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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)
RECENT VOLCANIC AREAS
Within and along the borders of the Colorado Plateau are several areas of geologically recent volcanic activity which have so modified both the appearance and the character of the country as to merit special treatment in this report. Although underlain by normal strata of the plateau type, lavas, cinder cones, and strata volcanoes have covered and masked the sedimentary rocks, thus obscuring the original topographic forms. Outstanding examples of such areas are the San Francisco Mountains and the White Mountains. A lesser example is found in the Mount Trumbull region.
San Franciscan volcanic field.—One of the largest, most spectacular, and probably the most interesting geologically, of the volcanic fields within the Colorado River watershed is that developed about San Francisco Mountain in the central part of Northern Arizona (Pl. 3, in pocket). This lava field covers an area of approximately 3,000 square miles. In addition, several hundred small cinder cones are scattered over this surface, and a series of five large strato-volcanoes form a row from east to west, dominating the landscape. San Francisco Mountain, the largest of the volcanoes, reaches an elevation of 12,611 feet above sea level or about 5,000 feet above the plateau surface.
The age of the various volcanic features in this area varies considerably, for the results of three distinct periods of activity can be recognized. The oldest of these, which was a time when very extensive basic lava flows poured forth on the surface of the plateau, dates back at least a million years (Pliocene). During the middle period the high peaks were formed of more viscous lavas. The last period, like the first, was featured by vast sheets of basalt spreading over the surface and by the development of small cinder cones, but it is most notable because of its recency. One cone, Sunset Crater, is less than a thousand years old.
The diversity of features represented and the excellent state of their preservation due to recency of origin combine to make this an outstanding area for illustrating volcanic phenomena. Most of these features are well known to scientists through the works of Robinson and Colton; some of them are familiar to the general public through the educational programs of the National Park Service and U. S. Forest Service, the agencies which control a large part of the area. Still, much remains to be learned about the various features and only a beginning has been made in utilization of the recreational possibilities of this area.
Features of special geological interest in the San Franciscan volcanic field, having potential value from an educational and inspirational standpoint, range from entire mountains that illustrate the results of remarkable or unusual processes to small detailed structures or products incidental to the volcanism. In the former category are Slate Mountain and Marble Mountain, which are excellent examples of semi-laccoliths small hills due to the partial doming up of once flat-lying layers of sedimentary rocks through the force of molten material being introduced from below. The structure in these hills is easy to see and presents an impressive story when understood, especially in Slate Mountain, where a road affording spectacular views makes many of the features readily accessible.
Sunset Crater, a recent cinder cone preserved as a national monument, already has received recognition as an outstanding natural feature, though only a beginning has been made in presenting its story so that the layman can see and understand it. Another cinder cone of unusual interest, and one that is little-known, is Red Mountain. Erosion has cut away the north side of this cone, clearly exposing to view the sloping layers of bright red cinders, thus demonstrating the manner in which it has been built up.
Among features of special interest on San Francisco Mountain are evidences of glacial activity including moraines and outwash plains, areas of obsidian or volcanic glass, and plugs or necks showing where molten material came up through the interior at several different times. Features of the adjoining volcanic field that deserve mention because of their unusual character are the "squeeze ups" and ice cave near Sunset Crater, the large lava tube known as Government Cave, and the Grand Falls of the Little Colorado formed by a lava dam.
White Mountain volcanic field.—Along the southeastern margin of the Colorado Plateau in Arizona, east of longitude 110°, the plateau surface and much of the area to the south is thickly mantled with volcanic rocks of relatively late geologic age. The highest elevations are reached at such important centers of eruption as Ord Peak and Mount Baldy, around 11,000 feet. Southward from here, the volcanic field includes the Blue Range and extends to the vicinity of Clifton and Morenci. Eastward, it goes far into New Mexico, while in the west, many lava flows extend down into the Apache Indian Reservation, especially in the area east of Eagle Creek and north of the Gila River, on the Natanes Plateau south of the Salt River, and down Forestdale Creek farther north.
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| Figure 27.—Sunset Crater in Arizona. |
The mantle of igneous rock forming this volcanic field varies greatly in thickness and in composition, and rests upon different types of surface from place to place throughout the region. Along the northern margin between Springerville and Snowflake, it consists of only a few relatively recent, flat-lying flows of basalt with small, partly-weathered cinder cones superimposed. Farther south toward Alpine, the volcanic series is much thicker and includes many deposits of soft tuff, but cinder cones are lacking. South of this area, where the scenic Coronado Trail reaches its highest elevations, deep canyons dissect the mountains and show that the landscape is formed of a great number of flat-lying basaltic flows with a total thickness of thousands of feet. Such vast outpourings of lava are unexcelled elsewhere in this region and represent a truly impressive volcanic history.
