The Hell Creek Formation

The panel's hedge has since been tested in Montana ash. Volcanic ash beds laid down through the Hell Creek and Fort Union rocks of northeastern Montana can be dated by argon, and in 2013 a Berkeley Geochronology Center team used them to tie the impact and the extinctions together to within 32,000 years, putting the boundary at about 66.04 million years rather than 65. Yet the same team wrote that the Chicxulub impact struck ecosystems already close to breaking, so the panel's second possibility, a journey hastened, still stands.

What the plaque says

The Hell Creek Formation 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.

Where it stands

More markers nearby

• Big Sky Country — steps away
• The Powder River Country — steps away
• Southeastern Montana — steps away
• Southeastern Montana — 1.7 mi