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A Survey of the Recreational Resources of the Colorado River Basin
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Geology
Recreational Benefits of Reservoirs
Canyon Lands of Southeastern Utah
Conservation of Recreational Resources
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A Survey of the Recreational Resources of the Colorado River Basin
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Chapter II:
GEOLOGY (continued)
EASTERN BORDER LANDS OF COLORADO
PLATEAU
The Green River Basin, the Uinta Mountain region, and the Colorado Plateau, which constitute by far the greatest and most characteristic part of the vast region drained by the Colorado River north of central Arizona, are relatively simple in topographic expression, physiographic expression, and physiographic history. Bordering this central region on the east and still within the Colorado drainage basin is a belt of unusual topography developed on rock complex in structure, composition, and relationship. Particularly noteworthy are the lands along the Upper Colorado, the Gunnison Valley, and the San Juan Mountains.
Upper Colorado River Valley.—In geologic make-up, scenery, and recreational resources, the valley of the Colorado River in north central Colorado is totally unlike its expression elsewhere. The landscape is that characteristic of the Rocky Mountains rather than of the plateau country. In the place of flat lands, vertically walled canyons, and cliff-bound mesas and plateaus, fairly simple and regular in topographic form and stratigraphic details, there is an aggregation of saw-toothed ridges, ragged peaks, abraded highlands, glacial cirques, and flat-floored "parks" whose origin and interrelations are difficult to interpret. The Upper Colorado and its tributaries flow in a succession of wide and narrow canyons and valleys with flaring sides, over plains, across anticlines, synclines, and faults, and from place to place trench schists, lavas, sedimentary rocks, and alluvium of varied composition and attitude. The larger tributaries have cut their way far into the granite core rocks along the Continental Divide—past the axis of the Rocky Mountain uplift—and thus bring into the Colorado drainage basin most of the water that falls on the lofty, glaciated Park Range, Gore Mountains, and the Sawatch Range, which culminate in the highest peaks of the Rockies. In the Sawatch Range about 25 peaks exceed 13,000 feet, and 10 exceed 15,000 feet. The Roaring Fork has cut from the granitic Sawatch mass the equally high Elk Mountains—a much deformed anticline of Carboniferous rocks—into which many dikes and sheets of igneous rock have been intruded. Because developed in rocks of different hardness, attitude, and stratigraphic sequence—flat lying and tilted sandstone, limestone, shale, and lavas—the remarkably sinuous streams flow swiftly between close-set vertical rock walls, between slopes gashed by ravines, or meander leisurely across wide alluvial flats. A traverse of Eagle River, the Roaring Fork, or the Colorado above the brightly colored canyon at Glenwood Springs reveals a succession of fascinating vistas, tumbling waters, quiet waters, pasture land, grain fields, farmhouses, and above the valley walls, snow capped mountains.
An outstanding feature of the Upper Colorado Valley is the White River Plateau, about 40 miles in diameter and 10,000 to 12,000 feet above sea level (Pl. 13, Sec. 6, in pocket) a flat-topped highland bordered by steep slopes and structurally an uplifted mass in which the deformation of its component strata is restricted to its rims. At its western edge the Grand Hogback, composed of thick Cretaceous and thinner Jurassic and Triassic strata with dips as great as 30 degrees, emerges from the adjacent Tertiary floor and stands as prominent ridges, 600 to 1,000 feet high. From the hogback across the plateau top, nearly horizontal Paleozoic rocks form the surface, and at their eastern limit are overlapped by steeply tilted Mesozoic beds, the remnants of strata that once overarched the plateau and joined those in the Grand Hogback. To attain their position and altitude, the sedimentary rocks inside the tilted borders of the plateau must have been uplifted en masse as much as 14,000 feet (early Tertiary time), then stripped down to a somewhat uneven surface over which lava in extensive sheets was poured out (Quaternary time). At the present time the plateau is a single topographic unit, the source of streams that flow to the Yampa, the White, and the Colorado, but as a regional structure once extended southwestward across the Colorado River, Eagle River, and Roaring Fork. Its major characteristic features in modified form are represented in Red Table Mountain and adjacent lava-capped mesas and its western terminus, the Grand Hogback, is traceable for 80 miles.
