Bedrock Geology of Ohio

From Ohio History Central
Revision as of 18:11, 24 April 2013 by (Talk)

File:Bedrock Map and Cross Section of Ohio.jpg
Bedrock Map and Cross Section of Ohio

The consolidated rocks that underlie Ohio are ancient and record several episodes of continental collision and mountain building, periodic inundation by warm, shallow seas, and extensive deltas and swamps. Throughout the Late Proterozoic (Precambrian) and Paleozoic Eons, Ohio was in equatorial latitudes and had a warm, tropical climate. The record of life during the Paleozoic Era, exquisitely preserved as fossils, is spectacular in Ohio rocks. In addition, these rocks have yielded abundant mineral resources that have been integral to the industrialization and prosperity of the state. No rock record is preserved in Ohio after the early part of the Permian Period, about 295 million years ago, and much rock was eroded away during this long interval until the beginning of the Pleistocene Ice Age about 2 million years ago.

PRECAMBRIAN
The Precambrian is the longest segment of geologic time, ranging from the formation of the earth 4.6 billion years ago to the beginning of the Cambrian Period about 542 million years ago. It is divided into the Archean Eon (4.6-2.5 billion years ago) and the Proterozoic (2.5 billion to 542 million years ago). Precambrian rocks underlie the state but are nowhere exposed at the surface because they are buried beneath much younger Paleozoic rocks. In western Ohio, Precambrian rocks lie at a depth of about 2,500 feet and dip eastward to a depth of about 13,000 feet in eastern Ohio. These rocks have been studied from samples taken from deep oil and gas wells and by remote geophysical methods.

Precambrian rocks beneath Ohio consist of igneous, metamorphic and sedimentary rocks. In western Ohio, Precambrian rocks are primarily granite and its fine-grained equivalent, rhyolite. These rocks have been radiometrically dated to about 1.5 billion years old. About 1.3 billion years ago, the continent began to split apart as the granite of western Ohio and adjacent areas was domed upward, forming the East Continent Rift Basin, a portion of which is present in western Ohio. As rifting progressed, the basin was filled with up to 20,000 feet of sediment. Rifting eventually stopped, resulting in a failed rift; that is, the continent did not completely split into two separate masses.

In central and eastern Ohio, Precambrian rocks are primarily igneous and metamorphic rocks about 800 to 900 million years old and represent the eroded roots of a mountain range, known as the Grenville Mountains, that stretched from Canada to Texas. At this time, Ohio was part of the eastern edge of the North American continent (called Laurentia) and the Grenville Mountains were the result of the collision of what is now northern South America with the Laurentian continent. This collisional event, and others elsewhere, resulted in the formation of a Late Precambrian supercontinent that geologists call Rodinia.

The boundary between these two regions, known respectively as the Granite-Rhyolite or Central Province on the west, and the Grenville Province on the east, is a north-south zone in west-central Ohio known as the Grenville Front. This 30-mile-wide zone consists of westward-thrust rocks that mark the western edge of a mountain-building event known as the Grenville Orogeny.

By the beginning of Cambrian time, the Grenville Mountains had been eroded to a gently rolling topography with only about 300 feet of relief. These rocks would be flooded by Paleozoic seas in Late Cambrian time and would form the foundation of Ohio.

CAMBRIAN PERIOD
The Cambrian Period began about 542 million years ago and ended about 488 million years ago. Cambrian rocks underlie the state, but like Precambrian rocks, they are nowhere exposed at the surface and have been characterized by deep drilling for hydrocarbons and by geophysical methods.

At the beginning of the Cambrian, the core of the North American continent, including Ohio, was emergent and consisted of a gently rolling surface covered by highly weathered granite and other Precambrian crystalline rocks. The Iapetus ocean had opened to the east and a series of down-dropped basins formed along the continental margin. One of these, the Rome Trough, formed on the southern edge of Ohio, stretching through Kentucky, West Virginia, and Pennsylvania. This basin accumulated a thick sequence of Cambrian rocks.

