This was once the bottom of an extensive series of Eocene lakes. The fossils record which species called this area home and what the climate was like they lived in some 50 million years ago. It was warmer and wetter than that area today. Think temperate to sub-tropical, with temperatures ranging from 15 to 20 degrees Celsius. The Green River Formation has layers and layers of fine-grain sedimentary rock particularly abundant in beautifully preserved fossil fish, eleven species of reptiles including a 13.5ft crocodile, an armadillo-like mammal, Brachianodon westorum, bats, birds and other freshwater aquatic goodies.