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Title: Prairie Peak and Plateau - A Guide to the Geology of Colorado
Author: Chronic, John, Chronic, Halka
Language: English
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STATE OF COLORADO
John A. Love, _Governor_
DEPARTMENT OF NATURAL RESOURCES
T. W. Ten Eyck, _Executive Director_
COLORADO GEOLOGICAL SURVEY
John W. Rold, _State Geologist and Director_
A. L. Hornbaker, _Mineral Deposits Geologist_
Richard H. Pearl, _Ground Water Geologist_
William P. Rogers, _Engineering Geologist_
Antoinette M. Ray, _Secretary_
MISSION OF THE COLORADO GEOLOGICAL SURVEY
The Colorado Geological Survey was legislatively re-established in
February 1969 to meet the geologic needs of the citizens, governmental
agencies, and mineral industries of Colorado. This modern legislation
was aimed at applying geologic knowledge toward the solution of today’s
and tomorrow’s problems of an expanding population, mounting
environmental concern, and the growing demand for mineral resources.
SPECIFIC LEGISLATIVE CHARGES:
“Assist, consult with, and advise state and local governmental
agencies on geologic problems.”
“Promote economic development of mineral resources.”
“Evaluate the physical features of Colorado with reference to present
and potential human and animal use.”
“Conduct studies to develop geologic information.”
“Inventory the state’s mineral resources.”
“Collect, preserve and distribute geologic information.”
“Determine areas of geologic hazard that could affect the safety of or
economic loss to the citizens of Colorado.”
“Prepare, publish, and distribute geologic reports, maps, and
bulletins.”
PRAIRIE
PEAK and
PLATEAU
A GUIDE TO THE GEOLOGY OF COLORADO
_by John and Halka Chronic_
[Illustration: Relief image of Colorado]
COLORADO GEOLOGICAL SURVEY BULLETIN 32
1972
Available from Colorado Geological Survey
1845 Sherman Street
Denver, Colorado 80203
Price—$2.00
ACKNOWLEDGMENTS
This guidebook was written at the request of the Colorado Geological
Survey to fulfill a long-felt need for a popular account of the state’s
geology and its relationship to Man.
The authors wish to thank those of their colleagues who have assisted at
various times in the preparation of this book. John Rold, Colorado State
Geologist, and William Weber, of the University of Colorado Museum
staff, made many helpful suggestions concerning the manuscript. John
Schooland, vice president of the Colorado Historical Society, generously
provided several pictures of early mining activities in Colorado.
Permission to reproduce drawings and paintings of fossils and
reconstructions of past environments was granted by the American Museum
of Natural History and the University of Colorado Museum. Drawings,
maps, and diagrams are largely the work of Robert Maurer, who also
designed the cover and title page.
[Illustration: Tilted dark red sedimentary rocks of the
Pennsylvanian-Permian Maroon Formation are well exposed in the cliffs of
Maroon Bells, southwest of Aspen. (Photo courtesy Hydraulic Unlimited
Mfg. Co.)]
CONTENTS
_Page_
Introduction 1
I Colorado’s Three Provinces 3
The Prairies 8
The Peaks 10
Front Range 11
Wet Mountains 16
Sangre de Cristo Range and Spanish Peaks 17
Park Range and Rabbit Ears Range 19
Gore Range 20
Tenmile and Mosquito Ranges 21
Sawatch Range 22
Elk Mountains and West Elk Mountains 24
San Juan Mountains 25
Uinta Mountains 26
The Plateaus 28
II Geologic History of Colorado 32
Precambrian Era 33
Paleozoic Era 38
Cambrian Period 39
Ordovician Period 40
Silurian Period 42
Devonian Period 42
Mississippian Period 43
Pennsylvanian Period 44
Permian Period 48
Mesozoic Era 51
Triassic Period 51
Jurassic Period 52
Cretaceous Period 56
Cenozoic Era 59
Tertiary Period 59
Quaternary Period 68
III Geology and Man in Colorado 74
Gold, Silver, and Other Metals 77
Boulder County 79
Central City and Idaho Springs 80
Georgetown, Empire, and Silver Plume 81
Leadville 82
Breckenridge 83
Fairplay 84
Silverton 85
Ouray 87
Aspen 88
Creede 89
Cripple Creek 90
Climax 91
Radium, Uranium, and Vanadium 93
Oil, Natural Gas, and Oil Shale 94
Coal 96
Construction Materials 97
Sand, Gravel, and Clay 97
Stone 99
Lime and Gypsum 101
Gems 102
Water 103
Surface Water 103
Groundwater 105
Caves 106
Springs 109
Environmental Geology 111
Glossary 114
Suggested Reading 119
Index 121
ILLUSTRATIONS
_Page_
Colorado’s three geologic provinces 2
Pikes Peak, seen from the Garden of the Gods 4
Rock classification (chart) 5
Stratigraphic column (chart) 7
Jurassic rocks in Colorado (map) 9
East face of Longs Peak 11
Rocky Mountain National Park (east-west profile) 12
Big Thompson Canyon, west of Loveland 13
Red Rocks Amphitheater, west of Denver 14
Colorado Springs area (map and cross section) 15
Joint systems in Precambrian rocks, Boulder Canyon 15
Spanish Peaks, southwest of Walsenburg 18
Hahn’s Peak, north of Steamboat Springs 19
Gore Range from the east 20
Aspen Mountain geology (map) 23
Mt. Sopris, south of Glenwood Springs 24
Ouray, in the San Juan Mountains 25
Steamboat Rock, Dinosaur National Monument 27
Grand Hogback, near Rifle (block diagram) 28
Mt. Garfield, near Grand Junction 30
Precambrian-Cambrian unconformity south of Ouray 34
Geologic map of Colorado 35
Black Canyon of the Gunnison National Monument 36
Precambrian-Cambrian unconformity, Glenwood Canyon 38
Cambrian fossils 39
Ordovician fossils 41
Devonian fossils 43
Mississippian fossils 44
Pennsylvanian paleogeography (map) 45
Fountain Formation northwest of Denver 45
Pennsylvanian fossils 46
Contorted Pennsylvanian rocks near Gypsum 46
Balanced Rock, Garden of the Gods 48
Permian reptile tracks 49
The Flatirons, near Boulder 50
Colorado National Monument 51
Morrison Formation, west of Denver 53
Dinosaur bones, found near Morrison 54
Dakota Sandstone hogback 56
Cretaceous fossils 57
Wolford Mountain, north of Kremmling 60
Eohippus, the “Dawn Horse” 61
Golden and South Table Mountain 62
Devil’s Staircase, near Spanish Peaks 63
Green River oil shale, west of Rifle 64
Florissant Fossil Beds National Monument 65
Pawnee Buttes, north of Fort Morgan 66
Fossil mammals, northeastern Colorado 67
Glacial lakes in Rocky Mountain National Park 68
Arapaho Glacier, west of Boulder 70
Pleistocene mastodons 72
Great Sand Dunes National Monument 73
Colorado Mineral Belt (map) 78
Sluicebox mining in early Colorado 81
Early-day Leadville 82
Gold dredge, Fairplay 84
Silverton, in the San Juan Mountains 86
Abrams Mountain, south of Ouray 87
Creede and its mines (map) 89
Cripple Creek, near Pikes Peak 90
Climax molybdenum mine (cross section) 91
Rampart Range quarry, near Colorado Springs 98
Yule Marble quarry, near the town of Marble 99
Lyons Sandstone quarry 100
University of Colorado Museum 100
Colorado-Big Thompson Project (cross section) 103
San Luis Valley (cross section) 105
Cave of the Winds, near Manitou 107
Mesa Verde cave and Indian dwellings 108
Glenwood Hot Springs 109
PRAIRIE PEAK and PLATEAU
Introduction
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.
Class Example Occurrence in Colorado
Sedimentary Sandstone Plains, plateaus, flanks of mountain
areas
Shale
Conglomerate
Limestone
Igneous Extrusive: Volcanic areas such as San Juan
Basalt Mountains, Spanish Peaks
Intrusive: Pikes Peak, Longs Peak, and most
Granite central mountain areas
Diorite
Metamorphic Marble Mountain areas
(from
limestone)
Quartzite
(from
sandstone)
Gneiss
(from
granite or
sandstone)
Schist
(from shale
or basalt)
Geologists arrange rocks in their chronologic sequence by studying the
fossils and minerals which they contain. The age of some rocks can be
determined with reasonable precision from ratios of radioactive
minerals and their fission products. The relative age of others can be
determined from their position, the fossils enclosed in them, and many
minor details of their structure.
The _stratigraphic column_ shown opposite may be thought of as a
calendar by which geologic events in Colorado can be arranged in their
proper order and related to events in the rest of the world.
Mississippian and Pennsylvanian Periods are American divisions;
elsewhere this time interval is known as the Carboniferous Period.
Other time terms are in worldwide use.
In the generalized geologic map of Colorado which accompanies Chapter
II, rocks are identified by the era in which they were formed. A more
detailed geologic map can be obtained from the U.S. Geologic Survey
map distribution center in the Federal Building, Denver.
[Illustration: Stratigraphic Column]
ERA Period Millions Distinctive fossils Events in Colorado
of years
ago
CENOZOIC
(Age of Mammals)
Quaternary Modern types of Development of present
animals and plants topography; glaciation in
mountains
3
Tertiary Mammals, flowering Uplift and mountain
plants building
70
MESOZOIC Dinosaurs and other
(Age of Reptiles) reptiles
Cretaceous Submergence, then uplift
135
Jurassic Desert, then submergence
180
Triassic Widespread floodplains
and deserts
225
PALEOZOIC
(Age of Fishes)
Permian First reptiles Widespread floodplains
and deserts
270
Pennsylvanian Swamp and forest “Ancestral Rocky
plants Mountains”
310
Mississippian Reef corals, sharks Partial submergence
350
Devonian Armored fish, first Probable submergence
insects
400
Silurian Corals and shellfish Probable submergence
440
Ordovician First fish Submergence
500
Cambrian First hard-shelled Gradual encroachment of
animals sea from west
570
PRECAMBRIAN “Lipalian Interval” Erosion to almost flat
surface or peneplain
Primitive Alternate episodes of
soft-bodied marine mountain building and
organisms erosion
3,600 plus
THE PRAIRIES
Beneath the flat prairies of eastern Colorado, sedimentary rocks form a
series of layers. Those near the surface are among the youngest rocks in
Colorado. We know this from the fossils they bear, fossils of large
mammals such as the hairy mammoth, which lived in early Quaternary time,
the bison, and many smaller mammals living today.
The layers below—sandstones, shales, and limestones—become progressively
older as one goes deeper. Most of them were formed originally on the
bottoms of shallow seas that covered this part of North America several
times during the history of the continent. In most places the layers are
horizontal or nearly so, but westward, as they approach the mountains,
they bend upward, gently at first and then more steeply. At the very
edge of the mountains, where they were dragged upward when the mountains
rose, their eroded edges appear at the surface.
The entire sequence of flat-lying rocks can be studied where they are
exposed along the mountain front or where streams and rivers have
dissected them. They are also known from cuttings and cores of oil and
water wells. Some parts of Colorado’s eastern plains have been drilled
so intensively in the search for oil and gas that we know a great deal
about the subsurface sedimentary rock and can even make maps showing the
distribution and character of the individual rock layers. From such
maps, the history of the region can be deduced. We know, for example,
that the area around Denver has subsided more in the past than has the
area near La Junta or Lamar; it is called the Denver Basin because of
its past history and not because it is a basin at present.
Although the plains of Colorado appear flat, they really slope gently
eastward. The rock layers near the surface slope eastward also, but the
deeper rock layers may not.
Near the western edge of the Plains Province, hills and valleys are
formed by differential erosion of hard and soft rock layers. Some hills,
such as Castle Rock, are topped with resistant sandstone; others, like
Mesa de Maya south of Trinidad and Table Mountain near Golden, are
capped with layers of basalt. Close to the mountains flat-topped
foothills result from partial dissection of former erosion surfaces as
the mountains, stabilized for a time, rose again, or as climatic cycles
changed. Examples of these dissected erosion surfaces can be seen north
and south of Boulder.
Far east of the mountain front, near the northern border of Colorado,
remnants of another, higher prairie surface stand as Pawnee Buttes.
Torrential erosion—spring floods and summer thunderstorms—has deeply
furrowed the prairie surface here and left these buttes as lonely
sentinels.
[Illustration: This map shows the distribution, character, and thickness
of certain Jurassic rocks in Colorado. These rocks are deeply buried
beneath the plains and are known there only from well samples. They have
been eroded from most mountain areas. They come to the surface along the
edges of the mountains and in the deeply incised canyons of the Plateau
Province.]
PRECAMBRIAN ROCKS
PALEOZOIC ROCKS
JURASSIC ROCKS
SANDSTONE
SHALY SANDSTONE
SANDY SHALE
SHALE
JURASSIC ROCKS COVERED WITH VOLCANICS OR NEVER DEPOSITED.
What lies below the sedimentary layers of the plains? The sedimentary
rocks are 5,000 to 10,000 feet thick. They lie on an almost horizontal
surface of much, much older rock, the Precambrian or “basement” rock.
This is igneous and metamorphic rock, much crumpled and folded, the
roots of long gone mountains which were beveled and leveled to an almost
flat surface or _peneplain_ perhaps a billion years ago.
We know little of the ancient basement rocks below the sedimentary
layers of the plains, for few wells penetrate this deep. What we do know
indicates that they are similar to rocks of the mountain masses to the
west, and are composed of granite, schist, and gneiss. They probably are
not rich in valuable minerals, however, for the mineral-rich veins of
the mountains came about as a result of uplift of the mountain areas.
THE PEAKS
Most of the individual ranges making up the Rocky Mountains in Colorado
are the result of highly localized movements of the crust as the entire
region was thrust upward from below. These movements broke the deep,
massive igneous and metamorphic rocks of the Precambrian basement, and
bent the more flexible Paleozoic and Mesozoic layered rocks above them
until they arched upward in a series of corrugations. The mountains thus
formed are known to geologists as _faulted anticlines_.
As the mountains rose, they were of course attacked by the forces of
erosion. The sedimentary layers were completely stripped from the crests
of many of the uplifts, so that Precambrian rocks were exposed. It is
these rocks which form the summits of the highest peaks of Colorado. As
with all rules, there are exceptions: the Spanish Peaks are volcanic,
and the crest of the Sangre de Cristo Range is composed of sedimentary
rocks.
The trend of most of the ranges in Colorado is north-south, swinging to
northwest-southeast near the southern end. Surprisingly, in the
northwestern corner of the state there is an east-west trending range,
the Uinta Mountains.
Fifty or more mountain ridges in Colorado have been named as separate
ranges. Of these, the most prominent, frequently visited ones will be
discussed here.
Front Range
The easternmost range of the Rocky Mountains is the longest continuous
uplift in the state. It is a relatively simple faulted anticline
extending from Canon City northward to the Wyoming border, where it
splits into two ridges, the Medicine Bow Mountains and the Laramie
Range.
[Illustration: Longs Peak challenges technical climbers with its
2000-foot vertical east face, the Diamond. This magnificent cliff is the
result of glacial action and freezing and thawing in homogeneous but
fractured granite. The small remnant of ice and snow at the lower left
is all that remains of the glacier. The flat summit may be part of an
ancient erosion surface formed toward the end of Precambrian time. (Jack
Rathbone photo)]
Along the highest portion of the range, from Pikes Peak to Rocky
Mountain National Park, the Paleozoic and Mesozoic sediments formerly
draped over the top of the range have long since been washed away,
leaving only the gneiss, granite, and schist of the mountain core. The
almost flat tops of Longs Peak, Mt. Evans, and Pikes Peak, and the
rolling upland traversed by Trail Ridge Road in Rocky Mountain National
Park are thought to be remnants of the 600-million-year-old erosion
surface that once existed at the top of the Precambrian rocks, and that
still exists below the sedimentary rocks of the Plains Province. This
surface, formed near sea level, has been raised 12,000 to 14,000 feet
within the Mountain Province.
Throughout most of its length, the Front Range displays some of the most
striking high-altitude scenery in the world. Particularly accessible
areas, well worthy of visits, are Rocky Mountain National Park, Berthoud
and Loveland Passes, Mt. Evans, and Pikes Peak. In these areas the
Precambrian rocks can be seen and studied, and the effects of glaciation
observed.
The granite, gneiss, and schist of the mountain core are shattered and
broken into blocks of various sizes. The breaks between the blocks are
called _joints_ if there is no apparent displacement between adjacent
blocks, and _faults_ where there is obvious displacement. The joints
frequently appear in parallel arrays or sets; there may be two or more
intersecting sets, giving a cross-hatched appearance to large exposures.
[Illustration: East-west profile across Rocky Mountain National Park,
through Grand Lake and Longs Peak, showing the inferred position of the
original surface of the anticlinal uplift of the Front Range. This
diagram is generalized, and faults are not shown. (USGS Bull. 730a)]
Restoration of surface which emerged from Cretaceous sea
Restoration of Dakota sandstone
MIDDLE PARK
_Grand Lake_
Longs Peak
Foothills
GREAT PLAINS
Sedimentary rocks
Granite and schist
Sedimentary rock of plains
_South Platte R._
[Illustration: Big Thompson Canyon, west of Loveland on U.S. highway 34,
is carved in almost vertical layers of Precambrian metamorphic rocks.
Gently dipping Late Paleozoic and Mesozoic sedimentary rocks of the
Fountain, Lyons, Lykins, and Morrison Formations can be seen in the
distance, capped by the Cretaceous Dakota Sandstone. (Floyd Walters
photo)]
The Precambrian rocks vary from place to place. Several irregular masses
of granite, called _batholiths_, make up portions of the range.
Batholiths are large intrusions of molten rock that cooled slowly at
great depth. The minerals in them form distinct crystals, often quite
large. The Pikes Peak Granite and the Boulder Creek Granite are
examples. Highly contorted and banded gneiss and schist are well exposed
elsewhere, particularly in the Idaho Springs-Central City-Black Hawk
region.
Along the flanks of the Front Range, the eroded edges of the sedimentary
rocks which once covered the range are exposed. These rocks are usually
tilted sharply against the mountains, as at Garden of the Gods, Denver’s
Red Rocks Park, and the Flatirons near Boulder. The Rocky Mountain
Association of Geologists has erected a plaque explaining the geology of
the Red Rocks area; look for it about half a mile northeast of the Red
Rocks Amphitheater. Tilted layers of Paleozoic and Mesozoic sandstones
form hogback ridges along the mountain front, and stand out clearly on
aerial photographs.
In some areas, particularly near Boulder, Coal Creek, and Golden, the
tilting of the sedimentary layers has been so extreme that the layers
are upside down. Basement rocks may even be thrust out above them.
[Illustration: Sandstones and conglomerates of the Pennsylvanian
Fountain Formation dip steeply toward the plains along the eastern edge
of the Rockies. Near Denver, erosion has carved these rocks into a
natural amphitheater, now the site of Red Rocks Amphitheater.
Precambrian granite forms the hill in the background. (Jack Rathbone
photo)]
Further north, near Loveland and Lyons, as well as further south at
Colorado Springs, irregularities in the uplift have caused abrupt breaks
in the generally smooth eastern edge of the range. Folds and faults in
these areas trend northwest, cutting across and offsetting the mountain
front.
[Illustration: South of Colorado Springs, between Fort Carson and the
NORAD installation in Cheyenne Mountain, Mesozoic rocks are faulted
against the mountain front. Paleozoic rocks are deeply covered by as
much as 3000 feet of Mesozoic sediments. They come to the surface about
10 miles further south.]
