Paleontology and geology
The Precambrian: All Precambrian rocks of Illinois are deeply buried below the surface. They consist primarily of volcanic and granitic intrusions, and thus do not contain fossils.
The Paleozoic: During much of the Paleozoic, the state was covered by a shallow sea, and the fossil record indicates a rich and diverse marine fauna from the Cambrian through the Early Carboniferous. During the Late Carboniferous, a large river delta system dominated most of Illinois. The swamps on this delta were inhabited by a tremendous variety of plants and animals, now referred to as the Mazon Creek flora and fauna. These swamps provided the carbon that today forms the coal beds mined in Illinois. There are no rocks of Permian age in the state.
The Mesozoic: There are very few Mesozoic rocks in Illinois. The state was mostly above sea level and subject to erosion during this time. There are small exposures of Cretaceous rocks, but the only fossils found have been some foraminifera and leaf fossils.
The Cenozoic: There are only sparse exposures of Tertiary rock in southernmost Illinois, and few fossils have been recovered from them. The Cenozoic is represented mainly by Quaternary glacial deposits covering most of the state. The exceptions are the areas to the west and in the southern part of the state, which were glacier-free. There is a rich Quaternary fossil record that includes giant beaver, mastodon, mammoth, and the stag moose.