Part 9

Tertiary Period (3-70 million years ago)

During the first part of the Tertiary Period, uplift began in earnest in Colorado and adjacent states. This uplift was part of the great Laramide Orogeny that built the Rocky Mountain chain from Alaska to New Mexico. The entire area rose above the level of the sea, and mountains were thrust up in a great series of north-south ranges that extended unbroken almost the length of the continent. Between the ranges, thick layers of gravel and sand, derived from the surrounding highlands, were deposited in intermontane basins. Occasional freshwater limestones and shales indicate the presence of lakes.

In Colorado, many details of the formation of the Rockies stand out in bold relief. The Front Range moved upward sharply, mostly as a linear block broken or faulted along both edges. Paleozoic and Mesozoic sediments along the margins of the block were steeply tipped and in some places even overturned, while in some localities Precambrian rocks were thrust out over the younger sediments.

Just east of the Front Range, especially in the area around Denver, the land remained lower and was the site of thick deposits of gravel and sand eroded from the range. The Denver Formation, the Arapahoe Conglomerate, and the Dawson Arkose are more than 2,000 feet thick in this area. These are delta and river sediments, all varying a great deal from place to place. Individual layers of sand or gravel are not continuous over extensive areas, but some, such as the Castle Rock Conglomerate, are very prominent locally.

Along the eastern margin of the Front Range west of Castle Rock and Sedalia, rocks deposited at this time are now folded steeply, indicating that the mountains continued to rise even as basin sediments were being deposited.

In southern Colorado, the Sangre de Cristo and Wet Mountains were also formed as upthrust blocks. Between them, the Huerfano Basin and adjoining Raton Basin received particularly rapid alluvial deposition. In the Raton Basin, quantities of vegetation were deposited in swamps and marshes, forming the thick coal beds which can now be seen in road cuts west of Trinidad and along the Raton Pass highway. Huerfano Basin deposits contain some of the earliest known horse remains, skeletons of a tiny four-toed horse called Hyracotherium (formerly known as Eohippus).

Other rising ranges provided material for alluvial deposition in North Park, Middle Park, South Park, and the San Luis Valley. Layers of basalt and volcanic peaks show that as the mountains rose, the crust cracked and allowed lava to rise to the surface in great quantities. Tertiary basalts are very much part of the Colorado landscape: some can be seen west of Granby, others in Table Mountains east of Golden. Near Boulder, Valmont Dike was intruded, though lava may not have reached the surface in that area. Spanish Peaks in southern Colorado, Mesa de Maya, the Rabbit Ears Range, Grand and Battlement Mesas, and many other volcanic features were formed at this time.

Most of the rich mineral deposits of Colorado are thought also to have been formed during the early part of the Tertiary Period. Solutions rich in gold, silver, zinc, lead, copper, and sulfides of iron seeped into joints and faults in the crust as the mountains were pushed upward. Ore minerals crystallized out, sometimes in veins in the ancient Precambrian igneous and metamorphic rocks, sometimes in Paleozoic sediments. These are further discussed in Chapter III.

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.

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.

During the remainder of the Tertiary Period, Colorado was the site of erosion rather than deposition. However, some stream material was deposited in the mountain valleys, and on the prairies wind-blown and stream-borne sands were spread thinly, interlayered with impure limestones deposited in ponds and lakes. In the San Luis Valley, deposition was probably more continuous than elsewhere, as the exit from the valley was blocked by volcanic flows. The deposits in this valley, sands and clays of the Santa Fe and Alamosa Formations, form a great artesian basin. The rich agricultural development of the valley is made possible by water wells tapping these formations.

Quaternary Period (3 million years ago to present)

The most significant feature of the Quaternary Period in Colorado, as elsewhere in the northern hemisphere, is the evidence of glaciation. During the first part of the Quaternary Period, known as the Pleistocene Epoch, great continental glaciers covered most of Canada and much of northern United States. The ice sheets did not extend southward as far as Colorado, but large valley glaciers developed in many of the mountain ranges of the state and left their traces in many mountain valleys.

