The Geologic Walk Through Time in the Natural Resources Park at the State Fair Grounds was originally intended for the front yard of the Ohio Governors Residence because the Heritage Garden, the state's official botanical garden of native plants, displays the varied ecosystems in the state. But the difference between 8000 and 800,000 yearly visitors quickly made us realize that the fairgrounds was a much better place for a permanent exhibit to encourage STEM education.
Geology is intimately linked to the Heritage Garden. A quick tour of the Heritage Garden takes you through the five major physiographic regions and all parts of the state. You can quickly see that the same plants do not grow everywhere.
Why is that? One big reason is soil composition. Soil composition depends on bedrock and glacial geology. Some plants prefer acid soils; others prefer that their roots be surrounded by alkaline soils.
What makes soils acid or alkaline? This is where bedrock geology comes into play. Believe it or not, over time rock crumbles or erodes away into soil and gives it pH characteristics. Limestones produce alkaline soils and sandstones produce more acid soils. So looking at the state's bedrock map can give the gardener a clue to what type of plants will do well in his backyard. Some say I -71 is the dividing line between alkaline limestone and acid sandstone in the state. While not totally accurate, it is a quick measure of the soil pH and what plants might be easiest to grow in your yard.
Where does the bedrock come from? The geologic history of Ohio is fascinating. This land mass started out 450 million years ago about 20 degrees south of the equator and has slowly worked its way north. Along this journey, the landmass has undergone many changes. The major ones are grouped into geologic time periods that produced different types of bedrock.
For almost 200 million years the landmass now known as Ohio was underwater. Pressure over millions of years slowly turned sand and crushed shells into sedimentary rocks formed in the ocean.
The Geological Walk Through Time discusses 9 geologic episodes. Most are represented in the Heritage Garden.
There are layers of bedrock from the Pre-Cambrian period (4.6 billion-505 million years ago) that can't be seen on the surface of Ohio except for stones, from large boulders to gravel, carried to Ohio by the glaciers. The ice sheet picked up pieces of Pre-Cambrian bedrock exposed at the surface in Canada, carried them south, and left them here when the ice melted. The Heritage Garden has some great examples of glacial gneiss, or Canadian bedrock along the Parkview Ave. fence.
The oldest bedrock exposed in the state formed during the Ordovician Period and can be seen in SW Ohio. The Cincinnati Arch formation is internationally famous for its fossils including a kind of trilobite that is Ohio's official state fossil. This limestone produces the base for alkaline soils and also provides good clay for pottery. The Ordovician Period was from 488 to 444 million years ago. A piece of Ordovician limestone can be seen in the Heritage Garden near the carriage house gate into the back yard.
During the Silurian Period (about 444 to 416 million years ago) the waters of a shallow warm sea were evaporating and produced the large salt deposits under the state. It also produced limestone. Examples can be seen near flowerbed opposite the Garden of he Lost in the Heritage Garden.
The Devonian Period (about 416-360 million years ago) finds Ohio just south of the equator still covered by a warm shallow sea that was teaming with life. Fossils of the giant, bone covered carnivorous prehistoric fish Dunkleosteus are found in the shale around Cleveland. The bedrock formed in this period is mostly shale and alkaline limestone. A streak of Columbus Limestone from this period runs lengthwise though the middle of the state. The fossil rich, glacial grooved alvar in the Heritage Garden's Lake Plains area is Columbus Limestone.
Mississippian Period (about 360-318 million years ago) has Ohio at the equator and the shoreline of a sea. This produces sandstone with its acid composition. During this period, the continents collide and the Appalachian Mountains are formed. This period's sandstone forms the land that abuts the foothills in eastern Ohio and is best seen in Hocking Hills. It almost mirrors the eastern most edge of the last glacial coverage of the state.
Many changes occurred during the Pennsylvanian Period (318-299 million years ago). Ohio was a vast swampy area slightly above sea level at the equator supporting giant tree-size plants such as horsetail, ferns and club mosses that later were buried, compressed and turned into coal. These plants can be seen in their current evolutionary form in the Heritage Garden. Shale, sandstone, thin layers of limestone and clay were deposited as the land rose and fell above and below sea level in repeated cycles. This is the bedrock under most of Appalachia Ohio and the Allegheny Plateaus. The Appalachian Garden in the Heritage Garden has a Blackhand sandstone (Mississippian Period) and a Pebbles dolomite limestone (Pennsylvanian Period) outcrop so it is easy to view the different vegetation each supports in the Heritage Garden's glimpse of the Allegheny Plateaus eco-region.