Gunnison Valley.—South of the Elk Mountains the landscape presents features not represented elsewhere along the eastern edge of the Colorado drainage basin. Above a general surface, developed on Cretaceous rocks at an average altitude of about 8,500 feet, rise low domes of metamorphic and igneous rocks—the uneroded remnants of a former peneplain. In the West Elk Mountains, masses of intrusive porphyry culminate in isolated peaks that rise as much as 1,500 feet above the upturned edges of the strata that once arched over them. The dominating surface features are thick, widespread accumulations of volcanic breccia and tuff—in one area continuously covering about 400 square miles—and extensive sheets of rhyolitic and andesitic lavas. Into the regional surface the Gunnison and its tributaries have sunk deep runways by cutting through sedimentary, igneous, and metamorphic rocks down to porphyritic granite.
The present scenic features of the Gunnison Valley record merely the last event in the long geologic history of the Rocky Mountains and the Colorado plateaus. In Cretaceous times the region was covered by the sea, in which sediments of great thickness accumulated, then near the beginning of Tertiary time was uplifted as a great arch the ancestral Rocky Mountains. During the long period of erosion that followed, the sedimentary rocks that once were continuous across its surface were stripped away, down to the core of the schist and granite, leaving an erosion surface of moderate relief; a broad belt that extends from the Sawatch Range westward about 70 miles, and southward merges with the foothill slopes of the San Juan Mountains. The latest major movement of the earth's crust was upward and brought the Sawatch Range and its bordering lands along the Gunnison to their present lofty position, and perhaps coincident with the uplift the newly exposed surface was flooded with lavas or buried beneath volcanic tuffs. Then followed a period of erosion whose length is the life span of the present-day streams. The progress of the streams in developing valleys and cutting channels in rocks of various types is faithfully recorded by the Gunnison River. For about 100 miles of its upper course (Tomichi Creek) the river occupies alternately shallow canyons in intrusive granite and open valleys in less-resistant Cretaceous and Jurassic rocks. Farther down it flows between vertical walls of dark-colored contorted schists and gneisses through the spectacular Black Canyon, 1,700 to 2,400 feet deep and in places not more than 50 feet wide at the bottom. The Pre-Cambrian rock in the canyon walls terminates abruptly at an eroded surface above which stratified rocks of Upper Cretaceous age form slopes. The division line between these unlike rock masses represents a hiatus in the geologic time scale of an estimated 400,000,000 years. That the Gunnison is a superposed stream that has persisted in the course established on a surface high above its present walls is shown by its entrenched meanders and its topographic position. It has continued to cut its path across highlands through hard Archean schists, seemingly ignoring the soft rocks in the adjacent, downfaulted lowlands.
San Juan Mountains.—The dominating topographic feature of southwestern Colorado is the San Juan Mountains, which lie athwart the Continental Divide and cover an area of about 6,500 square miles. (Pl. 13, Sec. 8, in pocket.) The western part of this lofty-peak-studded mass lies within the Colorado drainage basin and is the gathering ground for the waters carried northward directly to the Colorado by the Uncompahgre, San Miguel, and Dolores Rivers, and southward by the La Plata, Animas, Los Pinos, and Piedra Rivers which reach the Colorado River by way of the San Juan. Through the escarpment that walls in the mountains, these streams descend precipitously to the adjacent plateau lands. The average altitude of the main mountain mass including the western San Juan Mountains, the north-trending Cimarron and Mount Sneffels spurs, the Needles Mountains, and the somewhat isolated San Miguel Mountains is about 11,500 feet. Scores of peaks rise to heights of 13;000 feet, 13 exceed 14,000 feet, and the canyon floors lie 4,000 to 6,000 feet below the adjacent summits. The mountains consist almost entirely of Tertiary volcanic rocks; lavas, stratified tuffs, agglomerates, and breccias, 5,000 to 9,000 feet in thickness, with which are associated igneous intrusives. Mesozoic and Paleozoic formations are exposed in the deeper canyons, and along the edges of the mountain mass they appear in sequence beneath the volcanic cap, and dip north, west and south, and within short distances from the mountain center assume a horizontal position.
