The Bentonite Hills are located outside the park boundary on Bureau of Land Management land. The Bentonite Hills appear as softly-contoured, banded hills in varying hues of brown, red, purple, gray, and green. The hills are composed of the Brushy Basin shale member of the Morrison Formation. This layer was formed during Jurassic times when mud, silt, fine sand, and volcanic ash were deposited in swamps and lakes. Bentonite clay (altered volcanic ash) absorbs water and becomes very slick and gummy when wet, making vehicle or foot travel difficult or impossible. Repeated cycles of absorbing water and then drying results in a popcorn-like appearance on the clay's surface.
There are no trails through the Bentonite Hills.