The Morrison Formation is a rock unit that stretches across western North America, covering areas of the United States from Montana to Arizona. It formed during the Late Jurassic Period (162–145 million years ago), in the middle of the age of the dinosaurs. During the Jurassic Period, dinosaurs evolved from a group of small reptiles into the diverse, successful, and sometimes enormous animals that would rule the Earth until their extinction at the end of the Cretaceous Period.
The Morrison Formation represents one of the best-studied extinct ecosystems in the fossil record. Thousands of dinosaur specimens have been excavated from the Morrison Formation, including some famous dinosaurs like Stegosaurus, Diplodocus, Allosaurus, and Brachiosaurus.
