Hell Creek Formation

The Hell Creek Formation is the single most famous dinosaur-bearing geological formation in the world. Exposed across eastern Montana, western South Dakota, southwestern North Dakota, and northeastern Wyoming, it preserves the final 2 million years of the Mesozoic Era — the last dinosaurs to ever walk the Earth.

Geological Context

The Hell Creek Formation consists of interbedded sandstones, mudstones, and siltstones deposited in a river and floodplain environment along the western shore of the retreating Western Interior Seaway. During the Late Cretaceous, this region was a low, swampy coastal plain with meandering rivers, forests of conifers and angiosperms, and abundant wildlife.

The formation is capped by the K-Pg boundary — the thin clay layer that marks the asteroid impact 66 million years ago that ended the Age of Dinosaurs. In some exposures, you can place your hand on the exact boundary between the Cretaceous and Paleogene.

What You’ll Find

Vertebrate Fossils (Permit Required on Federal Land)

The Hell Creek is world-renowned for its dinosaur fauna:

• Tyrannosaurus rex — the apex predator. Teeth, bones, and occasional partial skeletons
• Triceratops — the most commonly found dinosaur in the Hell Creek. Horn cores, frill fragments, and teeth are relatively common
• Edmontosaurus — a large hadrosaur (duck-billed dinosaur). Bones and teeth
• Pachycephalosaurus — dome-headed dinosaur. Distinctive skull caps
• Ankylosaurus — armored dinosaur. Armor scutes and bone fragments
• Thescelosaurus — a smaller ornithopod

Non-dinosaur vertebrates include:

• Turtles (very common — shells preserve well)
• Crocodilians
• Champsosaurus (a crocodile-like reptile)
• Small mammals (teeth and jaws)
• Fish
• Pterosaurs (rare)

Invertebrate and Plant Fossils (Casual Collecting May Be Allowed)

• Freshwater bivalves and gastropods
• Leaf impressions and petrified wood
• Amber (rare)

Collecting in the Hell Creek

Important legal notes:

• Vertebrate fossils on federal land require a research permit — always, no exceptions
• Private land: Most productive collecting happens on private ranches. Landowners own the fossils and many welcome responsible collectors or run commercial dig programs
• Commercial operations: Several outfitters run guided dinosaur digs on private land in Montana and South Dakota. These are the most accessible way for hobbyists to experience Hell Creek fossil hunting.

Commercial Dig Programs

• Various operations near Jordan, Montana offer multi-day guided dinosaur digs
• Programs in Harding County, South Dakota provide similar experiences
• Prices range from a few hundred dollars for a day dig to several thousand for a week-long expedition
• You typically get to keep invertebrate and plant fossils; significant vertebrate finds are documented and may go to museums

Key Localities

• Jordan, Montana — The “T. rex Capital of the World.” The Hell Creek type section is near here, and more T. rex specimens have been found in this area than anywhere else.
• Marmarth, North Dakota — Rich exposures on private land, with several dig programs available.
• Harding County, South Dakota — The Hell Creek extends into the northwestern corner of the state.
• Lance Creek, Wyoming — The equivalent Lance Formation produces similar fauna.

The K-Pg Boundary

The Hell Creek Formation is one of the best places on Earth to study the end-Cretaceous mass extinction. The boundary is marked by:

• An iridium-rich clay layer (from the asteroid impact)
• A dramatic change in pollen (from diverse flora to fern-dominated “fern spike”)
• The disappearance of all non-avian dinosaurs above the boundary
• The overlying Fort Union Formation containing only mammals, turtles, and crocodilians — no dinosaurs

Fossils Found in This Formation