The geologic history of Ohio is represented in the rocks, sediments and geography of the landscape we see today. Many volumes have been written about the state's geology and exploration and research continue. Provided here is a brief summary of how Ohio was shaped by dynamic geologic forces. 

The table below is arranged by geologic period, starting with Earth's formation during what geologists refer to as the Precambrian Period and moving to the Quaternary Period and present day. Included in each period are descriptions of the environments, life forms, rock types that represent that period and the economic products produced from rocks of that period (mya = million years ago.)

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Precambrian Period (4.6 billion–542 mya)

Geologic Setting

Between 1.4 billion and 990 mya, volcanic activity, crustal rifting and filling of basins (formed by rifting) took place. Between 990 and 880 mya, a mountain range formed in eastern Ohio. Between 880 and 544 mya, these mountains were eroded, reducing the landscape to a gently rolling surface.

Life Forms

None known from Ohio. Algae and lower invertebrates found in areas other than Ohio.

Rock Types

Igneous, metamorphic, and metasedimentary rocks. Present only at great depths—2,500 to 13,000 feet beneath the surface.

Economic Products

No current production.

Cambrian Period (541–488 mya)

Geologic Setting

Marine seas gradually flooded the Precambrian land surface and covered Ohio by late Cambrian time, initially depositing sands, then silts, muds and limy muds. Ohio lay 10° south of the Equator. Toward the end of Cambrian time, limy sediments accumulated in a shallow marine sea in an arid climate.

Life Forms

Marine life abundant, but few fossils known from Ohio because knowledge is limited to core samples. Trilobites reached their peak diversity during late Cambrian time.

Rock Types

Sandstone, shale, and dolomite. Present in subsurface only.

Economic Products

Oil and gas (from sandstone and dolomite).

Ordovician Period (488–443 mya)

Geologic Setting

Similar to the modern Bahamas, Ohio lay 20 degrees south of the Equator, covered by a warm, shallow sea (deeper in eastern Ohio). The western part of Ohio, at times, emerged as low muddy islands. Limy sediments were dominant. Volcanic activity and mountain building to the east of Ohio produced periodic layers of ash over the entire state and muddy deltaic sediments in eastern Ohio. The sea deepened during later Ordovician time, covering all of the state. At the close of Ordovician time, continental glaciation in the southern hemisphere lowered sea level and the seas retreated.

Life Forms

Bryozoans abundant; also brachiopods, cephalopods, trilobites, horn corals, snails, clams, echinoderms, and graptolites. Ohio is world-famous for its fossiliferous Ordovician rocks in the Cincinnati area.

Rock Types

Limestone and shale form the surface rocks in southwestern Ohio, with dolomite in the subsurface.

Economic Products

Crushed stone (limestone). Oil and gas (limestone, shale); in 1884, the first giant U.S. oil field was discovered in Ordovician rocks in northwestern Ohio. Limestone in the Cincinnati area formerly was a source of building stone.

Silurian Period (443–416 mya)

Geologic Setting

At the beginning of Silurian time, Ohio was dry land and subject to erosion. Then warm, shallow seas returned. Ohio lay 20 degrees south of the Equator. Middle Silurian seas were deeper and covered Ohio; reef environments were common. Later during the Silurian, seas were shallower and formed evaporite (salt-bearing) basins in northern Ohio.

Life Forms

Coral reefs abundant; also echinoderms, clams, brachiopods, and cephalopods.

Rock Types

Limestone, dolomite, shale, and gypsum form the surface rocks in western Ohio. Salt and sandstone (eastern Ohio) are present in the subsurface.

Economic Products

Crushed stone for construction and cement, aglime, and oil and gas (limestone, dolomite, sandstone). Salt for road deicing. Historically, gypsum was mined for wallboard.

