undifferentiated rock units

Paleontology and geology

The majority of exposed bedrock in Illinois is Carboniferous in age. In the Early Carboniferous (Mississippian), changes in the size of glaciers near the South Pole caused sea level to fluctuate over Illinois. There were alternating periods when Illinois was underwater and periods when it was dry land. Limestones of the Early Carboniferous are found in the western and southern portions of the state, and yield marine fossils of crinoids, blastoids, bryozoans, corals, brachiopods, and sharks.

During the Late Carboniferous (Pennsylvanian), Illinois was dominated by a large river delta. Swamps covered much of the state, producing the coal deposits found in Illinois today. These swamps were inhabited by a tremendous variety of plants and animals, now referred to as the Mazon Creek flora and fauna. The fossils from Mazon Creek include ferns, seed ferns, extinct relatives of spiders, millipedes, and giant dragonflies, as well as jellyfish, shrimp, horseshoe crabs, clams, sharks, brachiopods, bony fishes, and the Tully monster.