Hell Creek and Lance Formations
The Hell Creek Formation (blue) and the Lance Formation (green) form a latest Cretaceous package of continental rocks across the western interior of North America.

The Hell Creek Formation is a thick package of continental sedimentary rocks deposited near the end of the Cretaceous Period, in the closing stages of the Mesozoic era. It crops out over a wide area of the northern Great Plains, especially in eastern Montana, southeastern Montana, western North Dakota, southwestern North Dakota, northwestern South Dakota, and parts of Wyoming.

These river‑laid sandstones, mudstones, and paleosols accumulated on the western margin of the northern Great Plains as rivers flowed eastward off the uplifting Rocky Mountains toward the retreating Western Interior Seaway and the developing Gulf of Mexico region. Together with the Lance Formation of Wyoming, the Hell Creek Formation preserves one of the most complete records of latest Cretaceous dinosaur fossils in western North America.

Stratigraphic Section
A stratigraphic column near Fort Peck, Montana, showing the Fox Hills Formation, the overlying Hell Creek Formation, and the Fort Union Formation across the Cretaceous–Paleogene boundary.

Stratigraphically, the Hell Creek Formation overlies the marine Fox Hills Formation and is in turn overlain by the Paleocene Fort Union Formation. The lower Hell Creek beds record the transition from shallow marine to fully terrestrial conditions along the western interior shoreline as global sea levels fell and the Western Interior Seaway withdrew toward the Gulf of Mexico.

The upper Hell Creek Formation is capped by the Cretaceous–Paleogene boundary clay, a distinctive thin bed enriched in iridium and shocked quartz grains. This boundary horizon, documented in Geological Society of America Special Papers, marks the moment of the Chicxulub impact and coincides with pulses of Deccan Traps volcanism, linking the Hell Creek fossil record directly to the global end‑Cretaceous extinction event that affected ecosystems from North America to South America.

Badlands Topography
Intense freeze–thaw and runoff erosion continually cuts into Hell Creek beds, exposing new dinosaur fossils each year along Hell Creek and other drainages.

Much of the Hell Creek Formation is exposed as rugged badlands, where soft mudstones and sandstones erode rapidly under the harsh climate of the northern Great Plains. This surface weathering is the reason paleontologists and amateur fossil collectors can find so many dinosaur fossils in shallow creek gullies and slopes.

Winter freeze–thaw cycles and intense summer thunderstorms in Montana, North Dakota, and South Dakota constantly strip away thin layers of rock. As a result, vertebrate fossils, plant fossils, and dinosaur bones weather out of the formation naturally, ready to be collected before they are destroyed by further erosion.

River and Floodplain Beds
Alternating sandstone channels and mudstone floodplain deposits preserve the original Late Cretaceous landscape in the Hell Creek Formation of Montana and the Dakotas.

Thick, cross‑bedded sand bodies in the Hell Creek Formation represent ancient river channels that migrated across the western interior floodplain. These rivers carried dinosaur carcasses and plant debris from upstream and buried them in sand bars and crevasse splays.

Overbank mudstones and paleosols record quieter intervals when fine sediment settled out on broad floodplains. In these settings, dinosaur skeletons, plant fossils, and invertebrate shells could be buried in low‑oxygen conditions, creating the rich fossil assemblages that paleontologists study today.

Popcorn Weathering
Smectitic clay‑rich mudstones in the Hell Creek beds swell and crack, creating a characteristic popcorn‑like surface that can damage shallow fossils.

Many Hell Creek mudstones are rich in smectite clays derived from volcanic ash. These clays expand when wet and contract as they dry, producing a bumpy “popcorn” weathering texture that is typical of Hell Creek outcrops.

While this weathering helps expose fossils, it can also fracture delicate bones and plant impressions near the surface. Paleontologists working in classic creek areas must routinely stabilize exposed dinosaur fossils before the next wet–dry cycle destroys them.

Carbonaceous Shale Beds
Dark, organic‑rich shales and low‑grade coal layers show that parts of the Hell Creek Formation were swampy wetlands with abundant plant biomass.

During the Maastrichtian, the Hell Creek region lay near the western shore of the Western Interior Seaway, in a warm climate with no polar ice caps. The coastal plain supported dense forests, riverine woodlands, fern prairies, and peat‑forming swamps typical of Late Cretaceous North America.

Thick carbonaceous shales and coal seams in the lower Hell Creek and overlying Fort Union Formation indicate extensive wetlands that later turned into peat and coal. These plant‑rich habitats sustained large populations of herbivorous dinosaurs, as well as turtles, crocodilians, and other vertebrates that depended on swamp environments.

Artocarpus lessigiana
Fossil leaves from the Hell Creek beds record a diverse angiosperm flora similar to modern broadleaf forests (Denver Museum of Nature & Science, specimen HC179).

