Oneota Dolomite

The Ordovician age Oneota Dolomite underlies all of Illinois except the northmost part of the state. It ranges in thickness from 100 feet in the north to over 500 feet in the south. This formation consists of fine- to coarse-grained, light gray to brownish gray, cherty dolomite that contains minor amounts of sand and, at its base, thin shaly beds.