Along the Coronado Trail south of Rose Peak, the volcanic rocks exposed to view are largely silicic or intermediate in composition, in contrast to the basic types farther north, and they are much folded and faulted. Furthermore, these lavas probably are considerably older than the basalts and at one time were covered by a southward extension of the basic lavas. Westward from this area across the great valley of Eagle Creek, the products of volcanism cover the surface for many thousands of square miles. Flat-lying sheets of relatively recent basalt, locally with beds of tuff, cover all of the Natanes Plateau and the area northward.
Examination of the volcanic rocks along the western margin of the field, that is, in the San Carlos and Fort Apache Indian Reservations, shows that strata of sedimentary rocks upon which the lavas rest are progressively younger and higher in altitude from south to north. In all of this area the underlying formations remain in a nearly horizontal position, having been uplifted vertically as in the Grand Canyon area. Near San Carlos, however, the basalts rest on a surface of Mississippian limestone. Near Fort Apache and White River the black lavas are on red Supai sandstone, and far to the north they are underlain by Permian limestone and later formations typical of the Grand Canyon and Navajo countries.
Geologic features of this volcanic field that have outstanding scenic interest include the deep canyons eroded by the Black and the White Rivers, the narrow gorge of the Blue River, and the black volcanic necks or plugs that rise spectacularly above the red sandstone east of Fort Apache and once served as feeders to lava flows long since removed.
Uinkaret and Shivwits Plateau volcanic fields.—The two westernmost plateau blocks north of Grand Canyon, known as the Uinkaret and Shivwits, are centers of relatively recent volcanic activity. Although these volcanic fields are small beside the San Franciscan and White Mountain fields to the southeast, they contain many elements of interest, especially in areas where the lavas and craters are in proximity to the rim of Grand Canyon. The outstanding features of these areas are discussed in this report, in the section on "Plateaus on north side of Grand Canyon."
BASIN AND RANGE PROVINCE
The term "basin and range" is applied to a type of country formed of long, usually parallel, mountain ranges separated by flat alluvial plains which are the result of mud, sand, and gravel filling former intervening valleys of considerable depth. West and south of the great plateau is a vast, low desert region, traversed by the lower part of the Colorado River and drained by its tributaries such as the Gila and Salt Rivers. Geologically speaking, this is but a part of the entire area commonly designated as the basin and range province, which includes the interior drainage of the Great Basin and parts of certain other watersheds.
Mountain ranges within the basin and range province are of many types whether classed according to structure or rock character. Most of them trend from northwest to southeast, though there are notable exceptions. Some of them are composed largely of stratified sedimentary rocks, some are granitic or metamorphic masses, and still others are dominantly of recent volcanic rock. A majority contain combinations of these materials. Certain of the ranges owe their present positions to the simple tilting of strata along nearly vertical fault planes, whereas others are the result of folding, overthrusting, or complex combinations of these processes. Various parts of the province can be classified and grouped in a rough way according to these characteristics. The treatment in this report follows such natural groups.
The principal subdivisions of the basin and range province as recognized here are (1) the sedimentary ranges of the Arizona-Nevada corner; (2) the desert ranges south of Lake Mead; (3) the east-west trending ranges of middle western Arizona; (4) the southeastern Arizona ranges; (5) the southwestern Arizona ranges; and (6) the Colorado River delta, and Salton sink area.
Arizona-Nevada corner.—That portion of the basin and range province that includes the southern part of Nevada, northwestern Arizona, and southwestern Utah differs from neighboring parts of the province in the Colorado River drainage in that most of its ranges are formed largely of the same sedimentary strata found in the adjoining Colorado Plateau. These ranges, in general, have a north-south trend, but they differ considerably in structure. Some are relatively simple, being formed by the tilting of rock layers through movement along normal faults, but others are extremely complex and involve folding, thrust faulting, and several stages of normal faulting.
The Muddy Mountains and the Spring Mountains represent the more complex types. Over-thrusting of rock strata on a huge scale is especially well illustrated and easy to see in the Muddy Mountains and might well be featured as an educational exhibit. Folding, compound normal and reverse faults forming wedges of strata, and examples of rock replacement near faults can be well illustrated in both ranges.
