Read Ebook: Prairie Peak and Plateau: A Guide to the Geology of Colorado by Chronic Halka Chronic John

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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.

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.

TERTIARY RED BEDS JURASSIC DAKOTA MANCOS DAKOTA Hahn's Peak PORPHYRY MANCOS DAKOTA PORPHYRY JURASSIC RED BEDS RE-CAMBRIAN

The range is crossed by Rabbit Ears Pass in the north; Gore Pass near Kremmling marks its southern end. Mt. Zirkel and Flattop Mountain 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 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 . 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 . With several other 14,000-foot summits, this range is the highest in the state. One group of peaks, known as the Collegiate Range forms a particularly imposing vista from U. S. highway 24 between Trout Creek Pass and Buena Vista. The Independence Pass highway between Leadville and Aspen penetrates the heart of the Sawatch high country.

The Sawatch Range as a whole is about 100 miles long 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 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.

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.

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 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.

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.

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.

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