastern coastal towns and cities such as Boston and New York City and Great Lakes coastal cities and towns as far south as Chicago and St. Louis, Missouri, and then followed the present course of the Missouri River up to the northern slopes of the Cypress Hills, beyond which it merged with the Cordilleran Ice Sheet. The ice coverage extended approximately as far south as 38 degrees latitude mid-continent.[2]

Description

Pleistocene north ice map
The maximum extent of glacial ice in the north polar area during the Pleistocene period included the vast Laurentide ice sheet in eastern North America.

This ice sheet was the primary feature of the Pleistocene epoch in North America, commonly referred to as the ice age. It was up to 2 mi (3.2 km) thick in Nunavik, Quebec, Canada, but much thinner at its edges, where nunataks were common in hilly areas. It created much of the surface geology of southern Canada and the northern United States, leaving behind glacially scoured valleys, moraines, eskers and glacial till. It also caused many changes to the shape, size, and drainage of the Great Lakes. As but one of many examples, near the end of the last ice age, Lake Iroquois extended well beyond the boundaries of present-day Lake Ontario, and drained down the Hudson River into the Atlantic Ocean.[3]

Its cycles of growth and melting were a decisive influence on global climate during its existence. This is because it served to divert the jet stream which would otherwise flow from the relatively warm Pacific Ocean through Montana and Minnesota to the south. That gave the Southwestern United States, otherwise a desert, abundant rainfall during ice ages, in extreme contrast to most other parts of the world which became exceedingly dry, though the effect of ice sheets in Europe had an analogous effect on the rainfall in Afghanistan, parts of Iran, possibly western Pakistan in winter, as well as North Africa.

Sea Ice off Baffin Island
The Barnes Ice Cap, containing remnants of the Laurentide Ice Sheet.

Its melting also caused major disruptions to the global climate cycle, because the huge influx of low-salinity water into the Arctic Ocean via the Mackenzie River[4] is believed to have disrupted the formation of North Atlantic Deep Water, the very saline, cold, deep water that flows from the Greenland Sea. That interrupted the thermohaline circulation, creating the brief Younger Dryas cold epoch and a temporary re-advance of the ice sheet,[5] which did not retreat from Nunavik until 6,500 years ago.

During the Pre-Illinoian Stage, the Laurentide Ice Sheet extended as far south as the Missouri and Ohio River valleys.

The ultimate collapse of the Laurentide Ice Sheet is also suspected to have influenced European agriculture indirectly through the rise of global sea levels.

Canada's oldest ice is a 20,000-year-old remnant of the Laurentide Ice Sheet called the Barnes Ice Cap, on central Baffin Island.

Ice Centers

During the Late Pleistocene, the Laurentide ice sheet reached from the Rocky Mountains eastward through the Great Lakes, into New England, covering nearly all of Canada east of the Rocky Mountains.[6] Three major ice centers formed in North America: the Labrador, Keewatin, and Cordilleran. The Cordilleran covered the region from the Pacific Ocean to the eastern front of the Rocky Mountains and the Labrador and Keewatin fields are referred to as the Laurentide Ice Sheet. Central North America has evidence of the numerous lobes and sublobes. The Keewatin covered the western interior plains of North America from the Mackenzie River to the Missouri River and the upper reaches of the Mississippi River. The Labrador covered spread over eastern Canada and the northeastern part of the United States abutting the Keewatin lobe in the western Great Lakes and Mississippi valley.[6]

Cordilleran Ice Flow

Cordilleran Ice Sheet covered up to 2,500,000 square kilometres (970,000 sq mi) at the Last Glacial Maximum. The eastern edge abutted the Laurentide ice sheet. The sheet was anchored in the Coast Mountains of British Columbia and Alberta, south into the Cascade Range of Washington. That is one and a half times the water held in the Antarctic. Anchored in the mountain backbone of the west coast, the ice sheet dissipated north of the Alaska Range where the air was too dry to form glaciers.[6] It is believed that the Cordilleran ice melted rapidly, in less than 4000 years. The water created numerous Proglacial lakes along the margins such as Lake Missoula, often leading to catastrophic floods as with the Missoula Floods. Much of the topography of Eastern Washington and northern Montana and North Dakota was affected.[6]

