undifferentiated rock units

Paleontology and geology

Most of the Silurian rocks in northern Illinois are dolostones, but further south, limestones, siltstones, and shale are more common. Illinois was almost continuously underwater in the Silurian, and most of the rocks formed as part of reef complexes. The northern reefs are called the Niagaran reefs and are highly fossiliferous. Corals, bryozoans, stromatoporoids, brachiopods, and crinoids were common members of these reef communities.