The Hell Creek Formation

By editor

Jordan, Garfield County, Montana, 66 million years ago

About 65 million years ago, the inland sea that had covered eastern Montana for most of the Cretaceous Period receded as the Rocky Mountains rose to the west. The shoreline moved east, and the coastal plain that replaced it was a warm, humid landscape of rivers and floodplains, dense with vegetation and populated by animals that would not survive the end of the period. The sediments deposited in those river deltas and floodplains became the Hell Creek Formation: tan sandstones, siltstones, and mudstones that preserve the last chapter of the age of dinosaurs.

The formation is named for Hell Creek, a tributary of the Missouri River in Garfield County, where Barnum Brown of the American Museum of Natural History collected the first Tyrannosaurus rex specimens in 1902 and 1908. Brown was one of the great fossil hunters of the early twentieth century, a man who traveled the badlands of Montana and Wyoming and the Gobi Desert of Mongolia in search of bones, and who found them in quantities that filled the museums of New York and London. The T. rex specimens he collected from the Hell Creek Formation are still among the most complete ever found. One of them, mounted in the American Museum of Natural History, has been seen by more people than any other dinosaur skeleton in the world.

The dinosaurs preserved in the Hell Creek Formation include Triceratops, the three-horned ceratopsian that is one of the most recognizable animals in the fossil record; Edmontosaurus, an immense hadrosaur that may have traveled in herds of thousands; Pachycephalosaurus, with its thick, domed skull; Ankylosaurus, armored from head to tail; and Tyrannosaurus rex, the largest terrestrial predator of its era. These animals lived in the coastal plain environment of the Hell Creek Formation during the last million years of the Cretaceous Period, and they were killed -- along with approximately three-quarters of all species on earth -- by the asteroid impact that ended the period approximately 66 million years ago.

The evidence for the impact is preserved in the Hell Creek Formation itself. The boundary between the Cretaceous Hell Creek Formation and the more recent Tertiary Fort Union Formation -- known to geologists as the K-Pg boundary -- contains a thin layer of iridium, a metal rare on earth but common in asteroids. The iridium layer is found at this boundary in rock formations around the world, and it is the primary evidence for the hypothesis that an asteroid impact caused the end-Cretaceous mass extinction. The asteroid, estimated to have been approximately 10 kilometers in diameter, struck what is now the Yucatan Peninsula of Mexico approximately 66 million years ago. The impact released energy equivalent to billions of nuclear weapons, ignited global wildfires, and sent enough debris into the atmosphere to block sunlight for months or years.

The Hell Creek badlands around Jordan, Montana are among the richest fossil beds in the world. The tan and gray sediments erode quickly in the dry climate of eastern Montana, exposing new fossils every year. Professional paleontologists from universities and museums work the badlands every summer, and amateur collectors find significant specimens regularly. The Charles M. Russell Wildlife Refuge, which covers much of the Missouri River breaks in this part of Montana, protects some of the most productive fossil beds in the region.

The Hell Creek Formation can be seen in road cuts along the highway, in the badlands of the Charles M. Russell Wildlife Refuge, and in Makoshika State Park near Glendive. Makoshika, whose name comes from the Lakota words for "bad land" or "bad earth," contains some of the most dramatic exposures of the formation in the state. The park's visitor center displays fossils collected from the formation, including a Triceratops skull and a partial T. rex skeleton.

The fossils excavated from the Hell Creek Formation have added immeasurably to the understanding of dinosaur biology, behavior, and ecology. The formation preserves not just bones but also skin impressions, stomach contents, and occasionally soft tissue. It preserves the last ecosystem of the Mesozoic Era, the world that existed before the asteroid ended it.

See also

• The Hell Creek Formation at Jordan, Garfield County
• Welcome to Dinosaur Country at Jordan, Garfield County
• Montana's Jurassic Park at Malta, Phillips County