CENOZOIC ERA
It is characteristic of earth history that the younger the rocks are, the more we know about them. This is because younger rocks lie near the surface, have not been disturbed as much by mountain building processes as have older rocks, and have not been affected as strongly by repeated erosion. Many of the events of the Cenozoic Era are documented in detail in the geology of Colorado, and these events have intimately influenced the scenery as we see it today.
The Cenozoic is the Age of Mammals. How it happened that mammals triumphed over reptiles is one of the mysteries of geology. Some scientists think that climatic changes—dropping temperatures and increases in rainfall—swung the balance in favor of the warm-blooded mammals. Others believe that cosmic ray bombardment during some unusual astronomical event may have destroyed many surface-living dinosaurs, while small burrowing mammals (as well as many small reptiles) were able to survive. Still others maintain that the superior intelligence and regulated body temperatures of mammals enabled them to win out in the battle for survival without the aid of climatic or cosmic change.
The names Tertiary and Quaternary, used for the two Cenozoic Periods, are holdovers from early studies in geology in which rocks were divided into Primary (very hard, crystalline rocks such as igneous and metamorphic rocks), Secondary (well consolidated layered rocks), Tertiary (layered rocks which are not fully cemented but which are nevertheless fairly well consolidated), and Quaternary (sediments in which the grains have not become cemented together).
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.
On Wolford Mountain, just north of Kremmling, Precambrian [granite] lies on top of Cretaceous shale. The older rocks were thrust up and over younger rocks during the Laramide Orogeny. The position of the [fault] shows clearly because trees prefer the granite soil above the fault to the shale below. (Jack Rathbone photo)
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).
Bones of Hyracotherium, the “dawn horse,” have been found northwest of Walsenburg in Early Tertiary sediments of the Huerfano Basin. (C. R. Knight painting, courtesy American Museum of Natural History)
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.
The town of Golden nestles between the Front Range and South Table Mountain. Tertiary [basalt] capping South Table Mountain covers beds of the Denver Formation. It thins to the right, or south, indicating that its source was probably to the north or northwest. Buildings in the right foreground are the Colorado School of Mines. (Jack Rathbone photo)
A series of almost vertical [dikes] radiate from West Spanish Peak. Surrounding sediments are Tertiary. Weathering and erosion along sets of [joints] in the largest dike have shaped it into the “Devil’s Staircase.” (Jack Rathbone photo)
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].
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.
Pawnee Buttes, about 40 miles north of Fort Morgan, rise like castles from the eastern Prairie Province. Remnants of Oligocene and Miocene sedimentary rock that once covered much of northeastern Colorado and adjacent states, they contain jaws, teeth, and other bones of primitive mammals. (Department of Highways photo)
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.
Remains of many now-extinct mammals have been found in Tertiary [sedimentary rocks] of northeastern Colorado, in the general area of Pawnee Buttes. Those illustrated are Oreodon from Oligocene strata and a “giraffe-camel” (Oxydactylus) from Miocene rocks.
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.
Mills and Jewel Lakes, in Rocky Mountain National Park, occupy small glacier-gouged basins in [Glacier] Gorge. The flat-topped peak at the upper left is Longs Peak, elevation 14,256 feet; Pagoda Mountain is in the center of the skyline. [Bedrock] in this area is Precambrian [granite], [gneiss], and [schist] at the Front Range “core.” (Jack Rathbone photo)
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.
A line of hikers approaches Arapaho [Glacier], west of Boulder. Movement of the glacier is evidenced by the crevasses apparent just below the snowfield in the dirty gray glacial ice. (H. H. Heuston photo)
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.
Several features of Colorado scenery changed with increasing aridity. The [glaciers] of course were gone or nearly gone. Streams were no longer the violent torrents they had been. Many mountain lakes, filled with sediment and vegetation, became instead mountain meadows. And the once fertile intermontane valleys became deserts.
During the last Ice Age, elephant-like mastodons roamed Colorado. As present-day erosion removes sediments, bones, teeth, and tusks are frequently exposed, especially in the Prairie Province. (C. R. Knight painting, courtesy American Museum of Natural History)
On the eastern side of the San Luis Valley, the Great Sand Dunes developed at this time. These dunes nestle against the Sangre de Cristo Range, where strong southwesterly winds blowing across the wide valley tend to funnel toward Mosca and Music Passes. These winds lift loads of sand from the lightly vegetated valley floor, and drop it as they rise over the mountains. Where the sand is dropped, the dunes have formed. They rise to about 700 feet above the valley floor, and cover about forty square miles. The low rainfall of the area, seven to eight inches per year, keeps vegetation from creeping over the dunes and makes them a most distinctive feature of Colorado, a lesson in geology in the making.
* * * * * * * *
Geologic processes in Colorado now seem to be much reduced from what they were a few thousand years ago. Reduction in rainfall has led to reduced erosion. Mountain-building, having reached a climax in Tertiary time, has declined markedly. However, we find evidence that volcanism has occurred within the last few thousand years and faulting within the last few hundred, and Colorado streams rise after sudden mountain storms to approximate the violent torrents of glacial times. Colorado’s scenery, fashioned during some three billion years of earth history, is ever changing.
