Bearpaw Shale, Late Cretaceous shale formation in Montana, Alberta and Saskatchewan, Canada and United States

The Bearpaw Formation, also called the Bearpaw Shale, is a Late Cretaceous (Campanian) marine geological formation exposed in Montana, Alberta and Saskatchewan. It consists mainly of dark grey shales, claystones, silty claystones and siltstones, with subordinate silty sandstones, calcareous and ironstone concretions, and thin bentonite beds. It was deposited in the Bearpaw Sea, part of the Western Interior Seaway, and is named after the Bear Paw Mountains in Montana.

The Bearpaw Sea was a readvance of the Western Interior Seaway that began around 74 million years ago, late in the Campanian, after Laramide uplift had slowed and the land subsided. When the Laramide orogeny resumed in the early Maastrichtian, the seaway withdrew south for the last time, and the Bearpaw marine sediments were covered by the continental beds of the Horseshoe Canyon Formation in Canada and the coastal Fox Hills Formation in Montana.

The formation's name comes from the Bear Paw Mountains in Montana. In south-central Alberta, ammonite shells from its shales are mined to make ammolite, an organic gemstone sold to jewellers. The same beds yield Placenticeras meeki, Placenticeras intercalare, Hoploscaphites and Baculites compressus.

Outcrops run from Montana through Alberta and Saskatchewan, east of the Rocky Mountains; the formation is named for the Bear Paw Mountains in Montana. Exposures are best read in river valleys and roadcuts where erosion has stripped the soil. In south-central Alberta, ammonite-bearing layers are worked for ammolite, so collecting there requires permission from the leaseholder.

Some ammonites in this formation, notably Placenticeras meeki and Placenticeras intercalare, are mined in south-central Alberta for ammolite, an organic gemstone made from their iridescent shells. The same beds also yield the bivalve Inoceramus and the baculite Baculites compressus.