RAMPART RANGE
Garden of the Gods
Ute Pass Fault
MANITOU SPRINGS
PIKES PEAK MASSIF
CHEYENNE MOUNTAIN
COLORADO SPRINGS
CROSS SECTION
Ute Pass Fault
Rampart Fault
Tertiary
Mesozoic
Paleozoic
Precambrian
[Illustration: West of Boulder, several intersecting sets of joints
pattern the Precambrian rocks above Boulder Creek. (John Chronic photo)]
The west margin of the Front Range is not as sharply defined as the
eastern margin. Prominent faults edge North, Middle, and South Parks,
however. The northern end of the range merges with the Medicine Bow
Mountains, where dips of sedimentary rocks seldom exceed 30 to 40
degrees. At its southern end, the Front Range plunges into the plains,
although a southwest-trending ridge connects it with the Wet Mountains.
Within the Precambrian core of the Front Range, many economic mineral
deposits have been found. These are discussed in Chapter III. Glacial
features of the Front Range are discussed in Chapter II in the section
on the Quaternary Period.
Wet Mountains
The Wet Mountains are the easternmost range of the Rockies south of
Canon City. Their crest has a distinct northwest-southeast trend, with
the north end offset about 25 miles westward from the south end of the
Front Range. The Canon City Embayment lies at the junction between the
ranges.
Though smaller and lower than the Front Range, the Wet Mountains include
many pleasant and easily accessible recreation areas and a number of
attractive streams and reservoirs. Greenhorn Peak, the summit of the
range, is 12,334 feet high. It is formed of Precambrian granite, as is
most of the crest of the range.
The structure of the eastern side of the Wet Mountains is similar to
that of the Front Range, except that there are more faults in the
sedimentary layers. The southern end plunges southeastward into the
plains. On the western side, westward-dipping sediments are completely
submerged in Cenozoic lava flows and debris from the mountains. Ore
minerals very like those of the Front Range occur near Silver Cliff, but
they have so far proved to be of little economic importance.
Sangre de Cristo Range and Spanish Peaks
The Sangre de Cristo Mountains are visible from many parts of
southeastern Colorado as a jagged, sawtoothed, snow-crested ridge on the
western skyline. They extend about 150 miles from the Arkansas River
near Salida southward into New Mexico.
Few mountain ranges form so impassable a barrier as the Sangre de
Cristos. Only at La Veta Pass does a highway cross the range. However,
old wagon roads, passable now by jeep or on foot, once existed across
Hayden, Music, Mosca, and Whiskey Creek Passes.
Often no more than twenty miles wide, the central portion of the range
is composed largely of red Late Paleozoic sediments like those exposed
in the Garden of the Gods and Red Rocks Park. These rocks are
intricately folded and faulted, but not metamorphosed. They include
sandstones, shale, conglomerates, and fossil-bearing limestones. The
northern end of the range is formed of Precambrian igneous and
metamorphic rocks.
Just west of La Veta Pass, Sierra Blanca stands as an outpost of the
range where its continuity is interrupted and its structure changed.
Huge blocks of Precambrian granite were here pushed upward and thrust
westward to form a cluster of peaks, several of which are over 14,000
feet in elevation.
Many prominent rock glaciers are present in the Sangre de Cristo
Mountains. They are composed of fragments of rock, lubricated by snow
and ice, creeping almost imperceptibly down the steep flanks of the high
peaks. One of these rock glaciers can be seen on the slope of Mt. Mestas
east of La Veta Pass; others are visible from Great Sand Dunes National
Monument.
South of La Veta Pass, an igneous intrusion along the axis of the range
changes the character of the Sangre de Cristos. This intrusion is harder
and has weathered more slowly than the rest of the range, and forms a
group of prominent peaks known as the Culebra Range.
On the west flank of the Sangre de Cristo Range, east of Villa Grove, a
prominent iron-mineralized area can be seen. Here the ghost mine of
Orient marks the site where iron ores were mined in the early days of
the Colorado Fuel and Iron Company. Nearby, an abrupt terrace along the
edge of the valley marks the position of a fault. Recent gravels are
involved in this fault, indicating that movement has taken place here
within the last few hundred years. A number of hot springs occur along
the base of the mountains nearby.
The Spanish Peaks, not structurally related to the Sangre de Cristos,
are visible from La Veta Pass highway. These two peaks represent a pair
of Cenozoic volcanoes, now deeply eroded and much reduced from their
former height. Numerous dikes radiating from the bases of these peaks
represent fissures which were filled with lava as the peaks formed.
The Great Sand Dunes, close to the Sangre de Cristo Mountains north of
Sierra Blanca, are discussed in Chapter II in the section on the
Quaternary Period.
[Illustration: Spanish Peaks, south of Colorado Springs and southwest of
Walsenburg, are twin mountains of volcanic and intrusive rock, the roots
of Tertiary volcanoes greatly worn down and reshaped by erosion. This
view looks southeast from near La Veta Pass, on U.S. Highway 160. (Jack
Rathbone photo)]
Park Range and Rabbit Ears Range
Bordering the western side of North, Middle, and South Parks, another
long north-south trending ridge extends from the Wyoming border toward
the center of Colorado. The northern part of this ridge, forming the
western boundary of the main mountain mass in the state, is called the
Park Range.
The structure of the Park Range is similar to that of the Front Range: a
huge linear corrugation in the earth’s crust, bounded by faults. Because
this area has fewer resistant sedimentary rock layers above the
Precambrian basement rocks, it is not prominently edged with upturned
sedimentary layers.
[Illustration: Hahn’s Peak, a highly eroded laccolith of rhyolite
porphyry, lies on the west side of the Park Range, along the eastern
margin of the Plateau Province. Placer gold was discovered here in 1865,
but the bedrock source of the gold was never found. (Jack Rathbone
photo) A geologic section shows the structure of the area.]
TERTIARY
RED BEDS
JURASSIC
DAKOTA
MANCOS
DAKOTA
Hahn’s Peak
PORPHYRY
MANCOS
DAKOTA
PORPHYRY
JURASSIC
RED BEDS
RE-CAMBRIAN
[Illustration: Hahn’s Peak]
The range is crossed by Rabbit Ears Pass in the north; Gore Pass near
Kremmling marks its southern end. Mt. Zirkel (12,180 feet) and Flattop
Mountain (12,118 feet) are the two high points of the range; these and a
number of unnamed peaks over 11,000 feet high are upward-faulted blocks
of Precambrian granite.
A rough ridge of volcanic country joins the Park Range with the Front
Range and effectively separates North Park and Middle Park. This is the
Rabbit Ears Range, named for a double-eared knob of Precambrian granite
near Rabbit Ears Pass on U. S. highway 40. Many Tertiary volcanic
features, including dikes and lava flows, can be seen along this ridge,
which is also traversed by Colorado state highway 125 between Granby and
Walden via Willow Creek Pass.
Gore Range
The Gore Range lies south of Gore Pass, along the Park Range trend. The
ridge of this range is low for about 15 miles south of Kremmling, but
the southern part of the range forms a spectacular high cluster of peaks
with many relatively inaccessible and rugged summits. Many of the peaks
in this remote country are as yet unnamed; the area has been set aside
as the Gore Range-Eagle’s Nest Wilderness Area. The Colorado River cuts
directly across the northern part of the Gore Range just west of
Kremmling, in a steep-walled canyon that is one of the wild scenic spots
of Colorado.
[Illustration: The southern part of the Gore Range, viewed from the
east, shows Precambrian granite and metamorphic rocks rising above
Cretaceous shale hills. The nearly horizontal crest of the range
probably represents the Precambrian erosion surface. (Jack Rathbone
photo)]
The Gore Range is, like the Front Range, a faulted anticline with
Precambrian rocks at its core. The red sedimentary rocks on the west
flank of the range, visible at Vail Pass and Vail ski area, are of the
same age as those in Red Rocks Park near Denver and the Garden of the
Gods near Colorado Springs. Paleozoic rocks are absent on the east flank
of the range, having been eroded from that area before Mesozoic
deposition. South of the Colorado River and north of the Wilderness
Area, Mesozoic rocks extend over the crest of the range.
The south end of the Gore Range is marked by Tenmile Gorge (U. S.
highway 6 between Frisco and Vail Pass). This gorge is a glacial valley,
carved during the Ice Age by a glacier more than 1,000 feet thick, along
a weak faulted zone in the range. A fault surface can be seen on the
east side of the valley.
From Vail Pass, or from the top of the Vail ski lift, other evidences of
glaciation can be seen—cirques and U-shaped valleys—testifying to the
former presence here of many large valley glaciers.
Tenmile and Mosquito Ranges
With scarcely a break, the Park Range-Gore Range structure continues
southward into the Tenmile and Mosquito Ranges. These high ridges
separate South Park from the upper Arkansas Valley, and include a
cluster of very high peaks, Quandary, Mt. Lincoln, Mt. Democrat, and Mt.
Bross, all over 14,000 feet in elevation.
Structurally, both the Tenmile Range and the Mosquito Range are highly
asymmetrical anticlines, gentle on the east and steeply faulted on the
west. Paleozoic sedimentary rock layers containing many fossils cover
large portions of the higher parts of these ranges, but two of the
highest peaks, Mt. Bross and Mt. Lincoln, are capped by the Lincoln
Porphyry, a Tertiary intrusive, while Quandary Peak is Precambrian
granite.
These mountains are highly mineralized, and have been extensively
explored and mined. The Climax Molybdenum Corporation operates an
especially large mine at Climax, and the New Jersey Zinc Company has a
large underground mine and mill at Gilman, on the western slopes of
Tenmile Range.
Buffalo Peaks, two highly eroded volcanic mountains near the south end
of Mosquito Range, are extrusions of lava and ash which have buried the
axis of the Mosquito uplift. They are major volcanoes related to a group
of small volcanic cones near Antero Junction, in South Park.
South of Buffalo Peaks, near Trout Creek Pass, the Mosquito Range loses
altitude rapidly and merges with the rough country called the Arkansas
Hills. Cinder cones, dikes, and other evidences of Tertiary volcanic
activity can be seen between Trout Creek Pass and Salida.
Sawatch Range
Bordering the Arkansas River valley on the west, the Sawatch Range
includes Colorado’s highest mountain, Mt. Elbert (14,417 feet). With
several other 14,000-foot summits, this range is the highest in the
state. One group of peaks, known as the Collegiate Range (Mts. Harvard,
Yale, Columbia, and Princeton) forms a particularly imposing vista from
U. S. highway 24 between Trout Creek Pass and Buena Vista. The
Independence Pass highway (Colorado 82) between Leadville and Aspen
penetrates the heart of the Sawatch high country.
The Sawatch Range as a whole is about 100 miles long (north to south)
and 40 miles wide. It is a great faulted anticline intruded by igneous
rocks. The high area north of Leadville shows that the Sawatch and
Mosquito Ranges are in reality one huge dome with a slight sag in the
middle. The ranges, though, are sharply separated topographically by the
deep valley of the Arkansas River. Precambrian rocks are near the
surface between the ranges, hidden only by a thin cover of stream
gravels. Near Leadville, some complexly faulted Paleozoic limestones lie
in the sag between the ranges.
At Mt. Princeton Hot Springs there is evidence of repeated faulting and
igneous activity. The rocks are strongly altered by hot water coming to
the surface through fissures and cracks.
On the west side of the Sawatch range, the old mining towns of Tincup
and Aspen grew up where limestone and sandstone layers, broken and
crumpled as the Sawatch Range rose, were mineralized by solutions rich
in gold and silver. The Aspen Mining District was studied extensively by
geologists of the U.S. Geological Survey, and their maps show almost
unbelievable complexity in the faulting of the rock layers which exist
there.
The north end of the Sawatch Range plunges under shales and sandstones
along the Eagle River east of Wolcott. Gypsum in the sediments here has
acted like putty: the layers of rock in which it was deposited have
become peculiarly crumpled, making the area along the Eagle River
(visible from U. S. Interstate 70) between Avon and Edwards hummocky and
irregular. Vegetation is unusually sparse here because of gypsum in the
soil.
About midway between Edwards and Wolcott, the Eagle River suddenly
changes direction and flows northward for about a mile before resuming
its former westward course. This sudden change is caused by a sharp
north-south fold in the sedimentary rocks on the northwestern flank of
the Sawatch Range. A magnificent series of roadcut and hillside
exposures along the highway here illustrates the close relation between
rock layers and river course. Within about a mile, the highway cuts
through rocks of Pennsylanian, Permian, Triassic, Jurassic, and
Cretaceous age, spanning a geologic time interval of more than 200
million years.
The south end of the Sawatch Range, at Monarch Pass, contains steeply
dipping Late Paleozoic limestones and coal beds. The coal has been mined
on a small scale; the limestone is now quarried for use as a flux in
iron smelters at Pueblo.
[Illustration: The area below the Aspen Mountain ski lift is highly
complex geologically. It is particularly well known because of
extensive prospecting and mining activity in the region.]
Elk Mountains and West Elk Mountains
The Elk Mountains and West Elk Mountains appear to be westward
continuations of the Sawatch Range. Structurally, however, they are not
faulted anticlines like most of the other ranges in Colorado, but are
composed of a series of layers of Paleozoic sediments thrust westward
over one another. These rocks, often crumpled and highly metamorphosed,
are cut by numerous sills, dikes, and other intrusions, many of which
have caused mineral enrichment locally.
At Maroon Bells, in the canyon of Maroon Creek, and at Redstone on the
Crystal River, these metamorphosed sediments are well exposed. Here, red
sandstones and shales have been altered to quartzites and slate. At
Marble, metamorphism of a thick limestone bed has produced white marble
of great beauty, known as Yule Marble. This decorative stone was
quarried extensively until about 1940. It was used in the Lincoln
Memorial and several other monumental structures; in the town of Marble
it has been used for the doorsteps of log cabins! The largest block
quarried, for the Tomb of the Unknown Soldier in Arlington National
Cemetery, measured 14 by 7.4 by 6 feet in the rough, and weighed 56
tons.
[Illustration: Mt. Sopris, south of Glenwood Springs, is an igneous
intrusion. (Jack Rathbone photo)]
Crested Butte, at the south end of the Elk Mountains, is a small
intrusive igneous mass called a _laccolith_. Hard and resistant to
erosion, it stands over 2,000 feet above the adjacent valley floor.
San Juan Mountains
The San Juan Mountains are the most extensive range in Colorado, and
also the most heterogeneous. Covering more than 10,000 square miles of
the southwestern part of the state, these mountains are formed mostly of
Tertiary volcanic rocks, the result of repeated outpourings of lava and
ash from a cluster of volcanoes. Water-laid gravels composed of volcanic
sand and pebbles are interlayered with basalts and ash beds; the total
thickness of these beds reaches many thousands of feet.
[Illustration: The mining town of Ouray, now also a tourist haven and
summer resort, nestles below Pennsylvanian sedimentary rocks of Ouray
Canyon. At the north end at town can be seen the Ouray Hot Springs
swimming pool. Gold, silver, lead, and zinc are still mined in this
area. (Jack Rathbone photo)]
The widespread volcanic activity which formed most of the range began in
mid-Tertiary time and continued for several million years. A few
Quaternary volcanic flows are known in the region, but there is no
active volcanism there at present.
The western side of the main range, including some of the highest peaks,
consists primarily of uplifted and faulted Paleozoic sedimentary layers.
These layers, highly dissected by erosion, can be seen near Ouray, at
Molas Lake, and at Durango. Large patches of Precambrian granite and
metamorphic rocks protrude through the sediments, as in the Needle
Mountains; they indicate that this part of the range is a faulted
anticline like many other Colorado ranges.
Early Cenozoic glacial deposits occur in some parts of the San Juans.
These are unusual features, as glaciation of this age is unknown
elsewhere in Colorado.
Three small ranges rise just west of the San Juans: the San Miguel,
Rico, and La Plata Mountains. Each consists of several small masses of
Tertiary igneous rock intruded into Paleozoic conglomerates, shales, and
limestones.
Mineralization has been intense in the San Juans; most of it took place
during the Late Tertiary volcanic period. Rich veins penetrate
Precambrian gneiss and granite, and Paleozoic limestones are often
enriched also. Several mines are still active near Ouray, Silverton,
Telluride, and Rico.
Uinta Mountains
The eastern end of Utah’s Uinta Mountains extends into Colorado. Unlike
other ranges in Colorado, these mountains trend east-west. Structurally,
the range is a faulted anticline. It is quite asymmetrical, however, and
is tilted and folded upward on the south, and overturned or
thrust-faulted on the north. Steeply dipping Mesozoic and Paleozoic
sediments on the south side of the range, sparsely vegetated and often
thrown into spectacular folds, are a prominent feature of northwest
Colorado scenery.
In Colorado the crest of the Uintas reaches an elevation of about 8,500
feet. It consists of Precambrian rocks, but these are not the igneous
and metamorphic rocks that characterize the Precambrian core of other
Colorado mountains. They are easily recognized as sediments—dark red
conglomerates, sandstones, and mudstones—virtually unmetamorphosed
though they were deposited nearly a billion years ago. Called the Uinta
Mountain Formation, these rocks are found only in this part of Colorado
and adjacent areas of Utah. They are probably related to similar
Precambrian rocks found in Montana and Canada.
At the east end of the Uintas two isolated uplifts, Cross Mountain and
Juniper Mountain, are faulted blocks of Paleozoic rocks standing like
islands in a sea of Cenozoic valley fill. They are the last outposts of
the Uinta anticlinal pattern as it wanes toward the southeast.
Dinosaur National Monument, a Uinta Mountain tourist attraction,
encompasses a vast area of wilderness on both sides of the Yampa River
in Colorado. Here many of the features of the east end of the Uinta
Mountain structure can be seen. A unique display of the world’s largest
fossils can be visited in the Utah portion of the Monument.
[Illustration: At their confluence in Dinosaur National Monument, the
Yampa and Green Rivers have carved Late Paleozoic sandstone into the
precipitous cliffs of Steamboat Rock. (William C. Bradley photo)]
THE PLATEAUS
The western quarter of Colorado is a region of flat-lying Paleozoic,
Mesozoic, and Cenozoic sedimentary rocks which have not been bent up
into mountains except in a few isolated instances. This area lies more
than a mile above sea level, however, and because of the gradient such
an elevation affords, it is deeply sculptured. The Colorado River and
its tributaries have sliced into the plateau surface, separating it into
many isolated tablelands or mesas. Some are capped with sedimentary
rock, others with Tertiary basalt.
[Illustration: The Grand Hogback is a good example of the type of
geologic structure known as a _monocline_. The hogback ridge is formed
by differential erosion, where soft layers wear away more easily than
hard layers.]
Simple folds and faults have given the mesas different elevations. Thus
the average elevation of the White River Plateau is 11,000 feet, that of
the Roan Plateau 9,500 feet, and that of Mesa Verde only 7,000 feet.
West of Durango the plateaus dip gently southward, as can be seen at
Mesa Verde. Igneous intrusions and extrusions have altered plateau
topography in some areas. West of Mesa Verde, for instance, an intrusive
stock forms a prominent dome in the Southern Ute Indian Reservation.
West of the northern Colorado mountains, and north and west of the White
River Plateau, a rolling upland extends from Colorado into Utah and
Wyoming. It is interrupted by the Uinta Mountains and a number of
smaller related uplifts such as Juniper Mountain and Cross Mountain.
South of the Uinta axis the area is known as the Uinta Basin.
The northern part of this area is structurally the south edge of the
Green River or Washakie Basin in Wyoming. The Rangely anticline, in the
northeastern corner of the Uinta Basin, is one of Colorado’s richest oil
fields; it is discussed in Chapter III.