Keewatin Ice Flow

Keewatin Ice flow has had four or five primary lobes identified ice divides extending from a dome over west-central Keewatin. Two of the lobes abut the adjacent Labrador and Baffin ice sheets. The primary lobes flow (1) towards Manitoba and Saskatchewan; (2) toward Hudson Bay; (3) towards the Gulf of Boothia, and (4) towards the Beaufort Sea.[7]

Labrador Ice Flow

Ice flowed across all of Maine and into the Gulf of St. Lawrence, completely covering the Maritime Provinces. The Appalachian Ice Complex, flowed from the Gaspé Peninsula over New Brunswick, the Magdalen Shelf, and Nova Scotia.[7] The Labrador flow extended across the mouth of the St. Lawrence River, reaching the Gaspé Peninsula and across Chaleur Bay. From the Escuminac center on the Magdalen Shelf, flowed onto the Acadian Peninsula of New Brunswick and southeastward, onto the Gaspe, burying the western end of Prince Edward Island and reached the head of Bay of Fundy. From the Gaspereau center, on the divide crossing New Brunswick flowed into the Bay of Fundy and Chaleur Bay.[7]

In New York, the ice that covered Manhattan was about 2,000 feet high before it began to melt in about 16,000 BC. The ice in the area disappeared around 10,000 BC. The ground in the New York area has since risen by more than 150 ft because of the removal of the enormous weight of the melted ice.[8]

Baffin Ice Flow

The Baffin Ice Flow was circular and centered over the Foxe Basin. A major divide across the basin, created a westward flow across the Melville Peninsula, from an eastward flow over Baffin Island and Southampton Island. Across southern Baffin Island, two divides created four additional lobes. The Penny Ice Divide split the Cumberland Peninsula, where Pangnirtung created flow toward Home Bay on the north and Cumberland Sound on the south. The Amadjuak Ice Divide on the Hall Peninsula, where Iqaluit sits created a north flow into Cumberland Sound and a south flow into the Hudson Strait. A secondary Hall Ice Divide formed a link to a local ice cap on the Hall Peninsula. The current ice caps on Baffin Island are thought to be a remnant from this time period, but it was not a part of the Baffin Ice Flow, but an autonomous flow.[7]

See also

References

  1. ^ Cohen, K.M.; Finney, S.C.; Gibbard, P.L.; Fan, J.-X. "International Chronostratigraphic Chart 2013" (PDF). http://www.stratigraphy.org/icschart/chronostratchart2013-01.pdf. ICS. Retrieved 15 June 2014. External link in |website= (help)
  2. ^ Dyke, A.S.; Prest, V.K. (1987). "Late Wisconsinan and Holocene History of the Laurentide Ice Sheet". Géographie physique et Quaternaire. 41 (2): 237–263.
  3. ^ Flint, R.F. 1971. Glacial and Quaternary Geology. Wiley and Sons, NY. 892 p.
  4. ^ Murton, J.B.; Bateman, M.D.; Dallimore, S.R; Teller, J.T.; Yang, Z. (2010). "Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean". Nature. 464 (7289): 740–743. Bibcode:2010Natur.464..740M. doi:10.1038/nature08954. PMID 20360738.
  5. ^ Broecker, W.S.; Denton, G.H. (1989). "The role of ocean-atmosphere reorganizations in glacial cycles". Geochimica et Cosmochimica Acta. 53 (10): 2465–2501. Bibcode:1989GeCoA..53.2465B. doi:10.1016/0016-7037(89)90123-3.
  6. ^ a b c d Geologic Framework and Glaciation of the Central Area, 1-1-2006; Christopher L. Hill; Boise State University, Boise, Idaho; 2006
  7. ^ a b c d Late Wisconsinan and Holocene History of the Laurentide Ice Sheet, 10.7202/032681ar; Arthur S. Dyke, Victor K. Prest; Geological Survey of Canada; Ottawa, Ontario; 1987; http://id.erudit.org/iderudit/032681ar
  8. ^ William J. Broad (5 June 2018). "How the Ice Age Shaped New York". The New York Times. Retrieved 24 February 2019. the ice was about 2,000 feet thick over Manhattan

