About 85 percent of what is now Illinois was covered by glaciers at least once during the Pleistocene Epoch (1.6 million to 10,000 years ago) of the Cenozoic Era. The glacial periods affecting Illinois are known as the pre-Illinoian, Illinoian and Wisconsinian. Only the extreme northwestern and extreme southern parts of the state along with Calhoun County and parts of Pike, Jersey, Monroe and Randolph counties were not glaciated. No one is sure what caused this ice age. It could have been due to a cyclic pattern of factors relating to the earth's orbit and tilt on its axis; shifts in the Gulf Stream in the Atlantic Ocean; reversals in the earth's magnetic field; volcanic activity; galactic dust clouds; or other reasons. The evidence does show that the glaciation occurred as the result of abrupt climatic changes, not gradual ones. Ice sheets began to grow from regions near the North Pole at this time when the summers were about 7 to 13 degrees Fahrenheit cooler than those of today, and the winter snows did not completely melt.
Low maximum temperatures, not low minimum temperatures were necessary for the glaciers to develop. Because the time of cooler conditions lasted tens of thousands of years, thick masses of snow and ice accumulated to form glaciers. As the glacial ice thickened with more snow, its great weight caused it to flow outward at the edges, often for hundreds of miles. As the ice sheets expanded, the areas in which snow accumulated also increased in size. Glaciers were able to continue to grow until the climate warmed enough so that the rate of melting was greater than the rate of expansion.
Pleistocene glaciers and the waters melting from them changed the landscapes that they covered. Some sections of the glaciers in northern Illinois were about 2,000 feet thick, while other areas of the state were covered by ice masses about 700 feet thick, still as tall as a 60-story building. The glaciers moved the land they overrode, leveling and filling many valleys. Moving ice carried colossal amounts of rock and earth, for much of what the glaciers wore off the ground was kneaded into the moving ice and carried along, often for hundreds of miles.
The continual floods released by melting ice carved new waterways, deepened old ones and partly refilled both with sediments as great quantities of rock and earth were carried beyond the glacier fronts. According to some estimates, the amount of water drawn from the sea and changed into ice during a glaciation was enough to lower the sea from 300 to 400 feet below its level today. Consequently, the melting of a continental ice sheet provided a tremendous volume of water that eroded and transported sediments. In most of Illinois, glacial and meltwater deposits buried the old hill-and-valley terrain and created the flatter land forms which would become the prairies. Glaciers left a mantle of soil and buried deposits of gravel, sand and clay over about 90 percent of the state.
Pre-Illinoian (1.6 million to 300,000 years ago) glaciers invaded Illinois from the west and east. There may have been several glaciers advancing into Illinois during this period, but not much evidence of them remains because it was so long ago and wind, water and other glaciers have mostly destroyed it.
The Illinoian stage glaciation was extensive in Illinois. At this time glaciers extended to the most southern point that they have ever reached in the northern hemisphere. That place was in Illinois, near Carbondale. About 85 percent of what is now Illinois was covered by this ice sheet.
The Wisconsinian glaciation started about 15,000 years ago and covered much of the northern and east-central parts of our state. The Illinois area of this glaciation would generally become the Grand Prairie natural division. The moraines and Lake Michigan in northeastern Illinois are all remnants of this glacial period. About 12,000 years ago the climate warmed, and the glaciers began to melt and retreat, forming large lakes. As the melting continued, the lake waters eventually eroded their banks and created enormous floods. The flood known as the Kankakee Torrent was mainly responsible for the deposition of sand along the Illinois River, where sand prairies developed.