By the time the Hell Creek Formation was deposited, flowering plants (angiosperms) had become a major component of the coastal‑plain vegetation. Fossil leaves show a variety of dicot taxa, including relatives of modern breadfruit and other broadleaf trees.

This angiosperm‑rich flora likely influenced dinosaur feeding strategies. High‑productivity plant communities would have supported large herds of hadrosaurids and ceratopsids, which in turn shaped the ecology of top predators such as Tyrannosaurus.

Araliaephyllum polevoi
Multi‑lobed leaves such as Araliaephyllum show that some Hell Creek forests would have looked surprisingly modern to today’s observer.

Many Hell Creek plant fossils belong to families that still exist, such as the Araliaceae (ivy and ginseng relatives). Leaves of Araliaephyllum preserve detailed venation patterns that help paleobotanists reconstruct canopy structure and climate.

These leaves often accumulated in quiet oxbow lakes and floodbasin ponds, where low‑oxygen conditions allowed delicate tissues to be preserved. The resulting plant fossils document the diversity of Late Cretaceous forests in the Hell Creek and related formations like the Scollard Formation and Frenchman Formation to the north in Alberta and Saskatchewan.

cf. Liriodendron
Fossil leaves related to modern tulip trees occur in the Hell Creek paleoflora (specimen HC208).

Magnolia pulchra
Early magnolias were a conspicuous part of Late Cretaceous forests in the Hell Creek and Lance formations (specimen HC066).

Members of the Magnoliaceae, including Magnolia and Liriodendron, are well represented in Hell Creek plant assemblages. These trees bore showy flowers and large seeds that would have attracted insects, birds, and small mammals.

Their presence helps paleontologists understand the structure of the Late Cretaceous canopy across the western interior and provides context for interpreting the diets of herbivorous dinosaurs that browsed these forests.

Nelumbium montanum
Lotus‑like aquatic plants flourished in the lakes and channels that overlay the Hell Creek beds and continue into the Fort Union Formation (specimen FU062).

Aquatic plants such as Nelumbium and various water ferns occupied ponds, oxbows, and abandoned river channels on the Hell Creek floodplain. Their remains are often found in laminated shales and fine siltstones.

These vegetated water bodies were important habitats for freshwater invertebrates, fish, amphibians, and semi‑aquatic reptiles. They also supplied food and cover for dinosaurs when they approached the water’s edge to drink or feed.

Dinosaur Community
Reconstruction of Hell Creek dinosaur fauna, showing ceratopsids, hadrosaurids, ankylosaurids, and tyrannosaurids living together in the Maastrichtian of the western interior.

The Hell Creek Formation preserves one of the last non‑avian dinosaur communities before their extinction. By this time, sauropods and stegosaurs had disappeared from the region, leaving ceratopsids, hadrosaurids, ankylosaurids, and tyrannosaurids as the dominant large‑bodied dinosaur groups.

This fauna represents a culmination of Late Cretaceous evolutionary trends: heavily armored herbivores, sophisticated chewing mechanisms in hadrosaurids, and extremely specialized apex predators like Tyrannosaurus rex at the top of the food chain.

Size Comparison
Scale diagram contrasting human height with Triceratops, Tyrannosaurus rex, and other archosaurs from the Hell Creek Formation.

Large dinosaur fossils from Hell Creek show that fully grown Tyrannosaurus rex could exceed 12 meters in length and weigh several tons. Its main prey species, such as Triceratops and Edmontosaurus, were similarly massive.

Living alongside these giants were smaller theropods, pachycephalosaurs, and ankylosaurs, filling other ecological niches. Together they formed a complex dinosaur fauna that has been intensively studied in Hell Creek and Lance Formation dinosaur census projects.

Non‑Avian Dinosaur Diversity
Silhouette chart showing the wide range of dinosaur taxa known from the Hell Creek Formation of eastern Montana and adjacent states.

In addition to iconic taxa like Tyrannosaurus and Triceratops, the Hell Creek Formation preserves ornithomimids, pachycephalosaurs, ankylosaurs, small dromaeosaurids, and other dinosaur groups. Many of these species are known from partial skeletons and isolated elements.

Continuing work by paleontologists in northeastern Montana, southeastern Montana, and southwestern North Dakota has expanded this taxonomic list, demonstrating that the Hell Creek dinosaur fauna was more diverse than once thought.

Large‑Bodied Dinosaur Abundance
Data from the Hell Creek Project and related studies summarizing the relative abundance of Triceratops, Tyrannosaurus, and other dinosaur taxa in the fossil record.

The Hell Creek Project, led by the Museum of the Rockies and partner institutions, carried out a large‑scale census of dinosaur fossils across sections of the Hell Creek and Lance formations. This work quantified the relative abundance of major dinosaur taxa.