Although surfaced with much younger sediments than the rest of the
Plateau Province, this area is structurally similar. On the whole,
sedimentary layers are relatively flat-lying, and where they are
uplifted they are deeply sculptured by streams and rivers. The
sedimentary rocks in this region contain uranium and placer gold in
addition to great oil and gas deposits. The southeastern part of the
Uinta Basin, usually called the Piceance Basin, is the site of a great
deposit of oil shale (see Chapter III). The term “basin” may here seem
unusual to the casual observer, for the oil shales occur on the Roan
Plateau at places well over 10,000 feet in elevation. However, the
entire region was basin-like—lower than the surrounding ranges—for many
millions of years, and during Tertiary time thousands of feet of valley
and lake deposits were laid down in it.
The White River Plateau, north of Glenwood Springs, is composed of
almost horizontal Paleozoic sedimentary rocks that fold downward sharply
along its south and west edges. The fold is 135 miles long and is
clearly marked by the Grand Hogback, the eroded edge of hard Cretaceous
and early Cenozoic rock layers. Shale and coaly layers involved in the
same fold have eroded more readily, leaving the resistant sandstone as a
prominent ridge.
The Uncompahgre Plateau, southwest of Grand Junction, is structurally
very like the White River Plateau. Its features can be well observed in
Colorado National Monument. It has been elevated several thousand feet
more than the Book Cliffs and Grand Valley areas to the north. Sharp
folding and faulting near the Colorado River at the north boundary of
the National Monument show that differential movement between the two
regions was sharp and localized.
A series of northwest-trending anticlines along the Utah border in
southwestern Colorado are of special geologic interest. They represent
peculiar structures in which salt and gypsum have played a major part.
These minerals were deposited in thick layers late in Paleozoic time;
subsequently they were covered by thousands of feet of sand, shale, and
limestone. Because of their low density and high plasticity they have
since crept upward along weak spots in the overlying sediments, often
contorting these rocks as they moved. Breaking through to the surface,
the salt and some of the gypsum washed away more rapidly than the
surrounding rock, leaving long faulted troughs such as Gypsum Valley and
Paradox Valley. In most of these structures the gypsum can still be
seen, although the more soluble salt has eroded away. Oil wells in this
part of Colorado and in adjacent parts of southeast Utah have penetrated
thousands of feet of evaporites, including pure salt, gypsum, and
potassium salts.
[Illustration: In the arid climate of the Colorado Plateaus, ledges of
well-cemented sandstone stand out sharply from slopes of shale or
mudstone. The Mesa Verde and Mancos Formations, Cretaceous in age, form
the slopes and top of Mt. Garfield near Grand Junction (Jack Rathbone
photo)]
The peculiar weathering characteristics of flat-lying sedimentary rocks
in an arid climate are well demonstrated in Colorado National Monument,
Mesa Verde National Park, and elsewhere in the Plateau Province. Those
fortunate enough to make a river trip through the Yampa or Green River
Canyons in northwestern Colorado or on the rivers of eastern Utah and
northern Arizona will have an unusually fine opportunity to observe
close at hand the weathering and erosion in this area. Resistant
sandstone and limestone layers break into sheer cliffs, often many
hundreds of feet high, while the softer layers of mudstone and shale
form gentle slopes and terraces. Vast arching caves often develop where
resistant layers are undermined—caves sometimes containing ancient
Indian dwellings.
II
Geologic History of Colorado
Astronomical and geologic evidence indicates that the earth was probably
formed as an immense blob of molten rock, held together and shaped into
spherical form by its own gravity. It may even have been gaseous at
first, cooling gradually to a molten state. After hundreds of millions
of years it became cool enough to begin to harden.
As the surface cooled, a crust formed, and lay like a blanket over the
liquid mass beneath. Convection currents—large-scale boiling
movements—stirred the molten interior, thrust portions of the crust
upward, and sucked other portions downward to be remelted. Some of the
lighter components, such as compounds of silicon and oxygen and
hydrogen, accumulated on the surface like froth on a kettle: the
continents were born. With further cooling the atmosphere and oceans
came into being.
Something can be told of the age of the continents. Measurements of
radioactivity in the most ancient rocks exposed at the surface today
indicate that the oldest known continental rock is between three and
four billion years old. Since the continents were formed, they have been
bent and shifted and broken by the pressures exerted against them by
convection in the interior. Parts of the continents at times have been
submerged below the level of the sea, even as they are today. Other
portions, lifted above sea level, were immediately attacked by the
wearing-down processes of erosion. The battle between mountain-building
forces and erosion has been a continuous one ever since the crust was
formed. Even now earthquakes give testimony to continued crustal
movement, storms still sweep across the continents and wash mud and
frost-loosened rocks into churning torrents, rivers still deposit great
floodplains and deltas, sediments accumulate slowly but persistently
upon the bottoms of the seas.
PRECAMBRIAN ERA
Only part of the earth’s very early history is represented in Colorado,
where the oldest known rocks are the gneisses and schists of the Idaho
Springs Formation, at least 1,800,000,000 years old. These rocks appear
to be the remains of ancient sediments, folded and metamorphosed into
vast mountain areas long before recognizable life inhabited the earth.
Precambrian rocks in Colorado are on the whole very poorly known. They
have, however, been studied in detail in the Front Range west of Denver
and Boulder, where they have been intensively explored for valuable
minerals. The lack of fossils in the oldest rocks makes their close
correlation difficult, but from studies of radioactive minerals
contained in these rocks, and of the relationships of the rock units
themselves, we can list them in order of their relative ages.
Note that the rock sequence given below reads chronologically from
bottom to top—a logical pattern in geology since younger rocks,
especially those of sedimentary origin, normally lie above older ones.
Recent studies indicate that the sequence may be much more complex than
shown here.
(youngest) Silver Plume Granite: light pinkish gray, fine-grained
granite.
Pikes Peak Granite: pink, coarse-grained granite.
Boulder Creek Granite: dark gray, faintly banded
granodiorite.
Coal Creek Quartzite: light gray quartzite with grains
ranging in size from fine sand to
boulders, with some interbedded
schist.
Swandyke Hornblende Gneiss: dark gray to black, strongly banded
gneiss.
(oldest) Idaho Springs Formation: gray to black schist and gneiss.
From a sequence such as this, it is possible to reconstruct some
features of Colorado’s early history. The first chapter of which we have
a record is the deposition of the Idaho Springs Formation, probably as
an accumulation of mud, sand, and limy mud in an ancient sea. Swandyke
deposition followed—the sediments were iron-rich, perhaps derived from
ancient volcanic materials. The original Coal Creek sediments were sands
and gravels, some of them quite coarse and therefore indicative of
near-shore deposition. The schist layers suggest that muds must have
been deposited also.
[Illustration: South of Ouray, Cambrian sandstones of the Sawatch
Formation lie almost horizontally across the vertical Precambrian
metamorphic rocks. (Jack Rathbone photo)]
Together these three formations represent some 40,000 feet of
sedimentary layers. Deposition of such a great thickness of mud, sand,
and lime must have taken a very long period of time. Details of the
geography of the continent during that period have of course been
obscured by later events, when these rocks were subjected to repeated
uplift, crumpling, folding, various degrees of remelting and
recrystallization, and erosion. But the ancient sediments must have been
derived from even more ancient highlands, either folded and faulted
mountains or volcanoes, and probably were deposited under water in broad
expanses of sea that covered portions of the continent.
[Illustration: Geologic map of Colorado. Geologic maps show the age of
rocks appearing at the surface, disregarding soil cover. A more detailed
geologic map of Colorado may be obtained from the U.S. Geological Survey
at the Federal Center in Denver.]
PRECAMBRIAN
PALEOZOIC
MESOZOIC
CENOZOIC SEDIMENTS
CENOZOIC VOLCANICS
Yampa River
White River
Fort Collins
South Platte River
Glenwood Springs
Denver
Colorado River
Grand Junction
Aspen
Gunnison River
Colorado Springs
Gunnison
Salida
Dolores River
Rio Grande
Arkansas River
La Junta
Walsenburg
Alamosa
Durango
The Boulder Creek, Pikes Peak, and Silver Plume Granites cut through the
metamorphic rocks, and are therefore younger. They represent pulses of
molten rock forced upward from deep within the crust, probably during
three separate episodes of mountain building. As each set of mountains
was formed, it was worn down, perhaps to low rolling hills, perhaps to
flat plains almost at sea level, and partially or entirely covered with
thick layers of sediment. Each time, another mountain building episode
followed, with new intrusions of granite and new metamorphism of the
pre-existing rocks.
Each succeeding period of metamorphism and mountain building further
changed the nature of the rocks involved, complicating the patterns of
folding and faulting, adding recrystallization to recrystallization,
until the oldest of the rocks bore little trace of their original
sedimentary nature. In general, the rocks that are oldest were most
altered by the repeated metamorphism, while the younger rocks were less
altered.
[Illustration: The Black Canyon of the Gunnison River is one of the
state’s deep and spectacular chasms. Canyon walls are of Precambrian
gneiss intruded by many dikes and highly fractured by later uplifts. The
flat upper surface of the Precambrian rocks represents an ancient plain
on which, during Jurassic time, the dinosaur-bearing Morrison Formation
was deposited. (John Chronic photo)]
The Precambrian Era ended with a long period of erosion, a period known
to geologists as the Lipalian Interval. During this time, over almost
the entire world there was no mountain building. The land lay sleeping,
subject only to the forces of erosion. The last mountains were flattened
nearly to sea level. Slow, sluggish streams and rivers carried sand and
mud toward the oceans—oceans in which perhaps primitive, soft-bodied
organisms, with no hard parts to be preserved as fossils, were beginning
to evolve.
On the continents, the time of intense metamorphism was over; most rocks
of later eras are preserved today in pretty much their original state.
The boundary between the Precambrian and later rocks is normally well
defined. It is visible at many places in Colorado: in Williams Canyon
near Colorado Springs, in Glenwood Canyon, near Red Rocks west of
Denver, just west of La Veta Pass, at the top of Royal Gorge and the
Black Canyon of the Gunnison. At most of these localities it is a
smoothly beveled surface, with highly contorted Precambrian rocks below
it and flat-lying Paleozoic sediments above it. Near Red Rocks and La
Veta Pass, the same relationship prevails, but the entire contact, and
the rocks above and below it, have been steeply tilted by the uplift of
the present mountains.
In portions of western North America, deposition late in Precambrian
time has left a series of flat-lying rocks between the contorted
Precambrian and later Paleozoic sediments. These rocks can be seen in
northwestern Colorado, where they form the dark red sedimentary core of
the Uinta Mountains.
PALEOZOIC ERA
Geologists have divided the second great era of geologic time into units
called Periods. The rocks deposited during a Period are called Systems,
but more often than not it is convenient to discuss them in terms of
easily recognized units of rock, called Formations. Formations are named
after areas in which they are well exposed.
The stratigraphic column given in Chapter I shows the Periods and
Systems in their correct order, and gives the age in years for each, as
determined by radioactivity methods. As you read, refer as often as
necessary to this column.
The geologic map on page 35 will help you locate areas where the rocks
discussed in the text are exposed, and will greatly facilitate your
understanding of the geology of the state.
[Illustration: The Cambrian Sawatch Sandstone lies almost horizontally
on Precambrian granite in Glenwood Canyon. In the foreground is the
Colorado River. (Jack Rathbone photo)]
Cambrian Period
(500-570 million years ago)
The first fossiliferous rocks in Colorado were deposited during the
Cambrian Period, at a time when over much of the world the seas were
creeping in across wide, level plains formed during the Lipalian
Interval. Colorado was not covered by these seas until quite late in the
Cambrian Period. Beach deposits progressively younger in age suggest
that the sea invaded from the west, and spread slowly eastward,
inundating most of the central part of the state but not the extreme
north or south.
The beach deposits, now called the Sawatch Sandstone because they are
well exposed in the Sawatch Range, are composed mostly of fine quartz
sand. They are colored with glauconite, a green mineral, and hematite, a
dark red mineral, so that the rock has a variegated appearance. The post
office at Manitou is built of this red and green rock, and good
exposures of it exist in Williams Canyon near Manitou, along U. S.
Highway 24 northwest of Manitou, near Red Cliff and Minturn, and in
Glenwood Canyon.
The sea which crept over Colorado at this time contained small
conical-shelled mollusks, brachiopods, and trilobites. Their shells can
occasionally be found in Cambrian rocks in Williams Canyon and in the
Sawatch and Mosquito Ranges. At two localities unusual fossils called
graptolites have been found in thin Upper Cambrian shales overlying the
Sawatch Sandstone.
[Illustration: These fossils can occasionally be found in Cambrian rocks
in central Colorado.]
Ordovician Period
(440-500 million years ago)
The sea deepened and widened as the Ordovician Period began, and a
series of limestones and dolomites was deposited, either on top of the
Sawatch Sandstone or, where the Sawatch had not been deposited, directly
on the Precambrian. These rocks are now called the Manitou Formation.
The fossils in these rocks are much more varied than those in the
Sawatch Sandstone: snails, echinoderms, sponges, cephalopods,
brachiopods, and trilobites are common. The Ordovician sea must have
teemed with life, as many rocks deposited at this time are more than
half composed of animal remains. In addition to hard-shelled animals
which formed fossils, there were probably abundant soft-bodied animals
such as jellyfish and worms, which left no record of their presence.
After deposition of the Manitou Formation, the seas receded slightly. A
new series of sands was deposited above the Manitou in central Colorado.
These now form the Harding Sandstone, a formation of unusual interest
because it contains remains of the earth’s earliest known vertebrates,
primitive jawless fish called Agnathids. In places in the Harding
Sandstone there are dense accumulations of the tiny polygonal armor
plates from these fish. Although no whole fish have been found, we can
reconstruct their appearance by comparing individual plates or groups of
plates with later, better known relatives.
Also present in great quantities in the Harding Sandstone are conodonts,
peculiar tiny brown tooth-like fossils. Relationships of the conodonts
are unknown; they may be parts of the Agnathids, or perhaps they
represent some entirely different group of animals, with no living
relatives.
After deposition of the sands of the Harding Sandstone, the sea deepened
locally and the Fremont Limestone, a massive gray crystalline limestone
containing many marine fossils, was deposited. Mollusks (some quite
large), brachiopods, and corals contributed their shells to the Fremont
Limestone. The chain coral _Catenipora_ and the horn coral
_Streptelasma_ may often be used to identify the formation.
The Fremont Limestone was deposited very late in the Ordovician Period.
Probably the seas were much more extensive then than present deposits
indicate; subsequent erosion has at several times erased the evidence in
uplifted areas.
[Illustration: These Ordovician fossils can be found in the Manitou
Formation in the Colorado Springs area.]
[Illustration: The earliest known fish remains come from the Ordovician
Harding Sandstone of central Colorado. These fragments of the protective
plates have been magnified about five times.]
[Illustration: Corals and coral-like organisms occur in the Ordovician
Fremont Limestone.]
Silurian Period
(400-440 million years ago)
Until very recently, no Silurian rocks or fossils were known in
Colorado, and it was thought that seas did not extend into the state
during this period. However, a few years ago good Silurian corals and
brachiopods were discovered near the northern edge of the state. They
occur in broken blocks and patches of Silurian limestone, mingled with
blocks of other sedimentary rocks and, oddly enough, with volcanic
material.
What seems to have happened here is that sedimentary layers of Silurian
age _were_ present over northern Colorado at one time. During some
subsequent period of volcanism, volcanic lavas penetrated these
sediments from below. Near the volcanic tubes, broken, angular fragments
of the surrounding sedimentary rocks were sometimes carried upward or
downward by the motion of the lava.
Much later, both the volcanic outpourings (if the lavas ever reached the
surface) and the sediments were stripped away by erosion, probably at a
time when mountains were rising in the area. Only the deep portions of
the tubes that fed the volcanoes were preserved. These tubes are called
diatremes, and thanks to the blocks of sedimentary rock in them we know
that there were indeed seas in Colorado during Silurian time, seas
containing the abundant life of a shallow marine environment very much
like that existing at the same time in Illinois, Iowa, and Indiana.
Devonian Period
(350-400 million years ago)
As far as we know now, Colorado was just a little above sea level during
most of Devonian time. Early and Middle Devonian deposits are lacking.
Late in the period, however, Colorado was widely inundated once more.
Embayments of a western sea covered most of the central part of the
state and an area in southwestern Colorado around Ouray.
Deposits formed in these embayments have been given several names.
Chaffee Formation is the name most commonly used in central Colorado;
Ouray Formation identifies rocks of the same age in southwest Colorado.
The Chaffee Formation has been subdivided into two well defined units,
the Parting Sandstone or Quartzite, and the Dyer Dolomite or Limestone.
Many ore deposits are associated with these rock units—notably deposits
of lead and zinc. The Parting Sandstone is frequently so well cemented
with silica that it is actually a quartzite; thin shale beds or
“partings” make it easy to recognize. It frequently contains remains of
fossil fish and distinctive beds of algae.
The Dyer Dolomite contains brachiopods and bryozoans, mollusks and
corals. Some of the best fossil hunting in Colorado is in Dyer beds
around the White River Plateau, where the fossils frequently weather out
of the rock as almost perfect specimens.
[Illustration: These Devonian brachiopods come from the White River
Plateau in western Colorado.]
Mississippian Period
(310-350 million years ago)
The sea continued to cover most of Colorado after the end of the
Devonian Period, well into Mississippian time. Mississippian rocks are
characteristically thick, massive gray limestones collectively called
the Leadville Limestone. This unit is well known as the host rock for
many Colorado ore deposits, notably those around the town of Leadville.
During Mississippian time the western sea, warm and rich in organisms,
covered much of North America. Brachiopods and corals flourished, as did
many other forms of life. The seas during part of this time extended
completely across Colorado to merge with seas that covered the
midwestern part of the United States.
Over all this vast area, as well as southwest into Arizona, the gray,
massive, fossiliferous Mississippian limestone is remarkably uniform and
easily recognized, although it is called by different names in different
areas.
Late in Mississippian time, the Colorado area rose slightly and the sea
in which the Leadville Limestone was deposited receded. An interval of
erosion followed. The surface of the limestone was dissolved and pitted,
tunnels and caves formed where running water etched deep into the rock,
and a reddish soil formed on the surface and in the hollows. This
portion of the limestone, which in some places also contains pebbles of
chert, is named the Molas Formation. Part of the Molas may be
Pennsylvanian in age.
[Illustration: Mississippian fossils from western Colorado show that
seas covered much of the state about 330 million years ago.]
Pennsylvanian Period
(270-310 million years ago)
As the Pennsylvanian Period began, the Colorado area continued to rise.
Earliest deposits of this age are fine-grained black shales and
sands—the Glen Eyrie Formation along the southern Front Range and the
Belden Formation in west central Colorado. Then, through millions of
years, mountain-building took place. Some areas rose more than others,
so that formerly flat-lying marine sediments were bent and broken, and a
series of high mountain ridges and deep basins were formed. Geologists
sometimes call these the Ancestral Rocky Mountains.
Although the pattern of the mountains changed repeatedly, the Ancestral
Rockies consisted principally of two large ranges. One range roughly
paralleled the present Front Range, but lay thirty to fifty miles
further west. The other extended from the San Luis Valley northwest
toward Colorado National Monument, including the area around the Black
Canyon of the Gunnison and the present Uncompahgre Plateau. Coarse
sediments washed off both sides of both ranges, and accumulated as
alluvial fans and valley fill along the mountain margins. These exist
today as the Fountain Formation of the eastern Front Range, the Minturn
Formation between the ancient uplifts, and the Hermosa Formation west of
the western uplift.
[Illustration: This paleogeographic map reflects the distribution of
land and sea during the early part of the Pennsylvanian Period and shows
where coarse sediments derived from the Ancestral Rockies were
deposited.]
