Ohio's Geologic Timeline

From Ohio History Central

Since the oldest sedimentary rocks in Ohio were deposited first, they lie below the youngest rocks. Therefore, if a layer of sedimentary rock lies on top of another layer of such rock, the top layer is younger than that beneath it. This holds true in Ohio EXCEPT in cases where a force has disturbed the layers after they were deposited.

Millions of years before present

Geologic Eras and duration

Geologic Periods and duration in years

Area of outcrop in Ohio and principal types of rock


Cenozoic Era

66+ million years

Pleistocene Epoch

1.5– 2 million years

Northwestern 2/3 of Ohio.

Unconsolidated sand, gravel and clay on top of bedrock.


Tertiary Period

62.5 million years

Not Present In Ohio

- - - - -

No Dinosaurs In Ohio


Mesozoic Era

179 million years

Cretaceous Period

78 million years


Jurassic Period

64 million years


Triassic Period

37 million years





<a name="paleozoic" id="paleozoic"></a>Paleozoic Period

67 million years

Permian Period

41 million years

Southeastern most slice of Ohio.

Shale, sandstone, coal, clay, limestone.


Pennsylvanian Period

34 million years

Eastern Ohio.

Shale, sandstone, coal, clay, limestone.


Mississippian Period

40 million years

East-central, northeastern and northwesternmost corner of Ohio.

Shale, sandstone, limestone.


Devonian Period

48 million years

Central, northeastern lake shore and northwestern Ohio.

Shale, limestone.


Silurian Period

30 million years

Western Ohio.

Dolomite, limestone, shale.


Ordovician Period

67 million years

Southwestern corner of Ohio.

Shale, limestone.


Cambrian Period

65 million years





Cambrian sandstone and shale.

Precambrian Period

3,400 million years

Precambrian sedimentary, igneous and metamorphic rocks present below the Cambrian rocks.

Following the Permian Period, which ended about 245 million years ago, a process known as uplift pushed Ohio’s bedrock upward. The force of this uplift is most noticeable in the western part of the state along a line running roughly from Cincinnati through Findlay to Toledo. As a result of the uplift’s being greatest in the western part of the state, the layers of bedrock were tilted downward to the east across the state.

Erosion and weathering of the exposed bedrock followed. Since the layers of rock had become tilted during the uneven uplift, the exposure was uneven. And different layers of rock were exposed in north-south bands across Ohio. Today the oldest rocks (Ordivician age) are exposed in southwestern Ohio and the youngest are exposed in the southeastern part of the state. The cross section of Ohio appearing with the map below shows this layering and exposure.

This uplift and wearing down of sedimentary rock also resulted in the removal of sediments that may have been laid down during the time period extending from the Triassic Period through the Tertiary Period. This helps to explain why we do not find dinosaurs in Ohio: the deposits that were laid down during the time when the great reptiles lived are not present in the state.

<img width="350" height="420" src="images/naturalHistory/rocks/geosystems.gif" alt="Map of surface rock" title="Map of surface rock" />

This map shows how the surface rock (termed bedrock) would appear if the loose materials such as soils, stream gravels and glacial deposits were stripped away. The diagram below the map represents a cross section of the state showing that the rocks of different ages are layered on top of one another. The older layers are beneath the younger layers.


  • Coogan, Allan H. Ohio’s Surface Rocks and Sediments; Chapter 3 in Fossils of Ohio, edited by Rodney M. Feldmann; Ohio Geological Survey, Bulletin 70, 1996.
  • Hansen, Michael C. "The Geology of Ohio — The Ordovician" Ohio Geology, Ohio Division of Geological Survey, Fall 1997.
  • Hansen, Michael C. "The Geology of Ohio — The Silurian" Ohio Geology, Ohio Division of Geological Survey, Fall 1998.
  • Hansen, Michael C. "The Geology of Ohio — The Devonian" Ohio Geology, Ohio Division of Geological Survey, 1999, No. 1.
  • Skinner, Brian J. & Stephen C. Porter The Blue Planet: An Introduction to Earth System Science; Wiley, New York; 1995.

See Also