Results indicate that Triceratops is the most common large herbivore, while Tyrannosaurus is unexpectedly abundant for an apex predator. These patterns provide insight into Late Cretaceous ecosystem structure just before the extinction event.

Tyrannosaurus rex from Hell Creek
Some of the most complete T. rex skeletons, such as “Sue” and “Stan”, come from the Hell Creek Formation and Lance Formation of South Dakota and eastern Montana.

The Hell Creek Formation of South Dakota, eastern Montana, and adjacent areas has yielded the most famous Tyrannosaurus rex specimens in the world. Skeletons collected for the American Museum of Natural History, the Museum of the Rockies, and other institutions have become icons of paleontology.

As a taxon, Tyrannosaurus rex combines massive skull strength with keen senses, enabling it to crush bone and hunt large ceratopsids and hadrosaurids. Its fossils are central to discussions of dinosaur biology, growth, and behavior in the latest Cretaceous fossil record.

The Nanotyrannus Debate
Skulls from the Hell Creek Formation have sparked debate over whether they represent a separate taxon (Nanotyrannus) or growth stages of T. rex.

Several small tyrannosaurid skulls from the Hell Creek and Lance formations were once assigned to the taxon Nanotyrannus lancensis. Some paleontologists have argued that this represents a distinct, more gracile tyrannosaur species.

However, many researchers now interpret these specimens as juvenile T. rex, consistent with a long growth series documented from multiple Hell Creek individuals. This interpretation has implications for understanding tyrannosaur ontogeny and ecological roles at different life stages.

Hell Creek Caenagnathids
Large, feathered oviraptorosaurian dinosaurs such as Anzu wyliei lived alongside tyrannosaurids and ceratopsids in the Hell Creek ecosystems.

Among the more bird‑like dinosaurs of the Hell Creek Formation are caenagnathid oviraptorosaurs, including Anzu wyliei, nicknamed the “Chicken from Hell.” These theropods had toothless beaks, long arms, and likely bore feathers.

Their fossils indicate omnivorous diets and suggest that Late Cretaceous dinosaur faunas included a variety of theropod feeding strategies, not just pure carnivory. Such finds broaden our understanding of dinosaur diversity in the Hell Creek and Lance formations.

Late Cretaceous Pterosaurs
Azhdarchid pterosaurs with enormous wingspans were part of the vertebrate fauna recorded in the Hell Creek Formation and equivalent units.

Fragmentary remains from the Hell Creek Formation suggest that large azhdarchid pterosaurs, similar to Quetzalcoatlus, lived above the Maastrichtian floodplains. These flying reptiles would have shared the skies with early birds.

Their bones are fragile and rarely preserved, but the available fossil evidence indicates that pterosaurs persisted in western North America right up until the Cretaceous–Paleogene boundary.

“Montana Today” Exhibit
The Milwaukee Public Museum uses a modern badlands diorama with a partially exposed T. rex skeleton to explain how fossils emerge from Hell Creek rocks.

To communicate the geology of the Hell Creek Formation to the public, several museums have built detailed dioramas. These exhibits often contrast the dry, eroded badlands of today with the lush floodplain environments of the Late Cretaceous.

By showing a fossil skeleton eroding out of a hillside, such displays highlight the role of natural processes in bringing dinosaur fossils to light, and they connect visitors directly to the fieldwork carried out in Montana and the Dakotas.

“Montana 65 Million Years Ago”
A companion diorama reconstructs the Hell Creek Formation landscape when dinosaurs like Triceratops and Tyrannosaurus rex were still alive.

Companion “living” dioramas place reconstructed dinosaurs within dense magnolia forests, fern undergrowth, and palmetto thickets representative of the Hell Creek paleoflora. These exhibits visualize how the formation’s plant fossils translate into real ecosystems.

By pairing these reconstructions with actual fossils, museums provide a bridge between the rock record and the dynamic Late Cretaceous ecosystems that once covered the western interior of North America.

Molluscan Shell Beds
Thick accumulations of bivalve and gastropod shells in the Hell Creek beds indicate long‑lived freshwater lakes and rivers.

Beyond dinosaurs, the Hell Creek Formation preserves abundant freshwater mollusks, including unionid clams and various snails. These invertebrates formed an important component of the floodplain food web.

Shell beds formed when storms, floods, or droughts concentrated mollusk remains in specific horizons. These layers now provide information about water depth, flow conditions, and the stability of aquatic habitats near the end of the Cretaceous.

Natural Microfossil Concentrators
Harvester ants in Harding County build mounds using small clasts, including tiny vertebrate fossils from the Hell Creek beds.

To study tiny vertebrates in the Hell Creek fauna—such as small reptiles, amphibians, and mammals—paleontologists collect microfossils. These fossils are often concentrated in ant hills built from sediment and small pebbles.

