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A Survey of the Recreational Resources of the Colorado River Basin



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Cover

Contents

Foreword

Introduction

Summary

The Colorado River Basin

Geology

Plant and Animal Life

Prehistory of Man

Recreational Benefits of Reservoirs

Potential Reservoirs

The Grand Canyon

Canyon Lands of Southeastern Utah

Dinosaur National Monument

Conservation of Recreational Resources

Life Zone Map

Bibliography





A Survey of the Recreational Resources of the Colorado River Basin
National Park Service Arrowhead


Chapter II:
GEOLOGY (continued)


COLORADO PLATEAU PROVINCE

Outline of geologic history.—The geologic history of the Colorado Plateau Province, now in vigorous process of destruction, is recorded in the composition, the sequence of deposition, fossil content, and geographic position of its constituent rocks. In broad outline it is the story of enormous masses of sedimentary rock raised high above sea level and dissected by the Colorado River and its many tributaries. Thus, in strong contrast with adjacent regions, its history is fairly simple and easy to read. In it six major events are recorded.

1. During Paleozoic and Mesozoic times—periods aggregating as much as 490,000,000 years—sandstone, limestone, and shale were deposited alternately on the ocean bed and on land, and progressively sunk, thus leaving each set of beds periodically not far above sea level.

2. Near the close of the Cretaceous era, approximately 60,000,000 years ago, the entire plateau country was uplifted. For the region as a whole, the raising of this enormous mass was accomplished without greatly modifying the original, almost horizontal attitude of its constituent strata. In places, however, the beds were warped into broad synclines, anticlines, and domes, and locally were folded into long narrow monoclines, conspicuously represented by the San Rafael Swell, the Kaibab Plateau, the Waterpocket Fold, and the Comb monocline.

3. Following this general uplift both the flat-lying rocks and the folded rocks were so completely worn down by streams that the surface again became substantially level.

4. On the surface produced by post-Cretaceous erosion, great thicknesses of Tertiary rocks were deposited over most of the plateau province, and upon them a master stream and subordinate streams were developed the ancestral drainage system of the Colorado River.

5. After a large part of the Tertiary sediments and some of the lavas had been laid down the plateau country was again uplifted—substantially to its present height and in places broken by faults. In consequence of the uplift, the gradients of drainage channels were greatly steepened; the streams became powerful agents of erosion.

6. During the past 20,000,000 years, substantially the present plateau landscape of canyons, cliffs, plateaus, mesas, terraces, and the amazing variety of minor land forms has been modeled, chiefly by stream erosion.

Great White Throne
Figure 20.—The Great White Throne. The Navajo sandstone in Zion National Park, Utah, is 2,280 feet thick.

Regional features of northern portion.—The central part of the Colorado drainage basin is unique in geologic history, topographic form, and scenic grandeur. Within it are displayed the oldest and youngest rocks exposed on the North American Continent, and the major subdivisions of the geologic time scale are represented in orderly succession. It is a vast expanse of plateaus, terraces, mesas, and cliffs that seem to have an unlimited range in form, size, and color.

Over an area of about 130,000 square miles in Utah, southwestern Colorado, northwestern New Mexico, and northern Arizona, plateaus that rise 5,000 to more than 10,000 feet above sea level are so numerous and prominent that the region well merits the various applied names—the Colorado Plateau province, the plateau province, the Colorado plateaus, or, considered as an areal unit distinct from surrounding regions, the Colorado Plateau. Outstanding features are the widespread Triassic, Jurassic, and Cretaceous strata in approximately horizontal position, the gigantic cliffs and the multitude of canyons that carry the perennial, intermittent, and ephemeral run-off. The sedimentary strata that lie above the meagerly exposed Paleozoic rock extend hundreds of miles with little change in character. They include series of shales 1,400 to 2,200 feet thick, of limestones 1,000 to 1,500 feet thick, of volcanic tuff 6,000 feet thick, and many single beds of sandstone 200 to 1,500 feet thick. In the walls of Zion Canyon the remarkable Navajo sandstone measures 2,280 feet—the thickest unit sedimentary bed so far known. In succession above the strongly predominant sedimentary rock extensive sheets of lava characterize the landscape in widely scattered areas. The edges of both these sedimentary and igneous beds are fully exposed in vertical, unscalable cliffs of seemingly interminable length that advance in headlands and retreat in bays, tower above the adjacent lower lands, and from their crests extend backwards to the bases of similar escarpments. Substantially the rocks in each of the great cliffs are of different geological age and are marked by distinctive colors—Lower Triassic, brown and light red; Upper Triassic, vermilion or brilliantly variegated; Jurassic, uniformly white or red, or regularly banded; Cretaceous, gray; Tertiary, pink, extending up and into white; and, the Tertiary and Recent capping lavas, black.

