Its difficult to believe now, but 65 million years ago the middle of our continent was a shallow sea. This area, when not underwater, was part of a hot, humid subtropical coastline of marshes, rivers, and river deltas bearing dense vegetation near the water courses, and grassy plains farther to the west. Fossils tell us that turtles, crocodiles, lizards, toads, fishes, small primitive mammals, and dinosaurs lived on this coastal plain. The dinosaurs included species like Tyrannosaurus, Triceratops, Edmontosaurus, Ankylosaurus, Ornithomimus, Thescelosaurus, and Pachycephalosaurus. Many of the most complete specimens of these species on display in the world were gathered here in Garfield and McCone Counties. The first Tyrannosaurus rex skeleton came out of these hills in 1902. In fact, half of the 30 some tyrannosaurs found in the world are from this area. The rock unit that these fossils come from is called the Hell Creek Formation and extends over much of eastern Montana and the Dakotas. Displays of the dinosaurs found in this area can be seen in the Garfield County Museum, the Fort Peck Interpretive Center at Fort Peck Dam, and the Museum of the Rockies in Bozeman.
Slide toward Today to fade the historical sheet and see the modern map underneath.