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The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square miles, including most of Canada and a large portion of the northern United States, multiple times during Quaternary glacial epochs. It last covered most of northern North America between c. 95,000 and c. 20,000 years before the present day. At times, its southern margin included the modern sites of New York City and Chicago, and then followed quite precisely 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 in the mid-continent.
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.
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. This 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.
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 is believed to have disrupted the formation of North Atlantic Deep Water, the very saline, cold, deep water that flows from the Greenland Sea. This interrupted the thermohaline circulation, creating the brief Younger Dryas cold epoch and a temporary re-advance of the ice sheet, which did not retreat from Nunavik until 6,500 years ago.
Some people have argued that huge influxes of fresh water stopped the feeding of the ice sheet and starved it, aiding the retreat that had already begun. This is a controversial position because it is known that the North Atlantic was very cold throughout glacial periods and it is likely that the anticyclone on top of the ice sheet helped sustain it through moist easterly winds which encouraged snow-bearing winds from the south.
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, located on central Baffin Island. Other inactive remnants of the ice sheet are also found beneath rocks around Deboullie Lake in northern Maine.