As sea level began to rise during the middle Cambrian, the transgressing sea reworked the weathered sediments forming a high-purity quartzose sandstone known as the Mt. Simon. This is the basal Cambrian unit in Ohio and ranges up to 400 feet in thickness. The precise age of the Mt. Simon is uncertain but it is probably Lower or Middle Cambrian.

As sea level continued to rise, carbonate sediments began to accumulate in eastern Ohio along with silty and sandy sediments. By the end of Middle Cambrian time, a wedge of sandy deltaic sediments began to spread from north to south across the central portion of the state. By Late Cambrian time and extensive and thick carbonate (Knox Dolomite) accumulated across the state. This unit continued to accumulate into Early Ordovician time.

Although Cambrian fossils cannot be collected in Ohio from outcrops, deep drill cores have yield remains of trilobites, brachiopods, echinoderms, and graptolites. These fossils date to the latter part of the Middle Cambrian.

Cambrian rocks have been important economically because of the abundant oil and gas deposits in some units. In particular, the Knox Dolomite has been productive. The Morrow County field, discovered in 1959, has produced approximately 38 million barrels of oil and 35 billion cubic feet of natural gas. The basal Cambrian Mt. Simon Sandstone has been used as a repository for disposal of liquid hazardous waste.

ORDOVICIAN PERIOD
The Ordovician Period began about 488 million years ago and ended about 444 million years ago. These are the oldest rocks in the state that are exposed at the surface, in southwestern Ohio along the axis of a positive structural feature known as the Cincinnati Arch or Platform, which formed in the Ordovician. The exposed rocks are from the upper part of the Ordovician System, except for limited exposures of Middle Ordovician rocks along the Ohio River. Older Ordovician rocks are present in the subsurface throughout Ohio.

The oldest Ordovician unit, the Knox Dolomite, began to be deposited in Late Cambrian time in a tidal-flat environment. During the early part of Ordovician time, deposition of the Knox ceased as the sea level fell as the land rose. It is thought that this gentle elevation of the land surface was a bulge (peripheral or forebulge) created by the beginnings of continental collision along the east coast of North America. This collision became a major mountain-building event later in the period and is termed the Taconic Orogeny. During this episode of emergence, deep erosion took place on the surface of the Knox Dolomite, forming a prominent unconformity. It was originally thought that his unconformity marked the boundary between the Cambrian and Ordovician Systems; however, later studies placed the boundary lower in the Knox sequence.

Although collision of island arcs and continental masses during the Taconic Orogeny took place far to the east of Ohio, this episodic mountain-building event had significant influence on Ohio. Shallow seas once again flooded the state and thick limestones were deposited in the quiet seas. Late in the period, streams eroding the Taconic Mountains carried sediment westward, building a major delta known as the Queenston Delta. The fine-grained clay and silt from the delta were carried out into the Ohio sea, forming thin beds of shale that alternated with thin limestone beds.

Large volcanoes associated with the island arcs had major eruptions as recorded by beds of volcanic ash preserved over a wide area, including Ohio. Some of these ash beds (known to geologists as bentonites) can be traced from the Mississippi River eastward to Europe and Russia. It has been estimated that these eruptions generated about 5,000 times the volume of volcanic ash as was produced by the eruption of Mt. St. Helens in 1980.

The sequence of alternating thin beds of limestone and shale exposed in southwestern Ohio was deposited in a warm, shallow sea and is known to geologists as the Cincinnatian Series. These beds are highly fossiliferous with exquisitely preserved marine invertebrate fossils. This rock sequence is so complete and well known that it is the official reference section for Upper Ordovician rocks in North America.

Many of the limestone and shale beds in Ordovician rocks of southwestern Ohio consist mostly of shells and other hard parts of marine invertebrates. Brachipods, clams, snails, cephalopods, bryozoans, echinoderms, and trilobites, among others, can be found at almost any outcrop in the area. Most sought after by collectors are the fossils of extinct marine arthropods known as trilobites. The most common one is Flexicalymene, which is often found in a rolled-up position. The most sought-after trilobite is perhaps Isotelus, a large trilobite that reached up to two feet in length. Isotelus was named the official state fossil of Ohio in 1985.

