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 a 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 an 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 to find and study these important fossils.

Description

The Hell Creek Formation is an important group of rocks from the Upper Cretaceous and Early Paleocene in North America. It was named after the rock 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 sits 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. This layer marks the end of the Cretaceous period, about 66 million years ago. It is famous for its fossils, which show life from that ancient time.

In North Dakota, the Tanis site may have preserved signs of the Chicxulub meteorite impact from long ago.

Paleobiology

The Hell Creek Formation is famous for its many animal fossils, including dinosaurs. It was where the eastern coast of Laramidia met the western shallows of the Western Interior Seway, so 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 shown 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. These layers formed at the end of the Cretaceous period and the start of the Paleogene period. They were created by rivers and deltas in a flat, forested floodplain. The area had a warm, temperate climate with lots of rain. This supported many plants such as trees, conifers, and ferns.

This formation, together with the Lance and Scollard Formations, shows parts of the western edge of an ancient shallow sea. This sea split North America when dinosaurs lived. The land was a coastal plain stretching west to the Rocky Mountains. It had swampy lowlands where many animals lived. The Hell Creek Formation is the most studied of these places. It shows a mix of forest and swamp plants, including flowering trees, conifers, and ferns. Fossil leaves found there help scientists learn about the climate of that time.

Fossil content

Main article: Paleobiota of the Hell Creek Formation

The Hell Creek Formation is well-known for its dinosaur fossils. The most common dinosaur families found there are Ceratopsidae, Tyrannosauridae, and Hadrosauridae. Other groups like Hypsilophodontidae, Ornithomimidae, Ankylosauridae, Pachycephalosauridae, Troodontidae, and Dromaeosauridae have also been found, although sometimes only as teeth.

The area also had many plants. It was a wet floodplain with woodlands. Flowers, ferns, mosses, shrubs, and vines grew there. The plant fossils show that there were many types of trees, like plane trees and palms. This suggests the climate was warmer and wetter than it is today. The forests had many different kinds of trees and plants, some of which no longer exist.