The Mississippian Period began about 359 million years ago and ended about 318 million years ago. Mississippian-age rocks crop out in a north-south band in east-central Ohio from the Ohio River almost to the Lake Erie shore and then eastward in northeastern Ohio. They are well exposed in much of this area because they consist primarily of erosion-resistant sandstones and sandy shales that form prominent cliffs.
Throughout most of the world, rocks of Mississippian age are defined as the lower part of the Carboniferous Period. However, in the United States Carboniferous-equivalent rocks are divided into the Mississippian and Pennsylvanian Systems.
During the Mississippian, Ohio was in equatorial latitudes and most of the state was covered by a shallow sea. There is some evidence that the Cincinnati arch or platform was emergent or nearly so for much of the period. During the early part of the period, sediments were being eroded from the Acadian Mountains to the east and from the Canadian shield to the north. Clay, silt, and fine-grained sand settled in the Ohio sea as the offshore portion of the Catskill Delta to the east. There was an abrupt change in sedimentation in the Late Devonian from the stagnant-sea, highly organic Ohio Shale to the gray and red Bedford shale. At the beginning of the Mississippian Period the sea covering Ohio briefly returned to a state of stagnation, similar to that of the Devonian Ohio Shale, and the black, highly organic Sunbury Shale was deposited. The Sunbury Shale is thin, about 20 feet in thickness, in contrast to thickness of hundreds of feet of Ohio Shale.
As the Early Mississippian progressed, coarser sands representing the offshore portions of delta lobes formed by many west-flowing streams built up thick deposits of sand and sandy shale.
One of the more interesting units of the Lower Mississippian is the Black Hand Sandstone. The nearly pure quartz sandstone reaches about 200 feet in thickness in the Hocking Valley and crops out to the north into Licking and Richland Counties. This erosion-resistant unit forms cliffs, rock-shelter caves, and waterfalls in scenic areas such as Hocking Hills State Park, the gorge of the Licking River, and Mohican State Park.
During Middle and Late Mississippian time mud and sand was no longer being carried into the Ohio sea in any quantity and there appear to have been episodes during which the sea withdrew periodically and erosion occurred. The Maxville Limestone is present in beds that are Middle Mississippian and Late Mississippian in age. Many of these beds show evidence of being deposited in shallow water and, in some cases, on tidal flats. The shallowing and withdrawal of the sea indicates a slow rise of the land surface in Ohio in a feature known as a forebulge. This marks the initial collision of North America and Africa that would produce the majestic Appalachian Mountains during the remainder of the Paleozoic.
Life during the Mississippian was diverse and flourished in the seas and was beginning do exert dominance of the land. However, the muddy seas that persisted across Ohio were not favorable for many bottom-dwelling invertebrates and many were not well preserved in these rocks. There are exceptions, especially in the Cuyahoga Formation where some localities in northeastern Ohio and in southern Ohio have a rich, although commonly not well preserved, assemblage of invertebrates. The Meadville Shale Member has produced fossils of at least 125 species.
Mississippian rocks have been important to the economic development of the state, although it is less today than in the past. The Black Hand Sandstone and Buena Vista Sandstone of the Cuyahoga Formation have been important building stones. The Buena Vista of southern Ohio is still quarried. Oil and gas have been produced in modest quantities from Mississippian rocks, particularly the Black Hand in the subsurface of southern Ohio. The Bedford Shale was once an important source of clay for bricks and tile.
Perhaps the most spectacular scenery in Ohio is due to thick, erosion-resistant sandstones of Mississippian age. The Black Hand Sandstone, in particular, forms high cliffs, large rock-shelter caves, and waterfalls in its areas of outcrop from Hocking County northward to Richland County. Hocking Hills State Park in Hocking County is noted for these features at separate park areas such as Old Man’s Cave, Ash Cave, Cedar Falls, and other areas. Black Hand Gorge, in Licking County, and Mohican State Park have spectacular outcrops of the Black Hand Sandstone.