A marine formation composed mostly of shale, it represents the last major expansion of the Western Interior Seaway before it completely receded from northwestern North America by the end of the Cretaceous Period. The seaway had previously divided North America in half before orogeny (mountain-building) in the west uplifted the land and forced the seaway to retreat. As the uplift slowed down or ground to a halt in the late Campanian stage, around 74 million years ago, subsidence of the land allowed the seaway to invade once more. This northern expansion is often called the Bearpaw Sea. When the Laramide Orogeny resumed in the early Maastrichtian, the seaway retreated to the south for the final time. Because the sea did not disappear all at once, but instead slowly withdrew to the south, the Bearpaw Formation is superseded by the terrestrial sediments of the Horseshoe Canyon in Canada, while in Montana the Fox Hills Sandstone represents a near-shore marine environment. The Fox Hills too would be replaced by the terrestrial sediments of the Hell Creek Formation in Montana by the late Maastrichtian.
It is famous for its well-preserved ammonite fossils. Other fossils found in this formation include many types of shellfish, bony fish, sharks, rays, birds, and marine reptiles like mosasaurs, plesiosaurs and sea turtles. The occasional dinosaur remains have also been discovered, presumably from carcasses washed out to sea.