| Eras |
|---|
| | Periods and their Duration in Millions of Years | | Important Physical Events | Important Organic Events |
|---|
| Cenozoic |
| | Quaternary |
| | | Recent | | Youthful land forms having high relief formed. | Dominance of man. |
| | | Pleistocene Epoch | 2 M.Y. | Period of glaciation; four great ice advances. | Heidelberg, Neanderthal, and Crô-Magnon man; extinction of large mammals. |
| | Tertiary |
| | | Pliocene Epoch | 10 M.Y. | Continuing world-wide land elevation. | Intermigration of North and South American mammals. Transformation of ape to man. |
| | | Miocene Epoch | 18 M.Y. | Cordilleras, Alps, Himalayas formed. Widespread vulcanism-basalt flows in northwestern United States. | Culmination of modern types of mammals. Apes appear in Old World. |
| | | Oligocene Epoch | 10 M.Y. | Land dominant; seas marginal. | Carnivores and ungulates develop into importance. |
| | | Eocene Epoch | 20 M.Y. | Extensive sedimentation; seas marginal. | Dawn of the dominance of mammals. Reptiles subordinate. |
| Cretaceous | 65 M.Y. | Widespread epicontinental seas. Laramide revolution at close of period—Rocky Mountains formed. | Climax and culmination of reptiles, especially dinosaurs; first flowering plants and grasses. |
|
| Mesozoic |
| | Jurassic | 38 M.Y. | Continent emergent; shallow seas on western North America. | Rise of birds and flying reptiles, first modern trees. |
| | Triassic | 35 M.Y. | Continent emergent; seas marginal. | Rise of dinosaurs, cycads, and ammonites. |
| Paleozoic |
| | Permian | 35 M.Y. | World-wide continental uplift and mountain building. Widespread glaciation. | Extinction of most Paleozoic fauna and flora. First modern insects. |
| | Pennsylvanian | 48 M.Y. | Continent alternately rising and sinking. | Great coal-forming forests, of ferns and seed-ferns. |
| | Mississippian | 35 M.Y. | Low lands and widespread submergence. | Culmination of crinoids, numerous sharks. |
| | Devonian | 40 M.Y. | Widespread submergence, local vulcanism. | First known land animals, first forests. |
| | Silurian | 28 M.Y. | Widespread submergence, local deserts. | First lung fishes and scorpions, abundant corals. |
| | Ordovician | 65 M.Y. | 60% of North America below sea. | Climax of invertebrate dominance, first vertebrate. |
| | Cambrian | 105 M.Y. | Widespread submergence. | First abundant invertebrate fauna, trilobites dominant. |
| Proterozoic | 700 ± M.Y. | Long periods of granite intrusion, sedimentation, and mountain building. | Bacteria and seaweeds present. Most invertebrates probably present, but remains are lacking. |
| Archeozoic | 800 ± M.Y. | World-wide intrusive igneous activity; some sediments. | Blue-green algae present, primitive one-celled plants and animals probably present. |