Ohio's Geologic Timeline
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 | |
|
0 |
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. | |
|
1.6 |
Tertiary Period 62.5 million years |
Not Present In Ohio - - - - - No Dinosaurs In Ohio | ||
|
66.4 |
Mesozoic Era 179 million years |
Cretaceous Period 78 million years | ||
|
144 |
Jurassic Period 64 million years | |||
|
208 |
Triassic Period 37 million years | |||
|
245 |
<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. | |
|
286 |
Pennsylvanian Period 34 million years |
Eastern Ohio. Shale, sandstone, coal, clay, limestone. | ||
|
320 |
Mississippian Period 40 million years |
East-central, northeastern and northwesternmost corner of Ohio. Shale, sandstone, limestone. | ||
|
360 |
Devonian Period 48 million years |
Central, northeastern lake shore and northwestern Ohio. Shale, limestone. | ||
|
408 |
Silurian Period 30 million years |
Western Ohio. Dolomite, limestone, shale. | ||
|
438 |
Ordovician Period 67 million years |
Southwestern corner of Ohio. Shale, limestone. | ||
|
505 |
Cambrian Period 65 million years |
Not Exposed In Ohio | Cambrian sandstone and shale. | |
|
570 |
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.
Resources
- 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.