Ohio's Geologic Periods
The Precambrian Period began when the Earth first became a solid planet. It lasted for close to 3,400 million years, ending about 570 million years ago. Because it was such a long period, some geologists have separated it into divisions.
The oldest record of Ohio’s geologic history is preserved in igneous and metamorphic rocks of this period. These rocks lie deep beneath the surface of Ohio. Geologists often refer to them as "basement rocks" and know them only from the drilling of deep wells. Precambrian rocks beneath the Ohio’s surface have been dated at more than 1 billion years old.
The first life on Earth appeared during the Precambrian Period.
The Cambrian Period began about 570 million years ago, and lasted for 65 million years. Its name comes from Cambria, the Roman name for Wales. During this period Ohio was part of a broad coastal plain, comparable to the modern Gulf Coast of the United States.
During the middle and later parts of the Cambrian Period, the sea slowly spread across and eventually covered the Precambrian rocks. The water deposited sand, mud and other sediments. Over a very long period of time these materials hardened into sedimentary rocks: sandstone, shale, limestone and dolostone.
As with the rocks of the Precambrian Period, Ohio’s Cambrian rocks lie beneath the surface of the state today, covered by other, younger layers of rock. During the Cambrian Period the different kinds of animals and plants expanded greatly. Trilobites were the dominat animals, and algae were the dominant plants.
The Ordovician Period began about 500 million years ago. It lasted for more than 60 million years. This period’s name comes from an ancient Celtic tribe that lived in Wales, the Ordovices.
During the early Ordovician Period the sea withdrew and erosion of the land followed. By Middle Ordovician time the sea had returned, submerging the eroded land surface. During this time, sediments hardened into limestone.
The Late Ordovician Period began about 445 million years ago. Late Ordovician shale and limestone in the Cincinnati area are the oldest rocks exposed at the surface of Ohio today. The Ordovician in Ohio ended with a receding ocean.
Geologists and amateur collectors find many wonderful fossils in southwestern Ohio. These include the popular trilobites as well as crinoids, brachiopods and many others. Fishes appeared during this period, the first animals with backbones.
The Silurian Period began about 440 million years ago, and lasted for 30 million years. The name for this period comes from another ancient Celtic tribe of Wales, the Silures. The sea again flooded Ohio, leaving sediments that mainly formed limestone and dolomite. During the last part of the Silurian Period Ohio had a shallow, marine environment. Periodically, the sea level lowered and the sea water evaporated. This formed the deep, thick salt beds that are mined today in the Cleveland area. During this period land scorpions were the first animals to appear on land. And we find the first examples of land plants. Also, geologists often refer to this as the golden age of fishes.
The Devonian Period is named for Devonshire, a county in southwestern England. It began about 410 million years ago and lasted for almost 50 million years.
Sediments from the marine waters that covered Ohio during this period formed numerous sedimentary rocks. These include limestone, dolostone, shale and sandstone.
The first land animals with backbones, amphibians, appeared during this period. They diversified into many forms. The first seed plants appeared also.
Mississippian & Pennsylvanian Periods
These two geologic periods together often are referred to as the Carboniferous Period. Their names come from the Mississippi River, the State of Pennsylvania, and the fact that the great coal ("carbon") beds were laid down during these periods. The Mississippian Period began about 320 million years ago. Together, the two periods lasted for more than 70 million years.
The Devonian marine basin was partly filled in during the early Mississippian Period. Later, erosion removed some of the first deposits. Pennsylvanian deposits were laid down by both marine and nonmarine waters. Rocks of these two periods are varied. They include sandstone, shale, coal, ironstone, flint and conglomerate.
Terrestrial environments developed during the Pennsylvanian Period and continued into the Permian Period that followed. During this period amphibians increased, the first winged insects appeared, and early reptiles appeared. For the first time, land plants began to overwhelm the landscape. Profuse growths of tree ferns, horsetails, reeds and rushes arose in and around Pennsylvanian swamps.
The Permian Period began about 285 million years ago. This geologic period lasted for a little more than 40 million years. Its name comes from the province of Perm in Russia.
Having begun in the late Pennsylvanian Period, progressively more terrestrial environments developed. Aquatic environments continued to develop: sea margins, rivers, lakes and deltas with peat (coal) swamps. Rocks from this period include shale, limestone and coal.
During the Permian Period insects and reptiles increased as amphibians decreased. And mammal-like reptiles appeared for the first time.
Ohio’s Geologic Gap
During the approximately 245 million years that followed the Permian Period there may or may not have been rocks formed in what now is Ohio. Nonetheless, natural forces erased all evidence of any rocks that might have been deposited during that time. Uplift of the bedrock, erosion, and weathering removed all traces, leaving a gap in the geologic record.
During this extended period of time, many changes occurred in animals and plants. Dinosaurs arose, flourished, and became extinct. The first birds appeared and developed into modern birds. Mammals appeared at about the same time as the first dinosaurs, developed slowly at first, then flourished. Flowering plants appeared and diversified rapidly.
Pleistocene Epoch — The Ice Age<img width="225" height="225" title="Image of a glacier" alt="Imge of a glacier" src="images/naturalHistory/rocks/glaciation.gif" />
About 1 million years ago, the first of several glaciers moved southward across what is now Ohio. At their greatest extent they covered approximately 2/3 of the state.
The glaciers scraped much of Ohio’s surface including its bedrock. Also, they left deposits of clay, sand, gravel and rock as they melted back.
During this period humans appeared in what we now call the "Old World." Sometime around the end of the Quaternary Period, groups of people migrated into the "New World," eventually reaching what later was to become Ohio.
As the last of the great glaciers melted back about 10,000 years ago, the Earth entered its most recent period of geologic history. New igneous rocks are being formed, such as when hot, volcanic lava cools. People can see this happening today at the Kilauea Volcano in Hawaii.
Also, new metamorphic rock is being formed where temperatures and/or pressures are great enough. And new sediments such as clay and sand are being deposited by water. If left undisturbed these will gradually be changed into sedimentary rocks.
During the past 10,000 years humans have flourished. They have come to occupy much of the Earth’s surface. Using numerous natural materials --- including rocks and ores --- they have maintained themselves with food, fiber and shelter.
- 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.
- Cincinnati, Ohio
- Cleveland, Ohio
- Cambrian Period
- Pleistocene Epoch
- Devonian Period
- Mississippian Period
- Pennsylvanian Period
- Trilobite Fossils
- Silurian Period
- Rocks and Ores
- Ordovician Period
- Permian Period
- Clay and Shale
- Clay and Shale
- Metamorphic Rock
- Quaternary Period
- Sedimentary Rock