Bentonite Outcrop



Wyoming bentonite is the result of chemical changes in ancient volcanic ash, which originated in western Wyoming and Idaho about 120 million years ago, during the late Cretaceous period. Prevailing winds blew the ash into a shallow sea which covered most of what is now Montana and Wyoming. Present day exposures of the ash deposits are mined to an average depth of 25 feet in the Big Horn basin, the Powder River basin, and along the edges of the Black Hills.