Ohio's Physiographic Regions
OHIO’S PHYSIOGRAPHIC REGIONS
Ohio’s landscape is surprisingly varied and has been favorable for settlement and population growth in many areas because of few natural barriers. Both Paleozoic sedimentary rocks and continental glaciations during the Pleistocene Ice Age have contributed significantly to the shape of the land surface. Most of the state falls into two main divisions, with several subdivisions. The Central Lowlands dominate the central and western parts of the state, where relief is low. The Appalachian Plateaus dominate the eastern and southern part of the state and are characterized by bedrock hills of moderate-to-high relief.
The Central Lowlands Province of western Ohio is underlain by limestone and dolomite, with some shale beds, mostly of Silurian age, but with a significant portion of bedrock of Ordovician age in the southwestern part. Outcrops of these rocks are few owing to the presence of sediments deposited on them by Pleistocene glaciers. Prior to advance of the glaciers, the area had moderate relief and was highly dissected by streams and rivers. However, advance of several glaciers across the area gouged and smooth the bedrock as they destroyed the drainage systems and filled the valleys with sediment. Today, these ancient preglacial valleys, some of which, such as the Teays River valley, are more than 400 feet deep but there is no hint of them on the nearly flat surface.
Most of this region can be placed in a subdivision known as Till Plains. Till refers to the heterogeneous mixture of clay, silt, sand and boulders deposited beneath the advancing glacier or dropped on the surface as the ice finally melted. Here and there, the flat plains are punctuated by low, hummocky arcuate bands of low hills that marked temporarily stable positions of the snout of the last glacier as it made its final retreat northward. These are termed recessional moraines. Surprisingly, the highest point in Ohio sits amid the Central Lowlands in Logan County, near Bellefontaine. This hilly area is underlain by erosion-resistant bedrock of Devonian age and is known as the Bellefontaine Outlier. Campbell Hill reaches an elevation of 1,549 feet and has claim as the highest point in Ohio.
Much of northwestern Ohio, and a narrow portion bordering Lake Erie, eastward to the Pennsylvania border, belongs to a subdivisions known as the Huron-Erie Lake Plains. This area has very low relief and appears nearly flat over large areas. This region marks the former extent of Lake Erie as the last (Wisconsinan) glacier was retreating from Ohio. There were numerous lake stages beginning about 14,000 years ago, and the beaches formed along their shorelines are present across this area as low ridges of sand. During the early days of settlement of Ohio, much of northwestern Ohio was swampy (Great Black Swamp) due to the poor drainage of the lakebed clays blanketing the region. The beach ridges served as passages through the swamps and some later became roads. Draining of the swamps in the late 1800s opened the area to settlement and agriculture.
The Central Lowlands of Ohio is in many ways the agricultural breadbasket of the state. Rich soils deposited by the Pleistocene glaciers and low relief have long supported extensive grain farms and livestock.
The eastern and southern part of Ohio is dominated by bedrock hills, mostly of Mississippian, Pennsylvanian, and Permian age. Streams have incised deeply into these rocks, which dip gently to the southeast, forming narrow valleys and a landscape with up to 300 feet of relief. Many hills are capped with erosion-resistant beds of sandstone. The western edge of the Appalachian Plateaus is marked by an escarpment formed by the Berea Sandstone.
The western and northern part of the Appalachian Plateaus was glaciated during the Pleistocene Ice Age, which resulted in the hills being of lower relief and the valleys more broad. This region is a subdivision termed Glaciated Allegheny Plateaus. The unglaciated portion of the Appalachian Plateaus is a subdivision termed Allegheny Plateaus and is made up of bedrock of Pennsylvanian and Permian age. The general boundary between the glaciated and unglaciated plateaus is marked by the Allegheny escarpment, which in many areas marks not only an abrupt transition in topography but also a transition in vegetation and agriculture. The poor, thin soils in the unglaciated portion have returned to forest whereas agriculture thrives in the glaciated portion. The highest topographic relief in the state is in far eastern Ohio, in Monroe, Belmont, and Jefferson Counties.
INTERIOR LOW PLATEAU
A small portion of the state, in part of Adams County and in a narrow band along the Ohio River in Brown, Clermont, and Hamilton Counties, is classified as the Outer Bluegrass Section of the Interior Low Plateaus. The area is underlain by bedrock of Ordovician and Silurian age and is unglaciated or has very thin, highly weathered deposits of early glaciations. The area is dissected by streams and has high relief. Sinkholes and small caverns are developed in the thick Silurian limestones in Adams County.