Further reading

External links

Cordilleran Ice Sheet

The Cordilleran ice sheet was a major ice sheet that periodically covered large parts of North America during glacial periods over the last ~2.6 million years. This included the following areas:

Western Montana

The Idaho Panhandle

Northern Washington state down to about Olympia and Spokane

All of British Columbia

The southwestern third or so of Yukon Territory

All of the Alaska Panhandle

South Central Alaska

The Alaska Peninsula

Almost all of the continental shelf north of the Strait of Juan de FucaThe ice sheet covered up to 2.5 million square kilometres at the Last Glacial Maximum and probably more than that in some previous periods, when it may have extended into the northeast extremity of Oregon and the Salmon River Mountains in Idaho. It is probable, though, that its northern margin also migrated south due to the influence of starvation caused by very low levels of precipitation.

At its eastern end the Cordilleran ice sheet merged with the Laurentide ice sheet at the Continental Divide, forming an area of ice that contained one and a half times as much water as the Antarctic ice sheet does today. At its western end it is currently understood that several small glacial refugia existed during the last glacial maximum below present sea level in the now-submerged Hecate Strait and on the Brooks Peninsula in northern Vancouver Island. However, evidence of ice-free refugia above present sea level north of the Olympic Peninsula has been refuted by genetic and geological studies since the middle 1990s. The ice sheet faded north of the Alaska Range because the climate was too dry to form glaciers.

Unlike the Laurentide ice sheet, which is believed to have taken as much as eleven thousand years to fully melt, it is believed the Cordilleran ice sheet, except for areas that remain glaciated today, melted very quickly, probably in four thousand years or less. This rapid melting caused such floods as the overflow of Lake Missoula and shaped the topography of the extremely fertile Inland Empire of Eastern Washington.

Deglaciation

Deglaciation describes the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume (IPCC AR5). Thus, it refers to the retreat of a glacier, an ice sheet or frozen surface layer, and the resulting exposure of the Earth's surface. The decline of the cryosphere due to ablation can occur on any scale from global to localized to a particular glacier. After the Last Glacial Maximum (ca. 21,000 years ago), the last deglaciation begun, which lasted until the early Holocene. Around much of Earth, deglaciation during the last 100 years has been accelerating as a result of climate change, partly brought on by anthropogenic changes to greenhouse gases.The previous deglaciation took place between approximately 22ka until 11.5ka. This occurred when there was an annual mean atmospheric temperature on the earth that increased by roughly 5 °C, which was also accompanied by regional high-latitude warming that exceeded 10 °C. This was also followed by noteworthy deep-sea and tropical-sea warming, between about 1-2 °C (deep-sea) and 2-4 °C (tropical sea). Not only did this warming occur, but the global hydrological budget also experienced noticeable changes and regional precipitation patterns changed. As a result of all of this, the world's main ice sheets, including the ones located in Eurasia, North America and parts of the Antarctic melted. As a consequence, sea levels rose roughly 120 metres. These processes did not occur steadily, and they also did not occur at the same time.

Dissected Till Plains

The Dissected Till Plains are physiographic sections of the Central Lowlands province, which in turn is part of the Interior Plains physiographic division of the United States, located in southern and western Iowa, northeastern Kansas, the southwestern corner of Minnesota, northern Missouri, eastern Nebraska, and southeastern South Dakota.The Dissected Till Plains were formed by pre-Wisconsin glaciations during the Pre-Illinoian Stage. Glacial scouring and deposition by the Laurentide ice sheet and the later accumulation of loess during the Wisconsin Stage left behind the rolling hills and rich, fertile soils found today in the region.

The region is also the western edge of the Corn Belt.

Geology of New England

New England is a region in the North Eastern United States consisting of the states Rhode Island, Connecticut, Massachusetts, New Hampshire, Vermont, and Maine. Most of New England consists geologically of volcanic island arcs that accreted onto the eastern edge of the Laurentian Craton in prehistoric times. Much of the bedrock found in New England is heavily metamorphosed due to the numerous mountain building events that occurred in the region. These events culminated in the formation of Pangaea; the coastline as it exists today was created by rifting during the Jurassic and Cretaceous periods. The most recent rock layers are glacial conglomerates.