Compared with adjacent regions, the San Juan Mountains are a deeply, though not intricately, dissected rock-surfaced plateau in which the forms of many peaks, ridges, and canyons have been modified by glacial erosion. Their present features record merely the latest major event in a long, complex geological history. As revealed by the composition and arrangement of the exposed rocks, an enormous basal mass of Pre-Cambrian and Cambrian schists, porphyries, and quartzite was worn down, then overlaid by Paleozoic sediments 2,000 to 4,500 feet thick—most of them deposited in a sea. Near the close of Paleozoic time the strata were folded, broken, locally uplifted, and then eroded to a surface of small relief, upon which were accumulated sediments of Mesozoic age to an estimated thickness of about 6,000 feet. Near the close of the Cretaceous period, when much of the region was below sea level, areal disturbances in the earth's crust, accompanied by volcanic activity, pushed the strata upward as a broad dome—the ancestral San Juan—far higher than the existing mountains. The uplift continued into early Tertiary time, attaining altitudes that permitted glaciation—a very rare episode in Pre-Pleistocene geologic history. Erosion also continued until the highland mass was reduced to a roughly level surface. Then followed the deposition of great beds of tuff and lava, for which the region is famous. With the cessation of volcanic activity, stream erosion once more became the dominant geologic force. On the widely spread volcanic rocks, streams tributary to the Colorado established their courses and renewed uplifts so increased their power that by the end of the Tertiary era the irregular mountain mass had been reduced to the San Juan peneplain, a generally even, regional surface above which rose residual peaks. Later uplifts introduced the present cycle of erosion, during which the extensive peneplain has been trenched by great canyons and its summit and edges modified in form. Even during Quaternary time the San Juan region has been subjected to almost continuous crustal warping, and sculpturing by vigorous streams has been interrupted only when the region was mantled by ice. During the Pleistocene epoch, except for the high peaks which doubtless were covered by snow, the summit lands were buried by ice to depths of 500 to 1,000 feet, and more than 30 ice streams 5 to 50 miles long flowed down the valleys to the mountain base. The effects of glaciation, repeated three times, are everywhere plainly recorded in ice-scoured surfaces, cirques, widened valleys, moraines, outwash plains, and scores of lakes. Durango, La Plata, Silverton, Ouray, Ridgeway, and several other villages are situated on glacial debris. Furthermore, the steepening of valley walls by glacial scour in conjunction with heavy rainfall has provided conditions exceptionally favorable for the transport of material en masse. In the San Juan Mountains more than 300 landslides, 100 rock streams, and many mud flows and soil creeps have been mapped; masses of moving debris that cover 1/2 to 2 square miles, extend up canyon walls to heights of 1,000 to 2,600 feet above the stream beds and some distance down valley—the Slumgullion flow for 6 miles.
Though the picturesque erosion forms characteristic of the plateau province are lacking, the San Juan landscape is truly magnificent. The grandeur of the desolate summit lands—bare rock flats, huge domes, lofty pyramids and needles—is matched by the majesty of the gigantic canyons. For the scientist the region has special interest. It reveals chapters in geological history and displays physiographic features not duplicated elsewhere. Here the whole story of glaciation is easy to read, and such features as the great rock streams and mud flows are fully displayed.
WESTERN BORDER LANDS OF COLORADO
PLATEAU
In topographic expression, the western border of the Colorado drainage basin is radically unlike the eastern border. In contrast with large eastern tributaries to the Colorado River, which generally rise along mountain crests at the Continental Divide and flow across Pre-Cambrian granite and Paleozoic rock, most of the western tributaries are short, carry little water, and occupy channels cut in sedimentary rocks of Mesozoic and Tertiary age. Along the Wyoming-Idaho boundary and in the Uinta Mountains some streams rise in highland valleys, but in Utah most of them originate on plateau escarpments, and in Nevada on flat lands or the slopes of scattered mountains. Across the tops of the high plateaus the drainage divide between the Colorado Basin and the Great Interior Basin is difficult to trace, and even more difficult where it lies on the slopes of Pine Valley, Snake, and Sheep Mountains, and crosses Charleston and New York Mountains. Southeastern Nevada is part of the Colorado drainage system with topographic features characteristic of the basin and range province.
In the area drained by the Virgin River, some of the little-known faulted mountainous masses are composed of Paleozoic rock; some, like Beaverdam and Virgin Mountains, of Pre-Cambrian schists overlaid by Paleozoic strata; and others, like the Muddy and Spring Mountains, of thick Paleozoic and equally thick Mesozoic formations. The character of this region is described in the section of this report concerning the basin and range province.
ARIZONA MOUNTAIN PROVINCE
This province, as described in this report, refers to the area along the south margin of the Colorado Plateau occupied by certain outliers which structurally resemble the plateau but which appear very different because they are eroded into rugged mountains.