The major features of the plateau topography are so consistent that in distant views the long stretches of even skyline give an impression of extensive flat surfaces that terminate in lines of cliffs. The plateau country in reality is intricately dissected and closer inspection reveals a ruggedness possessed by few if any other regions. Over large areas the canyons are so narrow, so deep, and so thickly interlaced that the region seems made up of gorges, cliffs, and mesas and platforms, intimately associated with a marvelous variety of minor erosion forms. Some of the topographic features are developed on a scale that in other regions would justify the term "mountains." However, the plateau is essentially devoid of high peaks and narrow, serrate ridges. Like a deeply engraved intaglio, its departures from the regional surface are downward rather than upward.

For making a land of canyons, the physiographic conditions in the plateau province are especially favorable. The region lies at high altitudes; its climate is generally arid; its major rock strata are thick, resistant, and lie nearly flat; its longer streams receive abundant water from the adjacent humid Rocky Mountains and are at the stage in their life history where their ability to grind up rock and to transport the resulting debris is greatest. Consequently, innumerable deep, narrow trenches are cut into strata that otherwise remain intact, and the great beds of sandstone and limestone terminate in vertical walls. In striking contrast to the wide open valleys, the graceful slopes, and the rounded hills of most other regions, the topography of the plateau lands is abruptly angular, even in minor details.

Though the Colorado Plateau is generally floored with horizontal or slightly inclined sedimentary strata, its surface has been roughened in places by folding, faulting, the building of volcanoes, and the intrusion of igneous rocks—geologic activities that have modified the otherwise normal drainage pattern and the orderly development of erosion features. Of the folds, many are merely "rock wrinkles" of slight importance, but the larger upwarps and downwarps have produced elongated domes 90 to 150 miles long and 40 to 75 miles wide that in the present topography stand 1,000 to more than 3,000 feet above the adjacent land. Particularly conspicuous are the mountainous upwarps that brought into existence the isolated Uncompahgre Plateau, the San Rafael Swell, the Kaibab Plateau, and the Waterpocket Fold, the Defiance upwarp, and the Escalante anticline. Most of these folds are steeper on their east sides where, in consequence of erosion, the strata that once overarched them are represented by remnant beds steeply inclined along their flanks. Few features in the plateau are more conspicuous than the upturned strata in the high, rugged hogbacks that characterize the Waterpocket, Comb, Echo, and East Kaibab monoclines. The larger folds, perhaps all the ridge-making folds, in the plateau country are the result of compressive forces directed horizontally from the east and confined to the sedimentary strata. Another type of upwarp in which the strata were raised by the intrusion of igneous rocks—laccoliths is also exceptionally well represented. The lofty Navajo, Henry, Abajo, La Sal, Ute, and Carrizo Mountains, and similar structures in the Rico and La Plata Mountains, are essentially igneous masses that once were completely overlain by sedimentary rocks. In all of them except Navajo Mountain, which retains its cover, erosion has exposed the igneous cores. Similar to the laccolithic mountains in manner of formation are the peculiar faulted domes in the vicinity of the La Sal Mountains, where the strata have been deformed by the intrusion and expansion of masses of salt and gypsum.

Colorado Plateau
Figure 21.—The Colorado Plateau. Like a deeply engraved intaglio, its departures from the regional surface are downward rather than upward. (Air photo)

Grand Gulch
Figure 22.—Grand Gulch in the Canyon Lands of southeastern Utah. (Air photo)

Kaibab Plateau
Figure 23.—East side of the Kaibab Plateau upwarp. (Air photo)

Navajo Mountain
Figure 24.—Navajo Mountain.

In the southwestern part of the plateau province, where conspicuous folds are rare, the strata have been deformed by faults so large in dimensions as to completely remodel the landscape. They have sliced the region into enormous earth blocks, realined streams, and formed cliffs comparable in height and length to the towering escarpments developed by erosion. In roughly parallel position the Paunsaugunt, Sevier, and Hurricane faults, each nearly 200 miles in length, and the shorter Grand Wash fault, extend from south central Utah into Arizona and southward across the Colorado River. Along the east side of these fractures the stratigraphic formations have been raised a few hundred to as much as 7,000 feet above their counterparts on the west. The effects of faulting are typically expressed in the Zion National Monument, where the sharply defined Hurricane Cliffs, produced by the Hurricane fault, separates the flat-lying cultivated plain about New Harmony and Kanarraville from the uninhabitable Kolob Plateau, which stands more than 3,000 feet higher.

