From the far, northernmost sides of the Octagon, there are clear views of the natural terraces down to Raccoon Creek. The geometric earthworks of the Ohio Valley region were always built on these high, second terraces above the rivers, where there were perfectly level surfaces plus deep, gravelly soils for good drainage. Dr. Brad Lepper:

The earthwork is up on this very, very high terrace, and it’s very, very near the edge. And there are some borrow pits of earth located nearby. There’s one over by the Country Club building. But it seems to be a very obvious and easy place to have gotten the earth was from that cliff face: rather than trying to dig a hole with a digging stick or a clamshell hoe, simply putting your stick into the ground and carving off the side of the bluff into the baskets, may have been one of the most important sources of earth for constructing the earthworks.