Bedrock Geologic Map of Illinois
Image courtesy Illinois State Geological Survey (fair use policy)
Illinois actually has almost no bedrock exposed at the surface, only a little at its south end, northwest corner, and along the west on the Mississippi River. Like the rest of the upper Midwest states, Illinois is covered with glacial deposits from the Pleistocene ice ages. (For that aspect of the state's geology, see the Quaternary map of Illinois page on this site.)
Beneath that recent veneer, Illinois is dominated by limestone and shale, deposited in shallow-water and coastal environments during the middle of the Paleozoic Era. The whole southern end of the state is a structural basin, the Illinois Basin, in which older beds around the rim dip downward beneath the younger central strata. In the northern part of Illinois these rocks are eroded away to expose older deposits from the Ordovician Period.
The bedrock of Illinois is richly fossiliferous. Besides the abundant trilobites found throughout the state, there are many other classic Paleozoic life forms represented, which you can see on the fossils page at the Illinois State Geological Survey site.
More Illinois resources on About.com:
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Illinois Archaeology
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