Glacial Lake Cape Cod

Glacial Lake Cape Cod was a glacial lake that formed during the late Pleistocene epoch inside modern Cape Cod Bay. After the Laurentide ice sheet retreated, glacial ice melt accumulated at the terminal moraine and blocked up the escape of glacial meltwater, creating the lake. Drainage from the lake occurred at Bass River, the location of the Cape Cod Canal and Orleans Harbor.

Glacial Lake Nantucket Sound

Glacial Lake Nantucket Sound was a glacial lake that formed during the late Pleistocene epoch inside modern Nantucket Sound. After the Laurentide ice sheet retreated, glacial ice melt washed over the terminal moraine of Cape Cod and the glacial meltwater settled in the modern day sound, creating the lake.

Ice sheet

An ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than 50,000 km2 (19,000 sq mi). The only current ice sheets are in Antarctica and Greenland; during the last glacial period at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of North America, the Weichselian ice sheet covered northern Europe and the Patagonian Ice Sheet covered southern South America.

Ice sheets are bigger than ice shelves or alpine glaciers. Masses of ice covering less than 50,000 km2 are termed an ice cap. An ice cap will typically feed a series of glaciers around its periphery.

Although the surface is cold, the base of an ice sheet is generally warmer due to geothermal heat. In places, melting occurs and the melt-water lubricates the ice sheet so that it flows more rapidly. This process produces fast-flowing channels in the ice sheet — these are ice streams.

The present-day polar ice sheets are relatively young in geological terms. The Antarctic Ice Sheet first formed as a small ice cap (maybe several) in the early Oligocene, but retreating and advancing many times until the Pliocene, when it came to occupy almost all of Antarctica. The Greenland ice sheet did not develop at all until the late Pliocene, but apparently developed very rapidly with the first continental glaciation. This had the unusual effect of allowing fossils of plants that once grew on present-day Greenland to be much better preserved than with the slowly forming Antarctic ice sheet.

Lake Bassano

Lake Bassano was a proglacial lake that formed in the Late Pleistocene during the deglaciation of south-central Alberta by the impoundment of a re-established drainage system and addition of glacial meltwater. It is associated with the development of through-flowing drainage within the Red Deer River basin in particular, and the South Saskatchewan drainage network in general. Approximately 7,500 square kilometers of the Bassano basin is covered with lacustrine sediments. These sediments are bordered by the topographically higher Buffalo Lake Moraine to the west, the Suffield Moraine to the east and the Lethbridge Moraine to the south.

The transmission of water through the basin was ultimately controlled by the regional topography and the position of the ice front. As the Laurentide Ice Sheet retreated, lower outlet channels were exposed. The lake levels at any given time were constrained by the elevation of the lowest drainage channel. As Glacial Lake Bassano, and the proglacial lake system as a whole developed, throughflow in individual channels waxed, waned and reversed, depending on the systemic controls.

Lake Chippewa

Lake Chippewa was a prehistoric proglacial lake. The basin is now Lake Michigan. It formed about 10,600 years before present (YBP). The lake occupied the depression left by the Michigan Lobe of the Laurentide ice sheet.

Lake Chouteau

Lake Chouteau was a glacial lake formed during the late Pleistocene along the Teton River. After the Laurentide ice sheet retreated, water melting off the glacier accumulated between the Rocky Mountains and the ice sheet. The lake drained along the front of the ice sheet, eastward towards the Judith River and the Missouri River.

The maximum advance of the Laurentide ice sheet blocked the drainages of north- and east-flowing rivers, forming glacial lakes along the margin of the ice. On the western Montana plains the Shelby lobe blocked the [Milk River (Alberta–Montana)|[Milk River]], creating glacial Lake Twin River. Tributaries of the Marias River were also blocked by the Shelby lobe, leading to the formation of glacial lakes Cutbank and Choteau. The Loma sublobe blocked the Missouri north of the Highwood Mountains, forming glacial Lake Great Falls. A lake also formed in the Musselshell River basin.