FOUNTAIN FORMATION
MINTURN FORMATION
HERMOSA FORMATION
[Illustration: West of Denver, the main line of the Denver & Rio Grande
Railroad tunnels beneath steeply dipping sandstones and conglomerates of
the Fountain Formation. (Jack Rathbone photo)]
[Illustration: Corals, brachiopods, and fusulinid Foraminifurida can be
found in the Pennsylvanian Minturn Formation at many places in the
Mountain and Plateau Provinces.]
[Illustration: In western Colorado, where vegetation is sparse, rock
structures are clearly defined. This photograph shows beds of the
Pennsylvanian Minturn Formation sharply folded, probably as a result of
the deformation of gypsum in underlying layers. (Jack Rathbone photo)]
In the Flatirons near Boulder, Red Rocks Park near Denver, and the
Garden of the Gods near Colorado Springs we see well exposed examples of
the Fountain Formation. The Minturn Formation is visible along the Eagle
River west of Wolcott, and along Gore Creek near Vail. The Hermosa
Formation forms striking red cliffs north of Durango. In the Sangre de
Cristo Mountains area, exceptionally great and rapid deposition took
place, and the Minturn Formation is very thick.
In west central Colorado, near the towns of Eagle and Gypsum, a large
basin formed. In it, gypsum and other salts were deposited as arms of
the sea were cut off from the main marine area. The unusual appearance
of the hills along the Eagle River, especially north of U. S. Highway
24, is caused by the presence of gypsum in the bedrock.
In a similar manner, the Paradox Basin was formed in southwestern
Colorado. Thousands of feet of gypsum, salt, and potash were deposited
here, probably also precipitated in restricted arms of the sea. These
minerals, the so-called evaporites, have since significantly controlled
development of the landscape in Gypsum Valley and other parts of this
region. (See The Plateaus in Chapter I and the section on Gypsum in
Chapter III).
Between the mountain masses and their surrounding alluvial deposits,
shallow seas repeatedly invaded the lowland areas of the state. Marine
fossils in some parts of the Minturn Formation bear witness to as many
as twenty marine cycles. Strangely, the Pennsylvanian Period appears to
have been cyclical in other parts of the United States as well, for
marine sediments are found alternating with nonmarine sediments in
Pennsylvania, Illinois, Kansas, Nebraska, and New Mexico. In middle
Pennsylvanian time, general uplift occurred in Colorado, and almost the
entire state was above sea level for the rest of the period.
Permian Period
(223-270 million years ago)
By the end of the Pennsylvanian Period, the mountains of the Ancestral
Rockies had been almost entirely removed by erosion, and the deep basins
were filled with sediments. Colorado was once more a great plain,
sloping gently to the northeast. In eastern Colorado, a shallow sea
gradually dried up, leaving some thin limestone and gypsum beds along
its margin. The western shore of this sea was edged with beaches and
sand dunes, preserved as the Lyons Sandstone. The buildings of the
University of Colorado, as well as many homes and other structures in
the Boulder-Denver area, are faced with this beautiful salmon-colored
sandstone.
[Illustration: Balanced Rock, in the Garden of the Gods northwest of
Colorado Springs, is an erosional remnant of iron-rich conglomerate and
sandstone. It remains while the rest of the surrounding layers are gone
because it is harder and more completely cemented together by silica.
The rock is part of the Late Paleozoic Fountain Formation. (John Chronic
photo)]
In the western part of the state, Permian deposits consist mostly of
shales and sandstones. The red color of these rocks, and the complete
absence of fossils in them, suggest that the environment in which they
were deposited was not marine, but was a vast, level mudflat subject to
alternating wet and dry periods. The shales and sandstones collectively
are called the Maroon Formation, named for Maroon Bells, near Aspen,
where they are dramatically exposed in the mountain cliffs.
[Illustration: Tracks of Permian reptiles called _Laoporus
coloradoensis_ occur in the Lyons Sandstone near Lyons. These are about
life size.]
During part of Permian time, a shallow sea extended from Idaho, Utah,
and Wyoming into the northwest corner of Colorado. In this sea was
deposited the Phosphoria Formation, a highly phosphatic limestone
containing only rare, poorly preserved molluscan fossils.
As the Paleozoic Era ended, Colorado was still flat and low-lying. By
this time land plants and animals had evolved, but if vegetation grew in
the Colorado area, or animals roamed it, they left few fossil remains.
Tracks of early reptiles have been found in the Lyons Sandstone. Dune
sandstones here and in adjacent areas suggest that desert conditions may
have prevailed, in which case Colorado would have been very similar,
scenically and climatically, to Sahara regions today.
[Illustration: Dark red Pennsylvanian and Permian conglomerates form the
Flatirons that overlook the University of Colorado campus at Boulder.
University buildings are faced with Permian Lyons Sandstone quarried
along the foothills of the northern Front Range. (University of Colorado
photo)]
MESOZOIC ERA
The Mesozoic Era, popularly known as the Age of Reptiles or Age of
Dinosaurs, is divided into three periods. The climate of the entire
earth appears to have been warmer then than it is at present, perhaps
because of a different distribution of land and sea areas, or because
continental areas were not as high and mountainous as they are just now.
Colorado was a rather low land area for most of the first two Mesozoic
periods; then a vast sea covered the entire state for the remainder of
the era.
[Illustration: The pink cliffs of Colorado National Monument are made of
Wingate and Entrada Sandstones. Underlying them, in the valley bottom,
Chinle shales form steep red slopes. (William C. Bradley photo)]
Triassic Period
(180-225 million years ago)
Saharan conditions continued to prevail in western North America during
the early part of the Mesozoic Era. In central Colorado, the lowest
Mesozoic deposits are the Triassic Lykins Formation, a series of soft,
bright red sandstones and shales. Where the Lykins is exposed along the
Front Range, its bright red color identifies it. Because of its
softness, it is often less prominent than adjacent rock layers in the
mountain foothills. The Lykins Formation includes some evaporites,
apparently derived from Permian evaporites washed into the Triassic
ponds and lakes which existed occasionally in this region.
Over almost the entire state, the rocks deposited at this time are very
similar. Formation names may differ—Lykins, Moenkopi, Chinle, Ankareh,
Wingate—but the rocks are almost universally fine-grained sandstones and
shales with a red or pink color. They represent ancient coastal plain,
dune, or delta deposits. Toward the western edge of the state they
coarsen, and contain layers of conglomerates similar to the Triassic
conglomerates of northern Arizona and Utah. These suggest that
mountain-building was taking place west of here at that time.
There are virtually no fossils known from Triassic rocks in Colorado,
although some fossil palm fronds have been found west of the San Juan
Mountains, in the southwestern corner of the state.
Jurassic Period
(135-180 million years ago)
During the Jurassic Period, Colorado was still a low, flat desert area
with intermittent streams flowing eastward over the surface of older
sediments. The Navajo Sandstone, formed from dune sands, was deposited
in the western part of the state. Streams flowing eastward from Utah
brought fine sediments—silts and muds—to western Colorado, forming what
is now the Carmel Formation. Near Canon City, coarse gravels bear
witness to local uplift in Jurassic time. Both these gravels and the
Carmel Formation were overlain by more dune sands, now hardened into the
Entrada Sandstone.
In Late Jurassic time the Colorado area, which had been predominantly
desert since Permian time, appears finally to have been submerged once
more. Fine calcareous muds of the Curtis Formation, containing
ammonites, belemnites, and other marine shellfish, show us that a
shallow sea transgressed from the west over the wind-blown sands. This
sea was, geologically speaking, of short duration—only a few million
years. Bounded on almost all sides by desert, it seems to have dried up,
depositing the gypsum that is now present in a thin layer along the
Front Range between Denver and Canon City in the Ralston Formation.
At about this time, however, the climate underwent a major change.
Deposits above the Ralston indicate an increasingly moist environment,
the environment in which the Morrison Formation was deposited over most
of Colorado and parts of the adjacent states of Kansas, Arizona, Utah,
and Wyoming. The Morrison Formation is exposed in many places, and is
characteristically composed of layers of fine, limy mud, brightly
colored in streaks of red, brown, green, and blue. In most areas it is
so soft that it becomes soil-covered; it is well exposed only in
roadcuts or where it is protected from erosion by a “caprock” of harder
sediments or lava. Spectacular outcrops can be seen in new roadcuts
along U. S. Interstate highway 70 just west of Denver.
[Illustration: In this roadcut along U. S. Interstate 70 west of Denver,
Jurassic and Cretaceous rocks are unusually well exposed in the Dakota
hogback. Green and purple shales represent the dinosaur-bearing Morrison
Formation. The Cretaceous Dakota Group forms the eastern, higher half of
the cut. Black layers are carbon-rich clays of the South Platte
Formation, frequently quarried locally for ceramic uses. (John Chronic
photo)]
Fossil dinosaur bones occur in great numbers in the Morrison Formation
near the towns of Morrison and Canon City and at several other places in
Colorado. Those at Canon City have been quarried extensively, and are
now mounted in a number of museums in the United States. At Dinosaur
National Monument, in eastern Utah and northwestern Colorado, many
excellent remains have been found; those in Utah can be seen in place in
the rock in a striking exhibit at the National Monument.
[Illustration: In an old painting, a paleontologist contemplates fossil
bones found near Morrison. The date is 1877. The bones are those of the
70-foot dinosaur _Apatosaurus_, more commonly known as _Brontosaurus_,
shown below in reconstruction.]
[Illustration: Apatosaurus]
Some of the dinosaurs known from the Morrison Formation reached 80 feet
in length. Both plant-eating and meat-eating types are known. In
addition to the bones themselves, gastroliths or gizzard stones can
frequently be found; these highly polished stones were as essential to
dinosaur digestion as gravel is to a chicken or a caged canary.
Along with the dinosaur fossils are found abundant remains of water
plants called charophytes. These plants formed tiny spiralled balls of
calcite as part of their reproductive activities; both the little balls
and the stalks of the plants themselves occur in many parts of the
state. In western Colorado, near Grand Junction, silicified shells of
freshwater snails can also be found in the Morrison.
Early in the 1900s vanadium, radium, and uranium were discovered in
Jurassic sandstones and mudstones of western Colorado. Extensive mining
in this area has revealed that these elements often become concentrated
by groundwater in organic material such as fossil plant stems or
dinosaur bones. The search for radioactive minerals has thus brought to
light many ancient fossil accumulations.
Cretaceous Period
(70-135 million years ago)
Early in Cretaceous time, marine conditions once more prevailed in
Colorado. This is indicated by a marked change in rock types from beach
and near-shore deposits to true marine sediments.
[Illustration: Between the Front Range and the Plains the Cretaceous
Dakota Formation forms a hogback ridge which can be traced for 200 miles
or more. The well-cemented sandstone resists erosion, and so remains as
a ridge when softer layers are stripped away. (Jack Rathbone photo)]
The sandstones derived from beach sands sometimes include coarse pebbles
of chert which can be traced to sources in Permian rocks of Utah and
Nevada. Occasionally the beach and near-shore deposits include marine
shells like oysters, indicating that there were brackish and salt water
lagoons and marshes along the shore. The Dakota Formation represents the
beach of the transgressive or advancing sea. This formation contains oil
in eastern Colorado, Nebraska, and Wyoming; the oil itself may have been
derived from decay of organic materials in swamps behind the beaches and
bars.
As the sea deepened in eastern Colorado, finer sediments were deposited.
These included the black muds of the Benton Shale, and the Niobrara
Limestone, a shallow-water deposit containing abundant shells of clams
(_Inoceramus_ and _Ostrea_) and ammonites and tiny one-celled animals
called Foraminiferida. Above the Benton and Niobrara Formations lie the
fine gray muds of the Pierre Shale. Several thousand feet thick, the
Pierre contains occasional beautifully preserved ammonite shells as well
as bones from fossil fish and swimming reptiles.
[Illustration: Cretaceous rocks in Colorado are rich in fossil
pelecypods. Each of the fossils illustrated above may grow to a much
larger size than shown.]
[Illustration: Shales of the Laramie Formation contain many recognizable
plant fossils.]
The rocks deposited in western Colorado at this time are markedly
different from those deposited in eastern Colorado. In the east,
deposits are fine and very limy, containing abundant shells and little
in the way of coarse debris. In the west, sandstones of the Mesa Verde
Formation dominate, and coal beds suggest marshy or swampy conditions
inshore from the ancient ocean. This is just the pattern we would expect
from a low-lying region bordering a shallow sea, a region similar
perhaps to the southeastern Atlantic and Gulf coasts of the United
States today.
Toward the end of the Cretaceous Period, the sea receded from Colorado.
Beaches and bars of the retreating sea left a sandstone layer which now
outcrops prominently east of the Front Range as the Fox Hills Sandstone.
Above lie interbedded sands and coals, the Laramie Formation. The
presence of coal above beach sands shows that the coal swamps moved
eastward as the sea retreated.
The exact age of the shoreline deposits and coal beds varies from place
to place in such a way as to indicate that the sea withdrew slowly and
irregularly. In general the shore moved eastward, but there are
localities such as North Park where deposition lasted much longer than
elsewhere. In some places no real beach was formed at the ancient strand
line.
In western Colorado, the end of Cretaceous time is marked by coarser
beds, indicating an increased rate of uplift in Utah. Conglomerates were
deposited in the beds of the McDermott Formation, now visible along the
Animas River south of Durango.
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.
[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:
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.
GOLD, SILVER, AND OTHER METALS
Colorado’s placer and lode sources of gold, which gave first impetus to
the series of mining booms in the state, were fantastically rich. Summit
and Lake Counties, for instance, each produced more than $5,000,000 in
placer gold between 1859 and 1867. During the same nine-year period,
more than $9,000,000 in lode gold was produced from Gregory Gulch, a
tiny canyon between Central City and Black Hawk. Other districts
rivalled or surpassed these figures.
Early in the game it was recognized that almost all the deposits
occurred along what came to be known as the “mineral belt,” a
fifty-mile-wide zone extending southwest from the Boulder region. Most
of the metals mined in the state come from this belt, but there are
three notable exceptions: Cripple Creek, Silver Cliff, and western
Colorado vanadium and uranium districts. In the first few years of the
Colorado rush, gold ores and placer gold were discovered only in the
northeastern part of the mineral belt. Gradually the belt was found to
extend further and further southwest: Tincup was discovered in 1861,
Silverton in 1870, Lake City in 1871, and Telluride in 1875. Aspen, on
the western edge of the belt, was not discovered until 1879, perhaps
because the area was difficult of access and lacked the easily
recognizable native gold.
In the northeast part of the mineral belt, gold and other minerals occur
in veins in Precambrian granite and gneiss. In the Leadville and Aspen
areas, ores are associated with altered Paleozoic limestones. At the
southwest end of the mineral belt, in the San Juan Mountains, ore veins
are found near or in Tertiary volcanic rocks. Native gold, gold-bearing
compounds, and other metallic ores in these veins originated where
mineral-rich solutions from deep within the earth penetrated fissures
and joints in the surrounding rock. Regardless of the age of the host
rock, almost all the ores of Colorado were deposited in the early or
middle Tertiary Period, about 35 to 70 million years ago.
Gold and silver are no longer mined extensively in Colorado, although
any summer Sunday will see weekend operators panning near mountain
streams or trundling rock from one-man mines. The recent rise in the
price of silver has encouraged many miners to reopen old shafts. The
most active mines in the state today are those producing molybdenum,
lead, zinc, and vanadium. (Vanadium, although a metal, usually occurs in
Colorado with radioactive minerals, and so is discussed with them rather
than with the metals.)
[Illustration: The Colorado mineral belt extends from Boulder County on
the northeast to San Juan County on the southwest. Almost all of the
prominent mining districts in Colorado lie along this belt. Cripple
Creek and Silver Cliff, however, lie far to the east of the general
trend.]
Telluride
Denver
Colorado Springs
Alamosa
BOULDER
Ward
Gold Hill
Boulder
Nederland
GILPIN
Central City
Black Hawk
JEFFERSON
Golden
CLEAR CREEK
Empire
Georgetown
Silver Plume
Idaho Springs
SUMMIT
Breckenridge
EAGLE
PITKIN
Aspen
GUNNISON
Tincup
CHAFFEE
PARK
Climax
Alma
Como
Fairplay
TELLER
Cripple Creek
FREMONT
OURAY
Ouray
Camp Bird
Ironton
SAN JUAN
Silverton
HINSDALE
Lake City
LA PLATA
Durango
MINERAL
Creede
CUSTER
Silver Cliff
All told, some 430 metal mining districts have been established as legal
entities in the state of Colorado. Each of these districts had the right
to draw up its own regulations concerning prospecting, claims, and
mining rights, within a framework established by the Federal government.
Only a few of the districts ever became really significant producers.
The geology and history of several of the leading areas are presented in
the pages that follow.
Boulder County
Gold Run, near Gold Hill, was the scene of one of the earliest strikes
in Colorado. Gold was found here in December 1858, and was sluiced from
stream sands and mined from veins early in 1859. Active placer mining
lasted only about a year, however, and lode mining dropped off rapidly
as near-surface oxidized ores were worked out. When a smelter was
erected at Black Hawk in 1868, and sulfide ores could be treated, there
was a revival of activity. In 1869 the Caribou and Poorman mines near
Nederland were discovered; they quickly became the most active mines in
the county. The Ward district opened soon after.
In 1872, a gold-silver telluride called petzite was found in veins at
Gold Hill. Renewed prospecting in this area resulted in location of
mines near Sunshine, Salina, and Magnolia. During the years that
followed, new mines appeared almost as fast as old ones were depleted.
In 1892, the peak year, more than $1,000,000 in gold and silver was
produced; total production has been about $25,000,000.
In 1900, a black mineral common in the Nederland area was recognized as
ferberite, an ore of tungsten, and a new rush to the area started.
During the next eighteen years Boulder County was the main tungsten
producer in the United States; about 24,000 tons of tungsten trioxide,
worth $23,000,000, were produced here. The ore was found in nearly
vertical veins six inches to three feet thick, in a lenticular area
about nine miles long extending from Nederland northeast to Arkansas
Mountain, four miles west of Boulder.
Boulder County is characterized by an abundance of small mines. Old
shafts, pits, and mine buildings can be found throughout the central
part of the county. Little mining is done here today; many of the towns
that once peppered these hills have fallen into decay or disappeared
entirely.
Central City and Idaho Springs
The Central City-Idaho Springs area was the principal metal mining
region in the state until the late 1880s. In 1858, rich placer deposits
were discovered in gravels and river terraces along both forks of Clear
Creek. Exploration upstream led to discoveries of rich oxidized quartz
veins at Central City, Black Hawk, and Idaho Springs. These veins, which
generally trend northeast-southwest, extend through the mountains in a
zone about six miles long and three miles wide between the two forks of
Clear Creek.
The ores filled a multitude of cracks and fissures in the Precambrian
bedrock. The veins are usually less than five feet thick, and are almost
vertical and often clustered in zones up to thirty feet wide. The
position of one of the vein systems may be seen clearly between Black
Hawk and Central City—the ore-bearing rock has been mined out, but a
series of collapsed tunnels marks the line where the veins crossed the
valley. A monument here commemorates the discovery of Gregory Gulch, one
of the richest localities in the state.
Several rich veins were mined in both directions—southwest from Central
City and northeast from Idaho Springs—until the mines met. The Argo
tunnel, marked by dilapidated buildings and extensive dumps on the north
side of Idaho Springs, connected the two districts; it was completed in
1904.
