Hell Creek Formation
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 by fluvial activity in river channels and deltas and occasional peaty swamp deposits. All this was along the low-lying eastern continental margin fronting the late Cretaceous 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, occurs as a distinct thin marker bedding above and occasionally within the formation, near its boundary with the overlying Fort Union Formation.
The world's largest collection of Hell Creek fossils is housed and exhibited at the Museum of the Rockies, in Bozeman, Montana. The specimens displayed are the result of the museum's Hell Creek Project, a joint effort between the museum, Montana State University, the University of California, Berkeley, the University of North Dakota and the University of North Carolina which began in 1998.
The fossils discovered include all types of late Cretceous dinosaurs, fish, amphibia, mammals (Multituberculates, Metatheria and Eutheria), and other reptiles turtles, crocodylomorphs, squamata (snakes & lizards).