Fossil Atlas formation guide
The Hell Creek Formation is a latest Cretaceous rock unit famous for preserving the last dinosaurs on Earth — including Tyrannosaurus rex and Triceratops — and the thin clay layer that marks the asteroid impact that ended their reign 66 million years ago.
Geology
Hell Creek is made of river-deposited sandstone and floodplain mudstone. Rivers flowing from the rising Rockies built the deposits. The formation is capped by the K-Pg boundary clay — rich in iridium, the element that marks the Chicxulub asteroid impact. Above it: no non-avian dinosaur fossils. Below it: a dense latest Cretaceous fossil record.
The extinction
Hell Creek is one of very few places where you can place your hand on the exact rock layer that separates the Mesozoic from the Cenozoic. Paleontologists have studied the K-Pg boundary here in extraordinary detail, analyzing fossil pollen, vertebrate microfossils, and geochemistry to reconstruct what happened in the days, years, and millennia after the impact.
Fossil Atlas
The Fossil Atlas Hell Creek hotspot maps selected records from the current site dataset. T. rex and Triceratops each have full specimen profiles with mapped records. The hotspot page includes a geologic timeline, notable animals, and a modern discovery map with source context where available.
Explore
FAQ
The Hell Creek Formation spans roughly 68 to 66 million years ago, covering the last two million years of the Cretaceous Period — the Maastrichtian stage. The top of Hell Creek is marked by the K-Pg (Cretaceous-Paleogene) boundary, a thin layer of clay rich in the element iridium, which is the chemical signature of the asteroid impact that caused the mass extinction. Below this boundary, dinosaur fossils are abundant. Above it, they are absent — non-avian dinosaurs did not survive the event.
Hell Creek is exposed across a broad region of the northern Great Plains in the United States. The best exposures are in eastern Montana — especially around Jordan, Fort Peck Reservoir, and the Charles M. Russell National Wildlife Refuge — as well as western North Dakota, northwestern South Dakota, and northeastern Wyoming. The formation was deposited by ancient rivers that flowed eastward from the rising Rocky Mountains into a shallow interior seaway that then covered much of the central United States.
Hell Creek is composed primarily of sandstone, mudstone, and claystone deposited by meandering rivers on a broad coastal floodplain. The sandstones represent ancient river channels — often filled with cross-bedded sand and gravel. The finer mudstones and claystones represent floodplain deposits where dinosaurs, mammals, and plants were buried. Some layers are rich in organic matter that has since turned into lignite (low-grade coal). The alternating sand and mud layers create a classic 'badlands' landscape of striped hills and gullies.
Hell Creek preserves the final chapter of the dinosaur era and is one of the best-known latest Cretaceous terrestrial formations in North America. Its importance comes from several kinds of evidence together: dinosaur fossils such as T. rex and Triceratops, the K-Pg boundary layer, small vertebrates, and plant fossils that help reconstruct the ecosystem before and during the extinction interval.
Sources
Source links show where Fossil Atlas gets record and curation context. They do not make this page an exhaustive scientific bibliography.
Caveat
This page summarizes the Hell Creek Formation based on published literature and the current Fossil Atlas dataset. Formation descriptions and age ranges are approximate. The K-Pg boundary summary is simplified; the actual extinction story is complex and still actively researched.
Fossil maps on linked pages show modern discovery locations for selected records. These are not ancient habitat or range maps.