Geology of the Grand Canyon: The Jurassic.
Beyond the Cretaceous, as we descend the stairway of terraces, the Jurassic comes to daylight. It forms a belt encircling the Cretaceous and outside of the latter. It is composed of two groups of strata; the upper consisting of red sandy shales with belts of impure limestone; the lower a great mass of white sandstone, nearly a thousand feet thick. The red shales contain abundant fossils, strongly characteristic of their Jurassic age, while the sandstone below is wholly barren of organic remains. The sandstone, however, is full of interest on account of its remarkable lithological characters. From summit to base, it is apparently one indivisible stratum. Here and there signs of a division are suspected, but closer scrutiny shows that they are produced by the contact of one plexus of cross-bedding with another, or by some other cause not affecting the dominant fact. It is remarkably homogeneous throughout its whole mass. On a near view of the rock faces they are seen to be covered with a wonderful filagree of cross-bedding. On every cliff and headland, on every butte or rocky knoll where this huge stratum is exposed, the rock faces are etched with an arabesque as beautiful as frostwork. Along hundreds of miles of linear extent, and over thousands of square miles of surface of the country, this graceful waving of myriads of curves is displayed. Cross-bedding is common enough in other regions and other formations, but nowhere in the world, I fancy, can such a profusion of it be seen.
The Jurassic sandstone is also conspicuous for its cliffs. Here every formation has its own style of architecture and sculpture, which are as distinctive as the lithological constitution, for upon that constitution the style depends. The Jurassic forms are characterized by a peculiar massiveness and boldness and by an extreme simplicity which is even severe. Its walls are quite plain, without horizontal or vertical mouldings, and the only decoration is the cross-bedding which becomes invisible at distances sufficient to render a general view of the fronts effective. A notable feature also is the absence of talus-or, if it be present, its small proportions. This simplicity usually gives a dull slumberous aspect to the escarpments, suggesting on a vast scale the structures of the Peruvian Incas. But it is not always so. Occasionally the austerity of these forms is relaxed or replaced by a strange kind of animation which sometimes becomes amusing. Looking southward from the brink of the Markágunt the eye is attracted to the features of a broad middle terrace upon its southwestern flank, named The Colob. It is a veritable wonderland. It lies beyond the Cretaceous belt and is far enough away to be obscure in its details, yet exciting curiosity. If we descend to it we shall perceive numberless rock-forms of nameless shapes, but often grotesque and ludicrous, starting up from the earth as isolated freaks of carving or standing in clusters and rows along the white walls of sandstone. They bear little likeness to anything we can think of, and yet they tease the imagination to find something whereunto they may be likened. Yet the forms are in a certain sense very definite, and many of them look merry and farcical. The land here is full of comedy. It is a singular display of Nature's art mingled with nonsense. It is well named the Colob, for the word has no ascertainable meaning and yet sounds as if it ought to have one.
Nor are these the only forms which the Jurassic discloses. Here and there blank faces of the white wall are brought into view as the sinuous line of its front advances and recedes. Isolated masses cut off from the main formation, and often at considerable distances from it, lie with a majestic repose upon the broad expanse of the terrace. These sometimes become very striking in their forms. They remind us of great forts with bastions and scarps nearly a thousand feet light. The smaller masses becomes regular truncated cones with bare slopes. Some of them take the form of great domes where the eagles may build their nests in perfect safety. But noblest of all are the White summits of the great temples of the Virgen gleaming through the haze. Here Nature has changed her mood from levity to religious solemnity, and revealed her fervor in forms and structures more beautiful than anything in human art. But we shall see more of this hereafter and from much more advantageous stand-points than the summit of the Markágunt. There only faint suggestions of the reality are given. We only perceive in imperfect detail some throngs of towers, snow-white above and red below, the bristling spires of ornate buttes, or a portion of the grand sweep of a wing-wall thrust out from some unseen facade. None of them appear in their full relations to the whole, and all of them are weakened, faded, and flattened by the distance.
[U. S. GEOLOGICAL SURVEY. A MIDSUMMERDAY'S DREAM.-JURASSIC.-ON THE COLOB. ANNUAL REPORT 1881, PL. XVI.]
At the border of the Jurassic the profile drops upon the summit of the Trias, but from the summit of the Markágunt nothing is visible in detail of that formation. The faces of the escarpments are turned away from us and only the crestlines are visible. The view from the Markágunt, however, is memorable because it is characteristic. To study the Trias we must leave the verge of that Plateau and descend the terraces to the southward.
On our way we may note several things of some importance. We may observe, first, that the strata all have a very slight dip to the north. This dip on the average is less than two degrees, but here and there inclinations as great as four or five degrees may be seen. This dip is very general throughout the terraces. Its effect is to make the altitudes of the higher or more northerly platforms less-or, conversely, to make the altitudes of the lower and more southerly terraces greater-than they would be if the entire series were horizontal. In the entire series of beds which are exposed, the aggregate thickness from the top of the Carboniferous to the summit of the local Eocene is not far from 10,000 feet, but the summit of the Eocene at present lies only about 5,000 to 6,000 feet above the Carboniferous platform of the Grand Cañon District. Thus, if the strata were horizontal, we should in ascending the terraces go up 10,000 feet, but the dip to the northward gradually carries down the horizons so that in crossing the edges of 10,000 feet of strata we only gain 5,000 to 6,000 feet in altitude. We find this same northward dip prevailing in the Carboniferous to the southward, and it is a feature of great moment in the studies which are to follow.
