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
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.
Illinois State Geological Survey. 1999. Pleistocene glaciations
in Illinois. Internet site. http://www.isgs.uiuc.edu/tours/outliers/pleist-g.htm
McClain, W. E. 1997. Prairie establishment and landscaping.
Technical Publication #2. Illinois Department of Natural Resources, Springfield,
Illinois. 62 pp. http://dnr.state.il.us/conservation/naturalheritage/florafauna/document.htm
Wiggers, R. 1997. Geology underfoot in Illinois. Mountain Press
Publishing Company, Missoula, Montana. 303 pp.