Bedrock Geologic Map of Ohio
Underneath a widespread cover of young glacial sediment laid down in the last million years, Ohio is underlain by sedimentary rocks older than 250 million years: mostly limestone and shale, laid down in gentle, shallow seas. The oldest rocks are of Ordovician age, in the southwest; overlying them in a sweep over to the southeast border are (in order) Silurian, Devonian, Mississippian, Pennsylvanian and Permian rocks.
Deep beneath these is the ancient core of the North American continent, sloping away to the Illinois Basin to the southwest, the Michigan basin to the northwest, and the Appalachian Basin to the east. The part that isn't sloping, in the western half of the state, is the Ohio Platform, buried some 2 kilometers deep.
Ohio produces a great deal of coal and petroleum as well as other mineral products such as gypsum and aggregate.
See a larger, 725x970 pixel version, which also includes an explanation and a cross section.
More Ohio resources on About.com:
About Cleveland
About Columbus
Ohio Maps
Ohio Geography, State Symbols & Facts
Ohio National Parks
Ohio State Parks
Ohio Campgrounds
Ohio Hunting
Ohio Skiing
Ohio Archaeology
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