Lake Circle

Lake Circle was a glacial lake that formed during the late Pleistocene epoch along the Redwater River in eastern Montana. After the Laurentide ice sheet retreated, glacial ice melt accumulated in the basin surrounded by the ridges of the preglacial valley and the retreating glacier.

Southwest of Nickwall are the remnants of a broad abandoned valley with long side slopes. The valley runs north from Redwater Creek to the Missouri River. The bottom is poorly drained and about 1 mile (1.6 km) in width. It lies 2,015 to 2,020 feet (614 to 616 m) above the sea level and 40 to 50 feet (12 to 15 m) above the Missouri River bottomland. The upland slopes are extensive, clear and flat. The valleys surrounding it are dissected with V-shaped coulees. The difference between the Redwater valley and those around it reflect stream erosion vs. lake sedimentation. The drift in the valleys, appears to be as left by the glacier in the previously created valleys. Using the dating of lake deposits near Great Falls, Montana, the Havre lobe of the Laurentide ice sheet dammed the ancestral Missouri River during the late Wisconsin Glacial Period.

Lake Glendive

Glacial Lake Glendive was a glacial lake on the lower Yellowstone River . It formed in the valley of Yellowstone, during the late Pleistocene epoch south of the Keewatin Ice Sheet. As the ice sheet retreated northward, the lake drained into the modern Missouri River.

Ice of the Keewatin Lobe of the Laurentide Ice Sheet advanced westward into the Missouri and Yellowstone river valleys. The Glasgow sublobe blocked the Missouri River west of present-day Fort Peck, Montana, and created Lake Jordan and glacial lakes Circle and Lambert were formed to the east. The Yellowstone lobe spread south past Intake, Montana, and formed glacial Lake Glendive. At its maximum the ice may have blocked the Little Missouri River forming glacial Lake Mikkelson. When the ice sheet began to retreat northward, the southwestern margin of abandonment its previous drainages and lakes formed in the depression along the ice margins. Melting of the Shelby and Havre lobes in western Montana led to the retreat of the ice into Alberta. By 16,200 B.C. the ice had created glacial Lake Carmichael in the area of the Cypress Hills. By 15,700 B.C. ice-free conditions may have existed in southwestern Saskatchewan north of Havre, Montana.

Lake Great Falls

Lake Great Falls was a prehistoric proglacial lake which existed in what is now central Montana in the United States between 15,000 BCE and 11,000 BCE. Centered on the modern city of Great Falls, Montana, Glacial Lake Great Falls extended as far north as Cut Bank, Montana, and as far south as Holter Lake. At present-day Great Falls, the Glacial Lake Great Falls reached a depth of 600 feet (183 metres).Approximately 1.5 million years ago, the Missouri River, the Yellowstone River and Musselshell River all flowed northward into a terminal lake. During the last glacial period, the Laurentide and Cordilleran ice sheets pushed these lakes and rivers southward. Between 15,000 BCE and 11,000 BCE, the Laurentide ice sheet blocked the Missouri River and created Glacial Lake Great Falls.About 13,000 BCE, as the glacier retreated, Glacial Lake Great Falls emptied catastrophically in a glacial lake outburst flood. The meltwater poured through the Highwood Mountains and eroded the hundred mile-long, 500-foot-deep (150 m) Shonkin Sag—one of the most famous prehistoric meltwater channels in the world.

Lake Hitchcock

Lake Hitchcock was a glacial lake that formed approximately 15,000 years ago in the late Pleistocene epoch. After the Laurentide ice sheet retreated, glacial ice melt accumulated at the terminal moraine and blocked up the Connecticut River, creating the long, narrow lake. The lake existed for approximately 3,000 years, after which a combination of erosion and continuing geological changes likely caused it to drain. At its longest, Lake Hitchcock stretched from the moraine dam at present-day Rocky Hill, Connecticut, to St. Johnsbury, Vermont (about 320 kilometres (200 mi)). Although the rift valley through which the river flows above Rocky Hill actually continues south to New Haven, on Long Island Sound, the obstructing moraine at Rocky Hill diverted the river southeast to its present mouth at Old Saybrook.

