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Geological Survey Bulletin 707
Guidebook of the Western United States: Part E. The Denver & Rio Grande Western Route |
PRINCIPAL DIVISIONS OF GEOLOGIC TIME a
| Era. | Period. | Epoch. | Characteristic life. | Duration according to various estimates. |
| Millions of years. | ||||
| Cenozoic (recent life). | Quaternary. | Recent. Pleistocene (Great Ice Age). |
"Age of man." Animals and plants of modern types. | 1 to 5. |
| Tertiary. | Pliocene. Miocene. Oligocene. Eocene. |
"Age of mammals." Possible first appearance of man. Rise and development of highest orders of plants. | ||
| Mesozoic (intermediate life). | Cretaceous. | (b) | "Age of reptiles." Rise and culmination of huge land reptiles (dinosaurs), of shellfish with complexly partitioned coiled shells (ammonites), and of great flying reptiles. First appearance (in Jurassic) of birds and mammals; of cycads, an order of palmlike plants (in Triassic), and of angiospermous plants, among which are palms and hardwood trees (in Cretaceous). | 4 to 10. |
| Jurassic. | (b) | |||
| Triassic. | (b) | |||
| Paleozoic (old life). |
Carboniferous. | Permian. | "Age of amphibians." Dominance of club mosses (lycopods) and plants of horsetail and fern types. Primitive flowering plants and earliest cone-bearing trees. Beginnings of backboned land animals (land vertebrates). Insects. Animals with nautilus-like coiled shells (ammonites) and sharks abundant. | 17 to 25. |
| Pennsylvanian. | ||||
| Mississippian. | ||||
| Devonian. | (b) | "Age of fishes." Shellfish (mollusks) also abundant. Rise of amphibians and land plants. | ||
| Silurian. | (b) | Shell-forming sea animals dominant, especially those related to the nautilus (cephalopods). Rise and culmination of the marine animals sometimes known as sea lilies (crinoids) and of giant scorpion-like crustaceans (eurypterids). Rise of fishes and of reef-building corals. | ||
| Ordovician. | (b) | Shell-forming sea animals especially cephalopods and mollusk-like brachiopods, abundant. Culmination of the buglike marine crustaceans known as trilobites. First trace of insect life. | ||
| Cambrian. | (b) | Trilobites and brachiopods most characteristic animals. Seaweeds (algae) abundant. No trace of land animals found. | ||
| Proterozoic (primordial life). |
Algonkian. | (b) | First life that has left distinct record. Crustaceans, brachiopods, and seaweeds. | |
| Archean. | Crystalline rocks. |
No fossils found. | 50+. | |
aThe geologic record consists of sedimentary
beds—beds deposited in water. Over large areas long periods of
uplift and erosion intervened between periods of deposition. Every such
interruption in deposition in any area produces there what geologists
term an unconformity. Many of the time divisions shown above are
separated by such unconformities—that is, the dividing lines in the
table represent local or widespread uplifts or depressions of the
earth's surface.
bEpoch names omitted; in less common use than
those given.
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Last Updated: 16-Feb-2007