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

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.

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.

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.

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.

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.

Stratigraphic Column

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.

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.

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.

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.

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.

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.

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

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].

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.

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

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.

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.

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.

[[This map in a higher resolution]]

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.

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.

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.

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].

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.

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.

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.

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.

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.

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.

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.

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.

These Ordovician [fossils] can be found in the Manitou Formation in the Colorado Springs area.

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.

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.

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.

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.

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

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)

Corals, [brachiopods], and [fusulinid] Foraminifurida can be found in the Pennsylvanian Minturn Formation at many places in the Mountain and [Plateau] Provinces.

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.

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.

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.

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.

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.

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.

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.

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.

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.