The bedrock geology of Ohio consists of flat-lying to gently dipping layers of sedimentary rocks that range in age from Upper Ordovician to Upper Carboniferous-Lower Permian, spanning some 200 million years of Earth history. Older sedimentary, igneous, and metamorphic rocks occur in the subsurface. At the surface, an irregular veneer of Quaternary-age sediments conceals most bedrock units occurring northward and westward of the glacial margin.
Explore the information below for noteworthy history of Ohio geology. For more information on specific time periods, visit the Geology of Ohio webpage.
Ohio's Hidden Geologic Past
The oldest rocks that make up Ohio are Precambrian and Cambrian in age, but they are not exposed at the surface and can only be studied from deeply drilled boreholes. The Precambrian aged rocks range from 1.4 billion to 542 million years ago (mya), and the Cambrian aged rocks range from 542 to 488 mya. During the Precambrian period, Western Ohio tried to split (rift) apart due to a volcanic upwelling from deep within the earth. Some time later continents collided with Eastern Ohio and formed a large mountain chain called the Grenville Mountains. Over the rest of the Precambrian and into the Cambrian time period these mountains eroded away filling in the former rift valley in western Ohio with sediments that turned into sandstone. The rocks of Eastern Ohio where the Grenville Mountains had once stood are made up of metamorphic rocks, which formed at great depths from immense heat and pressure deep within the mountains.
Ohio's Ancient Oceans
Following this violent recorded geologic history of rifting/volcanoes, and mountain building the rest of Ohio’s past has been relatively quiet. Starting in the late Cambrian period Ohio was covered by a shallow tropical sea for much of the Ordovician (488–443 mya), Silurian (443–416 mya), and Devonian (416–359 mya) Periods. Rocks deposited during these time periods, except for the Cambrian, can be found at the surface across different parts of Ohio. During these periods sea level periodically fluctuated, by deepening and shallowing, with occasional small islands forming at higher elevations in Western Ohio. For much of this time Ohio was home to reef forming corals and other organisms, which formed limestone much like in the Bahamas today. Near the end of the Silurian, the sea covering Ohio became separated from the worlds oceans and dried up leaving behind large salt deposits in Eastern Ohio. In the Devonian the sea level rose again and limestone forming conditions returned. In the late Devonian, continents collided East of Ohio, resulting in the formation of the ancestral Appalachian Mountains. This mountain building event caused Eastern Ohio to drop, resulting in a deepening of the sea, bringing about an end to the limestone deposition. These new mountains eroded, and rivers from the mountains brought sediment to Ohio, resulting in deeper water shales being deposited in Ohio. By the close of the Devonian, the rivers from the mountains deposited sediments filling in the sea until dry land began to spread from the mountains and into Ohio.
Ohio's Coastal and Deltaic Period
Once Ohio emerged from the sea and predominantly became dry land, it only rarely again was submerged by shallow sea incursions during the rest of its recorded geologic history in the Mississippian (359–318 mya), Pennsylvanian (318–299 mya), and Permian (299–251 mya) Periods. During these periods Ohio was like present day Louisiana, in that it had many rivers bringing in and depositing clay, silt, and sand across the landscape, with some of these sediments being deposited in vast delta deposits just offshore in the shallow sea. Another similarity to Louisiana is that during the Pennsylvanian and Permian Periods Ohio was covered with swampy forests, which eventually turned into vast coal deposits. Ohio was an ever-changing landscape as rivers migrated and changed course across the low plains, depositing different types of rocks depending on what the environment was like at the time.
Ohio's Recent Geologic History
By the early Permian Period Ohio had fully transitioned away from being near the coast of a shallow sea, which had filled in with sediment, and was now fully above sea level. Because Ohio was above sea level geologists think that Ohio transitioned from being a place where the deposition of rocks was occurring, to a place where erosion was taking place. No younger rocks have been discovered in Ohio dating from the later Permian, Triassic, Jurassic, Cretaceous, or Tertiary Periods of earth’s history (251–2.6 mya). This large gap in time is a complete mystery of Ohio’s history as no rocks are preserved from this time. It is possible that rocks were deposited during this time but were subsequently eroded away and are not preserved. After this gap in time during the Pleistocene Epoch (starting approximately 2.6 mya) massive glaciers of ice formed and came down from Canada bringing in and crushing up the rocks at the surface leaving behind a deposit of glacial till. These glacial deposits have not yet had enough time to become turned into rocks, but with time they may be preserved as rocks and added to the geologic history of Ohio.