From Ohio History Central
The Olentangy Shale is a gray to greenish-gray shale of Devonian age that averages about 30 feet in thickness. It was named for exposures along the Oentangy River in Delaware County. It lies above the Delaware Limestone and beneath the Ohio Shale. Fossils are moderately abundant in some areas but generally the Olentangy Shale is poorly fossiliferous.