A CENOZOIC 50 M.Y. MESOZOIC 200 M.Y. PALEOZOIC 500 M.Y. PRECAMBRIAN 4.5 B.Y. B Quaternary—Early man 2 Tertiary 65 Cretaceous 140 Jurassic 190 Triassic 220 Permian 280 Pennsylvanian 310 Mississippian 340 Devonian 390 Silurian 440 Ordovician 500 Cambrian 575 First abundant fossils Precambrian 2700 Oldest rocks in Yellowstone Beginning of the earth 4,500 M.Y. C PRINCIPAL EVENTS Holocene Glaciation, canyon cutting, thermal activity, eruption of Plateau Rhyolite Pleistocene Eruption of Yellowstone Tuff and associated lava flow; collapse of Yellowstone caldera; normal faulting 2 Pliocene Regional uplift; large-scale normal faulting and uplift of mountain ranges; deep erosion 12 Miocene Moderate erosion; possibly some volcanic activity 26 Oligocene ″ 37 Eocene Eruption and deposition of Absaroka volcanic rocks 54 Paleocene Laramide Orogeny—folding, faulting, uplift and erosion of mountain ranges; deposition of sand and gravel in subsiding basins 65 Cretaceous Deposition of sediments in oceans and along beaches and river flood plains ... Cambrian 570 M.Y. [Illustration: THE GEOLOGIC TIME SCALE—the “calendar” used by geologists in interpreting earth history. Column A, graduated in billions of years (B.Y.) and subdivided into the four major geologic eras (Precambrian, for example), represents the time elapsed since the beginning of the earth, which is believed to have been about 4.5 billion years ago. Column B is an expansion of part of the time scale in millions of years (M.Y.), to show the subdivisions (periods—Cambrian, for example) of the Paleozoic, Mesozoic, and Cenozoic Eras; column C is a further expansion to show particularly the subdivisions (epochs—Paleocene, for example) of the Tertiary and Quaternary Periods. The principal events in the geologic history of Yellowstone National Park are listed to the right of column C, opposite the time intervals in which they occurred. The ages, in years, are based on radiometric dating. Many rocks contain radioactive elements which begin to decay at a very slow but measurable rate as soon as the parent rock is formed. The most common radioactive elements are uranium, rubidium, and potassium, and their decay (“daughter”) products are lead, strontium, and argon, respectively. By measuring both the amount of a given daughter product and the amount of the original radioactive element still remaining in the parent rock, and then relating these