Although some of the limestone beds in the Cincinnati area were quarried for building stone and other uses, this industry is no longer active. Economically, Ordovician rocks in Ohio are most famous for oil and gas deposits. In 1884, oil and gas were discovered in the Middle Ordovician Trenton Limestone in northwestern Ohio. This marked the discovery of the first giant oil and gas field in North America, which eventually stretched from Toledo to Indianapolis. It is estimated that 100,000 wells were drilled, 76,000 of them in Ohio. The Trenton produced at least 500 million barrels of oil and 1 trillion cubic feet of gas before the field was depleted by 1910. Several American oil companies had their beginnings in this field.

The hilly terrain in the greater Cincinnati area is prone to landslides where an Upper Ordovician shale unit, the Kope Formation, is present. Highly weathered shale of the 220-foot thick Kope is prone to slope failure, particularly when wet.

SILURIAN PERIOD
The Silurian Period began about 444 million years ago and ended about 416 million years ago. Silurian rocks form the bedrock surface throughout much of the western half of the state, although outcrops are limited in many areas because of a thick cover of sediments deposited by glaciers of the Pleistocene Ice Age. These rocks are present in the subsurface beneath the eastern and northwestern portions of the state.

During the Silurian Period, Ohio was in tropical latitudes near the equator and the shallow sea that covered the state was dominated by the precipitation of limestone, dolomite, gypsum, anhydrite, and halite (salt). The dominance of chemical rocks and the minor amounts of clastic rocks such as shale and sandstone, indicate that the Taconic Mountains of the Ordovician were worn down and contributing only a little sediment to the sea in Ohio. Some of the limestone beds contain a diverse assemblage of marine invertebrate fossils.

The boundary between the Ordovician and Silurian Systems is marked by an unconformity, indicating a time of emergence and erosion. It is probable that this episode of emergence is related to a drop in sea level due to an extensive glaciation in the southern continental mass known as Gondwana.

An extensive series of reefs began forming in Ohio and adjacent areas late in the Silurian Period. As these reefs grew, they formed barriers that restricted the inflow of seawater across eastern Ohio, western Pennsylvania, Michigan, and portions of western New York. As the seawater evaporated in a hot, dry climate, it became increasingly salty and began to precipitate gypsum, anhydrite, and salt. Periodically, seawater refreshed the restricted basins and the precipitation cycle continued. The result was thick, economically important deposits of salt and lesser amounts of gypsum.

At the end of the Ordovician Period there was a major extinction of marine invertebrates that is thought to have been due to the lowering of sea level due to glaciation and the disappearance of shallow marine shelf habitats. As these habitats were reestablished with the rise of sea level in the Silurian, many new species of invertebrates appeared. The Silurian is sometimes referred to as the Age of Corals because these organisms built extensive reefs during this time. Many Silurian rock units in Ohio are dolomites in which recrystallization has destroyed most fossils. However, many of the limestones and shales, particularly in outcrop areas in southern Ohio, have abundant and often well-preserved fossils. Perhaps the best-known Silurian fossils from Ohio include a large pelecypod, Megalomoidea Canadensis, and the brachipods Pentamerus and Trimerella. Although not know from Ohio, the first well-established land plants appeared during the period and fishes began to diversify.

Silurian rocks in Ohio are of great economic importance. The western half of the state is dotted with quarries that extract limestone and dolomite that is used for road construction, commercial building, concrete, and agricultural and chemical lime. Rock salt (halite) was discovered by drilling beneath eastern Ohio in 1886. Two salt mines, about 2,000 feet beneath Lake Erie, mine rock salt for snow and ice control. These vast subsurface salt deposits have also been exploited by a process known as solution mining. Hot water is pumped into the salt bed where it dissolves the salt. The salty water, known as brine, is pumped to the surface where it is evaporated to recover the pure salt. Ohio has long been an important producer of salt. It is estimated that the salt deposits beneath eastern Ohio could supply the entire nation for 32,000 years.

