About 65 million years ago, as the inland sea receded and the Rocky Mountains rose, great rivers meandered through a warm, humid coastal plain and deposited the sediments that later became the Hell Creek Formation. Its tan sandstones, siltstones, and mudstones are common across eastern Montana and are especially visible in road cuts, the Charles M. Russell Wildlife Refuge, and Makoshika State Park. The formation frequently yields dinosaur fossils, including Triceratops, Edmontosaurus, Pachycephaosaurus, Ankylosaurus, and Tyrannosaurus rex. The boundary between the late Cretaceous Hell Creek Formation and the younger Tertiary Fort Union Formation provides evidence that a gigantic meteorite or asteroid struck the Earth about 65 million years ago, though it is not known whether that event caused the extinction of the dinosaurs or hastened it. Fossils excavated and studied in Montana from the Hell Creek Formation have greatly expanded knowledge of dinosaurs and their lives.