From Ohio History Central
The Pennsylvanian Period began about 318 million years ago and ended about 299 million years ago. Rocks of this geologic system are well exposed throughout a large, mostly unglaciated, area of eastern Ohio. Historically, these coal-bearing rocks have been of great economic importance to the state and preserve evidence of changing environments and a proliferation of both marine and terrestrial life.
By the beginning of the Pennsylvanian Period, erosion during the latter part of the Mississippian Period had carved deep, broad valleys across the state. This episode of downcutting was in response to uplift of the land as a forebulge formed east of the zone of collision of North America with Africa and the beginning of the Appalachian Mountains that would dominate eastern North America for the next 70 million years. Extensive glaciations in the southern hemisphere on the megacontinent of Gondwana, probably contributed to the lowering of sea level.
As the Appalachian Mountains began to rise, nearly pure quartz sand was carried into Ohio in the north from the Canadian Shield and in the south from the Appalachian uplands. These sands filled the valleys that had been cut into the Mississippian rocks forming thick, extensive beds that are now known as the Sharon Sandstone. The Sharon is present in three distinct areas that do not appear to be interconnected and plant fossils suggest that the Sharon is of slightly different age in these areas. In northeastern Ohio the Sharon Sandstone forms scenic cliffs principally in Geauga, Portage, and Summit Counties and is quarried for various construction and industrial uses. The second area of outcrop is in Jackson and Pike Counties in southern Ohio where it is quarried and forms scenic cliffs. The southern Sharon Sandstone appears to be older than the northeastern Sharon. The third area is a north-south oriented deposit in eastern Ohio that is entirely covered by younger Pennsylvanian rocks and is known only by drilling for oil and gas.
Soon after deposition of the Sharon Sandstone, shallow seas began to flood into Ohio from the southwest and deposited thin beds of limestone and shale. Marine waters were constantly being pushed back by an influx of sediment eroded from the rising Appalachian Mountains to the south and east. Thus, Ohio was a constantly fluctuating coastal area dominated by deltas built into the seas by sediment-laden streams and periodic rise and fall of sea level due to waxing and waning of continental glaciers in Gondwana and tectonic forces of mountain building. These circumstances resulted in multiple, thin, laterally discontinuous beds of limestone, shale, clay, sandstone, and coal in repetitive sequences.
The Pennsylvanian System in Ohio is characterized by beds of economically important bituminous coal, some of which are thick and extensive, some of which are thin and discontinuous. Indeed, early geologists referred to these rocks as the Coal Measures, although coal beds are only a small portion of the total thickness of these rocks. The coal beds formed in extensive coastal swamps in which grew lush vegetation. In many areas, again and again, plant material accumulated to sufficient thickness to form a bed of coal eventually. Inundation by the sea or a change in the course of a delta-building river buried the swamp and initiated a new cycle of deposition. The coal-forming coastal swamps flourished in the warm, moist climate near the paleoequator.
Pennsylvanian rocks in Ohio have been divided into four subdivisions, or groups, which are, in ascending (oldest to youngest) order: Pottsville, Allegheny, Conemaugh, and Monongahela. Each of these groups consists of numerous beds of shale, mudstone, sandstone, clay, marine or freshwater limestone, and coal that in most cases are comparatively thin and laterally discontinuous. More than 100 individual beds have been named within the Pennsylvanian rocks of Ohio.
Most of the beds in the lower half of the Pennsylvanian sequence are dark in color and represent deposition in marine embayments or low-lying lower delta plains. Near the middle of the Conemaugh Group, the rocks are dominated by red colors, marine limestones and shales become less common or absent, and freshwater limestones become more prevalent. This change, sometimes called the red-gray boundary, indicates a transition from marine-lower delta plain environments to upper delta plain environments. The redbeds represent oxidized sediments and soil zones and a change from a moist environment to seasonal wet-dry conditions. Some redbed units are prone to slope failure, especially when wet, and have caused costly landslides in southeastern Ohio. The freshwater limestones were deposited in lakes. By the end of the period, the basin was filled with sediment and the sea was pushed out of Ohio, never to return.
Life was abundant and diverse during the Pennsylvanian Period, both in the seas and especially on the land. Many of the marine limestones and shales, although only a few feet thick in most cases, contain abundant marine fossils of brachiopods, clams, snails, cephalopods, bryozoans, and rare trilobites, among others. Plant life flourished in the coastal swamps near the equator and was dominated by ferns, horsetail rushes, lycopods (scale trees), and conifers, among others. Some of the trees, such as the scale trees Lepidodendron and Sigillaria, reached heights of more than 100 feet. Trunks, roots, leaves, and reproductive structures are common in many shale beds and their highly altered remains formed the coal beds that have been so important to Ohio.
Fossils of vertebrates have been found in both marine and nonmarine rocks in Ohio. Marine limestones and shales contain teeth and fin spines of sharks and sharklike fishes and teeth and scales of bony fishes. Nonmarine rocks, principally lakes associated with coastal swamps, preserve remains of freshwater sharks and bony fishes, and skeletal elements of early amphibians and reptiles. One of these deposits, known as Linton, in Jefferson County, has produced numerous fossils of fishes, amphibians, and reptiles since the mid 1800