The Permian Period (299-251 million years ago) finds Ohio slightly above sea level most of the time in its cyclical movement above and below sea level. Gingko trees and large sail-backed reptiles began making an appearance. A large Gingko tree can be seen in the Heritage Garden's woodland area. The bedrock that formed during this time period was mostly sandstone and is the eastern most part of the state along the West Virginia border.
There was a period from 248-1.6 million years ago known as the Mesozoic and Tertiary or missing age. Mesozoic Ohio was well above sea level and erosion predominated so no bedrock from this period remains in the state. This was the time when dinosaurs probably lived here, but erosion prevented the fossilization of any of their remains. It was also the time that Ohio moved to its current position on the world map. Gingko trees were abundant. Conifer trees, such as dawn redwoods, appear on earth at the beginning of this period, while flowering trees, like deciduous magnolias, tulip and sassafras, appear towards its end. Examples of all these trees can be found in the Heritage Garden.
Today we are in the Pleistocene Epoch (1.6 million years ago to present day). It is most noted for the large ice sheets, often 1-2 miles thick in the Erie Basin, which covered most of the state on at least 3 occasions. The ice brought many granite rocks, sand and gravel with it from Canada and tended to smooth out the landscape that had been flattened by erosion. The last glacier left the state about 14,000 years ago. Wooly Mammoths, Mastodons, and Giant Sloths were among the animals that inhabited the state during the ice age. As the ice melted northward, finally opening up the Niagara Falls outlet to the sea, glacial lake levels dropped dramatically to the current level of Lake Erie and formed the Huron-Erie Lake Plains physiographic region. Extensive sand deposits of glacial Lake Warren now support the botanically rich Oak Openings Region of Northwestern Ohio. The melting glaciers carved waterways though the bedrock to create Ohio's current rivers, streams and oldest lakes. Spruce, Fir, White Cedar, Tamarack, Hemlock, and Northern Alder along with many other northern shrubs and trees could be found in Ohio's hills and valleys and around its bogs. See some of these trees around the bog and the in Allegheny Garden in the Heritage Garden.
A naturally occurring warming period followed the last glacier and allowed prairie plants from the west to grow over three quarters of the state and occupied much of the Till Plains. The Till Plains were formed by million of years of erosion accelerated by the rise of the Cincinnati Arch and later covered by rich glacial till. Ohio's Till Plains physiographic region provides the state with its western farmlands and is represented in the Heritage Garden by the prairie vista garden. As the weather cooled, deciduous forests previously forced eastward, recolonized much of the prairie lands and covered 90% of the state by the time the pioneers moved westward.
Some say the world is still in the Pleistocene Epoch and we are just in-between glaciers as the temperatures naturally fluctuate between hot and cold over long periods of time. No one knows if a new kind of bedrock will be created to add another layer of geologic history in the distant future.
What geology teaches us is that bedrock and glacial patterns have greatly influenced the land we call Ohio.
Millions of years in the making, Ohio's geologic past is usually buried beneath our feet. The Ohio Geologic Walk Through Time brings Ohio geology to the surface so visitors can see and touch the bedrock foundation of our great state. From sandstone, used for buildings, to coal, used for energy, to the salt used on icy roadways, the economic importance of our state's geology has influenced Ohio's history and is a valuable natural resource. Explore Ohio's past through millions of years and discover why Ohio rocks!
Visit the Geological Walk Through Time in the Natural Resources Park at the Ohio State Fairgrounds to learn more about Ohio's bedrock to learn more. Take a walk through time and see the bedrock formed in each period. Walk on the world's largest state bedrock map and find out what's beneath your county.
A visit to the Governor's Residence and Heritage Garden can help you understand just how much bedrock influences our lives. Free tours with a trained guide are available on Tuesdays. Call 614-644-7644 to make a reservation.