The “Patch,” a deep crater-like hole on Quartz Hill, about one mile
southwest of Central City, is an intriguing feature in this area. It was
produced by glory-holing, a mining technique in which a deep tunnel is
deliberately caved by blasting, so that ores above the tunnel can be
removed. This glory hole was dynamited below an irregular mass of highly
broken rock where many ore-rich veins converged. After the caving, ores
were taken out through the remaining part of the tunnel.
The principal ore minerals of Central City and Idaho Springs are native
gold, pyrite, sphalerite, galena, chalcopyrite, and tennantite.
Prospecting for uranium was carried out during the 1950s but no uranium
was ever mined here.
The area has produced almost $200,000,000 worth of gold, silver, lead,
zinc, and copper. A few mines still operate seasonally or on a small
scale, but tourists, many of them riding Jeeps across the mountainous
terrain to visit mines and ghost towns, are often more visibly active
than the mines.
Georgetown, Empire, and Silver Plume
A few miles southwest of Idaho Springs, another mining area had a
similar, though less productive, history. In 1859, placer and lode gold
were discovered near what is now Georgetown. Placer mining dominated
here between 1859 and 1863. Gravel and crushed rock from decomposed
quartz and sulfide veins were washed through sluiceboxes in the same way
as placer gravel, gold being caught in riffles or gunny sacking on the
bottoms of the troughs. The veins were found to be decomposed to depths
of about 40 feet; below this the gold occurred closely associated with
sulfides such as pyrite, sphalerite, galena, and chalcopyrite, from
which it could not easily be separated. However, smelters were developed
in 1866 for treatment of these sulfides, and gold, silver, lead, and
copper were recovered. Gradually, as the gold was worked out, silver and
lead became the important products of the mines.
[Illustration: Sluicebox mining was a common sight near the early gold
camps, where primary recovery was from placer deposits or decomposed
quartz and sulfide veins. (State Historical Society of Colorado photo)]
Leadville
Placer gold was discovered in 1859 in California Gulch, about seven
miles north of the present town of Leadville. The rush that followed was
short but sweet; the camp was called Oro—gold! About $5,000,000 was
produced from the placer mines within two years, though by 1861 the area
was all but deserted, for the easily won placer gold was gone.
[Illustration: Early-day Leadville sprawled among its mine dumps at an
elevation of 10,200 feet. The Sawatch Range, in the background,
contained many smaller mining communities, now deserted. Mt. Massive,
the state’s second highest peak, forms the crest of the continental
divide here. (State Historical Society of Colorado photo)]
In 1875 a smelter was erected a few miles downstream from Oro to process
cerussite—silver-rich lead carbonate—that occurred in the placer sands.
For years this mineral had been considered a nuisance because, being
much heavier than sand, it tended to separate out with the gold. The new
town of Leadville sprang up near the smelter and shortly afterward more
lode deposits were discovered south of the placer workings. From $63,000
in 1875, production climbed to $2,500,000 in 1878 and more than
$15,000,000 in the peak year of 1882.
Geologically, the ores of this district occur as Tertiary replacements
and veins in Ordovician, Devonian, and Mississippian limestones. The
“Blue” or Leadville Limestone, of Mississippian age, contains the
richest ore. Ore deposits were formed after the limestones had been
faulted and cracked extensively by mountain-building movements; the ores
themselves probably crystallized from molten or gaseous materials
involved in related igneous intrusions. River gravels and glacial debris
mask the true nature of the lode deposits, but studies in the mines show
that the fault systems along which ores are deposited trend north or
north-northeast.
The Leadville district is now experiencing its third mining boom as a
newly recognized lead-zinc orebody is being developed. Production is
expected to reach 700 tons of ore per day by 1971. Total production of
gold, silver, lead, zinc, and copper in the district has reached
$500,000,000.
Breckenridge
Breckenridge was also discovered in 1859, with placer gold the first
attraction. The placers gave out in 1862 after about $3,000,000 in gold
had been recovered. Earliest attempts to mine the rich silver and lead
veins of the district were in 1869.
As at Leadville, the sedimentary rocks of the area were intruded by
granitic masses in Tertiary time, but here the sedimentary rocks are
mostly Pennsylvanian sandstones and shales. These rocks were badly
faulted and broken during the intrusion, and the ores were deposited as
the granitic material cooled. The lode deposits occur mostly in small
veins well hidden by surface sands and gravels. Some of the veins
yielded exceptionally beautiful crystallized wire and flake gold,
specimens of which are on display at the Colorado School of Mines
library in Golden and in the Denver Museum of Natural History.
Dredging for alluvial gold was attempted in 1898 in the Breckenridge
district, but this method of extracting gold was not successful until
1905. A number of dredges operated between 1910 and 1925. These floating
behemoths shovel up gold-bearing gravels from the bottom and one side of
the pond on which they float, sort out the gold in giant sluiceboxes,
and spew out the leftover gravels in great arc-shaped heaps that can be
seen near Breckenridge and Fairplay and in a number of other valleys in
Colorado. They depend for their operation on a plentiful supply of water
and a shallow water table, but they can sift through quantities of
gravel at relatively low cost. All told, about $7,000,000 in gold has
been dredged from this district.
Fairplay
[Illustration: This gold dredge, still floating in its pond just south
of Fairplay, operated from 1941 to 1952. With chains of buckets like
those in the foreground, it dug gravel 70 feet below water level,
carving a 35-foot bank above water level; in effect it mined to a depth
of 105 feet. This dredge extracted nearly 115,000 ounces of gold from
about 33 million cubic yards of gravel (John Chronic photo)]
Another gold field discovered in 1859 was in the northwest corner of
South Park, along the headwaters of the South Platte River. Several
mining camps were established here. After early production of rich
placer deposits, claims were consolidated and large flumes constructed
so that gold could be recovered by hydraulic mining. In this type of
mining, streams of water from high-pressure hoses are directed at gravel
surfaces. The gravels are washed into long sluiceboxes, where gold is
caught in riffles. Hydraulic mining continued upstream from Fairplay
until about 1900.
In 1922 a dredge was constructed near Fairplay to process gravel along
the South Platte and in the valley floor. An even larger dredge,
constructed in 1941, operated until 1952, when rising labor costs
overrode the narrow margin on which it operated. At the time operations
ceased, the dredge was recovering about six cents in gold for each cubic
yard of gravel processed.
Placer gold has always been the principal mineral product of the
Fairplay area, but native gold also occurs in the surrounding mountains
in quartz veins, and many small mines were developed to extract it.
Sulfide ores were also mined; they contained silver, lead, and zinc as
well as gold. In the Mosquito Pass and Horseshoe Amphitheater areas,
there is renewed activity now because of the recent rise in the price of
silver.
Silverton
Gold was discovered in the San Juan Mountains of southwest Colorado in
1870. The earliest mine, near what is now Silverton, was located by a
group of prospectors sent out by Governor Pile of New Mexico Territory.
Since the site was on Ute Indian land, real mining did not begin until a
treaty allowing it was ratified in 1874.
Production in the Silverton district has been from veins in Tertiary
volcanic rocks within an elliptical area known as the Silverton
cauldron. Here the volcanic rocks, part of the several thousand feet of
lava flows and ash falls of the San Juan volcanic field, were cracked
and faulted by a second period of igneous activity. Ores formed in the
cracks and fissures.
In the 1870s the Silverton district was very remote, and difficulties
with transportation retarded activity there. In 1882, however, a
narrow-gauge railroad was built connecting Silverton with Durango, and
the problem of transporting ore out of the isolated mountain valley was
simplified. The railway still exists; a train makes daily passenger runs
during the summer—the only remaining operating narrow-gauge line in the
United States. The track follows the Animas River canyon, whose cliffs
and crags are dotted with long-abandoned mines, prospect holes, and mine
buildings, monuments to the tenacity and determination of the men who
mined here.
Production in this district was more than $22,000,000 in gold and
$20,000,000 in silver between 1874 and 1923. New activity is evident
here, as in other silver-rich areas of Colorado, because of recent
demand for silver, lead, and zinc.
[Illustration: Silverton lies in a remote mountain valley in the San
Juan Mountains. Silver, gold, lead, and zinc have been mined here since
1874. Storm Peak, composed of Tertiary volcanic rocks, forms the
backdrop; the narrow-gauge railroad track is visible in the foreground.
(Jack Rathbone photo)]
Ouray
Ouray was settled in 1875, when gold and silver deposits were found near
Mount Sneffels. Since 1877, mines in Ouray County have produced over
$35,000,000 in gold and $32,000,000 in silver. The district is still
quite active: in 1965, mines in this area produced more than $9,000,000
in gold, silver, copper, lead, and zinc, about a third of total Colorado
production of these metals for that year.
[Illustration: A few miles south of Ouray, along Uncompahgre Gorge, an
old mine clings to the slope below the Million Dollar Highway (U. S.
550). Abrams Mountain rises in the background. The Precambrian
Uncompahgre Quartzite outcrops up to about the road level; Miocene
Sunshine Peak Rhyolite caps the peak. (Jack Rathbone photo)]
A mile north of Ouray a prominent intrusive stock marks the center of
mining activity closest to Ouray. The richest deposits of the Ouray
area, however, lie about five miles southwest, near Mount Sneffels and
Red Mountain Creek. There, several large mines, including the famous
Camp Bird mine, have operated for many years, extracting ore from
hundreds of veins that underly the surface. Some of these veins are two
to four miles long. They are in Tertiary volcanic rocks of the San Juan
Formation. Quartz and calcite are the common gangue (non-economic)
minerals, and pyrite, sphalerite, galena, and chalcopyrite are the most
abundant ores. Most of the silver is in the galena; gold occurs in
streaks and nodules associated with quartz.
About ten miles south of Ouray, along the “Million Dollar Highway” (U.
S. 550), the Red Mountain district lies on the northwest edge of the
Silverton volcanic cauldron. It contains a number of small pipelike
bodies very rich in silver-copper and silver-lead ores. Following the
mid-Tertiary volcanism and ore intrusion, surface rocks in this area
were intensely oxidized: resulting iron oxides now form the gaudy reds
and yellows of Red Mountain and the slopes near Ironton. This
alteration, as well as the fact that much of the area is covered with
fallen rock, stream gravels, or glacial deposits, compounds difficulties
of locating the small though high-grade ore deposits.
The Idarado Mine, on the east side of U. S. highway 550 near Red
Mountain, used to produce ores from nearby volcanic pipes; now it
produces from veins some distance to the northwest. The area is
honeycombed with tunnels and shafts.
Aspen
Silver was found at Castle Creek and on Aspen Mountain in 1879. A group
of prospectors from Leadville, apparently after examining maps of the
Geological and Geographical Atlas of Colorado published in 1877,
explored along the line of Paleozoic limestones encircling the Sawatch
Range. As they had hoped, they found ores similar to those at Leadville
in rocks of the same age.
Mining began at Aspen in 1880. Here, as at Leadville, intrusion of
granite porphyry into or near the Leadville Limestone had broken and
deformed the layers, and ores were deposited in fissures and as
replacements during cooling of the intrusions. The intricacy of faulting
which controls the ore pockets in the limestone is well shown on the map
of Aspen Mountain in Chapter II.
Glaciation occurred in this area, and glacial deposits cover most of the
ore bodies and outcrops so that little bedrock is exposed. Mapping was
accomplished by extrapolating to the surface the bedrock patterns shown
in mine pits, shafts, and tunnels.
Aspen produced some of the richest silver ores in the world, and thrived
as a boom town for most of two decades. In 1888 the value of ores
produced reached over $7,000,000; the next year it topped $10,000,000.
After the silver crash of 1893 production declined rapidly; the last
mines were closed in the 1920s. Total production of silver, lead, zinc,
and copper reached about $100,000,000. There was virtually no gold in
the ores at Aspen.
Creede
[Illustration: Creede and its mines are located in an area of Tertiary
rhyolite and dacite, light-colored volcanic rocks.]
Happy Thought Mine
Amethyst Mine
West Willow Creek
AMETHYST FAULT
Last Chance Mine
Del Monte Mine
Commodore Mine
Jackpot Mine
Coppervein Mine
Bachelor Mine
BULLDOG MOUNTAIN FAULT
Kansas City Star Mine
Commodore Tunnel
Mustang Tunnel
Nelson Tunnel
Exchequer Mine
SOLOMON FAULT
CAMPBELL MOUNTAIN
Holy Moses #2
Holy Moses Mine
Ridge Mine
Solomon Mine
Monte Carlo Mine
Mollie S. Mine
East Willow Creek
Ramey Tunnel
Dora Belle Mine
Mammoth Tunnel
Homestake Mine
Mammoth Mine
MAMMOTH MOUNTAIN
Nancy Hanks Mine
Pipe Dream Mine
THE NARROWS
Windy Gulch
CREEDE
Willow Creek
The Creede district ranks as one of the most productive silver areas in
the United States. It came into being largely as a result of a discovery
by N.H. Creede in 1889. When exploring in this area, he was reported to
have exclaimed “Holy Moses!” on examining a rich piece of ore, thus
giving the name to the mine which initiated the rapid development of the
district. By the end of 1892 the Holy Moses and nearby mines had
produced ore valued at more than $4,000,000. The area was so rich that
it managed to survive 1893’s great decline in the price of silver; by
1920 almost $42,000,000 in gold, silver, lead, and zinc had been mined
there.
The ores, silver-bearing galena, sphalerite, native gold, pyrite, and
chalcopyrite, are in quartz or amethyst veins in faulted and shattered
Tertiary volcanic rocks. Nearly all the ore deposits lie along a complex
system of vertical faults, the Amethyst fault zone, which runs more or
less northwest-southeast through this region. Both the faulting and the
enrichment of the fault fissures are believed to have taken place in
mid-Tertiary time, shortly after deposition of the volcanic host rocks.
Cripple Creek
[Illustration: Cripple Creek, on the flanks of the Pikes Peak massif,
has produced more than $400,000,000 worth of gold. The Sangre de Cristo
Mountains are visible in the distance beyond the Arkansas River valley.
(Jack Rathbone photo)]
In 1890, two sheepherders stumbled on some richly mineralized rocks near
Cripple Creek. A boom developed immediately, for the rocks contained
both gold and silver. Since then, the area has produced more than
2,000,000 ounces of silver and nearly 19,000,000 ounces of gold.
Cripple Creek has produced almost half of all the state’s gold and
silver. The ores are located in or at the edge of a large mass of middle
Tertiary volcanic rocks which form an elliptical basin or _caldera_
several miles across. The caldera, surrounded by Precambrian gneiss and
granite of the Pikes Peak massif, was probably formed by collapse of a
volcanic center that had erupted through the older rock. The collapse
shattered the rocks around the basin margin, and subsequent volcanic
activity introduced mineral-rich solutions into the many faults and
fissures produced by the collapse. Tellurides of gold, silver, and
copper, as well as pyrite, sphalerite, galena, tetrahedrite, and other
minerals, are characteristic.
Climax
[Illustration: At Climax, the ore occurs scattered through the intrusive
Climax Granite Porphyry and the intruded Idaho Springs Formation.
Visitors can tour the surface workings during the summer months.]
Tertiary dikes
Shell of Climax stock
Core of Climax stock
Ore zone
Precambrian granite
Fault
Dykes
Molybdenum now ranks as the number one metal mined in Colorado. Over
$105,000,000 of “moly” was mined here during 1969, almost all of it from
the Climax Mine, the world’s largest single source of this metal. The
Climax deposit is located high on the west slope of Ten Mile Range in
central Colorado, about 100 miles southwest of Denver. It is in the
central part of the Colorado mineral belt, near the Mosquito Fault, a
prominent structural feature which extends about sixty miles along the
north-south trend of the mountains. Rocks on both sides of this fault
are intruded by Tertiary granite dikes, sills, and stocks. The Climax
Mine is in a stock just east of the fault, near the axis of a broad
anticline in Precambrian metamorphic rocks.
Ore minerals at Climax are molybdenite, huebnerite, and cassiterite;
pyrite is recovered also for the manufacture of sulfuric acid. The ore
is very low in metal content, containing only one-third of a percent of
molybdenum, 0.005% tungsten trioxide, and 0.0001% tin. The great size of
the ore body and efficient recovery by modern methods make Climax a
profitable mine, however. Production has risen each year since the mine
began operation.
Urad Mine near Berthoud Pass is a newly developed near-surface
molybdenum mine similar to Climax. Nearby at the Henderson Mine the ore
body is more than half a mile below the surface of the ground.
RADIUM, URANIUM, AND VANADIUM
Over a large area of the Plateau Province in western Colorado, Mesozoic
sedimentary rocks are locally stained bright yellow, orange, or green.
Such staining suggests mineralization, and radioactive compounds were
recognized here before 1900. At that time, however, there was little or
no market for them or for the vanadium frequently associated with them.
When Marie Curie required radium for experiments with her newly
discovered element, the raw materials were sent from western Colorado;
by and large, though, production of radium from these ores was
prohibitively expensive.
In 1905, vanadium was found to be effective in toughening steel. The
Vanadium Corporation of America was formed to mine the Colorado ore.
This company mines a rich zone in the Jurassic Entrada Sandstone, where
vanadinite occurs with carnotite and other uranium ores. In the early
days of vanadium mining, the uranium ores were discarded with other
gangue materials; now, of course, uranium is produced from them.
Since 1945, uranium production has been an important Colorado industry;
in 1969 about $17,500,000 worth was produced. Uranium occurs in the
state in two very different situations. In the Plateau Province, where
it was first discovered, it occurs in sedimentary rocks as patches of
pitchblende, carnotite, and a greenish yellow mineral called
schroekingerite. It is most abundant in the Triassic Chinle Formation
and the Jurassic Entrada and Morrison Formations, where it was probably
deposited by downward movement of rainwater from overlying uranium-rich
Tertiary volcanic rocks. Concentrations of uranium often occur in or
near organic matter such as coal, fossil bone, or petrified wood, so
mines tend to be located along rock layers carrying abundant organic
material.
Another type of uranium ore is found in the Mountain Province. Veins in
Precambrian rocks of the Front Range and several other ranges contain
pitchblende which seems to have been deposited by hot groundwater rising
through broken and fissured Precambrian rocks. Often exceedingly rich,
such ore is mined in the manner of most of Colorado’s metals. The
Schwartzwalder Mine, a few miles northwest of Golden, has produced more
ore of this type than any other mine in Colorado.
OIL, NATURAL GAS, AND OIL SHALE
Petroleum and natural gas have been found in large quantities in the
Prairie and Plateau Provinces in Colorado, as well as in smaller
quantities in North Park in the Mountain Province. They generally occur
in porous sandstone and limestone layers, where they have been trapped
by overlying finer-grained, less permeable layers in or near folds and
faults.
Several oil and gas seeps were found along the mountain front shortly
after the arrival of the earliest settlers. Near Canon City, on Oil
Creek, a plaque commemorates the first production:
Oil Creek—site of the first oil well in the west—second place in the
United States to produce petroleum from wells. In 1862 ... A. M.
Cassedy drilled an oil well 50 feet deep. By February, 1863,
production was one barrel a day. Later, several thousand gallons of
petroleum were produced by primitive methods, and kerosene and
lubricating oil were shipped by ox team as far as Denver and Santa Fe.
About twenty miles to the southeast, near Florence, the Cretaceous
Pierre shales were drilled in 1876. Oil was found in a system of
intersecting fractures and joints. Some of the early wells in the
Florence field are still producing, making this Colorado’s oldest and
longest producing field. It has yielded more than 10,000,000 barrels of
oil.
Small quantities of oil have been produced near Boulder since about
1900, also from Pierre sandstones and shales. In this area, wells were
located by “dowsing” or “witching,” as was fashionable at the time.
Several old rigs can be seen near Boulder Reservoir. As at Florence, oil
has been trapped in fractures of otherwise dense and impervious shale.
