Hell Creek Formation

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The Hell Creek Formation is an intensely-studied division of Upper Cretaceous rocks in North America. It is a series of freshwater clays, mudstones, and sandstones deposited during the latest part of the Cretaceous period by fluvial activity in fluctuating river channels and deltas and very occasional peaty swamp deposits along the low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway. The climate was mild^[1]. The famous iridium-enriched K-T boundary, which separates the Cretaceous from the Cenozoic, occurs as a discontinuous but distinct thin marker bedding within the Formation, near its uppermost strata.

It is named for exposures studied along Hell Creek, near Jordan, Montana.

The Hell Creek Formation occurs in badlands of eastern Montana and portions of North Dakota, South Dakota, and Wyoming. In Montana, the Hell Creek Formation overlies the Lennep Formation and is the uppermost formation of the Cretaceous period. "Pompey's Pillar" at the Pompeys Pillar National Monument is a small isolated section of the Hell Creek Formation.

The formation has produced impressive assemblages of invertebrates, plants^[2], mammals, fish, reptiles, and amphibians. A few bird and pterosaur fossils have also been found also. Teeth of sharks and rays are sometimes found in the riverine Hell Creek Formation, suggesting that some of these taxa were tolerant of fresh water then as now.

The large dinosaurs Edmontosaurus and Triceratops are by far the most common genera in the deposits. Rarer species include Torosaurus and the famous Tyrannosaurus rex,^[3]the armored Ankylosaurus, boneheaded Pachycephalosaurus and Stygimoloch and small ornithopods such as Bugenasaura and Thescelosaurus.

Commercial excavations bring Hell Creek fossils onto the market, usually dinosaur teeth, crocodylian osteoderm fragments, and dermal plates of fossil gars. A representative selection of Hell creek fossils can be seen at the Museum of the Rockies, in Bozeman, Montana.

[edit] Notes

^ The presence of crocodylians suggests a sub-tropical climate, with no prolonged annual cold.
^ Casts of Metasequoia seed cones have been found.
^ In the summer of 2001 a juvenile Tyannosaurus was named Jane and prepared for the Burpee Museum of Natural History, Rockford, Illinois: see Jane (dinosaur).