The wide river valleys south of Chillicothe, and also around Cincinnati, are part of an ancient, pre-glacial river system. These huge rivers flowed northward out of West Virginia and Kentucky, before turning west and entering the Mississippi basin. The biggest traces are here at Chillicothe, and south of here at Piketon. Historian Kevin Coleman explains:

The Teays River was the mother of all rivers in this area. It was actually the head waters of the Mississippi, but it bent around though Illinois and Indiana and Ohio, and its head waters were in western North Carolina. It actually flowed through the Appalachian Mountains because when it started, the mountains, at least the second version, which we have today, had not been formed yet. The New River Gorge is the head waters of the Teays, but because of three or four glacier advances, the whole area in Ohio, and a little farther south, has been completely re-plumbed. So where the Teays created what is now the Scioto Valley, and it flowed northward, now we have the Scioto, which is a smaller river flowing southward.

These ancient rivers left behind superb, wide, flat, well-drained terraces, that stand safely above the newer, smaller rivers. They became perfect building sites for the giant geometric earthworks, two millennia ago.