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.

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.

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.

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.

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.

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.