The remarkable unevenness of the regional surface produced by great faults has been intensified by the building of volcanoes and the outpouring of lavas during Tertiary and Recent times. Above the general surface of the Uinkaret Plateau in the southern portion of the province more than 150 ash and cinder cones rise to heights of 50 to 500 feet, and the tabular masses of lava that cap Trumbull, Logan, and Emma Mountains rise to heights of 1,000 to 2,000 feet. Similar igneous prominences characterize the adjoining Shivwits Plateau, and cones with attendant lava flows are conspicuous features of the St. George Basin, the Little Creek and the Kolob Terraces, and the valleys of Kanab and Johnson Creeks. Many craters are of such recent origin as to have retained their original features; others are worn to stumps or volcanic necks that, particularly in the Navajo country, rise sheer from the surface as towers of black rock. Likewise, some of the lava fields have been little changed, but others have been cut into fragments and, in the existing topography, are represented by the basaltic cap rock of many mesas and ridges and the thick sheets of andesitic lavas that cover Pine Valley Mountain, the Aquarius Plateau, and Grand Mesa.

Considered as a whole, the land forms that characterize the Colorado Plateau comprise a single physiographic province readily distinguishable from other parts of the North American Continent. However, the component plateaus vary so much in origin, altitude, degree of isolation, and amount of dissection that in geological literature they are classified by groups or sections. Certain areas of special scenic interest have been further segregated as national parks and national monuments.

Plateaus adjoining the Colorado River.—On both sides of the Colorado River from Lake Mead to Grand Junction, Colo., and for many miles along the tributary Green, San Juan, and Little Colorado Rivers, plateau topography is dominant. In the Grand Canyon section the Shivwits, Uinkaret, Kanab, Kaibab, and Coconino Plateaus terminate at the rim of the world's most spectacular gorge. In corresponding positions along Glen Canyon lie the moderately elevated Glen Canyon platform, the Paria, Rainbow, and Grand Gulch Plateaus, and the Kaiparowits Plateau which stands nearly 4,000 feet above the river at its base. The trenchlike Cataract Canyon separates Dark Canyon Plateau from the equally lofty Standing Rock Plateau drained by the Dirty Devil and San Rafael Rivers. Farther upstream the Colorado River is bordered on the north by the Roan Plateau and on the south by the Uncompahgre Plateau and Grand Mesa. In valleys tributary to the Colorado River, plateau topography persists. In Green River Valley, East Tavaputs and West Tavaputs Plateaus, parts of an otherwise continuous highland, are separated only by the profound Gray Canyon; in the San Juan Valley, Grand Gulch and Chaco Plateaus, and the plateau-like Mesa Verde are prominent features; and above the floor of the Little Colorado River, continued as the Rio Puerco, stands the Moenkopi, Manuelito, and Dutton Plateaus. In topographic expression these extensive river-border platforms, which lie at relatively low altitudes, are similar and all are remnants of high-lying lands but, in consequence of crustal deformation and varying stages of erosion, they display rocks of different ages and in somewhat different attitudes. Thus the eastward tilted Shivwits, Uinkaret, and Kanab Plateaus are outlined by faults, and the Kaibab Plateau is a sequence of bowed strata bordered by monoclines. As these four plateaus occupy the area of greatest uplift, and consequently of unusually vigorous erosion, they have been worn down the most—to rocks of Permian age that farther north in the plateau country are exposed only in canyons. The adjacent East Kaibab monocline and, a short distance beyond, the Echo monocline, have so lowered the strata that along Glen Canyon, rocks younger than the Triassic are buried. In fact, throughout the Colorado drainage basin the rocks in most of the plateaus are of Triassic, Jurassic, Cretaceous, or Tertiary age.

The Uncompahgre Plateau, 20 to 30 miles wide and nearly 100 miles long, with summit altitude of 9,000 to 10,000 feet, is a flat-topped anticline in which the core of Pre-Cambrian granite is largely covered by Triassic and Jurassic rocks, which are magnificently displayed in Colorado National Monument. In forming the plateau, the strata of Paleozoic age were stripped away from a former highland mass before younger sediments were deposited. Throughout its life the Uncompahgre seems to have determined the local drainage pattern. On approaching it, streams from the Rocky Mountains and the northern San Juan Mountains are deflected; they follow the edge of the upwarped highland. Even the powerful Colorado, which once made its way through the barrier, has chosen an easier course—a great curve around its north end. Its abandoned runway through the deep, narrow Unaweap Canyon, 53 miles in length, is now an automobile highway. It seems strange that this outstanding illustration of stream adjustment has received so little attention.

Grand Mesa, "the world's largest flat-topped mountain," is an isolated outlier of the Roan Plateau, 50 miles distant across the Colorado. In it the Tertiary and older rocks are preserved by a thick cap of basalt. Its glaciated lake-dotted surface covers an area of 900 square miles at an altitude of 10,300 feet—5,000 feet above the surrounding lowlands. North of it stands the smaller Battlement Mesa—similar in form and geologic history.

