Gold was discovered in the bed of the South Platte River in 1858.
Prospectors flocked to Colorado as they had flocked only a few years
before to California. They worked the sands and gravels of Cherry Creek,
Clear Creek, Boulder Creek, and California Gulch. Exhausting the placer
sands of the stream bottoms, they moved higher to mine gold-bearing
veins at Central City and Blackhawk. Mining camps sprang into existence
overnight, each heralding some new “strike,” each populated by a new
rush of fortune seekers. As lower areas were mined out, prospectors
moved yet higher—to Breckenridge, Gold Hill, and Empire, Aspen,
Leadville, and Cripple Creek. Silver was found as well as gold, then
iron, and later tungsten and molybdenum. The metallic ring of mining
tools echoed from Colorado’s peaks. Fortunes were made here. Legends
were born.
Prospectors and miners were not, however, the first people interested in
the rocks of Colorado. Earlier, bands of nomadic Cheyenne and Arapaho
Indians had searched Colorado’s hills for flint for arrowheads and
brightly colored clays for warpaint. Cliff-dwelling Pueblo Indians in
southwestern Colorado sought clay for their pottery and fossil seashells
for the magic of their medicine men. And from farther to the southwest,
Navajo tribesmen came to Colorado for turquoise.
From clay to gold, much of Colorado’s wealth has come from her
mountains. But after the rush to the mines, as veins were mined out and
placers worked over, as values and prices changed, her population sought
the riches of the prairies: fertile lands for agriculture, and in the
rock layers below, black gold—vast accumulations of oil and natural gas.
The tablelands and plateaus west of the mountains yield their wealth,
too. Here are valley farms, fed often by irrigation water, and ranch
country. Here is more oil, and in some areas precious metals and
uranium.
In recent years Colorado’s prairies, peaks, and plateaus have brought
new meaning to all America: the state now provides an attractive
playground for state residents and their visitors. Campgrounds, streams,
lakes, and high trails beckon in summer; barren slopes deep in winter
snow attract the skier. More and more, those who live in Colorado and
those who visit her seek to understand these mountains and hills and
prairies, to learn of her geologic origins and her far distant past. For
tourist and resident, casual visitor, ski enthusiast, Sunday picnicker,
for all those who have met Colorado and enjoyed her, this book is
written.
[Illustration: Topographically, scenically, and geologically, Colorado
can be divided into the three provinces shown here.]
PLATEAUS
UINTA MTS.
GREEN RIVER BASIN
Yampa River
Steamboat Springs
UINTA BASIN
White River
WHITE RIVER PLATEAU
ROAN PLATEAU
Glenwood Springs
Colorado River
Grand Junction
GRAND MESA
Gunnison River
UNCOMPAHGRE PLATEAU
Dolores River
PARADOX BASIN
MESA VERDE
MOUNTAINS
NORTH PARK
RABIT EARS RANGE
PARK RANGE
MIDDLE PARK
GORE RANGE
FRONT RANGE
ELK MTS.
Aspen
SAWATCH RANGE
Leadville
MOSQUITO RANGE
Fairplay
SOUTH PARK
WEST ELK MTS.
Gunnison
Salida
WET MTS.
SANGRE DE CRISTO RANGE
SAN LUIS VALLEY
Rio Grande
Alamosa
SAN JUAN MTS.
Ouray
Silverton
Durango
MESA DE MAYA
PLAINS
Fort Collins
South Platte River
Denver
GREAT PLAINS
Colorado Springs
Arkansas River
WET MT. VALLEY
HUERFANO PARK
La Junta
Walsenburg
I
Colorado’s Three Provinces
Scenically, Colorado is divided into three provinces: the Plains or
Prairies on the east, the Rocky Mountains bisecting the state from north
to south, and the Colorado Plateaus on the west. There are a number of
local variations of course, but by and large the provinces are clearly
defined. These three divisions will form the basis for our discussion of
the geology of Colorado, for the scenic differences are almost exactly
paralleled, and usually controlled, by differences in geologic
structure.
