Capital Reef May 9-15, 2021. James Kay Photography. Bentonite Hills (aka "Mars"). 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 period 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.