Paleontology and geology

The Uinta Mountains and several other large areas of uplift formed during the Early Tertiary. Periodically, extensive fresh water lakes covered central Utah. Fossils representing a diverse fauna of invertebrates, fish, reptiles, and birds that lived in and around this lake system are found in the Green River Formation in eastern Utah. By the Late Tertiary, the lakes had nearly vanished, and terrestrial sediments from this time contain fossils of large mammals, most notably the bizarre uintatheres. Volcanic activity increased, resulting in thousands of square miles of lava flows, tuffs, and intrusions. Uplift of the Colorado Plateau and the formation of the Basin and Range topography began in the Late Tertiary.