History
Geologic history During the early part of the Paleozoic Era, the area that would one day become Illinois was submerged beneath a shallow sea and located near the Equator. Diverse marine life lived at this time, including trilobites, brachiopods, and crinoids. Changing environmental conditions led to the formation of large coal swamps in the Carboniferous. Illinois was above sea level for at least part of the Mesozoic, but by its end was again submerged by the Western Interior Seaway. This receded by the Eocene Epoch. During the Pleistocene Epoch, vast ice sheets covered much of Illinois, with only the Driftless Area remaining exposed. These glaciers carved the basin of Lake Michigan and left behind traces of ancient glacial lakes and moraines. Pre-European American Indians of successive cultures lived along the waterways of the Illinois area for thousands of years before the arrival of Europeans. The Koster Site has been excavated and demonstrates 7,000 years of continuous habitation.
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