The conditions leading to Pleistocene glaciation are not fully understood. Climatic changes may have been initiated by a decrease in solar radiation, changing patterns of ocean currents, reduction of solar heating by volcanic dust, or an increase in general elevation of the land. As the climate became cooler and moister, snowfall increased in the north and at high altitudes. In areas where winter snowfall exceeded summer melting, glaciers developed.

In Colorado, glaciers formed along the crests of the Front Range, the Sawatch Range, the Elk Mountains and West Elk Mountains, the Sangre de Cristo and Mosquito Ranges, the San Juan Mountains, and the Park and Gore Ranges. Glaciation in Colorado was selective: in many places elevation was sufficient for glaciation, but snowfall apparently was not great enough. Where they did occur, the glaciers extended down to elevations of about 8,000 feet. There, temperatures became mild enough to melt the ice.

The mountain glaciers have left many tell-tale signs of their presence. Valleys above 8,000 feet are U-shaped, their upper ends bounded by horseshoe-shaped, steep-walled cirques. In the lower portions of the valleys, at elevations just above 8,000 feet, lie long lines of glacial debris known as moraines: terminal moraines forming crescents across the valleys to show where melting glaciers dropped their rocky loads; lateral moraines along the sides of valleys; medial moraines where glaciers from two valleys met. Terminal moraines, often forming effective barriers across the present streams, may act as dams, creating lakes such as Grand Lake in Rocky Mountain National Park.

There were at least three distinct glacial episodes in Colorado. This is known because careful studies of glacial debris in moraines reveal three different degrees of rock weathering. All three stages can be seen in or near Rocky Mountain National Park. The oldest is represented by a moraine about three miles west of Estes Park, where the Big Thompson River traverses a wide U-shaped valley before entering its narrow, unglaciated canyon. The next oldest is represented in terminal moraines further up the valley, at Aspenglen campground. The youngest is shown in a prominent terminal moraine about one mile west of the park entrance in Horseshoe Park.

A large lateral moraine separates Hidden Valley from the south side of Horseshoe Park, and an almost equally large lateral moraine is present on the north side of this valley. At Moraine Park, both sides of the valley are edged with lateral moraines also.

Studies in Rocky Mountain National Park have revealed many other details of glaciation in this area. These are described in Park Service brochures and guidebooks, in the museum at Park headquarters, and in informative roadside signs.

Several small glaciers are still present in the Colorado mountains, all in sheltered cirques above 11,000 feet. These may be remnants of the former larger glaciers, or new glaciers formed after a long warming episode. A hike to one of these glaciers is a rewarding experience for anyone interested in geology. Some of the more accessible are St. Mary’s Glacier west of Denver, Arapaho Glacier west of Boulder (the Boulder Chamber of Commerce sponsors a festive hike to Arapaho Glacier every August), and Tyndall Glacier in Rocky Mountain National Park.

The Ice Age brought drastic changes also to the landscape below 8,000 feet elevation. Heavily loaded with glacial debris, mountain streams disgorged coarse sands and gravels along the mountain front and in the intermontane basins. As the glaciers melted after each period of expansion, the swollen streams cut deeply into their former deposits and into much older rocks as well. Royal Gorge, the Black Canyon of the Gunnison, and many of the deep, colorful canyons of the Plateau Province were cut or at least deepened by these waters. The canyons along the east face of the mountains—Big Thompson, Boulder, Clear Creek, and others—were also deepened and sharpened by the rushing ice-fed torrents.

On the prairies, rivers dumping their loads of sand covered the older rocks. Sand dunes developed along the river channels. Bones and huge tusks of hairy mammoths were sometimes buried in these soft deposits; now they are occasionally revealed as the dune and river sands are washed or blown away by continuing erosion.

About 20,000 years ago, man arrived in Colorado. Soon after this, the water supply of the valleys diminished greatly, and erosion slowed down correspondingly. The climate gradually became semiarid to arid. Many features of the natural scene were much as they must have been a century ago, without the highways, dams, and television aerials of today. Buffalo and many smaller types of game roamed the plains and foothills; deer, elk, and bighorn sheep were plentiful in the mountains. Nomadic tribes camped and hunted in both mountain and prairie. In the western part of the state, homes could be built in the shelter of great caves, as at Mesa Verde, and game could be supplemented with corn and squash planted on plateau surfaces.