Mesa Verde, 250 square miles in area, is a portion of a widespread highland, in essence a promontory extending from the San Juan Mountains, from which it is separated by the broad valleys of the Mancos and Upper Dolores Rivers. In distant view, its surface, at an altitude of 8,000 feet, appears to be smooth and continuous and, in conformity with the dip of the strata, to slope southwestward. In reality the mesa is ragged. Except along its northern edge, it is so elaborately dissected by scores of south-flowing, box-headed canyons and gulches that the interstream spaces are reduced to narrow strips. The whole frayed highland has been likened to a "worm-eaten log." The mesa is surfaced with resistant sandstone about 1,000 feet thick (the Mesa Verde group) and underlain by the easily eroded Mancos shale, 1,800 to 2,000 feet thick, thus providing conditions favorable for the rapid making of great cliffs at the edge of the strata while their surfaces were but slightly abraded. The northern face of the mesa is an escarpment 1,500 to 2,000 feet high. In sequence of geologic events, a regional uplift permitted vigorous erosion of the ancient highlands; the more exposed and the softer rocks were stripped down to the massive sandstones. In late Tertiary times the mean surface was doubtless coextensive with the peneplained surface of the southwestern San Juan and adjacent areas in the plateau province. Renewed uplift and erosion brought the structure to its present form.

The high plateaus of Utah.—Of the 10 great plateaus that dominate the topography of south central Utah, the Pavant, Tushar, and Sevier are drained by westward-flowing streams; the Kaiparowits is the source of streams that go directly to the Colorado River; and the Wasatch, Fish Lake, Awapa, Aquarius, Paunsaugunt, and Markagunt Plateaus, and Pine Valley Mountains contribute water to the Colorado River and at the same time to the Great Basin. The plateaus that face the Colorado River are huge rock masses; nearly level platforms 25 to more than 500 square miles in area and bordered by terraced escarpments 4,000 to 5,000 feet high. From heights of 10,000 to 11,000 feet they overlook the lower series of plateaus along the Colorado River. Though the plateaus are now individual topographic units, it is believed that they once formed a continuous surface and that their present detachment is the result of faulting that has broken their once continuous surface into longitudinal, tilted earth blocks. The Paunsaugunt fault separates the Aquarius from the Paunsaugunt Plateau; the Sevier fault, the Paunsaugunt from the Markagunt; and the Hurricane fault, the Markagunt from the plateau-like Pine Valley Mountains. These long faults deformed all the sedimentary rocks and also the sheets of lava that overlaid them, some of them nearly 3,000 feet thick. This large-scale faulting accompanied by regional uplift introduced the long cycle of erosion still in progress. The Triassic, Jurassic, Cretaceous, and Tertiary rocks that form the stream-scoured fronts of the plateaus are but remnants. Corresponding formations appear south and east of Glen Canyon and doubtless once extended to the foothills of the Rocky Mountains. North of Grand Canyon, Triassic strata have been stripped back 30 to 80 miles, and the rocks of younger age still farther. To an extent not equalled elsewhere, the major erosion has been accomplished by wearing back the faces of cliffs rather than by abrasion and dissection of the plateau tops; erosion is directed against, rather than upon, the exposed strata. Because the individual rock layers differ in thickness and hardness, the protruding edges retreat at different rates, thus developing a zigzag vertical profile in which the resistant rocks appear as escarpments and the softer rocks as flats or gentle slopes. In other words, the plateau fronts are neither continuous nor broken curves: they are successive terraces that follow the contour for nearly 200 miles, and in distant view suggest a stairway whose ascending risers are the Chocolate Cliffs, Vermilion Cliffs, White Cliffs, and Pink Cliffs.

The high plateaus of Utah comprise a landscape famous alike for its esthetic appeal and its geological interest. Their crenelated skyline rims, their elaborately carved frontal escarpments, the canyon walls decorated with arches, bridges, and caves, and the picturesque towers and terraces in their foothill belts—all brightly colored—well merit the term "marvelous." Naturally, the unique landscape has attracted the attention of poets, prose writers, and artists, and its features have been reproduced in countless photographs in color. Each year thousands of persons go to Zion and Bryce Canyon National Parks and Cedar Breaks National Monument to view the spectacular scenery. To the scientist, the region is the most comprehensive text book on Mesozoic history and on processes involved in erosion. Fortunately, the significant features of structure, stratigraphy, and volcanism are fully exposed, and the fascinating story of geological events needs no profound study for interpretation. Few places in the world afford better opportunity to realize the power and persistence of the forces that have shaped the surface of the earth, for though displayed on an enormous scale the rock units show a certain simplicity of mass composition, form, and arrangement that makes their relations clear.

