undifferentiated rock units

Paleontology and geology

There are very few areas of Devonian rock exposed in Illinois, shown on this map in the west, east, and south. These rocks are limestones, dolostones, chert, and shale, showing that Illinois was mostly covered by the sea at this time. In the southern part of the state, the water was quite deep and little oxygen was present. Black muds (now shales) were deposited on this deep sea floor. The most common invertebrate fossils include rugose corals, brachiopods, and trilobites. Shark teeth are the most commonly found vertebrate fossils.