Paleontology and geology
The Precambrian: Most of the Precambrian in Colorado consists of igneous and metamorphic rocks as old as 2.7 billion years. The only Precambrian sedimentary rocks in Colorado are located in the extreme northwest corner of the state. No Precambrian fossils have been reported.
The Paleozoic: The Paleozoic was a time of fluctuating sea levels. Colorado lay near the equator during much of the Early and Middle Paleozoic (Cambrian through Devonian) and a variety of marine fossils indicates that a shallow, tropical sea covered the state during this time. At some point in the Middle Paleozoic (Silurian or early Devonian), the sea retreated for a time and the state was subjected to a period of erosion before marine waters returned in the Carboniferous. Mountain-building events in the Late Carboniferous raised a series of ranges in western Colorado, and both terrestrial and shallow marine environments existed in low-lying areas between these ranges. Fossils of sharks, trilobites, brachiopods, and crinoids can be found in the marine sediments, while fossils of some of the earliest conifers, tree lycopods, and huge horsetail Calamites occur in terrestrial deposits. The seas retreated in the Permian, once again exposing much of the state to erosion, and resulting in extensive dunes, where fossilized tracks of insects and unknown reptiles can now be found.
The Mesozoic: Much of Colorado lay above sea level during the Triassic and Jurassic. Extensive floodplains and coastal lowlands developed at the edge of a shallow seaway. Dinosaurs, other reptiles, amphibians, and conifers flourished in these environments. During the Cretaceous, the shallow seaway expanded across the state. Dinosaur footprints and fossil flowering plants are common in the forested and swampy coastal areas that formed along its margins. As the seas deepened, thick deposits of muddy and limy sediments accumulated on the sea floor. These sediments are rich in fossils, including single-celled algae, molluscs, fish, and the bones of large marine reptiles. Just before the end of the Cretaceous, the seas retreated, the Rocky Mountains began to rise, and dinosaurs roamed a landscape covered by broadleaf trees and palms.
The Cenozoic: During the Early Cenozoic (Tertiary), the Rocky Mountains continued to rise and tropical rainforests grew along its slopes. Mammals, crocodiles, and turtles roamed the area. Huge lakes formed in low-lying areas between the rising mountains. Oil shales formed in these lakes are the source of beautifully preserved fossils of fish, leaves, and insects. Volcanic eruptions spread debris and ash into the Denver Basin. Later in the Tertiary, Sequoia forests flourished in the cooling climate, and grasslands spread across much of the state. Mountain glaciers carved the spectacular peaks and valleys of the Rockies during the Late Cenozoic (Quaternary). True prairies appeared, and mammoths, camels, bison, horses, and flourished.