From Ohio History Central
The Cincinnati Arch, sometimes called the Ohio-Indiana Platform, is a north-south elongated area in the tri-state region of Ohio, Kentucky, and Indiana that branches in western Ohio to the west (Kankakee Arch) and the north-northeast (Findlay Arch). Paleozoic sedimentary rocks dip gently away from the Cincinnati Arch: to the east into the Appalachian Basin; to the west into the Illinois Basin; and to the north into the Michigan Basin. Thus, the Cincinnati Arch can be viewed as a stable area bordered by subsiding basins that accumulated sedimentary rocks. The oldest rocks exposed at the surface in Ohio (Ordovician) outcrop along the axis of the arch. Precambrian igneous and metamorphic rocks are nearest the surface along portions of the arch. Geologists indicate that the Cincinnati Arch formed during the Ordovician Period as surrounding basins began to sink.