From Ohio History Central
During Late Devonian time, about 350 million years ago, the inland sea that stretched across Ohio and adjacent areas to the north and south, became stagnated so that the bottom waters were foul and bottom-dwelling animals could not survive. However, the upper waters of this sea were oxygenated and supported plankton and a variety of early fishes. The organic matter from the dead plankton, as well as the dead fish, settled to the bottom of the sea along with particles of clay washed in from deltas and mountains to the east, forming a thick bed of sediment that eventually would become the Ohio Shale. This tough, cliff-forming shale is dark in color and crops out in a north-south band from the Ohio River northward through the central part of the state, then eastward along portions of the Lake Erie shore. It is rich in organic hydrocarbons (kerogen) and is considered an oil shale. Large, rounded, carbonate concretions are common in some zones of the Ohio Shale. Many of the concretions contain bony plates of armored fishes.