Examples of mountains developed with simple structure are the ranges through which have been carved Iceberg and Grand Wash Canyons on Lake Mead. These are particularly good illustrations of the effects of crustal disturbances, both because of the spectacular character of the ranges and because of their easy accessibility by boat. They also offer unusually good opportunities for illustrating to the layman the basic geologic principles involved in contrasting types of deposition, cyclic sedimentation, and differential erosion, all of which are shown especially well by the colorful beds forming the walls of these canyons.
Other ranges in this area which are relatively simple in structure, and therefore easy to understand, are the Frenchman, the Virgin, and the Beaver Dam Mountains. Steeply tilted layers of rock are readily apparent in all of these and usually attract the attention of the visitor. The Frenchman Mountains as seen from the air near Las Vegas Airport are especially striking.
In addition to large-scale structural features of the mountain ranges, this portion of the basin and range province contains an abundance of other geological features having interest and significance, many of which should be considered in developing a recreational program. Among these are (1) Gypsum Cave, Nev., where traces of early man were found associated with remains of extinct animals; (2) extensive lake deposits resting high above Lake Mead and illustrating clearly the former presence of a large natural lake in this area; (3) dikes of volcanic rock intruded into other rocks as illustrated especially well by the black dike in gravels by the highway south of Hoover Dam; (4) excellent illustrations of angular unconformities to be found in several places along Lake Mead and elsewhere in the region; (5) erosional controls in forming spectacular mesas of gravel as shown by the Temple, Napoleon's Tomb, and other hills near Lake Mead; (6) the remarkable erosional features developed in red sandstone at the Valley of Fire; and (7) deposits containing mammoth remains east of Las Vegas.
Desert ranges south of Lake Mead.—A large section of the basin and range province in northwestern Arizona contains north-south trending mountains composed partly of very ancient crystalline rocks and partly of geologically young (Tertiary) volcanic rocks in varying proportions. Sedimentary strata are absent. This type of desert topography extends over a large area from Lake Mead on the north to the vicinity of Topock on the south; from the Grand Wash Cliffs on the east to and beyond the Colorado River on the west. It is a region of rugged mountain ranges with mining operations in many places.
The Black Mountains are the largest desert range in this area. They are 100 miles long and reach heights of more than 5,000 feet at Mount Perkins and Mount Wilson. At their northern end, where they are cut through by the Colorado River, Hoover Dam has been constructed. From this locality southward, the range is paralleled on the west by the Colorado. Perhaps the most unusual element of the entire range is Fortification Hill a lava-capped mesa of light-colored fanglomerate, near Hoover Damn.
East of Black Mountains, across Detrital Valley, are three other prominent ranges of somewhat similar character. From north to south, they are the White, Cerbat, and Hualpai Mountains. The last named is the largest and highest, reaching an altitude of about 7,500 feet, and has an abrupt western front caused by a large fault. Already, advantage has been taken of the summit's cool climate and good views with the development of a county recreational area.
The broad geologic features are simple in most parts of this area where many flows of black basalt may be seen, one upon another or resting on an old eroded surface of granite and schist. On the other hand, details of structure in the older rocks are difficult to work out for they are much altered by metamorphism and intrusion. The region as a whole is inhospitable both because of the very hot summer climate and because of the barrenness of most of the ranges. Some geological features, such as a prominent lava dike that cuts through gravels along the highway to Hoover Dam, might be made interesting to visitors, but opportunities are limited.
East-west trending ranges of middle western Arizona.—In contrast to the north-south trend of most mountains throughout the basin and range province, those ranges forming a belt across middle western Arizona are oriented in an east-west direction. These mountains are largely of very ancient crystalline rocks, although some also contain remnants of sedimentary strata of Paleozoic age. Furthermore, late basalts are common in places. Most of these ranges have complex structure, resulting from a long history of crustal disturbances. Extensive overthrusting of great mountain blocks has been recognized in various localities.
Prominent among the east-west trending ranges of western Arizona are the Buckskin, Harquahala, and Huarcuvar Mountains in Yuma County, and the Vulture Mountains in Maricopa County. All of these are typical desert ranges of considerable interest from the standpoint of geologic structure and mineral wealth. The structure is very complex but in other respects there is nothing outstanding about the mountains. In general, they are relatively low in altitude.
The Mohave Mountains in southern Mohave County probably also should be included in this group. The western part of these mountains is formed of volcanic rocks and in them the "Needles"—irregular pinnacles of erosion near the Colorado River have been carved.
