Region 3: Topography of the Inland Basin

The Inland Basin is a large region that extends from southern Illinois to northern Alabama to central New York. Its rock formed primarily in the basins of inland seas many millions of years ago, and while its origin affects the way it is eroded, its topography is quite varied. In the Midwest, the region includes only eastern Ohio, southern Indiana, and the southernmost portion of Illinois. Most of the eastern half of Ohio is part of the Allegheny Plateau, which extends from northeastern Kentucky well into central New York. This portion of Ohio is further divided into glaciated and unglaciated areas (Figure 4.4). The glaciated northwest portion is quite flat, with relief features of little more than 30 meters (100 feet) high. The southeast portion of the state was never flattened by glaciers or buried in till, so relief here is several times greater, occasionally reaching up to 120 meters (400 feet). Millions of years of running water carving into the bedrock has resulted in high hills, steep cliffs, and gorges.

Figure 4.4: Inland Basin topography, showing the difference in elevation between glaciated and unglaciated areas.

The portions of southern Indiana and Illinois that were not glaciated have a landscape similar to that of southeastern Ohio: a modest degree of relief, but substantially more rugged than the northern (previously glaciated) portions of those states.

An important distinction is that, because much of the bedrock in the Inland Basin is carbonate, this area commonly displays karst features. Caves are not uncommon in Ohio, but many of them are located in the Central Lowland portion of the state. The bedrock of southern Indiana and Illinois is, in areas, riddled with limestone caves.