Title: Prairie, Peak, and Plateau: A Guide to the Geology of Colorado
Author: John Chronic
Halka Chronic
Release date: August 21, 2019 [eBook #60143]
Most recently updated: October 17, 2024
Language: English
Credits: Produced by Stephen Hutcheson and the Online Distributed
Proofreading Team at http://www.pgdp.net
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:
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
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.)
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.
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 | |||
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.
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.
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.