Hell Creek Formation

The Hell Creek Formation is a famous rock layer found in parts of Montana, North Dakota, South Dakota, and Wyoming. It dates back to the end of the time called the Cretaceous period, about 66 million years ago, and the very beginning of the next time period, the Paleogene. This area is very important for scientists because it holds many fossils from that time.

The rocks in the Hell Creek Formation were formed from mud, sand, and clay that settled in rivers, swamps, and deltas long ago. The climate back then was warm, and there were plants and animals that lived in tropical and temperate regions. One of the most famous features of this formation is a thin layer of rock enriched with a element called iridium, which marks the boundary between the Cretaceous and the following time period, the Cenozoic.

Many amazing fossils from dinosaurs and other creatures have been found in the Hell Creek Formation. The largest collection of these fossils is displayed at the Museum of the Rockies in Bozeman, Montana. Scientists from many universities have worked together since 1998 to find and study these important fossils.

Description

The Hell Creek Formation is an important geological formation made of rocks from mostly the Upper Cretaceous and some Early Paleocene in North America. It was named after the exposures found along Hell Creek near Jordan, Montana. This formation covers parts of Montana, North Dakota, South Dakota, and Wyoming. In Montana, the Hell Creek Formation lies on top of the Fox Hills Formation.

In 1966, the Hell Creek Fossil Area was named a National Natural Landmark by the National Park Service.

Geology

The Hell Creek Formation lies above the Fox Hills Formation and below the Fort Union Formation. The boundary between these layers marks the end of the Cretaceous period, about 66 million years ago. This layer is famous for its fossils, which show life from that ancient time.

In North Dakota, a place called the Tanis site may have preserved signs of the huge Chicxulub meteorite impact that changed Earth long ago.

Paleobiology

The Hell Creek Formation is famous for its many animal fossils, including dinosaurs. Because it was located where the eastern coast of Laramidia met the western shallows of the Western Interior Seway, both land and sea creatures were preserved. Scientists have found vertebrates such as pterosaurs, crocodiles, champsosaurs, lizards, snakes, turtles, frogs, and salamanders. They also discovered fishes and mammals.

Notable dinosaur discoveries include Tyrannosaurus and Triceratops. In 2000, the most complete hadrosaurid dinosaur, an Edmontosaurus, was found here and featured in a National Geographic documentary in December 2007. Fossils of birds, sharks, and rays have also been found, along with many invertebrates and plants.

Depositional environment

See also: Climate across Cretaceous–Paleogene boundary

The Hell Creek Formation is made of layers of clay, mudstone, and sandstone formed during the end of the Cretaceous period and the beginning of the Paleogene period. These layers were created by rivers and deltas in a flat, forested floodplain. The area had a warm, temperate climate with plenty of rain, supporting many kinds of plants such as trees, conifers, and ferns.

This formation, along with the Lance and Scollard Formations, represents parts of the western shore of an ancient shallow sea that split North America during the time of the dinosaurs. The land was a coastal plain stretching west to the Rocky Mountains, with swampy lowlands where many animals lived. The Hell Creek Formation is the most studied of these ancient environments, showing a mix of forest and swamp plants, including flowering trees, conifers, and ferns. Fossil leaves found there help scientists understand the climate of that time.

Fossil content

Main article: Paleobiota of the Hell Creek Formation

The Hell Creek Formation is famous for its many dinosaur fossils. Studies show that the most common dinosaur families there were Ceratopsidae at 40%, Tyrannosauridae at 24%, and Hadrosauridae at 20%. Other groups like Hypsilophodontidae, Ornithomimidae, Ankylosauridae, Pachycephalosauridae, Troodontidae, and Dromaeosauridae made up the rest, though some were found only as teeth.

Besides dinosaurs, the area was also home to many plants. The landscape was a wet floodplain with woodlands. Flowering plants, ferns, mosses, shrubs, and vines all grew there. The plant fossils show a mix of trees, such as plane trees and palms, which suggest the climate was warmer and wetter than it is today. The forests were made of many different types of trees and plants, some of which are extinct today.