From Ohio History Central
A small portion of the state, in part of Adams County and in a narrow band along the Ohio River in Brown, Clermont, and Hamilton Counties, is classified as the Outer Bluegrass Section of the Interior Low Plateau. The area is underlain by bedrock of Ordovician and Silurian age and is unglaciated or has very thin, highly weathered deposits of early glaciations. The area is dissected by streams and has high relief. Sinkholes and small caverns are developed in the thick Silurian limestones in Adams County.