Episode 127 – The Hell Creek Formation

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In the Upper Great Plains of the United States is a series of ancient sediments that have for more than a century been a premier source of information on the very end of the Age of Dinosaurs. This episode, we discuss the history, science, and significance of the Hell Creek Formation.

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The Latest Days of the Mesozoic

The Hell Creek Formation is a geologic formation (that is, a distinct layer of rock representing a particular time and environment) which comprises the last 1-2 million years of the Cretaceous Period – the end of the “Age of Dinosaurs.”

Map showing the extent of the Hell Creek Formation (and its sister, the Lance Formation). Hell Creek is exposed in Montana, North Dakota, and South Dakota, across an area known as the Williston Basin. Similar-aged formations are also found in Wyoming, Colorado, Alberta, and Saskatchewan. Image from Fowler, 2020.

The formation is named after a stream called Hell Creek in Montana. It was in the valley of Hell Creek that famed fossil hunter Barnum Brown became the first to formally study what he named the “Hell Creek beds.” When he first visited the valley in 1902, he found fossils of plants, invertebrates, and vertebrates, including the fossils that would go on to become the first official record of Tyrannosaurus rex.

Over the years, the Hell Creek Formation has become one of the best places in the world to study the end of the Mesozoic Era. The formation is rich in fossils, especially dinosaurs, making it a great place to research dinosaur evolution and ecosystem structure. Hell Creek is also one of the premier places to explore the end-Cretaceous mass extinction – in some places, the Hell Creek Formation sits directly underneath the famous K-Pg boundary, with its ample evidence of asteroid impact – and quite a lot of research in this region has been devoted to studying patterns of evolution and extinction before, during, and after the extinction event.

Hell Creek State Park in Montana. You can see the beautiful exposed geologic layers. There’s fossils in them hills!
Image by Vladimír Socha, CC BY-SA 4.0

Hell Creek in its Day

The Hell Creek Formation mostly consists of siltstones, mudstones, and sandstones that represent an ancient flat plain crossed by many rivers. These rivers ran west to east, originating from the early Rocky Mountains to the west and emptying into the Western Interior Seaway in the east. The finer sediments were generally deposited on the floodplain during high water levels, and sandstone channels mark the positions of the ancient rivers themselves.

During the latest Cretaceous, between 66-67 million years ago or so, this plain was home to an evergreen woodland with a lush plant community including abundant flowering plants alongside conifers, ferns, and others. Animal fossils from Hell Creek reveal a diverse ecosystem of freshwater invertebrates, insects, fish, amphibians, reptiles, mammals, and of course, dinosaurs, including some of the most famous dinosaurs of all.

A small sample of Hell Creek Formation reptiles.
Image by Durbed, CC BY-SA 3.0

Learn More

Some recent Hell Creek discoveries and studies include the giant dromaeosaur Dakotaraptor, analysis of Triceratops evolution, research on primate ancestors, and massive mosasaurs in the nearby seaway, just to name a few.

The Tanis Site in North Dakota is a section of the Hell Creek Formation that might preserve the day of the end-Cretaceous asteroid impact.

Saurian is a survival-simulation game that features an impressive recreation of the Hell Creek ecosystem.

The Hell Creek Formation and its Contribution to the Cretaceous-Paleogene extinction (technical, paywall)

If you enjoyed this topic and want more like it, check out these related episodes:

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