Bentonite is formed by the chemical alteration of minute glass particles that were once volcanic ash. The layers of this volcanic ash were deposited when volcanoes in what is now Montana erupted, spewing tonnes of this material into the air. This volcanic ash was carried by wind and water currents to form numerous beds all over most of the interior of North America.

Manitoba, is one of the few places where these layers are close enough to the surface and of sufficient thickness to be economically mined. As the bentonite is mined, fossils are discovered in the pits.