Oil and gas have also been produced from Silurian rocks. Of particular importance is a sandstone beneath eastern Ohio, known to drillers as the "Clinton." Large amounts of natural gas have been produced from more than 75,000 "Clinton" wells drilled in eastern Ohio.

Many museum-quality specimens of crystals of celestite, fluorite, sphalerite, galena, pyrite, and marcasite, among others, have been collected from pockets or vugs in Silurian limestones and dolomites, particularly in northwestern Ohio. Silurian rocks in Adams and Highland Counties in southern Ohio have produced mineral specimens.

In some areas of the state, thick, erosion-resistant limestones and dolomites form scenic cliffs and waterfalls. Of particular note are John Bryan State Park and Clifton Gorge State Nature Preserve in Greene County, where the Little Miami River and its tributaries have cut through the Cedarville Dolomite. Small solution caves are developed in these rocks in this area and in Silurian rocks on South Bass Island in Lake Erie.

DEVONIAN PERIOD
The Devonian Period began about 416 million years ago and ended about 359 million years ago. Devonian rocks crop out in a north-south band through the central part of the state and then eastward along the Lake Erie shore from Sandusky to Ashtabula. They are also present in northwestern Ohio although outcrops are few due to a thick cover of Pleistocene Ice Age sediments. In addition to being present in the subsurface of eastern Ohio, Devonian rocks crop out in an isolated area near Bellefontaine, in Logan County. This feature is known to geologists as the Bellefontaine Outlier because these Devonian rocks are about 30 miles from the nearest outcrop of rocks of similar age.

Ohio was in equatorial latitudes during the Devonian. The outcropping rock record representing earliest Devonian time is absent, with one minor exception, in Ohio as much of the state was apparently above sea level. Early Devonian rocks are present in the subsurface of eastern Ohio, indicating that the sea was nearby. The lone exception to the absent Lower Devonian rock record is a small lens of dark-gray shale that was discovered in Holland Quarry, in Lucas County west of Toledo, in the 1920's. This single exposure contained the remains of early fishes, eurypterids, and plants and represents a brackish-water embayment of the sea that was present in the Michigan basin. This quarry was reclaimed long ago.

In Middle Devonian time the seas once again flooded Ohio and limy sediment began to accumulate. The most prominent of these rock units are the Columbus Limestone and the Delaware Limestone. These rocks crop out through the central part of the state and are quarried extensively throughout this area. The Middle Devonian seas teemed with life as the Columbus Limestone, in particular, is noted for abundant corals, brachiopods, clams, snails, cephalopods, trilobites, and other invertebrates. Remains of fishes, although not common, are found in these rocks. Whereas the Columbus Limestone is a light-colored rock with little land-derived clay, the Delaware Limestone has a bluish color due to silt and clay derived from rising land areas far to the east. This influx of terrestrially derived sediment carried far offshore marks the beginning of another mountain building cycle as North America collided with the northern European continent. The overlying Olentangy Shale and its equivalent in northwestern Ohio, the Silica Shale, show even a greater influx of clay and silt deposited offshore. The Silica Shale is world famous for its well-preserved fossils. This orogeny is called the Acadian and culminated at the end of the Devonian. In addition, volcanic ash beds are preserved in Middle Devonian rocks in Ohio as evidence of volcanoes associated with continental collision to the east.

In the Late Devonian, a major change occurred. The clear, shallow seas with abundant bottom-dwelling invertebrates that had dominated Ohio during much of the preceding Paleozoic periods changed to a comparatively deep, stagnant sea. Black mud with much organic material accumulated to great thickness in a wide area west of the Acadian Mountains. In Ohio, this thick, black shale is called the Ohio Shale.

