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.
[Illustration: 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_).
[Illustration: 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.
[Illustration: 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)]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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)]
[Illustration: Mastodon]
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.
[Illustration: 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: