Though known primarily for its rolling farmlands, Ohio is a mosaic of geophysical provinces, defined by underlying geology, tectonic uplift and the erosive power of Pleistocene Glaciers. From a broad perspective, the State overlaps two major provinces; the northwestern 60% of Ohio lies within the Central Lowlands of North America while the southeastern 40% is occupied by the Appalachian Plateau. The border between these two geophysical regions runs SSW, from the northeast corner of Ohio to the Ohio River Valley at the southern edge of the State.
The Appalachian Plateau, composed of Carboniferous sandstones, limestones and shales, was lifted with the Southern Appalachian Mountains as North America and Africa collided during the formation of Pangea, some 300 million years ago. While the Plateau's western edge has been obscured by glacial erosion in northeastern Ohio, it stands out as a prominent escarpment through the southern half of the State, rising 500 feet above the adjacent lowlands. The Central Lowlands within Ohio are comprised of four sub-provinces: the Lake Plain, the Glacial Till Plains, the Ohio Valley and a small segment of the Interior Low Plateaus; the latter, unglaciated province, characterized by glades and karst landscapes, extends from Adams County, in southwestern Ohio, southward and then westward through Kentucky, western Tennessee, southern Indiana and southern Illinois.
The Lake Plain of northern Ohio is a swath of flat terrain south of Lake Erie, representing the post-glacial extent of Lake Warren, Lake Erie's larger predecessor; it is underlain with Devonian sedimentary rocks which have since been covered by glacial till and sandy lake deposits (though these rocks are exposed along the Lake Erie shoreline, in major river valleys and on Ohio's Lake Erie islands). South of the Lake Plain, the gently rolling farmlands of central and west-central Ohio occupy the Till Plains, molded by a number of post-glacial streams that flow southward to the Ohio River; beneath the thick layer of till are Silurian (western Ohio) and Devonian (central Ohio) sedimentary rocks, deposited in shallow seas from 400-350 million years ago. Finally, in the southwest corner of Ohio, ancient Ordovician limestones and shales, 500 million years old, have been sculpted into the hilly terrain of Greater Cincinnati by the erosive force of glacial meltwater and the continued action of post-glacial streams; harboring fossils of trilobites, brachiopods and other early marine invertebrates, these are the oldest exposed rocks in the State.
