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 the youngest rocks, of Pennsylvanian age (gray), occupy the center and successively older beds around the rim dip downward beneath them; these represent Mississippian (blue) and Devonian (blue-gray). In the northern part of Illinois these rocks are eroded away to expose older deposits of Silurian (dove-gray) and Ordovician (salmon) age.
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. See a gallery of Illinois geological attractions.
More Illinois resources on About.com:
About Chicago
About St. Louis
Chicago Travel
Illinois Maps
Illinois Geography, State Symbols & Facts
Illinois Campgrounds
Chicago Architecture
Illinois Hotels & Resorts
Illinois National Parks
Illinois State Parks
Illinois Camping Destinations
Illinois Scenic Roads
Illinois Archaeology