The lower portions of the sea were anoxic (without oxygen) and no organisms could live on the soupy sea bottom. However, the upper waters were clear and oxygenated and teemed with a variety of fishes, including some of the earliest, well-preserved sharks, ray-fined fishes, and predatory, armored fishes known as arthrodires. Among the largest arthrodires was Dunkleosteus, which reached nearly 20 feet in length and had jaws in which the bone was modified for puncturing and slicing prey.

One hypothesis suggests that the Ohio Shale sea stagnated when the rising Acadian Mountains to the east blocked the moist westerly trade winds, thus depriving areas west of this orographic barrier of rainfall. This prevented streams from carrying large amounts of sediment into the sea to dilute the organic matter produced by plankton in the upper waters. Late in the Devonian Period and continuing into the Mississippian Period, a large wedge of sediment was eroded from the Acadian Mountains. This feature is known as the Catskill delta.

Perhaps the most unusual feature of the Ohio Shale is the presence of large, spherical masses of limestone, called concretions. They occur in zones and are often abundant. Concretions in the lower part of the Ohio Shale can be up to six feet in diameter and exhibit distinct layers when broken open. In the upper part of the Ohio Shale, in the Cleveland area, the concretions tend to be flattened. These features grew within the shale, soon after its deposition, as the bedding planes in the shale bend around the concretion. It is thought that the concretion grew around an organic mass as many of them contain bones or other remains of fishes that lived in the Ohio Shale sea. Many of the spherical concretions are used as yard ornaments.

The dark, highly organic Ohio Shale is overlain by gray Bedford Shale, indicating an abrupt change whereby fine-grained clastic sediments were being carried into the Ohio sea as offshore delta deposits. Extensive studies of the Bedford suggest that these sediments were derived from the Catskill delta to the east and from the Canadian shield to the north. Portions of the Bedford are red in color, suggesting oxidation of the iron minerals.

Overlying the Bedford is the Berea Sandstone. This unit is composed of angular quartz grains and can reach a thickness of 200 feet locally in northeastern Ohio. Traditionally, the Bedford and Berea were considered to be of Mississippian age. Although fossils are sparse in these units, therefore making age determination and correlation difficult, researchers in recent years have placed the Bedford and Berea in the Upper Devonian.

Life was abundant and diverse during the Devonian, both in the seas and for the first time on land. Ohio rocks preserve a remarkable record of this biological diversity. In particular, the Columbus and Delaware Limestones of Middle Devonian age contain many species of well-preserved invertebrate fossils including corals, brachiopods, clams, snails, cephalopods, and trilobites, among others. Although not common, remains of fishes also occur in these rocks. The Devonian Period is known as the age of fishes and that appellation is in part derived from the remarkable fauna of bony fishes, sharks, and armor-plated arthrodires, some of which reached 20 feet in length, that are well known from the Ohio Shale and particularly the upper member of this unit, the Cleveland Shale. These fishes lived in the upper, oxygenated waters of the stagnant Ohio Shale sea and their remains are often found in the center of large carbonate concretions. Perhaps the best-known arthrodire is Dunkleosteus terrelli, a large, ferocious predator whose bite force is estimated to have been 80,000 pounds per square inch, more than any fossil or living fish and among the most powerful among all animals.

Devonian rocks in Ohio have been an important source of industrial minerals throughout much of the state's history. The Columbus and Delaware Limestones have been an important source of carbonate rock for agricultural lime, building stone, cement, road metal, and many other uses. The Bedford Shale was used for many years to make brick and sewer tile. The Berea Sandstone, informally known as the Berea Grit, was used by early settlers for grindstones and later as an important building stone. Many buildings in Ohio and indeed in the eastern United States were constructed from Berea Sandstone. It is still quarried in northern Ohio. Oil and gas are produced from Devonian rocks in eastern Ohio, particularly the Berea and Ohio Shale.

MISSISSIPPIAN PERIOD
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 locatities 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.

PENNSYLVANIAN PERIOD
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 eventually form a bed of coal. 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 Monongehela. 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 again.

Life was a