The Great Sand Dunes of Colorado were formed during Pleistocene and Recent time by deposition of quartz sand lifted from unconsolidated alluvial deposits in the San Luis Valley. The highest of the dunes rises 700 feet above the adjacent valley floor. (John Chronic photo)
III
Geology and Man in Colorado
Colorado’s first permanent settlers arrived in 1858, when gold was discovered in river sands near what is now the city of Denver. The ensuing gold rush, coming ten years after the rush to California, rivalled it in fury and brought sudden wealth to lucky miners and the adventurous merchants who grubstaked them. Several hundred mining towns or “camps” sprang into existence almost overnight, their sites determined by the geology of the mountain areas. The cities of Denver, Boulder, and Golden were established as milling and shipping centers for the products of the mines. In 1876 the now-wealthy area, previously part of Kansas Territory, became the State of Colorado.
For more than a hundred years Colorado’s minerals—products of her long and diverse geologic history—have influenced her development in many ways. The state’s early wealth, stemming from bonanzas in gold and silver, is evidenced by palatial homes, hotels, and public buildings constructed during the first few decades of mining activity. Some of these are still standing—the opera houses at Central City and Aspen, Central City’s famous Teller House, and the Grand Imperial Hotel at Silverton are examples.
Many of the stories and legends of Colorado’s gold camps are recounted in Stampede to Timberline, by Muriel Sibell Wolle, delightfully illustrated with sketches of old mining towns as they appear today. Mining in Colorado, published by the U. S. Geological Survey, also makes fascinating reading, as it contains many historical anecdotes and eyewitness accounts of gold-rush days.
Development of the metal-mining areas in Colorado followed a definite sequence. [Placer] gold was usually discovered first. Recovery of placer gold was followed by mining of gold from [veins] or “[lodes].” Although at first only [native gold] was mined, gold-bearing compounds such as telluride were soon recognized as an additional source, especially at Gold Hill, Cripple Creek, and of course the camp that came to be known as Telluride. As gold sources were depleted, silver, first produced as a byproduct, became of prime interest. Lead and zinc were in turn byproducts of silver mining. Other metals, notably copper, vanadium, tungsten, and iron, were produced later. Molybdenum is the Johnny-come-lately of the state’s mining industry, but is now the chief metal produced. A uranium boom in the 1950s brought a short rush to western Colorado and new vigor to the economy.
Oil was discovered near Canon City in 1862. The nearby Florence field and a small, shallow field near Boulder preceded much greater discoveries in the Denver Basin, the Uinta Basin, and southwest Colorado. Oil reservoirs, confined to areas of sedimentary rock, are found primarily in the Prairie and [Plateau] Provinces of the state, and recovery of the oil has done much to distribute population to these areas.
Coal is also restricted to sedimentary rock areas. Coal production in Colorado has waxed and waned with the years, but has provided fuel for export, for the railroads, for the manufacture of electric power, and for many of the state’s industries.
A good picture of present mineral production in Colorado can be obtained from the following summary for 1971, prepared by the Colorado Bureau of Mines:
| Product | Value |
|---|---|
| Molybdenum | $105,389,456 |
| Petroleum | 90,494,459 |
| Sand and gravel | 32,842,503 |
| Coal | 30,251,443 |
| Natural gas | 18,695,225 |
| Uranium | 18,048,692 |
| Vanadium | 15,863,554 |
| Cement | 13,377,520 |
| Zinc | 13,310,787 |
| Lead | 6,582,025 |
| Tungsten | 6,360,020 |
| Limestone and dolomite | 5,397,570 |
| Silver | 4,198,054 |
| Fluorspar | 3,887,210 |
| Copper | 3,875,976 |
| Stone | 1,961,279 |
| Gold | 1,832,791 |
| Clay | 962,986 |
| Iron | 880,047 |
| Pumice | 309,370 |
| Tin | 278,862 |
| Gypsum | 253,856 |
| [Pyrites] | 142,640 |
| All others | 1,091,927 |
| Total | $376,288,252 |
Colorado is now the nation’s leading producer of molybdenum, tin, and vanadium, and second in output of tungsten. In oil production it ranked twelfth among the states in 1968, but ninth in reserves, with 420,000,000 barrels of proven reserves on 1 January 1969. An as yet untapped source of oil lies in the oil shales of western Colorado.
As part of the natural environment, water plays a major role in man’s activities. Water problems in Colorado revolve mainly around the best use of runoff in a state whose major catchment basins are across the continental divide from her largest population centers and most fertile farm land. Groundwater, closely related to surface water distribution and movement, is a geological problem, and in Colorado as in other states many government and private geologists serve farm and industrial communities in the search for usable supplies.
CAUTION: Old mines are dangerous! They may contain water or deadly gases, or be on the verge of collapse. Keep away from abandoned prospect pits and mine shafts. WARN AND WATCH YOUR CHILDREN.