Devonian Period (416–359 mya)

Geologic Setting

Most of Ohio was dry land during early Devonian time, although the sea still covered eastern Ohio. Ohio was in equatorial latitudes. During Middle Devonian time, warm, shallow seas deposited limy sediments. Layers of muddy sediments and some volcanic ash during late Middle Devonian time signaled renewed volcanic activity and mountain building east of Ohio. During late Devonian time, the Ohio sea became stagnant; circulation was poor and the water was generally anoxic (lacking oxygen). Thick layers of black, organic-rich, uranium-bearing mud were deposited in these “stinking seas.”

Life Forms

Coral reefs, bryozoans, brachiopods, trilobites, cephalopods, clams, crinoids, and ostracodes. First major appearance of sharks and bony fishes during Middle Devonian. True land plants first appear.

Rock Types

Limestone, dolomite, shale, and sandstone form the surface rocks in northwestern Ohio, through central Ohio and along the Lake Erie shore.

Economic Products

Crushed stone for construction and cement, and aglime (limestone & dolomite). Synfuels and natural gas (from shale, sandstone, limestone).

Mississippian Period (359–318 mya)

Geologic Setting

During latest Devonian and early Mississippian time, dark organic muds gave way to fluvial and deltaic silts and sands. Ohio lay in equatorial latitudes. During late Mississippian time, a shallow sea deposited limy sediments. The seas retreated during latest Mississippian time, leaving a sparse rock record.

Life Forms

Brachiopods, clams, crinoids, and fishes. Land plants increasingly abundant.

Rock Types

Sandstone, siltstone, conglomerate, shale, and limestone form the surface rocks in northwestern and east-central Ohio.

Economic Products

Building stone and crushed stone for construction and industrial uses, and oil and gas (sandstone). Crushed stone for construction and cement (limestone). Common clay products (shale). Ohio has been a major producer of building stone from the Mississippian Berea Sandstone for more than 150 years.

Pennsylvanian Period (318–299 mya)

Geologic Setting

Ohio in Pennsylvanian time was a relatively flat coastal plain swamp in equatorial latitudes. Fluctuations in sea level resulted in alternating terrestrial, freshwater, and marine deposits.

Life Forms

Land plants abundant. Terrestrial and freshwater life included amphibians, reptiles, and freshwater clams. Marine life included crinoids, snails, cephalopods, brachiopods, and fishes. Trilobites were rare.

Rock Types

Sandstone, conglomerate, shale, clay, limestone, coal, flint, and ironstone form the surface rocks in eastern Ohio.

Economic Products

Building stone and crushed stone for construction and industrial uses such as glass manufacturing; oil and gas (sandstone). Crushed stone for construction and cement (limestone). Pottery and common clay products (shale, clay). Electric power (coal). During the 1800s, Ohio was a major iron-producing state.

Permian Period (299–251 mya)

Geologic Setting

During early Permian time, southeastern Ohio was a coastal-plain swamp. Ohio lay about 5 degrees north of the Equator. The swamp eventually was filled by an influx of deltaic sand and mud. Later Permian time was characterized by uplift and erosion.

Life Forms

Sparse freshwater fossils, such as snails, clams, and fishes. Marine fossils were rare. Sparse land fossils include plants, amphibians, and reptiles.

Rock Types

Sandstone, shale, freshwater limestone, and coal form the surface rocks in southeasternmost Ohio.

Economic Products

Crushed stone (limestone). Electric power (coal).

Mesozoic & Tertiary Periods (251–2.6 mya)

Geologic Setting

Ohio was above sea level and weathering erosion and nondeposition prevailed.

Life Forms

No known record, but dinosaurs probably roamed Ohio during the Mesozoic.

Rock Types

None.

Economic Products

None.

Quaternary Period (2.6 mya–Present)

Geologic Setting

During the Ice Age, Ohio is subjected to multiple glaciations. At times, two-thirds of the state was covered by mile-thick ice.

Life Forms

Many large mammals, such as mastodons, mammoths, giant beavers, and ground sloths, as well as modern plants and animals, including humans.

Rock Types

Glacial till, clay, silt, and sand and gravel form the surface sediments in northern and western Ohio.

Economic Products

Common clay products (clay). Material for road construction and cement (sand, gravel). Sand and gravel are also important groundwater aquifers.