The Bearpaw Formation, or Bearpaw Shale, is a Late Cretaceous geologic formation in Montana and parts of Alberta and Saskatchewan, named for Montana’s Bear Paw Mountains. It formed during the Campanian age when this region lay beneath the Bearpaw Sea, part of the Western Interior Seaway. Composed mainly of dark grey shale and other fine-grained marine sediments, it was later buried as the seaway retreated. The formation is especially important for its abundant marine fossils, including ammonites, shellfish, fish, sharks, rays, mosasaurs, plesiosaurs, and sea turtles, with rarer dinosaur remains. It remains a major record of the Western Interior Seaway in western North America.