Lake Hitchcock is an important part of the geology of Connecticut. It experienced annual layering of sediments, or varves: silt and sand in the summertime (due to glacial meltwater) and clay in the wintertime (as the lake froze). Analysis of varves along Canoe Brook in Vermont was conducted by John Ridge and Frederick Larsen, including radiocarbon dating of organic materials. Their research indicates that the lake formed sometime prior to around 15,600 years ago. Later, abrupt changes in sediment composition around 12,400 years ago appear to mark the initial breaching of the lake's dam. These varved lake deposits were later used by European settlers for brick-making. The lake was named after Edward Hitchcock, a geologist from Amherst College who had studied it.

Lake Jordan (Montana)

Lake Jordan was a glacial lake formed during the late Pleistocene along the Jordan River. After the Laurentide ice sheet retreated, water melting off the glacier accumulated between the Rocky Mountains and the ice sheet. The lake drained along the front of the ice sheet, eastward towards the Yellowstone River and Glacial Lake Glendive.

From the lake deposits near Great Falls, Montana, the Havre lobe of the Laurentide ice sheet dammed the ancestral Missouri River during the late Wisconsin Glacial Period.

Lake Merrimack

Lake Merrimack was a glacial lake that formed during the late Pleistocene epoch. After the Laurentide ice sheet retreated, glacial ice melt accumulated at the terminal moraine and blocked up the Merrimack River, creating the narrow lake. The lake extended from Manchester to Plymouth, New Hampshire. It is unknown when the lake was drained.

Lake Hitchcock is an important part of the geology of New Hampshire. It experienced annual layering of sediments, or varves: silt and sand in the summertime (due to glacial meltwater) and clay in the wintertime (as the lake froze).

Lake Stowe

Lake Stowe was a glacial lake that formed in Central Vermont approximately 15,000 years ago in the late Pleistocene epoch. After the Laurentide ice sheet retreated, glacial ice melt accumulated at the terminal moraine.The lake existed until the glacier had completely melted. Then it flowed out through the Lamoille River valley.The lake was named after Stowe, near where evidence of the lake was discovered.

Tyrrell Sea

The Tyrrell Sea, named after Canadian geologist Joseph Tyrrell, is another name for prehistoric Hudson Bay, namely as it existed during the retreat of the Laurentide Ice Sheet.

Roughly 8,000 years BP, the Laurentide Ice Sheet thinned and split into two lobes, one centred over Quebec-Labrador, the other over Keewatin. This drained Glacial Lake Ojibway, a massive proglacial lake south of the ice sheet, leading to the formation of the early Tyrrell Sea. The weight of the ice had isostatically depressed the surface as much as 270-280 m below its current level, making the Tyrrell Sea much larger than modern Hudson Bay. Indeed, in some places the shoreline was 100 to 250 km farther inland than at present. It was at its largest at roughly 7,000 years BP.Isostatic uplift proceeded rapidly after the retreat of the ice, as much as .09 m per year, causing the margins of the sea to regress quickly towards its present margins. The rate of uplift decreased with time however, and in any event was nearly matched by sea-level rise from the melting ice sheets. When the Tyrrell Sea "became" Hudson Bay is difficult to define, as Hudson Bay is still shrinking from isostatic rebound.

Wisconsin glaciation

The Wisconsin Glacial Episode, also called the Wisconsinan 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 Innuitian ice sheet, which extended across the Canadian Arctic Archipelago; the Greenland ice sheet; 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 (known globally as the Eemian stage) and the current interglacial, the Holocene. The maximum ice extent occurred approximately 25,000–21,000 years ago during the last glacial maximum, also known as the Late Wisconsin in North America.

This glaciation radically altered the geography north of the Ohio River. 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 in Lake Erie or in New York City's Central Park, 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 (the Bering Land Bridge) and move between North America and Siberia. As the glaciers retreated, glacial lakes were breached in great floods of water such as the Kankakee Torrent, which reshaped the landscape south of modern Chicago as far as the Ohio and Mississippi Rivers.

Continental glaciations
General
Landforms
North America
Eurasia and
Antarctica
Time periods
Africa
Asia
Europe
North America
South America
Summary

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