The Plains rise gently from an elevation of about 3350 feet at the
eastern border of the state to 5000 feet where they meet the mountains
150 miles further west.
Two major rivers cross the Colorado Plains: the South Platte River,
flowing northeastward from the Denver region, and the Arkansas River,
which leaves the mountains at Canon City south of Colorado Springs and
travels eastward across the southern portion of the state. Tributaries
of these two main river systems have etched the prairie surface, so that
much of eastern Colorado has a gently rolling, hilly appearance.
The Mountains rise abruptly along a north-south line at about 105° west
longitude. They reach elevations of over 14,000 feet at Pikes Peak,
Mount Evans, Longs Peak (all visible from far out on the plains), and
fifty other peaks further west. The ranges of the Colorado Rockies form
rank upon rank of ridges and peaks, roughly north-south in trend, about
100 miles across from east to west, extending from the northern to the
southern border of the state. Here, in mountain springs and lakes, are
born the rivers of Colorado: the Platte, the Arkansas, the Yampa, the
Colorado. Crags and cliffs tower above tree-covered slopes, the rocks
always a dominant part of the landscape. The continental divide runs
through the state along the summit ridges. West of the divide, all
streams flow to the Colorado River and the Pacific; east of it, streams
flow into the Mississippi or the Rio Grande, and thence to the Gulf of
Mexico.
West of the highest ranges, the country flattens out once more into the
Plateaus, which extend across western Colorado, southern Utah, and
northern Arizona. Here, the predominant land forms are flat-topped mesas
and deep canyons. Redrock walls shimmer in the brilliance of the western
sun, offset by deep purple shadows sometimes hiding ancient cliff
dwellings. Fragrance of pine and juniper mingles with the pungency of
sage. Narrow tracks lure the explorer. Despite the canyons, water is
scarce except along major river systems, for this is the beginning of
the desert west.
The scenic and geologic division of the state into three north-south
strips is not everywhere clearly defined. In southwestern Colorado, the
San Juan Mountains and the complicated uplifts surrounding Ouray and
Silverton are out of key with either mountain or plateau. They are best
considered part of the Mountain Province, however, although they extend
it far to the west. Other exceptions to these divisions occur also. The
Mountain Province is interrupted by four broad high-altitude valleys:
North Park, Middle Park, South Park, and the San Luis Valley. The Uinta
Mountains jut into the northwest corner of Colorado from adjacent Utah.
And the Paradox, Uinta, and Green River Basins protrude into the Plateau
Province, modifying its topographic character.
[Illustration: Pikes Peak rises to an elevation at 14,110 feet. Composed
of Pikes Peak Granite, the mountain is almost surrounded by younger
sedimentary rocks, including those of the Garden of the Gods, in the
foreground. (Floyd Walters photo)]
Before discussing the geologic nature of the three provinces, let us
review briefly two sets of geologic terms. The first set has to do with
the rocks themselves—What kind of rock is that?—but serves also to tell
something about the origin of the rocks. The second set is concerned
with time—When was that rock formed? Is it older or younger than
adjacent rock? How does it relate, time-wise, to geologic events in
other parts of the world?
These two sets of terms are presented in the charts that follow. If you
are unfamiliar with geologic terminology, refer to these charts as often
as you need to while you read this book, as well as to the glossary on
pages 114-118.
Geologists divide rocks into three main groups, depending on their
modes of origin.
_Igneous rocks_ originate from molten material, cooling deep below the
surface of the earth (intrusive igneous rocks) or flowing out and
hardening at the surface (extrusive igneous rocks).
_Sedimentary rocks_ are formed from broken or dissolved bits of other
rock, washed by wind and water and deposited as layers of fragments or
as chemical precipitates. They often contain fossil plants or animals.
_Metamorphic rocks_ are pre-existing rocks (igneous or sedimentary)
changed by heat, pressure, or chemical action.
Examples of these three classes of rocks are given in the accompanying
figure. Many varieties of all three classes occur in Colorado.