Like the Grand Canyon province, this province is formed of large upraised crustal blocks or segments. Here, however, the rocks forming the surface are not flat-lying sedimentary types of Paleozoic age as at Grand Canyon, but granite and metamorphic rocks of earlier age. The result is that erosion has sculptured them into forms very different from the characteristic mesas and buttes of the plateau. Outstanding examples of these outliers are the granite masses around Prescott, Ariz., the Mazatzal and the Sierra Ancha Mountains. Between these are some large valleys, including those of the Verde River and Tonto Creek, which have distinctive character and are of exceptional interest from a geologic standpoint.
Prescott Area.—The mountainous region about the town of Prescott, in central Arizona, is largely composed of granite. It weathers into rugged hills and irregular surfaces that are in marked contrast to topographic features of the plateau area farther north. This granitic area extends eastward to the Verde Valley, north to Chino Valley, west to the Aquarius Cliffs, and south as far as Yarnall Hill. Lavas cover and conceal the granite across some rather extensive portions of the region, locally modifying the general appearance of the area. For the most part, however, the vast granite mass dominates the landscape.
Along the eastern borders of the Prescott Mountain area, faulted blocks, formed of sedimentary strata resting on granite, are preserved, indicating that this entire region once was covered with strata such as now form the plateau province. An enormous amount of erosion, therefore, must have stripped away the overlying sediments here, leaving the great granite core to form the present land surface. Peculiar rock forms resulting from this erosion, as illustrated by the Granite Dells, are among the most interesting features of the area.
From a geologic standpoint, the most significant elements of the Prescott area are those that have to do with ore deposits. Great ore bodies have been brought up by faulting, especially along the border areas, and many mines have been developed in this region as a consequence.
Mazatzal and Sierra Ancha Mountains.—South of the Mogollon Plateau and east of the Verde Valley is a portion of central Arizona that differs geologically from other parts of the State. It consists of two great mountain blocks—the Sierra Anchas on the east and the Mazatzals on the west—separated by the Tonto Basin. The ranges which extend for many miles along north-south axes are eroded into jagged, irregular skylines, resembling the mountains of the basin and range province to the south. Structurally, however, they appear to be more closely related to the plateau, having been raised vertically as great blocks, the upper parts of which were long ago removed by erosion. Very ancient quartzites (Algonkian) form the present mountain summits, whereas in the plateau to the north, rocks of corresponding age, if they occur at all, lie buried at great depths.
The height of the mountains (Aztec Peak in the Sierra Anchas, 7,400 feet), the abrupt rise above the surrounding valley floors, and the ruggedness of the terrain combine to make the Sierra Ancha Mazatzal Mountain area one of the wildest, least accessible parts of Arizona. Vegetation is dense over wide areas and wildlife is abundant. In general, this area is one of the most favorable places for recreational development of the type that involves hunting, camping, and hiking.
Among features of outstanding geologic interest in this area are the asbestos deposits high in the Sierra Ancha Mountains. They appear as a conspicuous white line when seen from a distance. Similar asbestos deposits are found in other parts of this area, notably in Salt River Canyon.
Between the Sierra Ancha and Mazatzal Mountains, near the town of Payson, Ariz., is a natural bridge which has been developed in limestone through the work of solution. It is large enough to have a small orchard on top and should attract considerable interest.
Verde Valley.—The walled-in Verde Valley differs in physiographic character from other large valleys in Arizona, yet it is one of the most delightful because of the large permanent stream flowing down its length. On three sides the valley is enclosed by cliffs forming the margins of upraised blocks of the Colorado Plateau. Details of geologic structure have not yet been determined in this area, but a series of great vertical faults apparently controlled its early development. Erosion has since greatly modified the escarpments on each side and partly obliterated evidence of the crustal movements involved. Lavas formed a dam across the river in one place and extensive deposits of limestone are the result of deposition in a very large lake that once covered much of the valley. Because of this complex history, many geologic features of unusual interest are represented in the Verde Valley.
Unlike the dry plateau country of northern Arizona or the mountain ranges to the south, Verde Valley is destined, both because of its character and location, to be developed as an important source of water and of agricultural produce. Among the best known features of this area deserving consideration and more study are Verde Hot Springs, Soda Springs, fossil tracks of lions and other prehistoric mammals, salt deposits and prehistoric salt mines, and lake deposits with mollusks.
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Last Modified: Mon, Sep 6 2004 10:00:00 pm PDT
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