Prairie, Peak, and Plateau: A Guide to the Geology of Colorado

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

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

Hahn’s Peak

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.

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