Some gas is produced and is used by local farms.
More recently, oil was found far beneath the surface in the northern
part of the Prairie Province. Here, in the Denver Basin, oil is produced
from several levels in the Dakota Sandstone. The oil has accumulated in
lenses of beach sand deposited along the shoreline of the Cretaceous
sea. The general trend of the shoreline, and of the oil fields, is
northeast-southwest. The shore appears to have been similar to Georgia’s
present coastline: a swampy tidal zone separated from open sea by
lagoons, sandy bars, and clean sand beaches.
Individual oil pools in the Denver Basin are small, but there are many
of them; they lie nearly a mile below the surface, under much of Morgan
and Logan Counties and adjacent parts of Nebraska. Exploratory and
development drilling keeps total oil production at about 50,000 barrels
a day. Oil and gas produced here is piped to Denver and other Colorado
cities.
In southeastern Colorado, oil and gas occur in late Paleozoic limestones
and sandstones similar to those which outcrop at the edge of the Wet
Mountains. Prospecting by geophysical methods and by drilling has
revealed several small, rich accumulations, one of which is thought to
contain about 30,000,000 barrels of oil.
The Rangely field, in northwestern Colorado, is the most productive
field in the state. Located in the northeastern part of the Uinta Basin,
it is an outstanding example of an anticlinal field, where oil is
trapped in a large, gentle dome. The shape of the dome shows up well on
the surface; rock layers can be seen dipping outward in all directions
from the town of Rangely. Oil was found by drilling on the crest of the
dome. At first, oil was produced from fractures in the Cretaceous Mancos
Shale at less than 1,000 feet depth. Later, deeper drilling showed that
oil had also accumulated in the Permian Weber Sandstone, at 5,000 to
7,000 feet. At present this field is producing about 28,000 barrels of
oil a day, but the figure is dropping each year as the field is
depleted.
Oil and gas are produced in southwestern Colorado from the eastern edge
of the Paradox Basin and the northern edge of the San Juan Basin. In the
Paradox Basin, oil comes from Pennsylvanian limestone mounds or reefs.
Production in the Colorado part of the basin has been at most a few
thousand barrels per day; more is produced in adjacent Utah. In the San
Juan Basin, gas and oil are trapped in thin porous layers of Cretaceous
and Pennsylvanian sandstone, between impervious layers of shale. Most of
the production is in New Mexico, although some oil comes from the
Colorado part of the basin.
The greatest known potential oil resource in the world lies in the oil
shales of western Colorado. The richest of these shales cover an area of
1,600 square miles north of the Colorado River, south of the White
River, and just east of the Colorado-Utah line. The oil shales are part
of the Tertiary Green River Formation, which extends over much of
northwest Colorado, northeast Utah, and southern Wyoming. Oily material
called _kerogen_ is locked in these rocks, too solid to flow out of the
fine pore spaces of the shale. To free it the shale must be mined,
finely crushed, and heated until the kerogen converts to liquid oil.
This is an expensive process, and as yet production of petroleum from
the oil shale has not been possible at a cost which will compete with
production of oil and gas from wells. The United States Bureau of Mines,
as well as a number of oil companies, have sought for more than fifty
years to discover a less expensive method for extracting oil from the
shale. No doubt at some time in the future a competitive technique will
be developed, or a growing shortage of other oil will bring world prices
to a level with which present production techniques can compete.
Oil and gas production in Colorado is decreasing at present, even though
great efforts are being made to find new oil pools. Petroleum
prospecting and wildcat drilling are carried out in as yet unproductive
basins in the Plateau Province, in intermontane basins such as the San
Luis Valley, and on the Plains. Known reserves will continue to provide
the state with significant income for many years to come, and if oil
shale recovery becomes profitable. Colorado’s hydrocarbons will become
the most prominent of her commodities.
COAL
Coal resources of Colorado amount to about 60 billion tons. Only one per
cent of this has been mined. Thousands of tons are now being produced
daily from large mines in central, southern, and northwestern parts of
the state.
Colorado’s coal deposits were formed during late Cretaceous and early
Tertiary time, when seas were receding from this region and the land was
rising. They represent accumulations of leaves and other plant material
in swamps and flood plains similar to those now found in the delta of
the Mississippi River and in the swamps of southeastern United States.
Almost all Colorado coal is bituminous or soft coal.
Coal was recognized early in Colorado history by settlers along the
mountain front, and was mined west and north of Denver in the 1860s.
Several large underground mines still operate in this district,
supplying local power plants, but production does not compare with that
of the Walsenburg-Trinidad area in southern Colorado or the Hayden area
in northwest Colorado.
The Walsenburg-Trinidad region, part of the Raton coal field, has
produced coal since the building of the Santa Fe Railroad in the early
1870s. For many years coal from these mines moved the Santa Fe trains
and many of the numerous smaller railroads that served Colorado’s cities
and mining camps. The location of the mines helped to determine the
location of the Colorado Fuel and Iron Company smelter in Pueblo. Now,
most southern Colorado coal is used to produce electric power. Many
small mines, miles away from the power plant west of Trinidad, are
deserted.
A large coal-burning power plant has recently been built between Hayden
and Steamboat Springs, just west of the Yampa River. Here, some of the
extensive coal deposits can be seen in road cuts along U. S. highway 40.
Until conversion to diesel fuel became almost universal in North
American railroads, mines of this district produced coal for
locomotives.
In the heyday of the gold and silver mines, coal was also mined near
Coalmont, in North Park, and Como, in South Park. Coal from these areas
was used for fuel in nearby mining towns and ranches, and for the
narrow-gauge railroads that penetrated the mountains here.
At Anthracite, near Crested Butte, high-grade anthracite coal was mined
for a time. Identical in origin with other Colorado coal, the anthracite
of this region was hardened by heat and pressures from Tertiary igneous
intrusions forcing their way into local sedimentary rocks during
post-Cretaceous mountain building.
A multitude of other coal camps are scattered about Colorado: Cokedale,
Delcarbon, Coaldale, Roncarbo, Carbondale, and Cardiff stand out because
of their suggestive names. These early small camps are, like their
metal-mine cousins, largely deserted today.
CONSTRUCTION MATERIALS
Sand, Gravel, and Clay
Sand, gravel, and crushed rock rate high among geologic products in
Colorado; more than $27,000,000 worth of these materials were produced
in the state in 1969. Highway and construction activities have brought
recent expansion in the number and size of quarries and gravel pits.
Increasingly, Coloradoans are insisting that quarries and pits be
excavated only where they will not mar the natural beauty of the
landscape, and many old pits are now being filled in. Unfortunately, the
scars left by some quarries—such as that on the Rampart Range near
Colorado Springs—are difficult to erase.
Clay of good quality occurs in Cretaceous deposits in many parts of
Colorado, most frequently in the Dakota or Laramie Formations. In the
area around Golden, the Coors Porcelain Company for many years mined
clay for use in pottery and low temperature ceramic ware. Scars from
this mining can be seen along the mountain front north and south of
Golden, and deep clefts within the town, just west of Colorado School of
Mines, testify to the amounts of clay that have been removed. Colorado
clay is not pure enough to be used in high temperature ceramics, and the
present use for it is in the manufacture of common tiles and bricks.
A recent development in Colorado is the use of Cretaceous Pierre shales
in manufacturing lightweight aggregate for building. The shale is mined
between Golden and Boulder, near Colorado highway 93. In the nearby
plant, it is pulverized and then heated in a large rotating cylinder
until the surface of each particle fuses. Then the particles are quickly
cooled. The resulting product is much like cinder, light in weight and
yet strong. It can be mixed with cement for use in construction work
requiring a great strength-to-weight ratio, or made into concrete
blocks.
[Illustration: Quarrying of Paleozoic limestones and dolomites along the
east flank of the Rampart Range northwest of Colorado Springs has badly
defaced a prominent mountain backdrop. Recent seeding efforts by quarry
operators are returning the exhausted part of the quarry to its original
lightly vegetated condition, and hopefully, as the quarry is depleted,
the scar will disappear. (John Chronic photo)]
Stone
In Colorado, as in most parts of the world, building stone for local use
is quarried locally. Two of the state’s stones, however—Yule Marble from
the Crystal River Canyon, and Lyons Sandstone of the Front Range—have
been more widely used.
The Yule Marble, or Yule Colorado Marble, was produced by metamorphism
of Leadville Limestone in an area intruded by the Treasure Mountain
Granite, thirty-five miles south of Glenwood Springs. This exquisite
marble, which has graced many famous monuments and buildings (among them
the Lincoln Memorial and the Tomb of the Unknown Soldier), is known for
its almost uniform snowy whiteness and regular, fine crystallization.
Although its beauty, massive character, and uniformity made it a
sought-after ornamental stone, quarrying was economically marginal
because of the remoteness of the site. In spite of this, nearly
$7,000,000 worth of the marble was produced before the quarry closed in
1940.
[Illustration: Pure white marble was quarried for many years at the Yule
Colorado Marble Quarry, about three miles southeast of the village of
Marble. (U. S. Geological Survey photo)]
The Lyons area, north of Boulder, provides pink, hard, even-grained
sandstone which splits readily into slabs or flagstones. These are used
in the Denver-Boulder area for sidewalks and patios as well as for
facing buildings. Quarries owned by the University of Colorado provide a
constant supply of handsome facing material and flagstone for new
university buildings, although in recent years the high cost of stone
construction has limited its use on the campus.
[Illustration: Lyons Sandstone is quarried near Lyons, Colorado. The
salmon-colored sandstone splits along surfaces defined by slight
differences in size and arrangement of the sand grains. (John Chronic
photo)]
[Illustration: Most of the buildings of the University of Colorado are
faced with Permian Lyons Sandstone, which is widely used for buildings
and flagstones throughout the Boulder-Denver area. The University
Museum, shown here, was established in 1902, and contains over a million
scientific specimens, including many Colorado fossils and minerals.
Exhibits in the Hall of Earth portray Colorado’s geologic history.
(Tichnor Bros. photo)]
The Lyons Sandstone was deposited as beach and bar sand along the edge
of a sea which lay east of the Front Range in Permian time. After
deposition, the sand was deeply buried and compacted. Now tilted up
along the Front Range uplift, it comes to the surface along the east
side of the range. Only between Fort Collins and Boulder does the stone
have the desirable combination of hardness, thin-beddedness, and color
which makes it desirable for ornamental use. The pink color of the Lyons
Sandstone is derived from iron oxides, mostly hematite, disseminated
between the sand grains. Dendrites (often erroneously called fossil
ferns or plants) ornament some slabs; they were formed by
crystallization of manganese dioxide from groundwater as it slowly
percolated through the rock.
Lime and Gypsum
Outcrops of the Cretaceous Greenhorn and Niobrara Limestones provide
most of the cement materials in Colorado. A number of plants along the
mountain front, including a completely automated and dust-free one near
Lyons, provide the major population centers with millions of tons of
cement each year.
Colorado is richly endowed with gypsum, useful in cement and plaster
manufacture and for ornamental stone and sculpture. Along the eastern
front of the mountains, gypsum occurs in the Triassic Lykins Formation;
in the Mountain Province, it is abundant in Pennsylvanian sedimentary
rocks. Particularly high-quality Pennsylvanian gypsum is quarried at the
town of Gypsum, west of Eagle.
The Colorado portion of the Paradox Basin, in the Plateau Province,
contains immense deposits of Pennsylvanian gypsum. Here, rocks near the
surface have been pushed up into sharp northwest-trending faulted
anticlines by upward movements of gypsum and salt from depths of several
thousands of feet. The soluble salt and gypsum cores of these structures
have been washed away more rapidly than the surrounding layers of
sandstone and shale, leaving depressions such as Gypsum Valley, Paradox
Valley, and Sinbad Valley, on the crests of the anticlines. Red and
yellow Triassic sandstones and shales, especially the Chinle Formation
and the Wingate Sandstone, dip away from these valleys. Exploratory
wells indicate that vast masses of salt and gypsum are present beneath
the surface, and may extend to depths greater than 10,000 feet.
GEMS AND ORNAMENTAL STONES
More than thirty different gems and ornamental stones are known to occur
in Colorado. Amazonstone, amethyst, garnet, tourmaline, aquamarine,
topaz, lapis lazuli, quartz crystal, smoky and rose quartz, sapphire,
several varieties of agate, zircon, and other attractive stones are
gathered within the state, mainly in the Mountain Province. Turquoise is
known at several places in the volcanic area of southern Colorado.
Alabaster is mined along the northeastern mountain front near Fort
Collins and Loveland. Localities of interest to gem hunters are
described in _Colorado Gem Trails and Mineral Guide_, by Richard M.
Pearl.
Gem Village, in southwestern Colorado on U. S. highway 160 between
Durango and Pagosa Springs, is a favorite stopping place for tourists
wishing to see or buy colorful and attractive Colorado stones such as
petrified wood, agatized dinosaur bones, chalcedony, and jasper.
WATER
Although not all aspects of water and water supply are geologic, water
is an important geologic agent, determining the shape of the surface,
the distribution of minerals, and the location of caves. Water used in
Colorado comes entirely from precipitation within the state, as all of
Colorado’s rivers flow from Colorado outward toward the surrounding
lower-elevation states.
Surface Water
[Illustration: A cross section through the Front Range northwest of
Denver shows the redistribution and use of western slope water in
eastern Colorado through the Colorado-Big Thompson Project. This project
has cost about $160,000,000, but it is repaying the investment many
times over by providing electric power and increasing farm production.]
Moisture carried by prevailing westerly or northwesterly winds falls
primarily on Colorado’s western slope, although at some times of year
precipitation may come from the northeast or southeast. West of the
continental divide, where population is sparse, there is a surplus of
water. East of the divide, where more than 90 per cent of the population
lives, water is in desperately short supply. The high and largely
unpopulated Mountain Province receives by far the greatest proportion of
precipitation, while agricultural areas of the Prairie and Plateau
Provinces receive much less. Needless to say, the major problem
involving water in Colorado is how to move it from areas where it is
abundant to areas where it is needed.
In many parts of the state, complex water laws and complicated
irrigation canals and water systems were developed soon after the area
became settled. Gradually but inevitably, water resources have been
transferred from the western slope to the eastern. However, such
transfer must be undertaken with due regard for the rights of downstream
users, notably California, Arizona, and New Mexico.
One of the largest water movement schemes in the state is the
Colorado-Big Thompson Project. Water that otherwise would flow into the
Colorado River is piped from Grand Lake through the Alva B. Adams tunnel
under the high mountains of Rocky Mountain National Park, and into the
Big Thompson drainage near Estes Park. It then travels through a series
of reservoirs and tunnels into the South Platte River basin, where it is
used for irrigation and household water. The water is pumped up the
western gradient of this system by electric power produced as it flows
down the eastern slope. Surplus electric power serves the
Colorado-Wyoming area.
Another large project is the Denver Water Board’s Dillon Reservoir
Project, in which western slope water collected at Dillon is pumped
twenty-three miles under the continental divide through the Harold D.
Roberts tunnel to the North Fork of the South Platte River for use by
the city of Denver. The exit point of this tunnel can be seen a few
miles west of Grant along U. S. highway 285. This project is
continuously growing as Denver’s water needs mount.
In each of these projects, engineering geologists played a prominent
part in locating dams and tunnels that would not leak or fail, and that
could collect and transport a maximum amount of water during the
high-runoff spring season for distribution through the rest of the year.
Fortunately for geologists, the tunnels and bores necessary to the
projects allowed them to learn a great deal about the structure of the
interior of the high mountains, and helped to improve their
interpretation of earth history in this most interesting region.
The necessity for storing irrigation water along the eastern mountain
front has led to the creation of hundreds of new lakes in the region.
Although water levels vary with the season, many of the lakes provide
opportunities for water sports and recreation for the burgeoning inland
population.
Two large dams have recently been built in western Colorado for another
purpose: to control the flow of water in the Colorado River drainage
basin. Electric power for western Colorado also comes from these dams.
One of the dams is on the Gunnison River at Curecanti, upstream from the
Black Canyon of the Gunnison National Monument, and the other is on the
Frying Pan River near Ruedi. The latter was completed over the
objections of geologists, who believed that the extensive gypsum
deposits underlying the damsite would cause its failure. Cement pumped
deep into the rocks in the vicinity has so far prevented serious
rupture.
There is strong resistance by conservation groups to the construction of
more dams on Colorado River drainage, primarily because the Colorado and
its tributaries pass through many irreplaceable canyons, some of them
parts of National Parks and Monuments, that are very much a part of our
western heritage.
Groundwater
[Illustration: In the San Luis Valley, runoff from the San Juan and
Sangre de Cristo Mountains sinks into layers of sand in the Alamosa
Formation. Flowing along the sand layers toward the center of the
valley, it provides artesian water for irrigation of valley farmlands.]
SAN JUAN MOUNTAINS
LIMIT OF FLOWING WELLS
HUBBARD’S WELL
OTTOWAY’S WELL
ALAMOSA WELL
GEORGE NEWSOM’S WELL
CALKIN’S WELL
LIMIT OF FLOWING WELLS
Moraine
Alluvial Slope
SANGRE DE CRISTO MOUNTAINS
Sands, lava beds, gravels, conglomerates, etc.
Alamosa formation
Granites
WEST
SANTE FE FORMATION
SANTE FE FORMATION
EAST
Groundwater is extremely important to Colorado, especially in the
Prairie Province and the San Luis Valley. Below these two areas lie a
number of distinct and productive groundwater aquifers, several of them
artesian. In Otero County, for example, there are five major aquifers:
three separate Quaternary gravel deposits, the Cretaceous Dakota
Sandstone, and the Cheyenne Sandstone Member of the Purgatoire
Formation, also Cretaceous. All these aquifers are characterized by
their high porosity and permeability, which allow water to flow rapidly
through them. Wells in the younger, shallower aquifers produce as much
as 2,000 gallons per minute; those in the older, deeper aquifers produce
about eighty gallons per minute, some of it with an artesian “head.”
The San Luis Valley supports intensive agriculture, made possible by a
great artesian water supply. A thick series of soft interlayered clays
and sands, the Alamosa Formation, slopes down toward the center of the
basin from the surrounding mountains. Water entering the sandstone beds
at the mountain edges flows through the sand layers held there by the
impermeable clay beds. By the time it reaches the center of the valley,
it has developed considerable hydrostatic head, and the water rises in
wells without pumping. Unfortunately, both the irrigation water and the
soils in the San Luis Valley are highly alkaline. Constant evaporation
from the irrigated fields has concentrated the alkali near and on the
surface, rendering some of the land less usable than it was originally.
Caves
Colorado has many caves, most of them carved by underground water in
Paleozoic limestone. The Cave of the Winds at Manitou is the only one in
the state which has been developed as a tourist attraction. It is in
highly faulted Ordovician and Mississippian limestone near the mountain
front, where the faulting, coupled with the high relief, has accelerated
solution of the rock by allowing groundwater to percolate downward
rapidly. The cavern was probably carved during the Pleistocene Ice Age,
when surface water and groundwater were much more abundant than at
present. Deposition of stalactites and stalagmites has occurred within
the last few thousand years, as supplies and movement of water have
decreased.
Spanish Cave, above timberline on Marble Mountain in the Sangre de
Cristo Range, is probably the nation’s highest limestone cave. It is in
thick folded and faulted Pennsylvanian reef limestone, at an elevation
of over 12,000 feet. The cave has many intricate passageways branching
from its main vertical tubes and channels.
Fulford Cave, south of Eagle, is in the Mississippian Leadville
Limestone of the northern part of the Sawatch Range. Many other caves
are situated south of Fulford, near Woods Lake, where the limestone is
widely exposed and highly dissected.