Harvester ants inadvertently bring fossil teeth and bones to the surface while constructing their mounds. By sieving abandoned ant hills, researchers can recover a diverse microvertebrate assemblage that complements the larger dinosaur record.

Microfossil localities in the Hell Creek and Lance formations have produced dozens of mammal taxa, including multituberculates, early marsupials, and basal placental mammals. These animals were typically small, nocturnal, and insectivorous or omnivorous.

Their fossil record across the Cretaceous–Paleogene boundary shows that many mammal groups survived the extinction event and diversified further in the overlying Paleocene Fort Union Formation, helping define the transition from the Late Cretaceous to the Paleocene in western North America.

At the top of the Hell Creek Formation, the fossil‑rich Maastrichtian succession ends abruptly at the Cretaceous–Paleogene boundary layer. This thin horizon records the impact of a roughly 10‑kilometer‑wide asteroid at Chicxulub on the Yucatán Peninsula.

The impact triggered wildfires, global dust and sulfate aerosols, and a sharp collapse of photosynthesis. In the Hell Creek region and across the northern Great Plains, plant communities, non‑avian dinosaurs, and many other organisms disappeared from the fossil record, marking one of the most dramatic mass extinction events in Earth’s history and closing the Mesozoic era worldwide—from North America to Europe, Africa, and South America.

Where is the Hell Creek Formation located?

The Hell Creek Formation is located in the northern Great Plains of North America, mainly in eastern Montana, western North Dakota, northwestern South Dakota, and northeastern Wyoming. Smaller outcrops also appear in neighboring areas where Late Cretaceous rocks were preserved.

Why is the Hell Creek Formation so important in paleontology?

Hell Creek and the closely related Lance Formation form one of the clearest windows into the last million years of the Cretaceous Period. They yield world‑famous dinosaur fossils, well‑preserved plant assemblages, and a continuous stratigraphic section through the Cretaceous–Paleogene boundary in the northern Great Plains, making them key reference sections for the end of the Mesozoic era.

What dinosaurs are found in the Hell Creek Formation?

The formation is best known for dinosaurs such as Tyrannosaurus rex, Triceratops, Edmontosaurus, Ankylosaurus, pachycephalosaurs, ornithomimids, small dromaeosaurids, and other theropods. These taxa represent one of the last non‑avian dinosaur faunas in North America before the K–Pg extinction.

What kinds of fossils are found in the Hell Creek Formation?

Fossils include dinosaur skeletons, crocodilians, turtles, amphibians, fish, freshwater mollusks, plant remains, microfossils of small mammals and reptiles, and trace fossils such as burrows and coprolites. This broad fossil record allows scientists to reconstruct both the fauna and flora of the latest Cretaceous.

How old is the Hell Creek Formation?

Radiometric ages and biostratigraphic data indicate that the Hell Creek Formation spans roughly 68 to 66 million years ago, during the Maastrichtian stage of the Late Cretaceous, immediately before the global Cretaceous–Paleogene extinction event.

What caused the extinction of the Hell Creek dinosaurs?

Most researchers agree that a combination of the Chicxulub asteroid impact and large‑scale volcanism in the Deccan Traps of India drove rapid climate change at the end of the Cretaceous. In the Hell Creek region, these global events caused ecosystem collapse, leading to the extinction of non‑avian dinosaurs while some smaller vertebrates survived into the Paleocene.

What was the environment like in Hell Creek during the Late Cretaceous?

During the Maastrichtian, sea levels were falling and the Western Interior Seaway was retreating toward the Gulf of Mexico. The Hell Creek area was a warm, humid, subtropical coastal plain with rivers, floodplains, swamps, and lakes, supporting dense vegetation and abundant vertebrate life.

Can you collect fossils in the Hell Creek Formation?

Fossil collecting regulations vary by land ownership. On U.S. federal and many state lands, vertebrate fossil collecting is restricted to permitted scientific projects. On some private lands in Montana, North Dakota, South Dakota, and Wyoming, owners may allow supervised fossil collecting, often through commercial or educational digs, but collectors must always obtain explicit permission and follow local laws.

Is the Hell Creek Formation unique to North America?

The Hell Creek Formation itself is restricted to the northern Great Plains of North America, but equivalent Late Cretaceous formations exist on other continents, including units in South America and Asia. Together, these formations help paleontologists compare how dinosaur faunas and ecosystems differed around the world at the end of the Mesozoic era.

Where can I see Hell Creek fossils on display?

Specimens from the Hell Creek Formation and related units are held in major museums such as the Museum of the Rockies, the American Museum of Natural History, the Denver Museum of Nature & Science, and numerous regional museums across Montana, North Dakota, South Dakota, and Wyoming. Many of these institutions display complete dinosaur skeletons and interpretive exhibits about the K–Pg extinction.