Geology of the Grand Canyon: Chapter Ii. Geography Of The Grand CaÃÂon District.
CHAPTER II. GEOGRAPHY OF THE GRAND CAíON DISTRICT.
The Grand Cañon District-the region draining into the Grand and Marble Cañons-is the westernmost division of the Plateau Province. Nearly four-fifths of its area are situated in Northern Arizona. The remaining fifth is situated in Southern Utah. Let us turn our attention for a moment to the portion situated in Utah. It consists of a series of terraces quite similar to those which we have already seen descending from the summit of the Wasatch Plateau to the San Rafael Swell like a colossal stairway. At the top of the stairs are the broad and lofty platforms of the High Plateaus of Utah; at the bottom is the inner expanse of the Grand Cañon District. The summits of the High Plateaus are beds of Lower Eocene age. Descending southward we cross, step by step, the terminal edges of the entire Mesozoic system and the Permian, and when we reach the inner floor of the Grand Cañon District we find that it consists of the summit beds of the Carboniferous series patched here and there with fading remnants of the Permian.
Far beyond the remotest limits of vision stretches the great expanse of Upper Carboniferous beds, flecked with Permian outliers, and rising or falling to form the broader inequalities of level in the surface of the region. The terraces of Southern Utah are the border land between the High Plateaus on the north and the Carboniferous platform of the Grand Cañon on the south, and may be regarded as the appanage of either district. Their nature and meaning may become clearer by glancing a moment at the District of the High Plateaus.
Let us conceive a right-angled triangle in which the acutest angle is regarded as the apex and the shortest side as the base. Place the apex about 25 miles east of Mount Nebo, the great mountain which marks the southern end of the true Wasatch Range; place the right angle 170 or 180 miles due south of the apex, and the other acute angle about 100 to 110 miles due west of the right angle. This figure would include pretty nearly all of the summit areas of the High Plateaus. Consider now the north-and-south side reaching from the apex to the right angle. It runs along the crest lines and terraces which look down eastwardly upon the San Rafael district and upon other enormously eroded districts further south. The base reaching westward from the right angle to the other acute angle runs among the terraces which descend from the southern termini of the High Plateaus to the Grand Cañ District. The hypothenuse looks north northwestward over a portion of the Basin Province. The High Plateaus themselves are large remnants of Mesozoic and Tertiary strata which have been spared in the enormous denudation which has eaten out the heart of the Plateau Province. Their preservation has been largely due to extensive outpours of lavas, which have overspread most of their summits, and the energy of the eroding agencies has there spent itself upon the more obdurate materials of volcanic origin.
Starting from the right angle and reaching out south-southeast is a rather lofty mass, named the Kaiparowits Plateau. It reaches to the Colorado River, where it is cleft asunder by the mighty gorge of the Glen Cañon, but resumes its course on the other side, extending into Arizona, where it spreads out. It is composed of Cretaceous strata. Its western flank forms a part of the eastern boundary of the Grand Cañon; or more restrictedly of the Marble Cañon, district. Here again is the same old arrangement-terraces with their marginal cliffs rising from the Carboniferous platform step by step to the Middle Cretaceous-the cliffs all looking westward over the great region from which the former extensions of the strata they terminate have been swept away.
Thus we may note that the northern and eastern boundaries of the Grand Cañon District are cliff-bound terraces. Crossing the district either longitudinally from north to south, or transversely from east to west, we find as we approach the southern or western border that the Carboniferous platform ascends very gradually. There are broad and feebly marked (sometimes well marked) undulations, or ups and downs; but, on the whole, the country gains in altitude as we approach its western and southern limits. At last it terminates in a giant wall plunging down thousands of feet to the platform of a country quite similar to that of the Great Basin of Nevada. Those who have traveled on the Central Pacific Railway will recall the features of that very desolate region which lies between Great Salt Lake and the Sierra Nevada; and all those features are repeated and their desolation intensified in the dreadful region which lies west and south of the Grand Cañon District.
The district may be conveniently divided into parts. The northernmost portion is the area comprising the southern terraces of the High Plateaus. A description of these sufficient for preliminary purpose has already been given. At the foot of these terraces stretches away to the southward the great Carboniferous platform of the heart of the district. That portion of the platform which lies north of the Colorado River may be subdivided into five distinct plateaus. Naming them in regular order from west to east, they are: 1, the Sheavwits; 2, the Uinkaret; 3, the Kanab; 4, the Kaibab; 5, the Paria. These five plateaus are separated from each other by natural boundaries, which are for the most part quite distinct. These boundaries are great faults or dislocations of the main platform, which have produced cliffs by hoisting the platform on one side of the fault line or dropping it on the other. To show how these dislocations have affected the topography, the reader is referred to the east and west section delineated in the accompanying diagram. [U. S. GEOLOGICAL SURVEY. SECTION FROM NORTH TO SOUTH ACROSS THE GRAND CAíON DISTRICT. ANNUAL REPORT 1881. PL. XIII.] [U. S. GEOLOGICAL SURVEY. SECTION FROM NORTH TO SOUTH ACROSS THE GRAND CAíON DISTRICT. ANNUAL REPORT 1881. PL. XIII.] (Pl. XIII.) Immediately south of the Paria Plateau extends the Marble Cañon, with a southwestward course. Directly across the remaining four plateaus winds the Grand Cañon, of the Colorado. South of the river is a vast expanse of nearly flat surface, but little diversified, called the Colorado Plateau. Upon its southern borders rise abruptly a group of great volcanic piles called the San Francisco Mountains, the largest or dominant cone being of an impressive order of magnitude. There are still other portions beyond, but the entire region south of the Colorado has been reconnoitered rather than surveyed; and though we have a general knowledge of pretty nearly the whole of it, our knowledge of details is not as yet precise.
