| Laurentide ice sheet | |
|---|---|
 The maximum extent of North American ice sheets during theLast Glacial Maximum | |
| Type | Continental |
| Location | North America |
| Highest elevation |
|
| Lowest elevation | Sea level |
| Terminus |
|
| Status | Remnants:Barnes Ice Cap,Penny Ice Cap[1] |
TheLaurentide ice sheet (LIS) was a massivesheet of ice that covered millions of square miles, including most ofCanada and a large portion of theNorthern United States, multiple times during theQuaternary glaciation epochs, from 2.58 million years ago to the present.[2]
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 fiveGreat Lakes and the hosts of smaller lakes of theCanadian Shield. These lakes extend from the easternNorthwest Territories, through most of northern Canada, and the upperMidwestern United States (Minnesota,Wisconsin, andMichigan) to theFinger Lakes, throughLake Champlain andLake George areas ofNew York, across the northernAppalachians into and through all ofNew England andNova Scotia.
At times, the ice sheet's southern margin included the present-day sites of coastal towns of theNortheastern United States, and cities such asBoston andNew York City and Great Lakes coastal cities and towns as far south asChicago andSt. Louis, Missouri, and then followed the present course of theMissouri River up to the northern slopes of theCypress Hills, beyond which it merged with theCordilleran Ice Sheet. The ice coverage extended approximately as far south as 38 degrees latitude mid-continent.[3]
This ice sheet was the primary feature of thePleistocene epoch in North America, commonly referred to as theice age. During thePre-Illinoian Stage, the Laurentide Ice Sheet extended as far south as theMissouri andOhio River valleys. It was up to 2 mi (3.2 km) thick inNunavik,Quebec,Canada, but much thinner at its edges, wherenunataks 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 andglacial 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-dayLake Ontario, and drained down the Hudson River into the Atlantic Ocean.[4]
Its cycles of growth and melting were a decisive influence on globalclimate during its existence. This is because it served to divert thejet stream southward, which would otherwise flow from the relatively warmPacific Ocean throughMontana andMinnesota. That gave theSouthwestern 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 inEurope had an analogous effect on the rainfall inAfghanistan, parts ofIran, possibly westernPakistan in winter, as well asNorth Africa.
Its melting also caused major disruptions to the global climate cycle, because the huge influx of low-salinity water into theArctic Ocean via theMackenzie River[5] is believed to have disrupted the formation ofNorth Atlantic Deep Water, the very saline, cold, deep water that flows from theGreenland Sea. That interrupted thethermohaline circulation, creating the briefYounger Dryas cold epoch and a temporary re-advance of the ice sheet,[6] which did not retreat fromNunavik until 6,500 years ago.
After the end of the Younger Dryas, the Laurentide Ice Sheet retreated rapidly to the north, becoming limited to only the Canadian Shield until even it became deglaciated.[7] 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 in remnants of the LIS: theBarnes Ice Cap and thePenny Ice Cap.
During theLate Pleistocene, the Laurentide ice sheet reached from theRocky Mountains eastward through theGreat Lakes, intoNew England, covering nearly all ofCanada east of the Rocky Mountains.[8]Three major ice centers formed in North America: theLabrador,Keewatin, andCordilleran. 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 theMackenzie River to theMissouri River and the upper reaches of theMississippi River. The Labrador covered spread overeastern Canada and the northeastern part of theUnited States abutting the Keewatin lobe in the westernGreat Lakes andMississippi valley.[8]
The Keewatin ice dome has had four or five primary lobes identified ice divides extending from a dome over west-centralKeewatin (Kivalliq). Two of the lobes abut the adjacent Labrador and Foxe-Baffin ice domes. The primary lobes flow (1) towardsManitoba andSaskatchewan; (2) towardHudson Bay; (3) towards theGulf of Boothia, and (4) towards theBeaufort Sea.[9]
The Labrador dome flowed across all ofMaine and into theGulf of St. Lawrence, completely covering theMaritime Provinces. The Appalachian Ice Complex, flowed from theGaspé Peninsula overNew Brunswick, theMagdalen Shelf, andNova Scotia.[9]The Labrador flow extended across the mouth of theSt. Lawrence River, reaching theGaspé Peninsula and acrossChaleur Bay. From the Escuminac center on theMagdalen Shelf, flowed onto theAcadian Peninsula ofNew Brunswick and southeastward, onto the Gaspe, burying the western end ofPrince Edward Island and reached the head ofBay of Fundy. From the Gaspereau center, on the divide crossing New Brunswick flowed into the Bay of Fundy and Chaleur Bay.[9]
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 theenormous weight of the melted ice.[10]
The Foxe-Baffin ice dome was circular and centered over theFoxe Basin. A major divide across the basin, created a westward flow across theMelville Peninsula, from an eastward flow overBaffin Island andSouthampton Island. Across southern Baffin Island, two divides created four additional lobes. The Penny Ice Divide split theCumberland Peninsula, wherePangnirtung created flow toward Home Bay on the north and Cumberland Sound on the south. The Amadjuak Ice Divide on theHall Peninsula, whereIqaluit sits created a north flow intoCumberland Sound and a south flow into theHudson Strait. A secondary Hall Ice Divide formed a link to a local ice cap on theHall 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.[9]
TheCordilleran ice sheet covered up to 1,500,000 square kilometres (580,000 sq mi) at theLast Glacial Maximum.[11] The eastern edge abutted the Laurentide ice sheet. The sheet was anchored in theCoast Mountains ofBritish Columbia andAlberta, south into theCascade Range ofWashington. That is one and a half times the water held in theAntarctic. Anchored in the mountain backbone of the west coast, the ice sheet dissipated north of theAlaska Range where the air was too dry to form glaciers.[8]It is believed that the Cordilleran ice melted rapidly, in less than 4000 years. The water created numerousProglacial lakes along the margins such asLake Missoula, often leading to catastrophic floods as with theMissoula Floods. Much of the topography ofEastern Washington and northernMontana andNorth Dakota was affected.[8]
TheInnuitian ice sheet, centered on theQueen Elizabeth Islands, was connected to the northern part of the LIS.[12]
the ice was about 2,000 feet thick over Manhattan
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