Central lowlands. (geological map)
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 pre-glacial 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.