The entire expanse of the Grand Cañon platform within the terraces consists of a flat surface, interrupted at wide intervals by cliffs or sharp flexures produced by the great displacement which traverse the land in a prevailing north to south direction.* On the whole, it is a smooth country in comparison with the other districts of the Plateau Province. It is not dissected or honeycombed by the ramifications of innumerable side gorges, as is the case with most of the other districts, for the Grand Cañon has on the north side only one lateral or tributary cañon of any considerable length, and this tributary has but few branches. In truth, one of its remarkable features is the paucity of lateral chasms. The southern side is not more diversified than the northern, and I think we may say that it is somewhat less so. Between the great cliffs of displacement and between the foot of the terraces and the brink of the chasm the country is not more uneven than the Great Plains of Western Nebraska and Kansas, and not much more so than Indiana and Illinois. But the traveler is seldom out of sight of the palisades reared at the fault-lines, or of the gigantic and gorgeously colored walls which bound the terraces of the High Plateaus.
The Grand Cañon will be described at some length in subsequent chapters. Here will be noted only those more general features which may be made to appear on the map. It crosses transversely the four western plateaus of the district, while the Marble Cañon traverses the eastern or fifth plateau. The two cañons are only nominally separated, for there is no gap between them. The Marble Cañon begins at the base of the eastern terraces. The Colorado River, after traversing the central mesas of the Plateau country in a series of profound chasms, at length emerges from the Echo Cliffs of Triassic and Permian age. Here for an instant the river is in comparatively open country. But within a mile or two it begins to sink another chasm in the Carboniferous rocks, and in the course of 65 miles the depth steadily increases until it becomes about 3,500 to 4,000 feet. This is the Marble Cañon. It is a gorge of very simple form, and its width is about twice as great as its depth. Its course is at first southwest, but gradually deflects to the southward. Its lower end is arbitrarily fixed at the junction of the Little Colorado or Colorado Chiquito, a stream coming in from the southeast and entering by a lateral chasm as deep as the main gorge itself. Below this junction the river turns westward, the walls grow rapidly higher, the great chasm widens out to six or eight times its width in the Marble Cañon, and the valley of the river is filled with buttes as large as mountains and wonderfully sculptured. Here the river enters the Kaibab, and its walls soon attain the altitude of about 6,000 feet. After a tortuous course of sixty miles in a prevailing northwest direction the river passes out of the Kaibab and at once changes its trend to west southwest. It passes without a break from the Kaibab to the Kanab Plateau. Here its depth diminishes to about 5,000 feet and its topography changes in character, becoming more simple. Preserving its new features and the directions of its course throughout the Kanab and Uinkaret plateaus, it at length crosses the Hurricane fault where the whole platform of the country suddenly drops more than a thousand feet, corresponding diminishing the height of the walls. But the lost altitude is steadily and rather rapidly regained as the river enters the Sheavwits Plateau. Soon after crossing the fault the river turns abruptly to the south, and after describing a great curve in the heart of the Sheavwits platform it turns northwestward again. The great chasm suddenly terminates in the face of the giant wall which forms the western boundary of the Sheavwits Plateau. Here the river emerges through a mighty gateway a mile a depth, and is almost in open country, its banks dropping at once to altitudes of only a few hundred feet. This is the western bound, not only of the Grand Cañon District, but of the Plateau Province itself. Thenceforth the course of the Colorado to the ocean is through and across that dismal, torrid sierra region, which is the southward extension of the features of the Great Basin.
For convenience of discussion the Grand Cañon is divided into four divisions, (1) the Kaibab, (2) the Kanab, (3) the Uinkaret, (4) the Sheavwits, divisions. The last three are much alike in all their features and dimensions. The Kaibab division is a little deeper, notably wider, and very much grander and more diversified than the others. The total length of the Grand Cañon, as the river runs, is about 218 miles, and its depth varies from 4,500 to 6,000 feet, averaging 5,000. Its width, from crestline to crestline, varies from 41/2 to 12 miles-the widest portions being always the grandest.
It is also necessary to advert to the tributaries of the Colorado lying within the district. Upon the northern side there is but one now entering the Grand Cañon; but there are on this side two others, one of which, named the Paria, enters at the head of the Marble Cañon, and the other, the Virgen, which enters it about 40 miles west of the lower end of the [U. S. GEOLOGICAL SURVEY. SECTION FROM EAST TO WEST ACROSS THE GRAND CAíON DISTRICT. ANNUAL REPORT 1881. PL. XIV.] [U. S. GEOLOGICAL SURVEY. SECTION FROM EAST TO WEST ACROSS THE GRAND CAíON DISTRICT. ANNUAL REPORT 1881. PL. XIV.] Grand Cañon. Kanab Creek joins the Colorado on the north side in the heart of the great chasm. These three streams, the Virgen on the west, Kanab Creek in the middle, and the Paria on the east, all have their sources in the terraces of the High Plateaus. They are very important factors in the problem of reconstructing the history of the region.
On the south side of the river are (1) the Colorado Chiquito or Little Colorado, entering at the foot of Marble Cañon, (2) Cataract Creek, entering near the middle of the Kanab division, (3)Diamond Creek, joining at the elbow of the great bend is the Sheavwits division. These, too, have their bearing upon the general problem.
|1||* It will be noted here that cliffs may be produces (1) by faults lifting the country on one side of the fault-line or depressing it on the other; (2) by the denudation of the strata in front of the cliff. The former are called Cliffs of Displacement; the latter, Cliffs of Erosion.|