The Hell Creek Formation is an intensively-studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation includes portions of Montana, North Dakota, South Dakota, and Wyoming. In Montana, the Hell Creek Formation overlies the Fox Hills Formation. “Pompey’s Pillar” at the Pompeys Pillar National Monument is a small isolated section of the Hell Creek Formation. It is a series of fresh and brackish-water clays, mudstones, and sandstones deposited during the Maastrichtian and Danian (respectively the end of the Cretaceous period and the beginning of the Paleogene) by fluvial activity in fluctuating river channels and deltas and very occasional peaty swamp deposits 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–Paleogene boundary, which separates the Cretaceous from the Cenozoic, occurs as a discontinuous but 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.