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).

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.

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.

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 a 'new' roadcut in 1974 (photo at right). Weathering and erosion make it more difficult to see these layers in older roadcuts.

Some important characteristics of local formations, include:

• bedding contacts
• cracks
• ripple marks

Learn more about:

• fossils of the Cincinnatian Series
• sediment formation