Fairy Cave, northeast of Glenwood Springs, is the best known of the many
caverns in the Paleozoic limestones that form the southern flanks of the
White River Plateau.
[Illustration: In Cave of the Winds near Manitou, Paleozoic limestones,
cracked and tilted by uplift of the Front Range, have been honeycombed
by ground water. Calcite stalactites hang from the ceiling, while
stalagmites grow up from the floor. (Cave of the Winds Company photo)]
In the Plateau Province another type of cave is formed not so much by
groundwater as by weathering of the flat-lying alternating beds of
massive resistant sandstone and less resistant, thinly bedded mudstone
and shale. Where the resistant layers are undermined, great arching
caves develop. These are best observed at Mesa Verde National Park,
where many of them once sheltered Indian communities. They can also be
seen in Colorado National Monument and along the Colorado River and
several of its major tributaries.
[Illustration: Along the edge at Mesa Verde, caves in Cretaceous Mesa
Verde sandstone were used for shelter by Indians. Springs near the bases
of the caves, which provided the Indian communities with water, probably
contributed to the undermining of the sandstone cliffs. (Colorado
Department of Highways photo)]
Springs
The multitudes of mineral and hot springs in Colorado are a fascinating
and interesting facet of the Mountain Province. Some are located along
major faults, where the rocks are so broken and shattered that
groundwater can move freely toward the surface. Colorado Springs,
Manitou Springs, and Eldorado Springs are on the fault complex that
forms the east edge of the Front Range. Glenwood, Juniper, Steamboat,
and Poncha Springs are on well defined faults also.
[Illustration: Glenwood Hot Springs flow from Pennsylvanian shales of
the Belden Formation, where sedimentary layers are faulted by the sharp
upward tilting against the south side of the White River Plateau. Behind
the hotel and on the right can be seen the Mississippian Leadville
Limestone, cut by the Colorado River. (From a painting by William H.
Jackson, courtesy of Colorado State Archives and Public Record)]
Many other springs do not seem to be controlled so strongly by faulting,
but owe their presence to sources of volcanic or magmatic heat which
exist near to the surface of the ground. Some springs of this type issue
from Precambrian granite, or Cenozoic volcanic rock, while others flow
from sedimentary rock layers. Waunita Hot Springs and Pagosa Springs,
although near volcanic rocks, reach the surface through porous
sandstones and shales of Cretaceous age. Mt. Princeton Hot Springs comes
from alluvium but its heat source is the intrusive igneous rock which
makes up part of the adjacent mountain.
Springs of another general type are also present in Colorado where
aquifers, generally sandstones, are dissected by erosion. These springs,
usually not highly mineralized or warm, are most often found in the
Plateau Province. Such springs are frequent at the bases of the great
sandstone cliffs of Mesa Verde and Colorado National Monument.
Manitou’s carbonated springs, which attract many tourists, have their
origin in the arrangement and nature of the rocks through which the
water flows. Water from the Pikes Peak region, slightly acid from its
contact with the granitic rock, flows into the Manitou limestone all
along Ute Pass fault, which extends from Cheyenne Mountain northwest to
Woodland Park. Descending through channels along the fault, the water
becomes pressurized. Because of its pressure and its acid content, it
partly dissolves the calcium carbonate of the limestone, and from then
on carries carbon dioxide in solution. As the water comes to the surface
at the low point of the fault exposure, near the west edge of Manitou,
the pressure is released and the carbon dioxide effervesces, just as a
bottle of soda water effervesces when the cap is removed.
ENVIRONMENTAL GEOLOGY
The preceding part of this chapter mentions many ways in which man’s
destiny in Colorado has been shaped by geologic factors. Early
Coloradoans settled near gold and silver placers, later ones near mines
that produced ores of other metals, or in the towns that sprang up
around the mills and smelters that processed these ores. Our present
distribution of population is partly a heritage from these first
settlements, partly a result of later discoveries of oil, gas, and
radioactive minerals, and partly a response to the state’s extreme
topographic variation, which controls and delineates agricultural areas
and transportation routes.
In recent years, man has begun to appreciate the fact that he may
benefit in other ways from knowledge about geology. A new geology has
developed—_environmental geology_—which may be defined as the total of
all geological conditions and influences affecting the life and
development of man.
Environmental geology is a broad science, concerned not only with the
location of cities and towns, but with the uses people make of the land
and its economic products, and with the relationship between the
geological character of the land and the present and future location of
roads, dams, bridges, factories, homes, recreation facilities, sanitary
land fills, and even sewage plants.
Two aspects of environmental geology which are particularly pertinent to
Colorado’s residents are discussed below.
_Landslides_ and slumping rock or earth are a frequent menace to
Colorado’s development in the Mountain Province. Often activated by
heavy rains or deep manmade cuts, they can cause—and _have_ caused—much
damage to roads, buildings, and other works of man.
The flanks of North and South Table Mountains, near Golden, are mantled
by thick landslide debris; intermittent movement of the individual
slides has repeatedly affected the railroad, irrigation ditches, and
roads. As many as six different slides have moved within a single year.
In one slide area, asphalt road material is estimated to be thirteen
feet thick; successive layers of pavement have been laid one on top of
another to keep the street up to grade.
Landslides and landslide-prone areas may not be obvious to the untrained
eye. Each year buildings and roads are constructed on unsuitable rock
and soil foundations, in places where some degree of land slip is almost
inevitable. Building in such areas is risky, but sometimes worth the
risk; if condition are less than ideal, risks can be reduced by
specialized types of construction.
_Floods_ are a perennial threat to much of the state, because of the
high relief of the drainage basins and the torrential nature of the
spring and summer rainfall. Their damaging effects were realized early
in Colorado’s history, when canyons were used as highways and railroad
routes.
Colorado’s most expensive flood was probably the flood in the South
Platte River basin south of Denver in 1965, which caused $508,000,000
worth of damage and drowned six people. The losses can be attributed to
man’s failure to realize the significance of the South Platte drainage
routes and flood plains. Homes, shopping centers, and many other
buildings occupied—and still occupy, as of 1971—land that has been
intermittently flooded for many years. The following description of this
flood, by H. F. Matthai of the U. S. Geological Survey, may help to
convey some warning to residents or potential residents of the South
Platte valley and other river valleys in Colorado:
“The morning of June 16 was most pleasant, but conditions changed
rapidly shortly before noon. A tornado touched ground 15 miles south
southeast of Denver about 1 p.m. Within the next hour, another unroofed
30 homes in the little town of Palmer Lake, 40 miles south of Denver.
About 2 p.m., a dense mass of clouds descended and concealed the top of
Dawson Butte, 7 miles southwest of Castle Rock; and the little light
remaining faded until it was dark black and frightening, according to
some people. A nearby rancher’s wife described the intense quiet as
awesome, but the calm did not last very long.
“The deluge began, not only near Dawson Butte, but also at Raspberry
Mountain, 6 miles to the south, near Larkspur. The rain came down harder
than any rain the local residents had ever seen, and the temperature
dropped rapidly until it was cold. The quiet was shattered by the
terrible roar of wind, rain, and rushing water. Then the thudding of
huge boulders, the snapping and tearing of trees, and the grinding of
cobbles and gravel increased the tumult. The small natural channels on
the steep slopes could not carry the runoff; so water took shortcuts,
following the line of least resistance. Creeks overflowed, roads became
rivers, and fields became lakes—all in a matter of minutes.
“The flow from glutted ravines and from fields and hillsides soon
reached East and West Plum Creeks. The combined flow in these creeks
have been described as awesome, fantastic, and unbelievable; yet none of
these superlatives seem adequate to describe what actually occurred.
Large waves, high velocities, crosscurrents, and eddies swept away
trees, houses, bridges, automobiles, heavy construction equipment, and
livestock. All sorts of debris and large volumes of sand and gravel were
torn from the banks and beds of the streams and were dumped, caught,
plastered, or buried along the channel and flood plains downstream. A
local resident stated, ‘The banks of the creek disappeared as if the
land was made of sugar.’
“The flood reached the South Platte River and the urban areas of
Littleton, Englewood, and Denver about 8 p.m. Here the rampaging waters
picked up house trailers, large butane storage tanks, lumber, and other
flotsam and smashed them against bridges and structures near the river.
Many of the partly plugged bridges could not withstand the added
pressure and washed out. Other bridges held, but they forced water over
approach fills, causing extensive erosion. The flood plains carried and
stored much of the flood water, which inundated many homes, businesses,
industries, railroad yards, highways, and streets.
“The flood peak passed through Denver during the night, and the
immediate crisis was over by morning; but those in the inundated areas
were faced with a Herculean task. The light of day revealed the nature
of the destruction—mud in every nook and cranny, soggy merchandise,
warped bowling alleys, drowned animals, the loss of irreplaceable
possessions, to name a few types. The colossal cleanup job, which would
take months, began.”
Hydrogeological studies by the U. S. Geological Survey and Corps of
Engineers give knowledgeable estimates of flood danger for different
populated areas of the state, and recommend that homes, roads, and other
structures be placed above likely flood levels.
GLOSSARY
Alluvial fan. A cone-shaped mass of sediment built by rivers or streams
as they issue from mountains onto more level ground.
Alluvium. Stream deposits formed in recent geologic time, composed of
sand, gravel, and stones.
Ammonite. One of a large group of extinct mollusks related to the living
chambered _Nautilus_. Ammonite shells, usually cone-shaped or
coiled, are divided into many chambers by crenellated septa.
Angular unconformity. A surface separating tilted or folded layers of
rock from overlying less disturbed layers.
Anticline. An upward fold or elongated arch in rock layers.
Aquifer. A rock layer that is water-bearing.
Artesian water. Groundwater that is under sufficient pressure to rise
above the level at which it is encountered in a well. It does not
necessarily rise completely to the surface.
Basalt. An extrusive igneous rock, fine-grained and dark colored,
composed mainly of calcium-rich feldspar and the black mineral
pyroxene.
Basement. A name commonly applied to metamorphic or igneous rocks
underlying the sedimentary rock layers.
Batholith. A large body of intrusive igneous rock, 40 square miles or
more in outcrop area, which extends downward to an unknown depth.
Bedrock. The solid rock which underlies soil, sand, clay, or other loose
surface material.
Belemnite. The cigar-shaped internal shell of an extinct marine mollusk
similar to a squid.
Brachiopod. One of a large group of marine shelled animals having two
unequal, bilaterally symmetrical shells.
Bryozoa. A large group of tiny colonial marine animals that secrete
calcareous or horny coverings in a great variety of shapes.
Caldera. A large basin-shaped depression caused by explosion or collapse
around a volcanic center.
Cassiterite. A heavy, brown to brownish black mineral composed of tin
and oxygen (SnO₂) that is an ore of tin.
Cephalopod. A marine mollusk with a head surrounded by tentacles. Squids
and octupuses belong to this group, as do fossil forms having
straight or coiled shells divided into numerous interior chambers.
Chalcopyrite. A reddish-gold colored ore of copper (CuFeS₂).
Cirque. A deep, steep-walled recess in a mountain, caused by glacial
erosion at the head of a valley.
Concretion. A nodular or irregular concentration of minerals such as
calcite or limonite, formed by precipitation of the mineral from
groundwater around a nucleus.
Conglomerate. A rock containing coarse fragments of an older rock,
usually as rounded water-worn stones or pebbles.
Conodont. One of a group of tiny dark brown tooth-like fossils thought
to be dermal or dental parts of some extinct group of fish.
Diatreme. A volcanic vent or pipe drilled through rocks by the explosive
energy of gas-charged molten rock, now containing igneous rock and
often altered or unaltered fragments of the surrounding rock.
Dike. A vertical or nearly vertical sheet of igneous rock which cuts
across the structure of adjacent rocks.
Diorite. An intrusive igneous rock composed of sodium-rich feldspar and
dark minerals, with only small amounts of quartz.
Dip. The angle at which a layer of rock is inclined below the
horizontal.
Dome. A roughly circular upfold in which the rock layers dip outward in
all directions from the center.
Dowsing. Searching for underground water or ore with a divining rod,
usually a forked stick supposed to locate spots where the desired
substance may be found under the surface.
Echinoderm. One of a large group of marine invertebrate animals, most of
which have pentagonal symmetry and a skeleton of many calcite
plates. Many forms are spiny. The group includes starfish and sea
urchins.
Evaporite. Chemical sediments precipitated when water (usually sea
water) evaporates.
Extrusive rocks. Igneous rocks formed when molten rock material is
ejected onto the surface. Synonymous with volcanic rocks.
Fault. A break in the rocks in which there has been displacement of the
two sides relative to each other.
Fault block range. A mountain range bounded on two or more sides by
faults.
Feldspar. A group of light-colored aluminum silicate minerals that are
major constituents of igneous rocks. They contain potassium, sodium,
and calcium in differing proportions.
Fold. A bend in rock layers.
Foraminiferida. One-celled marine animals with microscopic, perforated,
many-chambered calcium carbonate shells, often called forams.
Fossil. The remains or traces of an animal or plant which has been
preserved in the rock.
Fusulinid. One-celled marine animals (forams) with shells which look
like a grain of wheat in shape and size, frequently abundant in
Colorado Pennsylvanian rocks.
Galena. A heavy gray metallic mineral (PbS), often cubic in form, that
is the most important ore of lead.
Gangue. Nonvaluable minerals occurring in veins with ore minerals.
Glaciation. Alteration of the earth’s surface by erosion and deposition
by glacier ice.
Glacier. A body of ice originating on land by recrystallization of snow,
and showing evidence of movement by flowing.
Gneiss. A coarse-grained metamorphic rock usually banded with streaks of
darker, finer-grained rock.
Granite. An intrusive igneous rock consisting essentially of sodium or
potassium feldspar and quartz, often speckled with dark-colored
minerals.
Graptolite. Extinct marine organisms without known close living
relatives, with small black sawblade-like chitinous hard parts
preserved as fossils.
Hematite. A steel gray or metallic grayish black or reddish gray mineral
(Fe₂O₃) that is an important ore of iron.
Hogback. A sharp-crested ridge formed by a resistant layer of steeply
dipping rock.
Huebnerite. A heavy reddish brown mineral (MnWO₄) that is a major ore of
tungsten.
Igneous rocks. Rocks formed by solidification from a molten state,
either at the surface (extrusive) or below the surface (intrusive).
Intrusive rocks. Igneous rocks formed when molten rock material
solidifies without reaching the surface.
Joint. A fracture in the rock, along which no discernible movement has
taken place.
Kerogen. Solid bituminous material in oil shales.
Laccolith. A lens-shaped mass of igneous rock intruded into layered
rocks.
Lava. Fluid or molten rock such as that which issues from a volcano.
Lode. A rock mass, often a vein, containing valuable minerals.
Massif. A mountainous mass that has relatively uniform geologic
characteristics and which may embrace a number of peaks.
Mesa. A flat-topped mountain bounded on at least one side by a steep
cliff.
Metamorphic rock. Rock formed by alteration of pre-existing rock,
especially by great temperatures and pressures.
Mollusk. Any one of the large group of invertebrate animals which
includes the snails, clams, octopuses, squids, and their extinct
relatives.
Molybdenite. A soft bluish gray, metallic mineral (MoS₂) that is a major
ore of molybdenum.
Monocline. A steplike fold in otherwise horizontal or gently dipping
rock layers.
Moraine. An accumulation of unsorted rock material built up by the
action of glacier ice.
Native gold. Gold occurring in nature uncombined with other elements.
Peneplain. A land surface worn down by erosion to a nearly flat or
broadly undulating plain.
Petzite. A heavy black or steel gray metallic telluride ore of gold and
silver (Ag₃AuTe₂).
Placer. A sand or gravel deposit containing particles or nuggets of gold
or other heavy valuable minerals.
Plateau. An elevated, comparatively flat surface of land, usually larger
than a mesa, sometimes composed of many mesas, and often dissected
by deep stream valleys.
Porphyry. An igneous rock, usually intrusive, which contains conspicuous
large crystals in a fine-grained matrix.
Pyrite. A brass-yellow metallic mineral (FeS₂) that is an important
source of sulfur. It is commonly known as fool’s gold.
Reef. A moundlike limestone structure built in the sea by sedentary
organisms such as corals.
Rhyolite. A light-colored volcanic rock with quartz and feldspar as the
principal constituents.
Schist. A metamorphic rock characterized by parallel orientation of
flat-grained minerals like mica.
Sedimentary rocks. Rocks formed of fragments of other rock transported
by wind or water, or formed by precipitation from solution.
Sphalerite. An amber-yellow to black mineral (ZnS) that is an important
ore of zinc.
Stalactite. A cylindrical or conical deposit of calcite hanging from the
roof of a cavern, formed by evaporation of water droplets containing
calcium carbonate.
Stalagmite. Columns or ridges of calcite rising from the floor of a
cavern, formed by water containing calcium carbonate dripping from a
stalactite.
Stock. A mass of igneous intrusive rock that covers less than 40 square
miles, has steep sides, and extends to an unknown depth.
Tennantite. A metallic gray mineral that contains copper, iron, and
arsenic, and is an ore of copper.
Tetrahedrite. A brittle, dark gray to black, metallic mineral containing
copper, iron, zinc, and silver.
Trilobite. One of a primitive group of extinct marine crustaceans,
related to crabs and lobsters, having segmented bodies divided by
longitudinal grooves into three lobes.
Unconformity. A surface separating layers of rock, formed by a period of
nondeposition or erosion.
Vein. A crack or fissure filled with mineral material, often with
valuable ore minerals.
SUGGESTED READING
There are thousands of scientific articles and books on Colorado
geology, and many new ones appear each year. Following is a selection of
books and booklets which we believe will be most useful and interesting
in extending your knowledge of the state’s geology.
Donnell, John R., editor, 1960, GEOLOGICAL ROAD LOGS OF COLORADO. Rocky
Mountain Association of Geologists, Denver. Itineraries for a number
of geological trips along Colorado highways and byways.
Eckel, Edwin B., 1961, MINERALS OF COLORADO, A 100-YEAR RECORD. U. S.
Geological Survey Bulletin 1114.
Emmons, S. F., Cross, Whitman, and Eldridge, G. H., 1896, GEOLOGY OF THE
DENVER BASIN IN COLORADO. U. S. Geological Survey Monograph 27. The
classic early treatment of the surface geology around Denver, with
many historic illustrations.
Hansen, Wallace R., 1965, THE BLACK CANYON OF THE GUNNISON TODAY AND
YESTERDAY. U. S. Geological Survey Bulletin 1191. A readable account
of this unusual national monument near Montrose.
Hansen, Wallace R., 1969, THE GEOLOGIC STORY OF THE UINTA MOUNTAINS. U.
S. Geological Survey Bulletin 1291. The eastern part of this range
is in Colorado.
Henderson, C. W., 1926, MINING IN COLORADO, A HISTORY OF DISCOVERY,
DEVELOPMENT AND PRODUCTION. U. S. Geological Survey Professional
Paper 138.
Lee, W. T., 1917, THE GEOLOGIC STORY OF THE ROCKY MOUNTAIN NATIONAL
PARK, COLORADO. U. S. National Park Service Publication. An old
report, not adequately superseded.
Lovering, T. S., and Goddard, E. N., 1950, GEOLOGY AND ORE DEPOSITS OF
THE FRONT RANGE, COLORADO. U. S. Geological Survey Professional
Paper 223. A comprehensive study of mineral-bearing areas in the
Front Range.
Lohman, S. W., 1965, THE GEOLOGIC STORY OF COLORADO NATIONAL MONUMENT.
Colorado and Black Canyon Natural History Association, Grand
Junction.
