<P> The Great Lakes are estimated to have been formed at the end of the last glacial period (the Wisconsin glaciation ended 10,000 to 12,000 years ago), when the Laurentide Ice Sheet receded . The retreat of the ice sheet left behind a large amount of meltwater (see Lake Algonquin, Lake Chicago, Glacial Lake Iroquois, and Champlain Sea) that filled up the basins that the glaciers had carved, thus creating the Great Lakes as we know them today . Because of the uneven nature of glacier erosion, some higher hills became Great Lakes islands . The Niagara Escarpment follows the contour of the Great Lakes between New York and Wisconsin . Land below the glaciers "rebounded" as it was uncovered . Since the glaciers covered some areas longer than others, this glacial rebound occurred at different rates . </P> <P> A notable modern phenomenon is the formation of ice volcanoes over the lakes during wintertime . Storm - generated waves carve the lakes' ice sheet and create conical mounds through the eruption of water and slush . The process is only well - documented in the Great Lakes, and has been credited with sparing the southern shorelines from worse rocky erosion . </P> <P> The Great Lakes have a humid continental climate, Köppen climate classification Dfa (in southern areas) and Dfb (in northern parts) with varying influences from air masses from other regions including dry, cold Arctic systems, mild Pacific air masses from the West, and warm, wet tropical systems from the south and the Gulf of Mexico . The lakes themselves also have a moderating effect on the climate; they can also increase precipitation totals and produce lake effect snowfall . </P> <P> The most well - known winter effect of the Great Lakes on regional weather is the lake effect in snowfall, which is sometimes very localized . Even late in winter, the lakes often have no icepack in the middle . The prevailing winds from the west pick up the air and moisture from the lake surface, which is slightly warmer in relation to the cold surface winds above . As the slightly warmer, moist air passes over the colder land surface, the moisture often produces concentrated, heavy snowfall that sets up in bands or "streamers". This is similar to the effect of warmer air dropping snow as it passes over mountain ranges . During freezing weather with high winds, the "snow belts" receive regular snow fall from this localized weather pattern, especially along the eastern shores of the lakes . Snow belts are found in Wisconsin, Michigan, Ohio, Pennsylvania, and New York, United States; and Ontario, Canada . </P>

How many modern-day states of the us border the great lakes