The Eocene Green River Formation includes great thicknesses of oil shale, an untapped petroleum reserve containing perhaps three trillion barrels of oil. The richest part of the oil shale is a dark brown layer called Mahogany Ledge, visible here on cliffs just west of Rifle. If placed in a campfire, fragments of this shale release enough oil to burn with a yellow, smoky flame. (Jack Rathbone photo)
Further to the north and west, the Uinta Mountains rose. They are a fault-block range, but they lie at right angles to the general north-south trend of the Rocky Mountains. South of them the Uinta Basin, one of the largest of the intermontane basins, received shaly deposits in a great lake which existed here for probably several million years. The lake extended over some 100,000 square miles, and during its existence great quantities of tiny organisms lived in its waters. Oily material from these organisms was deposited in the mud of the lake sediments, particularly in the eastern end of the basin, there to remain trapped in a great oil-shale deposit. [Fossil] fish, crayfish, algae, and many forms of insect and plant life have been found as fossils in these lake shales.
West of Pikes Peak, another lake formed, dammed by a [lava] flow from a nearby volcanic field. Fine volcanic ash falling into this lake preserved the trunks and leaves of many plants as well as abundant insects, fish, and occasional mammal bones. These are now protected and exhibited in Florissant [Fossil] Beds National Monument. The fossil plants, among them redwoods, poplar, hackberry, and pine, suggest a climate warmer than the present one, and have been taken to indicate that regional uplift to the present altitude had not yet occurred.
Another rich deposit of [fossil] insects and plants occurs near Creede. Other lake deposits in South Park contain ash layers with fossil algae and snails.
Large petrified trunks of redwoods and other trees can be seen at Florissant [Fossil] Beds National Monument, west of Colorado Springs. (John Chronic photo)
In southwestern Colorado, extensive Tertiary [lava] flows, ash falls, and river deposits form the eastern part of the San Juan Mountains, the largest volcanic area in the state. Mineral collectors are attracted to this region by the many excellent localities for agate and other siliceous stones.
Still another center of Tertiary volcanism was located in what is now Rocky Mountain National Park. Specimen Mountain, northwest of Trail Ridge, was an active volcano about 30 million years ago, shedding ash and [lava] over much of northern Colorado. The [rhyolite] which now caps the hill west of Iceberg Lake, on Trail Ridge Road, was derived from this volcano, but is now separated from it by the deep glaciated valley of the Cache la Poudre River and Milner Pass.
Volcanic ash at times drifted far eastward and blanketed the surface of the plains, burying specimens of many animals and plants. The White River Formation, extending from northeast Colorado northward into South Dakota, is formed of such drifting ash. Many now-extinct mammals have been excavated from this formation.
Sometime after the mid-Tertiary episode of violent volcanic activity, Colorado was uplifted to its present altitude. This was a general uplift, raising the plains and [plateau] areas as well as the mountains. The uplift was not an abrupt process, but continued for perhaps ten million years. It raised the entire state 3,000 to 5,000 feet above its previous level.