Southeastern Arizona ranges.—Satisfactory generalizations concerning the many mountain ranges of central and southern Arizona are difficult to make. These mountains have in common a general north-south or north-west-southeast trend and most of them rise abruptly to considerable altitudes from the low, flat, alluvial plains of the desert. A majority are composed chiefly of sedimentary strata that are steeply tilted, but there are notable exceptions. The Santa Catalinas, as exposed today, are mostly metamorphic. The Galiuro and some others are largely volcanic.
Each separate mountain range has a distinct and, in most cases, complicated structural history. Some involve folding, and some have their fronts raised along huge normal faults. Cross faults may or may not be present, and overthrusts are responsible for some of the ranges. In brief, a description of these ranges, adequate to illustrate the origin of each, would require a series of individual treatments and, in many instances, sufficient information is not yet available.
The more prominent of the mountains in this region are the Pinal, Mescal, Dripping Springs, Apache, Santa Catalina, Santa Rita, Wetstone, and Huachuca. Mining operations are, or have been, active in a majority of these. Other types of development are negligible except in the foothills and most development is handicapped by the general lack of permanent water.
Of the more prominent scenic attractions are such high peaks as Mount Graham (10,100 feet), Mount Turnbull (7,800 feet), Mount Lemmon (9,500 feet), and Baboquivari (7,500 feet). Also of note are the great steepwalled canyon of the Gila River (2,200 feet) in the Mescal Mountains and the weird monuments of erosion at Chiricahua National Monument. Several caverns, including Colossal Cave and Onyx Cave, are known, but none of outstanding character.
Southwestern Arizona ranges.—In the southwestern portion of Arizona is found the driest, hottest part of the State. The nearly parallel desert ranges that slant diagonally across this area from the northwest to southeast differ from those to the north in their orientation and from those to the east in being smaller and less covered with vegetation. Furthermore, they are almost entirely lacking in sedimentary rocks such as occur in the adjoining regions.
Mountain ranges in this region are of two principal types those composed dominantly of ancient granites and schists, and those of relatively recent volcanics. Because these two varieties of rock weather very differently, mountains developed from each have distinctive shape and topographic expression. On the other hand, some ranges are combinations of both types of rock, with volcanic materials partially covering and concealing the older granites. In such cases, the resulting mountain types have mixed characteristics.
In the extreme southwestern part of this area the mountains are almost all of the granitic type. They include the Tinajas Altas, the Gila Mountains, the Cabeza Prieta, Mohawk, and Sierra Pintas. All are low and characteristically weather into blocks. In the northeastern part of the area other ranges, especially the Sierra Estrella and the Maricopa, are also of this type. Elsewhere volcanic rocks predominate and tend to make the ranges rougher and more impressive. Especially rugged and massive are the Kofa Mountains, northeast of Yuma, which rise with a sheer western front high above the alluvial plain at their base and which have several narrow, spectacular gorges cut into them, including Palm Canyon. Other smaller mountains of the group are the Castle Dome and Plomosa in the west and Growler and Sand Tank farther east.
The Whipple Mountains on the California side of Lake Havasu just to the northwest of Parker Dam are scenically spectacular because of their brilliant coloring, picturesque canyons and peaks of deeply eroded volcanic rocks, and the striking desert vegetation.
The Eagle Tail Mountains in the north central part of this area form a narrow, prominent ridge that is remarkable because of its picturesque peaks known as Eagle Tail and Court House Rock. These mountains are formed of a fine succession of early volcanic rocks.
One of the most interesting geologic features of this region is the Pinacate lava field at the international boundary southeast of Yuma. It is part of a very large area of volcanism extending southward into Mexico and is significant because of its recency. Lava is 50 to 100 feet thick with well-defined margins. Many small craters are present, all of them apparently very young. Through this rough but awe-inspiring terrain the famous Camino del Diablo passes.
Colorado Delta and Salton Sink.—The delta of the Colorado River is formed in, and controlled by, one of the most remarkable structural troughs on the earth's surface. It is a depression resulting from the subsidence of a long narrow crustal block between faults or breaks, comparable to the trough of the Dead Sea and Jordan Valley in Palestine. Extending for more than one hundred miles in a north-south direction, it is submerged at one end beneath the Gulf of California and is occupied at the other by the Salton Basin which is partly below sea level. In the central part, where the Colorado has for ages been piling up its load of sediment, the history has been that of the constructional activities of the river versus the downsinking of the trough.
