Paleontology and geology
Early Cretaceous rocks in Utah were formed in terrestrial environments, and recently dinosaur and mammal fossils have been found in significant numbers. During the Middle Cretaceous, shallow marine seas again inundated Utah, leaving behind sands and thick accumulations of muds. These marine deposits contain the fossils of large clams, fish, marine reptiles such as plesiosaurs, and the occasional dinosaur carcass that was washed out to sea. This was to be the final invasion of marine waters onto North America. Late Cretaceous rocks formed in a wide range of terrestrial environments. The fossil record from this time includes lizards, mammals, eggs, and dinosaurs, including Tyrannosaurus rex.