It is a series of fresh and brackish-water clays, mudstones, and sandstones deposited during the end of the Cretaceous period and the beginning of the Palaeogene. It was laid down in river channels and deltas, and occasional peaty swamp deposits. along the low-lying eastern margin of the Western Interior Seaway.
The climate was mild, and the presence of crocodilians suggests a sub-tropical climate, with no prolonged annual cold. The famous iridium-enriched Cretaceous–Palaeogene boundary, which separates the Cretaceous from the Cenozoic, is a distinct thin bedding. It is above and occasionally within the formation, near its boundary with the overlying Fort Union Formation.
The fossils discovered include all types of late Cretceous dinosaurs, fish, amphibia, and other reptiles such as turtles, crocodylomorphs, and squamata (snakes & lizards).
