The cliffs mark the escarpment of the Highland Rim where it transitions to the Central Basin at their foot. These rock outcroppings are primarily limestone underlain by shale, and because the shale erodes more quickly, it leaves overhangs and rock shelters below the harder limestone. The limestone contains inclusions of chert, a flinty stone used for flintknapping to make cutting and penetrating tools. At this point the cliff forms the perimeter of the enclosure, though the mound wall soon begins again and turns away from the river and cliffs to tie back into the other river valley. The soil here at the edge of the Highland Rim and within the enclosure is a layer of fine, windblown sediment called loess with pockets of iron rich clay. Although these soils at this elevation are not conducive to agriculture, they did promote a prairie ecosystem with its plants and wild game.