Navajo Country.—The Navajo country, about 25,000 square miles in area, extends from the Little Colorado River eastward across Arizona into New Mexico and from the Rio Puerco northward to the San Juan and Colorado Rivers. Within this large area the landscapes are notably diversified. At its center lies the extensive, little-dissected Black Mesa, from which most of the major drainage passes southward through canyons in the high bordering cliffs and continues across relatively flat lowlands as shallow washes 40 to 60 miles long. East of Black Mesa the dominant topographic features are the up-arched Defiance Plateau and the lofty, narrow, cliff-walled Chuska Mountains, and the isolated Carrizo Mountains, which rise abruptly above the surrounding lowlands. The western part of the Navajo country adjoining the famous Painted Desert is substantially the surface of a chain of low-lying plateaus deeply trenched by the Moenkopi and Navajo Creeks. At the north, overlooking the San Juan and Colorado Rivers, are the Tsegi Mesas—enormous blocks of red sandstone separated by profound canyons—and west of them is the conspicuous Navajo Mountain, whose dome-like summit (altitude 10,416 feet), 7,000 feet above the floor of Glen Canyon, is the highest land within an area of many thousand square miles.

There seems reason to believe that the prominent, detached high-lying sedimentary rocks in the Navajo country are remnants of once widely extensive formations that owe their positions to a regional uplift near the close of the Cretaceous period and that, during Tertiary and Recent times, the landscapes of pre-Tertiary times have been completely remodeled, chiefly by stream erosion. From a former general surface that stood perhaps 9,000 feet above sea level, the region has been worn down to an average altitude of about 5,000 feet, parts of it to less than 4,000 feet, and in consequence nearly all the Tertiary, most of the Cretaceous, large parts of the Jurassic, and some of the Triassic and Permian strata have been swept away. The volcanoes have been reduced to stumps, the lava flows to their feeding dikes, and deep-seated folds have been uncovered. Coincident with the removal of enormous masses of sedimentary and igneous rock, differential erosion has produced prominent topographic inequalities; flat surfaces that end abruptly at the base of terraced escarpments; mesas that rise at various stratigraphic levels above plateau surfaces, and streams that follow roundabout courses, in and out of canyons, across folds and hogbacks, with seeming disregard for rock composition, structure or regional slopes. Continuous vigorous erosion has exposed complete sections of the Permian, Triassic, Jurassic, and Cretaceous rocks and partial sections of the Tertiary and Carboniferous. In fact, the type sections for many Permian and Mesozoic formations are in the Navajo country.

Furthermore, in consequence of regional denudation, igneous rocks, particularly intrusives, are unusually well displayed seemingly extraneous features of a landscape modeled almost wholly in sandstone. Rising abruptly from brightly colored surfaces are countless black spires, serrate ridges, walls, and irregular masses of basaltic rock, agglomerate, and tuff. Within four volcanic fields, or widely dispersed, about 50 volcanic necks, 100 to 700 feet high, and 60 dikes, 10 to 900 feet long, have been mapped. In places, lavas cap mesas and protrude as palisade walls. Of the two huge laccoliths, Carrizo Mountain has been stripped down to its igneous core, but Navajo Mountain retains its original cover of sandstone. Thus the present topography of the Navajo country bears little resemblance to its earlier expression. The once fairly continuous surface is represented by a bewildering array of erosion forms among which the dwindling streams follow their tortuous courses. The outstanding elements of the landscape are mesa, butte, volcanic neck, canyon, and wash repeated indefinitely. Natural bridges, windows, arches, alcoves, cliff caves, and miniature erosion features of great variety and rare beauty stand as ornamental carvings on the larger architectural forms.

Bryce Canyon NP
Figure 25.—Elaborately carved frontal escarpment, Bryce Canyon National Park, Utah.

Gypsum Canyon
Figure 26.—Gypsum Canyon at mouth of Fable Valley in the Canyon Lands of southeastern Utah.

In its structural make-up the Navajo country is a region of rock flexures; the faults are few and of small dimensions. Characteristically northward-trending synclines and anticlines—both broad and narrow domical upwarps, and sharply delineated monoclines follow one another in succession and, in places, overlap or abut against each other. Ten major folds and eight minor folds, in addition to scores of local flexures, have been noted in the region between the Puerco and San Juan Rivers. In origin, most of these structural features are associated with the regional uplift from the Cretaceous sea. For a time they were concealed by the deposition of Tertiary sediments but since they have been exposed to erosion they have governed in various degrees the adjustment of streams and the development of the regional and the local topography. Some of the folds have been widely effective in preserving and obliterating the sedimentary rocks. Thus the central part of the broad de Chelly upwarp has been stripped down to Permian rock but at its eastern edge all of the Mesozoic formations are present in the steep Defiance monocline and extend long distances beyond. The Tusayan downwarp preserves Cretaceous strata that have been removed from surrounding areas. Likewise, the symmetrical Gallup syncline has prevented the destruction of a commercially valuable coal field.

The Navajo country, substantially the Navajo Indian Reservation, is the home of approximately 56,000 Navajo Indians, most of them nomadic sheepmen who follow their flocks from place to place in search of forage and water, and of 3,700 Hopis who occupy ancient villages on mesa tops and derive their support from cultivated crops, chiefly corn. A prehistoric pueblo population is represented by cliff houses, especially well preserved in Canyon de Chelly and Tsegi Valley. The region is known chiefly to professional geologists, archeologists, and graziers. The absence of paved automobile highways and the restrictions imposed by the Bureau of Indian Affairs are deterrents to general travel. Passable roads connect the administration centers but reach few places of special scenic interest. In fact, some areas of considerable size await exploration.

San Juan Basin.—At the eastern edge of the plateau province southward from the foothills of the San Juan Mountains at Durango, Colo., the land surface descends into the San Juan Basin—a topographic and structural depression that occupies about 12,000 square miles of northwestern New Mexico and adjacent parts of Colorado drained by the San Juan River system. The southern part of the basin is floored with nearly flat-lying Cretaceous sandstones, above which rise a few conspicuous mesas capped by rocks of Tertiary age; the northern part by southward and westward dipping Cretaceous sandstones overlain by Tertiary rocks that include shales, conglomerates, sandstones, and as much as 2,000 feet of volcanic debris. The strata in the basin have been deformed by several small folds and faults. The basin is conspicuously walled in on the west by the Hogback Ridge—a huge monocline, in which the Cretaceous beds are abruptly bent downward at angles exceeding 30 degrees. (Plate 13, Sec. 9, in pocket.)

Regional features of southern portion.—As defined in this report, the southern portion of the Colorado Plateau is that area, extending both north and south from the Grand Canyon and southeastward to the Arizona-New Mexico boundary, in which the rock sequence is chiefly of Paleozoic age. The plateau province to the north and east, including both the Navajo country and the canyon region of southern Utah differs from this southern portion chiefly in the character and sequence of the rocks. In both regions strata are flat-lying with the result that the processes of erosion have sculptured innumerable straight-walled canyons, mesas, and tablelands. In the southern area, however, the rocks are of an older group (Paleozoic) than those upstream (Mesozoic) and basic differences in these strata have profoundly affected various aspects of the landscape.

Contrast between physical features of the Grand Canyon district and those of the basin and range province to the south and west is great. The characteristically horizontal strata of the former are in marked contrast to the steeply tilted or much-folded layers of the other. Flat-topped hills and level skylines are replaced by jagged ranges and irregular peaks. Even where the same strata that form the walls of Grand Canyon are represented in mountains to the south and west, features of the landscape are totally different because of differences in rock structures.

The southern portion of the plateau province is divisible into 10 principal sections, separated one from the other by major lines of displacement such as faults or folds. Each section is in reality a block or segment of the earth's crust raised more or less vertically to its particular elevation high above sea level. North of the Colorado River at Grand Canyon are five of these segments or plateaus as shown on Plate 13, Section 10 (in pocket). Two others lie south of Grand Canyon, and an eighth extends southeastward forming the southern margin of the Colorado Plateau in this area. (Pl. 13, Sec. 12, in pocket.) Finally, near the New Mexico-Arizona boundary two additional sections are assigned to this province because of the similar rock types involved, even though the strata are more folded than elsewhere, making dome structures. (Plate 13, Sec. 11, in pocket.)

Within the plateau province and along its borders are areas of varying size, the surface features of which have been formed through the agencies of volcanism. Some of the features are spectacular; all of them have profoundly modified the general appearance and character of the landscape. Because they differ so from other features of the southern plateau district, a separate treatment will be given the volcanic areas.

Plateaus on the north side of Grand Canyon.—The general character and geologic structure of the plateau blocks that form the north wall of Grand Canyon have been well-known since the early surveys made by Powell, Dutton, and others 60 to 70 years ago. The remarkable manner in which great segments of the earth's crust have been elevated along faults or fractures to form a series of steplike platforms, rising from west to east (Pl. 13, Sec. 10, in pocket), has been the subject of much speculation and has greatly influenced the development of ideas concerning the causes of mountain uplift.

Approached from the west along the Colorado River, the plateau province begins in Arizona, not far from the Nevada line, where a great cliff has been formed as a result of vertical movement of the area to the east along the Grand Wash fault. This upraised block is known as the Shivwits Plateau and extends from west to east for 30 miles. Its flat surface terminates abruptly against the face of a cliff formed by the Hurricane fault, and east of this is the higher Uinkaret Plateau. Two other faults, the Toroweap and Kanab, mark the western boundaries of successively higher plateaus still farther to the east. These are the Kanab and Kaibab. Finally the eastern end of the Kaibab Plateau, which has elevations up to 9,000 feet, is reached where the normally flat-lying rock strata are bent steeply downward in a monocline to the low level of the Marble platform. These four great blocks or plateaus form the northern rim of Grand Canyon, which is more than 100 miles long in air-line distance.

The Kaibab Plateau is by far the best known of the areas bordering Grand Canyon on the north. This is partly because it is more accessible and therefore better known than the plateaus farther west, and partly because its higher altitude affords a more attractive summer climate. Significant, however, is the fact that while many of the finest views of Grand Canyon are to be had from points along the margin of the plateau, such views cover only one part of Grand Canyon, whereas many superlative views of other and different parts can be obtained only from the more western areas. Furthermore, though the climate of the Kaibab favors summer recreation, it prohibits any appreciable winter usage, whereas the lower areas to the west can be visited during almost any season.

Geologically each of the plateau blocks is a modified repetition of the next. It is composed of essentially flat-lying strata such as are seen in the walls of Grand Canyon, but varying in proportion and in detailed character so that over many miles the surface expression of the rocks is quite distinct. Thus, the wide, green bench of shale that forms the well-known Tonto platform of eastern Grand Canyon gradually disappears westward, whereas the broad, red esplanade of western Grand Canyon is a negligible feature in the east as a result of the thinness of the Hermit shale there. The significant point, therefore, concerning the Grand Canyon as a great spectacle with unusual inspirational value is that it is distinctly different in different parts, yet large and impressive throughout.

Notable exceptions to the general uniformity in geological features of the plateau are those developed by volcanic activity. In both the Uinkaret and Shivwits divisions, extensive flows of lava and numerous small craters and cinder cones blanket large parts of the surface, thus giving a new character and added interest to the region. In Toroweap Valley on the Uinkaret Plateau, this display of volcanism is especially fine and constitutes an exhibit of exceptional value which has as yet been but little appreciated. The freshness and recency of some flows, the symmetry of many craters, the spectacular manner in which lavas have cascaded over canyon walls, examples of lava dikes leading upward from the depths, and, above all, the complex interrelationship between erosion of the canyon on the one hand, and the piling up of volcanic matter on the other, make this area outstanding as a place for stimulating the mind and furnishing inspiration.

The Hualpai Plateau.—The southwesternmost section of the Colorado Plateau, bounded by the Colorado River on the north, the Grand Wash and Aubrey Cliffs on the west and east, respectively, and the Juniper Mountains on the south, is commonly termed the Hualpai Plateau. The Hualpai Indian Reservation occupies a large part of this area.

In geologic structure the Hualpai Plateau is similar to the adjoining plateaus. It is formed of essentially flat-lying strata of sedimentary rock that have been uplifted as a block or series of blocks along great fault lines that traverse the region with a general north-south trend. The largest of these faults are the Grand Wash and Aubrey, although others of considerable magnitude also cross the region.

The surface of the Hualpai Plateau, about 5,000 feet in elevation, is somewhat lower than that of neighboring plateaus because the several upper most formations in the others have here been stripped away by long erosion. On the other hand, the surface of the Hualpai is everywhere 2,000 to 3,000 feet higher than the desert valleys to the west and south, causing it to have a temperate climate and to be favorable for year-round recreational use.

The geological features of this region are primarily those of the Grand Canyon, which forms its northern boundary. Surface rock over most of the area is massive gray limestone of the uppermost Redwall (Mississippian), although locally beds of red Supai sandstone (Permian) remain above this, and elsewhere Devonian strata have been faulted up to the surface. Where the river has cut through these strata, forming Grand Canyon, there is a drop of about 3,000 feet and the canyon walls have the appearance of being nearly sheer. This accounts for many fine and spectacular views along the rim, as at Quartermaster Canyon and Bridge Canyon in the west and above Granite Park farther east.

Geology of the Hualpai Plateau is similar and related to that of eastern Grand Canyon, yet it is different enough to be worthy of special consideration. In addition to the broad aspects of stratigraphy and structure, there are such special features as Rampart Cave, where prehistoric animals, notably the extinct ground sloth, are buried in great numbers. The green and purple shales at Columbine Falls also contain quantities of well-preserved Cambrian trilobites.

Coconino Plateau.—The Coconino Plateau, extending from Grand Canyon south to the San Franciscan volcanic field (see Pl. 13, Sec. 12, in pocket) and from the Aubrey Cliffs east nearly to the Little Colorado, is the best known of the plateau blocks within the Grand Canyon province. It is composed of essentially flat-lying strata with resistant Kaibab limestone of Permian age forming the surface. Along the eastern margin the strata are folded into a monocline, causing them to dip under more recent rocks of the Painted Desert. Elsewhere in this plateau and along its western margin, strata have been broken by faults of varying magnitude. In general, however, the geologic structure is very simple.

