From Ohio History Central
Clay is composed of very fine-grained clay minerals, is not layered and has a plastic consistency. Clay deposits in Ohio were formed as ancient soils beneath coal beds (underclays) of Pennsylvanian age or deposited in glacial lakes during the Pleistocene Ice Age. Shale is a fine-grained rock composed of clay minerals but differs from clay primarily because it is bedded (layered).
Ohio ranks fifth in production of clay and shale with a total value of about $11.9 million from 2.24 million tons of annual production. Clay is produced in 19 counties and shale in 18 counties. Pennsylvanian-age underclays are present in the eastern half of the state whereas production from Pleistocene deposits is from the western part of the state.
Ohio has long been a center of the clay and shale industry. During the 1900s and early 1900s many companies produced brick, common clay products such as sewer tile, and decorative ceramic pottery. Although many of these companies have ceased operation, Ohio clays and shale are still used to produce brick, tiles, ceramics, as a component of cement, and in the manufacture of paper, cosmetics, and many other products. Much of the clay mined is used as liners for landfills and ponds.