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.
| Class | Example | Occurrence in Colorado |
|---|---|---|
| Sedimentary | Sandstone | Plains, [plateaus], flanks of mountain areas |
| Shale | ||
| [Conglomerate] | ||
| Limestone | ||
| Igneous | Extrusive: [Basalt] | Volcanic areas such as San Juan Mountains, Spanish Peaks |
| Intrusive: [Granite] [Diorite] | Pikes Peak, Longs Peak, and most central mountain areas | |
| Metamorphic | Marble (from limestone) | Mountain areas |
| 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.
Stratigraphic Column
| ERA Period | Millions of years ago | Distinctive [fossils] | Events in Colorado |
|---|---|---|---|
| CENOZOIC | |||
| (Age of Mammals) | |||
| Quaternary | Modern types of animals and plants | Development of present topography; [glaciation] in mountains | |
| 3 | |||
| Tertiary | Mammals, flowering plants | Uplift and mountain building | |
| 70 | |||
| MESOZOIC (Age of Reptiles) | Dinosaurs and other reptiles | ||
| Cretaceous | Submergence, then uplift | ||
| 135 | |||
| Jurassic | Desert, then submergence | ||
| 180 | |||
| Triassic | Widespread floodplains and deserts | ||
| 225 | |||
| PALEOZOIC | |||
| (Age of Fishes) | |||
| Permian | First reptiles | Widespread floodplains and deserts | |
| 270 | |||
| Pennsylvanian | Swamp and forest plants | “Ancestral Rocky Mountains” | |
| 310 | |||
| Mississippian | [Reef] corals, sharks | Partial submergence | |
| 350 | |||
| Devonian | Armored fish, first insects | Probable submergence | |
| 400 | |||
| Silurian | Corals and shellfish | Probable submergence | |
| 440 | |||
| Ordovician | First fish | Submergence | |
| 500 | |||
| Cambrian | First hard-shelled animals | Gradual encroachment of sea from west | |
| 570 | |||
| PRECAMBRIAN | “Lipalian Interval” | Erosion to almost flat surface or [peneplain] | |
| Primitive soft-bodied marine organisms | Alternate episodes of mountain building and erosion | ||
| 3,600 plus | |||