The layers of Ohio's bedrock are slightly tilted, with the youngest rocks at or near the surface in the eastern half of the state and the oldest rocks exposed in the west. The oldest rocks, including the beds of southwestern Ohio near Cincinnati, were formed during the Late Ordovician Period. These formations, known as the Cincinnatian Series, are approximately 200 meters (600 feet) thick and are composed of alternating thin to very thin bedded, gray, fossiliferous limestone and blue-gray shale (or mudstone). To learn more about Cincinnatian Series limestone, click here.Limestone and shale layers
The proportion of shale to limestone varies greatly within the Cincinnatian Series. There are generally larger amounts of shale in the lower and upper parts of the Series. Single limestone beds are not continuous layers across the shale, but generally extend only for a few meters to several tens or hundreds of meters, pinching out within the shale as in the graphic below.
The Cincinnatian Series is divided into several separate packages or sets of layers, called formations. The Waynesville Formation (or Waynesville Shale) exemplifies the concept of a formation.The Waynesville Formation
The Waynesville Formation is blue-gray shale (or mudstone) with minor, interbedded thin limestone layers. It contrasts markedly with the overlying Liberty Formation, which has more equal amounts of shale and limestone in its layers.
The thin limestone layers within the shale, characteristic of the Waynesville Formation, are best seen in fresh exposures like this photograph of a 'new' roadcut in 1974. Weathering and erosion make it more difficult to see these layers in older roadcuts.
Follow the links below to learn more about local geology.
- Learn about some important characteristics of local formations, including bedding contacts, cracks, and ripple marks.
- Find out what kinds of fossils can be found in the Cincinnatian Series.
- Learn how these limestone layers and fossils formed.