undifferentiated rock units

Paleontology and geology

During the Ordovician, Illinois lay in the tropics south of the equator, and a warm shallow sea covered the state. Later in the Ordovician, sandstones, such as the St. Peter Sandstone, were deposited over much of the state. Fossils from this time period include corals, encrusting algae, trilobites, brachiopods, bryozoans, snails, and nautiloids that thrived in the warm waters. This map indicates the presence of Ordovician rocks over much of northern Illinois.