Hell Creek Formation

The Hell Creek Formation is a series of rocks where fossil dinosaurs are found. The strata are mostly Upper Cretaceous and some lower Paleocene rocks in North America. 
The name comes from Hell Creek, near Jordan, Montana. The formation includes portions of Montana, North Dakota, South Dakota, and Wyoming. 
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 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. This is 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. The project began in 1998.
The fossils discovered include all types of late Cretceous dinosaurs, fish, amphibia, and other reptiles such as turtles, crocodylomorphs, and squamata (snakes &amp; lizards).
Mammals are plentiful in the Hell Creek Formation. They include multituberculates, metatherians, and eutherians.