Paleontology and geology
The Precambrian: There are no Precambrian rocks at the surface in Ohio, and thus, little is known about this time.
The Paleozoic: Cambrian rocks, known from drill cores in Ohio, indicate that a shallow sea flooded the state during that time. By the Ordovician, muds from the emerging mountains in the northeast were deposited in this sea, and its waters teemed with brachiopods, bryozoans, corals, and crinoids. The Silurian ocean was largely mud-free across Ohio and large coral-sponge reefs separated the shallower waters across much of the state from the deeper waters of marine basins to the north and east. Corals, brachiopods, and stalked echinoderms are common fossils in Ohio’s Silurian and Early Devonian rocks, but by the end of the Devonian, these oceans became oxygen-poor and supported relatively little ocean-floor life. During the Early Carboniferous, sediments from the eroding Appalachian Mountains to the east formed extensive marine deposits of muds and silts. The Late Carboniferous brought the development of large deltaic systems and only rare occurrences of oceanic conditions. Coal-forming swampy environments were an important part of these vast deltas, and plant fossils are common. Permian rocks indicate that Ohio was fully terrestrial at this time. Lakes, rivers, and other habitats dominated the landscape; fossils of ferns and horsetails are common.
The Mesozoic and Cenozoic: There are no Mesozoic rocks preserved in Ohio, indicating that this was primarily a time of extensive erosion. This situation continued into the early part of the Cenozoic; consequently, there are no Tertiary rocks preserved in the state.
The Cenozoic: The Early Quaternary record in Ohio is one of massive glacial advances and retreats. At the time of the last glaciation, ice was as much as one mile thick in the area of present-day Cleveland. During these glacial times, mammoths, mastodons, ground sloths, giant beavers, and musk oxen roamed the area.