Hell Creek

The Hell Creek Formation in Montana and the Dakotas holds the youngest dinosaur beds in the world. It is also the one place where you can walk up through the rock and see exactly where the dinosaurs stop.

What is there

The beds were laid down by rivers and swamps on a coastal plain along the western edge of the inland sea that split North America in two. The climate was subtropical. The fauna is the best known of the Late Cretaceous: Tyrannosaurus rex, Triceratops, Edmontosaurus, Ankylosaurus — and beneath them mammals the size of rats, crocodiles, turtles and the first flowering plants.

Timeline

68 million years ago

the uppermost Hell Creek beds begin to accumulate

66.04 million

the asteroid strikes the Yucatán

that same day

tsunamis, firestorms and glowing debris across the continent

weeks after

sunlight blocked by dust; photosynthesis stops

66.0 million

the clay layer with iridium settles over the last dinosaur fossil

66.0 million

the fern spike: pollen samples show ferns taking over entirely

65.5 million

the mammals begin to grow

A boundary you can touch

Directly above the last dinosaur bones lies a thin layer of clay, seldom more than a couple of centimetres. It contains iridium — an element rare in the Earth's crust and common in asteroids. Luis and Walter Alvarez proposed in 1980 that the layer was dust from an impact, and the proposal was rejected for over a decade. The crater was found in 1991, beneath the Yucatán.

Just above the clay comes the fern spike: pollen from flowering plants almost vanishes, and fern spores suddenly make up over ninety per cent. That is the signature of an ecosystem stripped bare and starting again, and the same signature follows forest fires and volcanic eruptions today.

The argument that is not settled

Were the dinosaurs in decline before the impact? Some studies find fewer species in the last couple of million years; others hold that this only reflects how much rock survives. Hell Creek is the main arena for the question precisely because it has the most finds close to the boundary.

Who owns a fossil

The formation lies largely on private land, and in the United States fossils belong to the landowner. That has made Hell Creek the site of a continuing dispute: specimens sell at auction for tens of millions of dollars, and a fossil in private hands cannot be checked by other researchers. "Stan", a T. rex found here, sold for 31.8 million dollars in 2020.