The rock sequences that we observe on the Manitoba escarpment were deposited in that warm shallow sea during the Late Cretaceous (80-90 m.y.) and consist mainly of marine shales with some units containing thin bands of bentonite. Bentonite is a type of clay formed by the alteration of volcanic ash by seawater and is used in industrially to absorb impurities in liquids. In the Morden-Miami area these deposits started to be mined commercially in the 1930’s. Most of the creamy white to yellow bentonite that we see today is exposed at old quarries and pits or in gullies where streams have cut through the overlying blanket of glacial sediments deposited during the last ice age.