Eastern Grand Canyon, of which the Coconino Plateau forms the southern rim, is the outstanding feature of this area. Included is the most frequently visited and best known portion of Grand Canyon. As part of a national park, its recreational aspects have been carefully studied and partly developed.

Throughout the Coconino Plateau are many minor features of geologic significance some of which, through proper development as places of interest to the visitor, would add to the recreational assets of the area. Included in this category are solution phenomena formed in the surface limestone by rain and snow waters. Examples are the Citidel fissure and Doney fissure northeast of the San Francisco Mountains, Bottomless Pits near Walnut Canyon, and the caverns south of Ashfork. Another type of feature is represented by Grand Falls of the Little Colorado. It is formed where an ancient lava flow blocked the stream course and caused the river to drop over a limestone cliff, making an inspiring sight, especially in flood season. Farther downstream is the canyon of the Little Colorado, remarkable for its proportions. It stands out in a region of canyons as one of the narrowest and most sheer-walled. Excellent views into its depths may be obtained at a few places along the Grand Canyon-Cameron highway, but much more might be done to take advantage of the canyon's inspirational and educational possibilities. Salt Springs near its mouth, where Hopi Indians obtained their main supply of salt for years, is a little-known place of considerable historic interest.

East of the San Francisco Mountains toward Winslow is Meteor Crater—one of the most remarkable features in the United States and one that is internationally known. Today, a preponderance of evidence seems to favor the theory that this pit is the result of impact and explosion of a meteorite from outer space, but whether this be true or, as some scientists have suggested, a volcanic steam explosion was responsible, the crater is unique in character.

The Mogollon Plateau.—The Mogollon Plateau is a southward and a southeastward extension of the Coconino Plateau. These two sub-provinces are similar in geologic structure and age, but are separated by the San Francisco volcanic field. (See Pl. 13, Sec. 12, in pocket.) In both, the surface is formed largely of resistant limestone of Permian age and the strata are essentially horizontal, being disturbed only by a few breaks or normal faults and by monoclinal folds. The Mogollon Plateau ends abruptly on the south and west where high escarpments form what is known as "the Rim." Beyond this, low valleys give a very different aspect to the country.

Most of the Mogollon Plateau is covered with forests which, together with the cool summer climate, make it favorable for recreational development. Springs are not abundant but are scattered throughout the area. Relatively small, but narrow and attractive little canyons such as Chevelon, Black, Clear Creek, and others, dissect its surface. Cutting back into it from the adjoining valleys to the south and especially to the west are some large and colorful canyons that are very scenic. One of these—Oak Creek Canyon—is readily accessible by a main road and is a very popular resort area offering fishing, swimming, and exceptional scenery. Some of the others, such as Beaver Creek and Fossil Creek, are attractive and interesting, but less known.

Of special geologic significance in the Mogollon Plateau area are coal deposits located in the eastern part of the area near to the rim, and "the Sinks" which are to the north of this area. "The Sinks" consist of 30 or 40 bowl-shaped depressions in limestone, ranging from a few yards to about one hundred yards in diameter, formed by the solution work of water. Lesser features of interest are the spectacular dikes of black lava that extend through red sandstone in the walls of Oak Creek Canyon and the examples of columnar jointing in lava at the head of this canyon.

Defiance Plateau.—Immediately west of the Arizona-New Mexico line from the vicinity of Houck on Highway 66 almost to the border of southern Utah is an upwarped area named the Defiance Plateau. Strata of Paleozoic and earlier age, comparable to those on the Mogollon and Coconino Plateaus to the south and west, appear here at the surface, folded into a large anticline along a north-south axis. (See Plate 13, Sec. 11, in pocket.) Near the center of the fold, in Quartzite Canyon, very ancient quartzites project through the red shales and sandstone, showing that for vast ages an old land mass remained in this area while seas came and went around its borders.

The magnitude of the uplift, together with the massive character of the sandstone involved, resulted in the erosion of numerous deep and spectacular canyons in the flanks of the anticline. Most notable are Canyons de Chelly and del Muerto, although lesser canyons such as Nazlini are also colorful. From a geological standpoint these and related features of erosion compete with the antiquity and history of the quartzite core for major interest.

Southwest of Fort Defiance is a natural bridge of spectacular character carved in the red de Chelly sandstone. It is one of many scenic features sculptured by erosion along the flanks of this plateau.

Zuni Mountains.—The Zuni Mountains of New Mexico are interesting geologically because they represent an uplifted area the surface features of which have been considerably modified through erosion. Structurally the area is similar to the Defiance Plateau to the northwest in Arizona in that there is a massive core of very ancient rock flanked by younger strata. In the Zunis, granite forms the core and red sandstone of Permian age has been deposited around the margins. This structure is significant from the standpoint of historical geology for it represents a positive area or region that stood above sea level during millions of years (Paleozoic) while marine waters from several directions periodically advanced toward and retreated from it.

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