Pearl, Richard M., 1956, NATURE AS SCULPTOR: A GEOLOGIC INTERPRETATION
OF COLORADO SCENERY. Denver Museum of Natural History Popular Series
No. 6, Revised Edition.
Pearl, Richard M., 1969, EXPLORING ROCKS, MINERALS, FOSSILS IN COLORADO.
Swallow Press, Revised Edition.
Pearl, Richard M., 1971, COLORADO GEM TRAILS AND MINERAL GUIDE. Swallow
Press, 3rd Edition.
Powell, John Wesley, 1876, REPORT ON THE GEOLOGY OF THE EASTERN PORTION
OF THE UINTA MOUNTAINS AND A REGION OF COUNTRY ADJACENT THERETO. U.
S. Geological and Geographical Survey of the Territories. One of the
earliest accounts of geology in Colorado, written by the explorer of
the Colorado River and the father of the U. S. Geological Survey.
Rabbit, Mary C., and others, 1969, THE COLORADO RIVER AND JOHN WESLEY
POWELL. U. S. Geological Survey Professional Paper 669. A resumé of
part of Powell’s work and a good discussion of the geologic history
of the entire Colorado River, which begins near Grand Lake.
Richmond, Gerald M., 1965, GLACIATION OF THE ROCKY MOUNTAINS. A part of
THE QUATERNARY OF THE UNITED STATES, Princeton University Press. A
summary of current knowledge of glaciation in Colorado and
surrounding areas.
Rodeck, Hugo G., editor, 1964, NATURAL HISTORY OF THE BOULDER AREA.
University of Colorado Museum Leaflet No. 13. Contains articles on
geology and biology.
Untermann, G. E., and Untermann, B. R., 1954, GEOLOGY OF DINOSAUR
NATIONAL MONUMENT AND VICINITY, UTAH—COLORADO. Utah Geological and
Mineralogical Survey Bulletin 42. A detailed study of the eastern
Uinta Mountains.
Weimer, Robert J., and Haun, John D., editors, 1960, GUIDE TO THE
GEOLOGY OF COLORADO. Geological Society of America, Rocky Mountain
Association of Geologists, and Colorado Scientific Society, Denver.
A concise summary of many aspects of Colorado geology, this guide
includes several geological itineraries and many reference listings.
Wolle, Muriel Sibell, 1949, STAMPEDE TO TIMBERLINE, Sage Books. An
excellent account of early mining activity in the state, with many
fine drawings of the early settlements.
INDEX
A
Abrams Mountain, 87
Alamosa, 35
Alamosa Formation, 67, 105, 106
Alma, 78
Ancestral Rocky Mountains, 44, 45
Animas River, 58, 86
Ankareh Formation, 52
Antero Junction, 21
Anthracite, 97
Arapahoe Conglomerate, 60
Arapaho Glacier, 70, 71
Arkansas Hills, 21
Arkansas Mountain, 79
Arkansas River, 3, 22, 35, 90
Arkansas Valley, 21
Aspen, _Front._, 1, 22, 35, 50, 74, 77, 78, 88-89
Aspen Mountain, 23, 88
Avon, 22
B
Battlement Mesa, 62
Belden Formation, 44, 109
Benton Shale, 57
Berthoud Pass, 12, 92
Big Thompson Canyon, 12, 71
Big Thompson River, 69, 103, 104
Black Canyon of the Gunnison, 36, 37, 44, 71, 105
Black Hawk, 14, 77, 78, 79, 80
Blue River, 103
Book Cliffs, 29
Boulder, 8, 14, 33, 45, 47, 48, 50, 71, 74, 75, 94, 98, 99, 100,
101
Boulder County, 78, 79
Boulder Creek, 1, 15, 71, 103
Boulder Creek Granite, 14, 33, 35
Boulder Reservoir, 103
Breckenridge, 1, 78, 83-84
Bross, Mt., 21
Buena Vista, 22
Buffalo Peaks, 21
Building stone, 24, 48, 50, 99-101
C
Cache la Poudre River, 66, 103
Cambrian, 7, 34, 39
Camp Bird, 78, 88
Canon City, 3, 11, 16, 52, 53, 75, 94
Canon City Embayment, 16
Carbondale, 97
Carboniferous, see Mississippian, Pennsylvanian
Cardiff, 97
Carmel Formation, 52
Carter Lake, 103
Castle Creek, 88
Castle Rock, 8, 61, 112
Castle Rock Conglomerate, 60
Cave of the Winds, 106, 107
Caves, 31, 106-108
Cenozoic (see also Tertiary, Quaternary), 7, 16, 18, 26, 28, 29,
59-73, 109
Central City, 1, 14, 74, 77, 78, 80
Chaffee Formation, 42
Cherry Creek, 1
Cheyenne Mountain, 14, 15, 110
Cheyenne Sandstone, 105
Chinle Formation, 51, 52, 93, 101
Clay, 75, 97-99
Clear Creek, 1, 71, 80
Climax, 21, 78, 91-92
Climax Granite Porphyry, 91, 92
Coal, 23, 75, 96-97
Coal Creek, 14, 15
Coal Creek Quartzite, 33
Coaldale, 97
Coalmont, 97
Cokedale, 97
Collegiate Range, 22
Colorado National Monument, 29, 31, 44, 51, 108
Colorado River, 3, 20, 21, 28, 29, 35, 39, 103, 104, 105, 108, 109
Colorado Springs, 14, 15, 35, 37, 48, 97, 98, 109
Columbia, Mt., 22
Como, 78, 97
Construction materials, 97-102
Copper, 74, 75, 80, 81, 83, 89, 91
Creede, 65, 78, 89-90
Crested Butte, 24, 97
Cretaceous, 7, 12, 20, 23, 29, 30, 53, 56-58, 94, 95, 97, 98, 101,
105, 108, 110
Cripple Creek, 1, 74, 77, 78, 90-91
Cross Mountain, 26, 29
Crystal River, 24, 99
Culebra Range, 17
Curecanti, 105
Curtis Formation, 52
D
Dakota Formation, 12, 51, 53, 56, 94, 97, 105
Dawson Arkose, 60
Dawson Butte, 112
Delcarbon, 97
Democrat, Mt., 21
Denver, 3, 8, 14, 33, 35, 37, 45, 47, 48, 52, 53, 60, 74, 83, 94,
96, 99, 100, 104, 112, 113
Denver Basin, 8, 75, 94, 95
Denver Formation, 60, 62
Devonian, 7, 42-43, 83
Dillon, 104
Dinosaur National Monument, 27, 53, 55
Durango, 25, 28, 35, 47, 58, 78, 85, 102
Dyer Dolomite, 42
E
Eagle, 47, 101, 106
Eagle River, 22, 47
Edwards, 22
Elbert, Mt., 22
Eldorado Springs, 109
Elk Mountains, 24, 69
Empire, 1, 78, 81
Englewood, 113
Entrada Sandstone, 51, 93
Environmental geology, 111-113
Eocene, 64
Estes Lake, 103
Estes Park, 69, 104
Evans, Mt., 3, 12
F
Fairplay, 78, 84-85
Fairy Cave, 107
Flattop Mountain, 19
Floods, 112-113
Florence, 74, 94
Florissant Fossil Beds National Monument, 65
Fort Carson, 15
Fort Collins, 35, 101, 102
Fountain Formation, 12, 14, 45, 47, 48
Fox Hills Sandstone, 58
Fremont Limestone, 40, 41
Frisco, 21
Front Range, 11-16, 33, 35, 44, 50, 51, 52, 56, 60, 61, 68, 69,
70, 71, 93, 99, 101, 103, 107, 109
Frying Pan River, 105
Fulford, 107
Fulford Cave, 106
G
Garden of the Gods, 4, 14, 15, 47, 48
Garfield, Mt., 30
Gas, natural, 1, 28, 75, 94-96
Gems, 75, 102
Gem Village, 102
Georgetown, 78, 81
Gilman, 21
Gilpin County, 78
Glen Eyrie Formation, 44
Glenwood Canyon, 37, 39
Glenwood Springs, 24, 29, 35, 99, 107, 109
Gold, 1, 22, 29, 74, 75, 77-91
Golden, 8, 14, 62, 74, 78, 83, 93, 98, 111
Gold Hill, 1, 74, 78, 79
Gore Creek, 47
Gore Pass, 19, 20
Gore Range, 20-21, 35, 69
Gore Range-Eagle’s Nest Wilderness Area, 20, 21
Granby, 20, 62
Granby Lake, 103
Grand Hogback, 28, 29
Grand Junction, 29, 30, 35, 55
Grand Lake, 12, 69, 103, 104
Grand Mesa, 35, 62
Grand Valley, 25
Gravel, 75, 97-99
Great Sand Dunes National Monument, 17, 18, 73
Green River, 27, 103
Green River Basin, 4
Green River Canyon, 31
Green River Formation, 64, 95-96
Greenhorn Formation, 101
Greenhorn Peak, 16
Groundwater, 76, 105-106
Gunnison, 35
Gunnison, Black Canyon of the, 36, 37, 44, 71, 105
Gunnison River, 35, 36, 37, 105
Gypsum (mineral), 22, 30, 75, 101-102
Gypsum (town), 47, 101
Gypsum Valley, 30, 47, 101
H
Hahn’s Peak, 19
Harding Sandstone, 40, 41
Harvard, Mt., 22
Hayden, 96, 97
Hayden Pass, 17
Hermosa Formation, 45, 47
Hidden Valley, 69
Horseshoe Amphitheater, 85
Horseshoe Park, 69
Horsetooth Reservoir, 103
Huerfano Basin, 35, 61
I
Ice Age, see Pleistocene
Iceberg Lake, 66
Idaho Springs, 14, 78, 80
Idaho Springs Formation, 33, 92
Independence Pass, 22
Iron, 1, 17, 74
Ironton, 78, 88
J
Jewel Lake, 68
Juniper Mountain, 26, 29
Juniper Springs, 109
Jurassic, 7, 9, 23, 36, 52-55, 93
K
Kremmling, 19, 20, 60
L
La Junta, 8, 35
Lake City, 77, 78
Lake County, 77
Lamar, 8
Landslides, 111-112
La Plata Mountains, 26
Laramide Orogeny, 59, 60
Laramie Formation, 57, 58, 97
Larkspur, 112
La Veta Pass, 17, 18, 37
Lead, 74, 75, 80, 81, 82, 83, 85, 86, 89
Leadville, 1, 22, 77, 78, 82-83
Leadville Limestone, 43, 44, 83, 88, 99, 106, 109
Lime, 75, 101
Lincoln, Mt., 21
Lincoln Porphyry, 21
Lipalian Interval, 7, 36, 39
Littleton, 113
Logan County, 95
Longs Peak, 3, 11, 12, 68
Loveland, 14, 102
Loveland Pass, 12
Lykins Formation, 12, 51, 52, 101
Lyons, 14, 49, 99, 100, 101
Lyons Sandstone, 12, 48, 49, 50, 99, 100, 101
M
Magnolia, 79
Mancos Shale, 30, 95
Manitou, 39, 106, 107, 110
Manitou Formation, 40, 41, 110
Manitou Springs, 109
Marble, 24, 99
Marble Mountain, 106
Maroon Bells, Front., 24, 50
Maroon Creek, 24
Maroon Formation, 50
Mary’s Lake, 103
Massive, Mt., 82
McDermott Formation, 58
Mesa de Maya, 8, 35, 62
Mesa Verde, 28, 29, 35
Mesa Verde Formation, 30, 58, 108
Mesa Verde National Park, 31, 71, 108
Mesozoic (see also Triassic etc.), 7, 10, 11, 12, 14, 15, 26, 28,
51-58, 60, 93
Mestas, Mt., 17
Middle Park, 4, 16, 35, 61
Million Dollar Highway, 87, 88
Mills Lake, 68
Milner Pass, 66
Minturn, 39
Minturn Formation, 45, 46, 47
Miocene, 66, 67, 87
Mississippian, 6, 7, 43-44, 83, 106, 109
Moenkopi Formation, 52
Molas Formation, 44
Molas Lake, 25
Molybdenum, 1, 74, 75, 76, 77, 91-92
Monarch Pass, 23
Morgan County, 95
Morrison, 53, 54, 55
Morrison Formation, 12, 36, 51, 53, 55, 93
Mosca Pass, 17, 73
Mosquito Pass, 85
Mosquito Range, 21, 22, 35, 39, 69
Mountain Province, 3, 4, 10-27, 35, 46, 93, 94, 101, 102, 103,
109, 111
Mt. Bross, 21
Mt. Columbia, 22
Mt. Democrat, 21
Mt. Elbert, 22
Mt. Evans, 3, 12
Mt. Garfield, 30
Mt. Harvard, 22
Mt. Lincoln, 21
Mt. Massive, 82
Mt. Mestas, 17
Mt. Princeton, 22
Mt. Princeton Hot Springs, 22, 110
Mt. Sneffels, 87, 88
Mt. Sopris, 24
Mt. Yale, 22
Mt. Zirkel, 19
Music Pass, 17, 73
N
Navajo Sandstone, 52
Nederland, 78, 79
Needle Mountains, 26
Niobrara Formation, 57, 101
North Park, 4, 16, 35, 58, 61, 94, 97
O
Oil, 1, 29, 30, 75, 76, 94-96
Oil Creek, 94
Oil shale, 95-96
Oligocene, 66, 67
Ordovician, 7, 40-41, 83, 106
Orient, 17
Oro, 82
Otero County, 105
Ouray, 4, 25, 26, 34, 42, 78, 87-88
Ouray Formation, 42
P
Pagoda Mountain, 68
Pagosa Springs, 102, 109
Paleozoic (see also Cambrian etc.), 7, 10, 11, 12, 14, 15, 17, 21,
22, 24, 26, 27, 28, 30, 34, 37, 38-50, 60, 64, 77, 95, 98,
106, 107
Paradox Basin, 4, 47, 95, 101
Paradox Valley, 30, 101
Park Range, 19-20, 35, 69
Parting Sandstone, 42
Pawnee Buttes, 9, 66, 67
Peak Province, see Mountain Province
Peat, 75
Pennsylvanian, _Front._, 6, 7, 14, 23, 44-47, 48, 50, 83, 95, 101,
106, 109
Permian, _Front._, 7, 23, 48-50, 95, 99, 100, 101
Petroleum, 1, 29, 30, 75, 76, 94-96
Phosphoria Formation, 50
Piceance Basin, 29
Pierre Formation, 57, 94, 98
Pikes Peak, 3, 4, 11, 12, 15, 65, 90, 91, 110
Pikes Peak Granite, 4, 14, 33, 36
Plains Province, see Prairie Province
Plateau Province, 3, 4, 9, 28-31, 35, 46, 71, 75, 93, 94, 96, 101,
103, 107, 110
Platte River, 3
Pleistocene, 7, 8, 25, 59, 68-73, 105, 106
Plum Creek, 112
Poncha Springs, 109
Prairie Province, 3, 8-10, 12, 35, 66, 72, 75, 94, 96, 103, 105
Precambrian, 7, 10, 11, 14, 15, 16, 17, 19, 20, 21, 26, 33-37, 40,
60, 64, 68, 77, 87, 91, 92, 93, 109
Princeton, Mt., 22
Pueblo, 23, 97
Pumice, 75
Purgatoire Formation, 105
Pyrites, 75
Q
Quandary Peak, 21
Quaternary, 7, 8, 25, 59, 68-73, 105, 106
R
Rabbit Ears Pass, 19
Rabbit Ears Range, 20, 35, 62
Radium, 93
Rampart Range, 15, 97, 98
Rangely, 29, 95
Raspberry Mountain, 112
Rattlesnake Reservoir, 103
Raton Basin, 61
Raton Pass, 61
Red Cliff, 39
Red Mountain, 88
Red Mountain Creek, 88
Red Rocks Park, 14, 37, 47
Redstone, 24
Rico, 26
Rico Range, 26
Rifle, 64
Rio Grande, 35
Roan Plateau, 28, 29, 35
Rocky Mountain National Park, 11, 12, 66, 68, 71, 104
Roncarbo, 97
Royal Gorge, 37, 71
Ruedi, 105
S
St. Mary’s Glacier, 71
Salida, 17, 21, 35
Salina, 79
Sand, 75, 97-99
Sangre de Cristo Range, 10, 17-18, 35, 47, 61, 69, 73, 90, 105,
106
San Juan Basin, 95
San Juan County, 78
San Juan Formation, 88
San Juan Mountains, 4, 25-26, 35, 52, 65, 69, 77, 85, 86, 87, 88,
105
San Luis Valley, 4, 35, 44, 61, 67, 73, 96, 106
San Miguel Range, 26
Santa Fe Formation, 67, 105
Sawatch Range, 22-23, 35, 39, 69, 82, 106
Sawatch Sandstone, 34, 39, 40
Sedalia, 61
Shadow Mountain Reservoir, 103
Sierra Blanca, 17, 18
Silurian, 7, 42
Silver, 22, 74, 77-91
Silver Cliff, 16, 77, 78
Silver Plume, 78, 81
Silver Plume Granite, 33, 35
Silverton, 4, 26, 74, 77, 78, 85-86, 88
Sinbad Valley, 101
Sneffels, Mt., 87, 88
Sopris, Mt., 24
South Park, 4, 16, 21, 35, 61, 65, 84, 97
South Platte River, 1, 3, 35, 85, 103, 104, 112, 113
Spanish Cave, 106
Spanish Peaks, 10, 18, 62
Specimen Mountain, 66
Springs, 17, 22, 109-110
Steamboat Springs, 97, 109
Summit County, 77
Sunshine, 79
Sunshine Peak Rhyolite, 87
Swandyke Gneiss, 33
T
Table Mountain, 8, 62, 111
Telluride, 26, 74, 77, 78
Tenmile Gorge, 21
Tenmile Range, 21, 91
Tertiary, 7, 15, 20, 21, 25, 26, 29, 59-67, 73, 77, 83, 87, 88,
89, 90, 91, 92, 93, 95, 96, 97
Tin, 75
Tincup, 22, 77, 78
Trail Ridge Road, 12, 66
Treasure Mountain Granite, 99
Triassic, 7, 23, 51-52, 93, 101
Trinidad, 8, 61, 96, 97
Trout Creek Pass, 21, 22
Tungsten, 1, 74, 75, 79
Tyndall Glacier, 71
U
Uinta Basin, 4, 29, 64, 75, 95
Uinta Mountain Formation, 26
Uinta Mountains, 4, 10, 26-27, 29, 35, 37, 64
Uncompahgre Gorge, 87
Uncompahgre Plateau, 29, 35, 44
Uncompahgre Quartzite, 87
Urad Mine, 92
Uranium, 1, 29, 80, 93
V
Vail, 21, 47
Vail Pass, 21
Valmont, 62
Vanadium, 74, 75, 93
Villa Grove, 17
W
Walden, 20
Walsenburg, 35, 61, 96
Ward, 78
Water, 76, 103-110
Waunita Hot Springs, 109
Weber Sandstone, 95
West Elk Mountains, 24, 35, 69
Wet Mountains, 16, 35, 61, 95
Wet Mountain Valley, 35
Whiskey Creek Pass, 17
White River, 35, 95
White River Formation, 66
White River Plateau, 28, 29, 35, 43, 107, 109
Williams Canyon, 37, 39
Willow Creek Pass, 20
Willow Creek Reservoir, 103
Wingate Formation, 51, 52, 101
Wolcott, 22, 47
Wolford Mountain, 60
Woodland Park, 110
Woods Lake, 107
Y
Yale, Mt., 22
Yampa River, 3, 27, 31, 35, 97
Yule Marble, 24, 99
Z
Zinc, 74, 75, 80, 82, 83, 85, 86, 89
Zirkel, Mt., 19
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