Part 1

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.

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.

Class Example Occurrence in Colorado

Sedimentary Sandstone Plains, plateaus, flanks of mountain areas Shale Conglomerate Limestone Igneous Extrusive: Volcanic areas such as San Juan Basalt Mountains, Spanish Peaks Intrusive: Pikes Peak, Longs Peak, and most Granite central mountain areas Diorite Metamorphic Marble Mountain areas (from limestone) Quartzite (from sandstone) Gneiss (from granite or sandstone) Schist (from shale or basalt)

Geologists arrange rocks in their chronologic sequence by studying the fossils and minerals which they contain. The age of some rocks can be determined with reasonable precision from ratios of radioactive minerals and their fission products. The relative age of others can be determined from their position, the fossils enclosed in them, and many minor details of their structure.

The _stratigraphic column_ shown opposite may be thought of as a calendar by which geologic events in Colorado can be arranged in their proper order and related to events in the rest of the world. Mississippian and Pennsylvanian Periods are American divisions; elsewhere this time interval is known as the Carboniferous Period. Other time terms are in worldwide use.

In the generalized geologic map of Colorado which accompanies Chapter II, rocks are identified by the era in which they were formed. A more detailed geologic map can be obtained from the U.S. Geologic Survey map distribution center in the Federal Building, Denver.

ERA Period Millions Distinctive fossils Events in Colorado of years ago