Era Period Epoch Years Major before geological present events time Cenozoic Quaternary Recent 10,000 Cascade Range and Sierra Nevada uplifted Pleistocene 1,000,000 Tertiary Pliocene 12,000,000 Miocene 26,000,000 Oligocene 38,000,000 Eocene 54,000,000 Rocky Mountains formed Paleocene 65,000,000 Mesozoic Cretaceous 136,000,000 Jurassic 195,000,000 Triassic 250,000,000 Appalachian folding Paleozoic Permian 280,000,000 Pennsylvanian 320,000,000 Mississippian 345,000,000 Devonian 395,000,000 Silurian 440,000,000 Ordovician 500,000,000 Cambrian 570,000,000 Precambrian [Illustration: Summer thunderstorm in the Rincon Mountain Section. The Catalina Mountains rise in the background.] climate: the vital factor Climate is the chief arbiter of life on earth. Each plant and animal, including man, has tolerance limits for heat and water below or above which it dies. The Sonoran desert, with its low and uncertain rainfall and high summer temperatures, thus presents one of earth’s most taxing environments. For most desert plants and animals, the main problem presented by this climatic combination is a scarcity of water. Why, we might ask, is this part of the globe so dry?