The Laurentide ice sheet was a massive sheet of ice that covered hundreds of thousands of square miles, including most of Canada and 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 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.
Up to two miles thick in Nunavik but 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 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 Atlantic Ocean initially via the Susquehanna River and 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 Dryas cold epoch and a brief re-advance of the ice sheet, which did not retreat from Nunavik until 6,500 years ago.
Some people have controversially 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 anticyclone on top of the ice sheet helped sustain it through moist easterly winds which encouraged snow-bearing winds from the south.
During the Kansan Glaciation the Laurentide Ice Sheet extended even further south, reaching as far as Douglas County, Kansas and almost as far as St. Louis, Missouri.