Hell Creek Formation

The Hell Creek Formation is made up of fresh and brackish-water clays, mudstones and sandstones deposited by rivers and streams along a coastal plain bordering the ancient Western Interior Seaway. It dates to roughly 68 to 66 million years ago, spanning the Maastrichtian and earliest Danian stages, and the iridium-enriched Cretaceous-Paleogene boundary marking the extinction of the non-avian dinosaurs occurs within or near its upper layers. The formation extends across parts of Montana, North Dakota, South Dakota and Wyoming, and is named after Hell Creek near Jordan, Montana, where it was first studied. It is renowned for exceptional fossil preservation, containing abundant remains of Tyrannosaurus and Triceratops alongside pterosaurs, early mammals and diverse plant material, and the Museum of the Rockies in Bozeman, Montana holds the world's largest collection of Hell Creek fossils. This entity documents the Hell Creek Formation specifically as a geological formation, a latest Cretaceous coastal-plain sedimentary sequence spanning the K-Pg boundary.