About the Morrison Formation:
The Morrison Formation is a rock unit from the Late Jurassic (155 million years ago-148 million years ago). It extends throughout the Western United States and often contains fossils of dinosaurs such as Camarasaurus, Diplodocus, Apatosaurus, Stegosaurus, Allosaurus, and others. Although the Morrison was deposited in a wide variety of environments, it is the river channels that are most likely to contain dinosaur bones. Quiet water lake and pond deposits are best for preservation of the small delicate skeletons of salamanders, frogs, lizards, and others. In order for a fossil to form, the body must be buried quickly before weather conditions and scavengers have a chance to scatter or otherwise destroy the bone. In the park you will also see "Rainbow Beds" of the Morrison, which are mostly crumbly grey and maroon mudstones. They are pretty to look at, but do not often contain dinosaur fossils.