The badlands are composed of water-deposited layers of volcanic ash, interbedded with thin layers of shale, sandstone, and river gravel. The alteration of the ash has converted it into a claylike substance that is called bentonite.Hard and strong when dry, bentonite will, however, absorb water like a sponge and with enough moisture will disintegrate into a fine flowing mud. Thus it is that in this semiarid region, with its long dry season and its torrential summer rains, the bentonitic beds are rapidly cut into turreted ridges, conical hills, and small steep-walled canyons and ravines.

Petrified Forest National Park