Another point to be noted is that the strata slowly diminish in thickness from west to east. The attenuation, however, is ordinarily very slow and gradual, and the observer would have to travel many miles along the escarpments exposing the edges of the strata before he became aware of it. It is most noticeable in the Trias, and in the sequel this will be more fully discussed. The meaning of this attenuation of the strata towards the east is as follows.
It is a common fact that the greatest thickness of a group of strata is usually found near the shorelines of the mainlands from which their materials came. As we recede from these ancient shorelines we generally find that the strata diminish in thickness, at first quite rapidly, but afterwards more slowly. The materials deposited near the shores are, in many cases, of coarser texture than those deposited at a distance from them. This is not always true of every distinct bed, but if we consider any group of strata with many members we shall usually find it true of the group as a whole. In the case of the Mesozoic strata of the terraces, they are remnants of beds deposited in a sea or bay, the shoreline of which lay to the westward and northwestward. The position of this shoreline, no doubt, varied during the Mesozoic periods, now advancing and now receding; but in general terms its means position appears to have been nearly along what is now the boundary of the Basin Province. The Great Basin was then dry land, undergoing denudation, and its detritus was washed down on this side into the sea, where the Mesozoic strata of the Plateau Province accumulated. The position of this ancient shoreline in the Sierra country south of the Great Basin and west of the Grand Cañon district we do not as yet know; the presumed location not being explored as yet. This attention of the strata and their relation to the shoreline of the mainland, from which they were in great part at least derived, is another important factor which must be kept in mind in the course of the discussion.
It will be well to bestow also a glance at the distribution of the more important drainage channels. The western portion of the terraces is [Fig. 5.-Entrance to the Parú-nu-weap.] drained by the branches of the Virgen River. Upon the Colob heads the northern fork of the Virgen, sometimes called the Mu-kún-tu-weap, sometimes Little Zion River. It flows due south. East of this is the eastern fork, called the Pa-rú-nu-weap. Both branches have their sources at the base of the Pink Cliffs (Eocene), and at length unite to form the [U. S. GEOLOGICAL SURVEY. THE PARUNUWEAP. From Powell's Exploration of the Colorado River. ANNUAL REPORT 1881. PL. XVII.] Virgen. Their channels are surely very wonderful freaks of nature. The Parúnuweap, after collecting its several filaments on the slopes of the Cretaceous terrace, at length begins to burrow into the Jurassic, cutting a very deep and remarkably narrow gap in the white sandstone, and then into and through the Trias. For many miles it flows in a mere cleft barely fifty feet wide at the bottom and sometimes narrower, and attaining a depth of more than 2,500 feet. In scouring down its channel into the sandstones the stream did not cut always vertically, but swayed from side to side, so that now great bulges of the wall overhang the bottom of the abyss, and in some places shut out the sky overhead. The Mukúntuweap, or Little Zion fork, is even more remarkable. For a conriderable distance this stream also runs in a profound and exceptionally narrow chasm, but it at length widens out, and just where it joins the Parúnuweap is a scene which must ultimately become, when the knowledge of it is spread, one of the most admired in the world. Of this hereafter. Below the junction of the forks the Virgen flows westward, and passes out of the terraces and out of the plateau Province. At length it joins the Colorado.
East of the drainage area of the Virgen is that of Kanab Creek. It heads in the broad valley of Upper Kanab, which occupies an indentation of the southern margin of the High Plateaus between the Markágunt and Paunságunt. The bulk of the drainage passes through the upper cañon of Kanab Creek, and at length emerges upon the desert to the southward. Further on it sinks another chasm in the Carboniferous, which becomes a mighty side gorge of the Colorado, and unites with the Grand Cañon in the middle of the Kanab division.
Still east ward is the great amphitheater which gives rise to the branches of the Paria. This stream flows southeastward and ultimately enters the Colorado at the head of the Marble Cañon.
In these three subordinate drainage basins of the terraces it is well to notice some features of importance, common more or less to all, but most distinctly seen in Kanab Creek. They all run contrary to the dip of the strata. The summits of the terraces dip to the northward, while the streams run southward. They thus form each a chain of cañons. Thus, Kanab Creek with its upper tributaries flowing in open valleys soon begins to cut into the Jurassic, and its gorge, ever deepening, at length becomes nearly a thousand feet in depth. Suddenly the cañon walls swing to right and left to form the mural front which terminates the Jurassic terrace; and the river, now at the summit of the Trias, is once more in open country; but only for a short distance, for it soon begins to cut into the Trias, forming a great cañon as before. The same process is repeated and the river flows out of its Triassic chasm into the open again, while its walls swing in either direction to form the terminal escarpment of the Triassic terrace.
The three streams just mentioned are not the only drainage channels in the terraces, though they are the principal ones, and sooner or later gather the greater part of the drainage. There are many cañons in the terraces, and they all have the same relation to the cliffs and to the dips of the strata. They cut into the terraces and emerge from them at the bases of their several cliffs. All except the three first mentioned are dry, carrying no streams except spasmodic floods during heavy rains and the melting of the snows. Many of them are actually filling up, the floods being unable to carry away all the sand and clay which the infrequent rains wash into them.
It is through the dry and partially refilled chasms that we may easily descend from the High Plateaus to the Carboniferous platform of the Grand Cañon district. To study the Trias, we may best go to the little village of Kanab and prepare for a journey along the base of the Vermilion Cliffs.