- Laurentide ice sheet
Ice Sheetwas a massive sheet of ice that covered hundreds of thousands of square miles, including most of Canadaand a large portion of the northern United States, between c. 95,000 and c. 20,000 years before the present day. Its southern margin included the modern sites of New York Cityand Chicago, and then followed quite precisely the present course of the Missouri Riverup to the northern slopes of the Cypress Hills, beyond which it merged with the Cordilleran Ice Sheet.
Up to two miles thick in
Nunavikbut much thinner at its edges where nunataks were common in hilly areas, this ice sheet was the primary feature of the North American ice age.
Its cycles of growth and melting were a decisive influence on global
climateduring its existence. This is because it served to divert the jet streamwhich would otherwise flow from the relatively warm Pacific Oceanthrough Montanaand Minnesotato 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 Europehad an analogous effect on the rainfall in Afghanistan, parts of Iran, possibly western Pakistanin winter, as well as North Africa.
Its melting also caused major disruptions to the global climate cycle, because the huge influx of low-
salinitywater into the Atlantic Oceaninitially via the Susquehanna Riverand later via the Saint Lawrence is believed to have disrupted the circulation of water from south to north in the Atlantic, creating the brief Younger Dryascold epoch and a brief re-advance of the ice sheet, which did not retreat from Nunavikuntil 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 began. 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
anticycloneon top of the ice sheet helped sustain it through moist easterly winds which encouraged snow-bearing winds from the south.
Pre-Illinoian Stagethe Laurentide Ice Sheet extended even further south, reaching as far as Douglas County, Kansasand almost as far as St. Louis, Missouri.
The ultimate collapse of the Laurentide Ice Sheet is also suspected in influencing European agriculture indirectly through the rise of global sea levels.
Canada's oldest ice is a remnant of the Laurentide Ice Sheet called the
Barnes Ice Capon central Baffin Islandbeing 20,000 years old.
Glacial history of Minnesota
*cite journal |journal=University of Exeter |title=Noah's Flood Kick-started European Farming? |accessdate=2007-11-20 |url=http://www.sciencedaily.com/releases/2007/11/071118213213.htm
* [http://www.museum.state.il.us/exhibits/ice_ages/laurentide_deglaciation.html The Retreat of Glaciers in North America] (MPEG-Video)
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