Characteristics of Local Bedrock

The bedrock of southwestern Ohio contains certain characteristics that can tell you a lot about the environment in which the sediments that later became our local bedrock were deposited; these characteristics include:

  • bedding contacts
  • desiccation cracks
  • ripple marks

Bedding Contacts

Bedding contacts are the boundaries between adjacent beds of sedimentary materials representing changes in the nature of sediments over time. An example of a bedding contact is the surface separating a sandstone layer from an overlying shale layer. Bedding contacts can be gradational (gradual) or sharp (abrupt). As such, bedding contacts tell you how quickly the layers formed.

photos showing the difference between gradual and abrupt separations between 2 layers

If the contact is not very well defined and appears gradational, that means that sediments were deposited gradually. This type of contact reflects the occurrence of gradual changes in a benthic community over time.

If the contact is sharp, that means that there was a rapid change in sediment type. One example of a cause of rapid change is the storms that frequently destroyed benthic communities. These disturbances removed all fine matter, leaving the coarse to settle down. This would later cause a sharp bedding contact.

Most bedding contacts in the Cincinnatian Series are gradational; however, some are sharp and diastemic.

Desiccation Cracks

Desiccation cracks occur in several of the formations. These features are the result of drying of fine-grained sediments due to subaerial exposure in tidal flats. Finding mudcracks in a bedrock outcrop tells you that the sediments forming the rocks were very close to sea level after their deposition and before their lithification.

Ripple Marks

Ripple-marked beds occur in all of the formations of southwestern Ohio. These features are the result of the action of waves in relatively shallow water. The presence of ripple marks in a sedimentary layer tells geologists that the sediments in that layer were deposited within the reach of waves, within a few feet or tens of feet from the water's surface.