From Ohio History Central
Ohio’s youngest rocks are present in southeastern Ohio and belong to a division called the Dunkard Group by geologists. Similar rocks are present in adjacent areas of West Virginia and Pennsylvania. These rocks consist of alternating beds of sandstone, shale, coal, and non-marine limestones and represent deposition on an upper delta plain. The sediments that formed Dunkard rocks were eroded from the rising Appalachian Mountains to the east and southeast. These rocks represent the final filling of the Ohio basin and pushed the persistent seas of the Paleozoic out of Ohio, never to return. Coal has been mined from Dunkard rocks and limestone and sandstone have been quarried.
There has long been a debate among geologists if Dunkard rocks are Late Pennsylvanian or Early Permian in age. The difficulty in establishing an age is because of the lack of marine index fossils. Plant and vertebrate fossils found in Dunkard rocks could belong to either geologic system. In recent years, the Dunkard Group has been placed in the Pennsylvanian.