Although much of the Colorado delta is in Mexico and therefore beyond the scope of this report. Its effect on that part of California within the Salton Basin has been profound. There is evidence to indicate that despite recent subsidence, concurrent building of the delta has successfully kept the upper end of the trough blocked off from the Gulf of California. As a result, the Salton Sea is about 250 feet below sea level and has a depth of 24 feet, yet the river delta is built up to slightly above sea level.
Numerous features of unusual geologic interest are found in this area, but there is some question as to what extent they may be developed for educational or other purposes because of the inhospitable character of much of the delta area. Mud volcanoes occur on the west side of Volcano Lake in Mexico, and hot springs issue forth along the San Jacinto fault. At Travertine Point, a spur from the Santa Rosa Mountains that projects toward the Salton Sink from the west shore, are seen evidences of former levels of the lake. North of the delta and west of Yuma is an extensive area of large, well-developed sand dunes.
PLAN RECOMMENDED FOR DEVELOPMENT OF CERTAIN
RECREATIONAL FEATURES IN THE PLATEAU PROVINCE
In writing of the remarkable scenic features of the Colorado Plateau, Capt. C. E. Dutton stated in 1884: "Great innovations, whether in art or literature, in science or in nature, seldom take the world by storm. They must be understood before they can be estimated, and must be cultivated before they can be understood." This penetrating analysis of the problem that is faced in obtaining maximum recreational value from a region of exceptional and unique scenic character is the basis of a plan that follows.
Information available to visitors in the plateau region should not be confined to unrelated ideas concerning the immensity, the beauty, or the marks of erosion presented by such spectacles as Zion, Bryce Canyon, Rainbow Bridge, or Grand Canyon. Such features are truly great and inspiring, and normally have a profound influence on the thinking of those who see them, but as a source of mental stimulation they are surpassed in greatness by the history of the earth which is illustrated through them. To one who has not studied the geological processes this history will not at once be apparent, but with a very little explanation it may become understandable to any person of normal intelligence and will thereafter remain a source of great pleasure and inspiration.
Earth history as represented in the Colorado Plateau is not significant merely because of the chronological record that it represents, but also because it serves as a means of conveying certain great concepts to the human mind. These include the magnitude of geologic time, the reality of crustal movements and mountain uplifts, the evolution of life, and the never-ending processes of deposition, erosion, and volcanism. Such concepts are responsible for great mental stimulation and lead, through thought, to attainment of one of the highest forms of recreational enjoyment.
Events representing the history of the earth undoubtedly are more closely and simply illustrated by the record of the rocks in the Colorado Plateau than anywhere else in the world. In order that this history be skillfully and artfully presented to visitors of this region, emphasis must be placed on original materials. Facts must be dealt with and presented in a way that will guide people's thoughts toward a realization of the principles. By way of illustration, consider the result of calling attention to sea shells and corals in rock layers now some thousands of feet above the sea. In most cases, a concept of crustal movement and uplift will come to the visitor, evolved through his own thinking and not because of a theory advanced to him by some scientist. The thrill and satisfaction of discovery will be his.
From the preceding analysis it appears that the task of the planner is, first, to lead people to the best possible illustrations through a skillful development of roads and trails and, second, to make available, without forcing upon them, data necessary to a correct interpretation and correlation of facts. In the Colorado Plateau the normal flat-lying attitude of the rock layers and the orderly succession of beds according to age greatly simplify this problem.
There are three principal routes used today by visitors crossing the plateau region and, fortunately, each of these routes offers opportunity for seeing evidence of the normal succession of events in geologic history. One of the routes is from Grand Canyon northward, through Zion and Bryce. Another (which probably will be much used in the future) is from Grand Canyon northeastward through Tuba City, Kayenta, and Monument Valley. The third is from Grand Canyon eastward by way of Petrified Forest and Gallup. Visitors traveling any of these routes from Grand Canyon will pass through rocks from oldest to youngest or, taking the routes in reverse order, will start with the youngest layers and end with the oldest. Thus a unified story may be obtained and the history made appealing if it is properly explained along the route by wayside observation stations, markers for exhibits-in-place, and literature covering the routes.
It is clear that at each place of interest and significance along any of the main routes of travel not only the local features of geology, such as fossils or rock structures, should be brought to notice as they appear in natural position, but also their place in the general scheme of things should be emphasized. This must be done primarily by means of charts and diagrams. Details of museum and exhibit development, of roadside signs, and of circulars needed to take people from one unit to another over these routes should be so planned as to have an orderly and appealing sequence.
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Last Modified: Mon, Sep 6 2004 10:00:00 pm PDT
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