Some 225,000 years ago, most of what is now Illinois was buried under
the ice of the Illinoian glacier. Two earlier glaciers had covered large
parts of Illinois, and another, known as the Wisconsinan glacier, came
into the state later, about 50,000 to 70,000 years ago (fig. 16).
The relatively small glaciers in the United States today, such as those
in the northern Rocky Mountains, are concentrated in valleys and are
called valley glaciers. The glaciers that covered Illinois were parts of
huge ice sheets that extended over much of the North American continent
and are called continental glaciers. They spread over most of Canada,
then pushed southward to bury New England and a great area in the
north-central part of the United States north of the Ohio and Missouri
Rivers.