About 65 million years ago, the Western Interior Seaway receded as the Rocky Mountains rose, pushing the shoreline further east. Great rivers meandered through the coastal plain in a warm and humid climate, depositing sediment which would later become known as the Hell Creek Formation. The tan sandstones, siltstones, and mudstones are common throughout eastern Montana, but can be best seen in road cuts along the highway, the Charles M. Russell Wildlife Refuge, and in Makoshika State Park. Dinosaur fossils are frequently found in the Hell Creek Formation and include Triceratops, an immense hadrosaurs called Edmontosaurus, the thick-skulled Pachycephaosaurus, the armored Ankylosaurus, and the awesome Tyrannosaurus rex to name just a few. The boundary between late Cretaceous Hell Creek Formation and the more recent Tertiary Fort Union Formation provides evidence that a gigantic meteorite or asteroid struck the Earth about 65 million years ago. But it is not known if that event caused the extinction of the dinosaurs or just hastened their journey to oblivion. Fossils excavated and studied in Montana from the Hell Creek Formation have added immeasurably to our knowledge about the dinosaurs and their lives.
Slide toward Today to fade the historical sheet and see the modern map underneath.