Geology of the Grand Canyon: Weathering.
The work of corrasion is limited to the cutting of narrow gashes in the strata, and the grinding up of the fragments brought into the river channels. The widening of these cuts into the present configuration of the chasm is the work of weathering. The common notion is that “solid rock“ is but little affected by any natural agents such as water and air, and though it is acknowledged that water and carbonic acid exert a certain nominal solvent and chemical action upon rock material, yet these are usually esteemed so feeble that even the enormous periods of time which the geologist invokes seem quite insufficient to warrant us in ascribing to them any very important effects. Our observation upon the works of human construction which have been exposed for many centuries to the action of the elements confirms this notion. The structures of Egypt, Greece, and Italy have been thus exposed for periods which are nearly or even exactly known. They bear evidence that this action is a real one, and that their final dissipation would, in the event of indefinite exposure, be a mere question of time. But they also indicate that so far as their own materials are concerned the process is exceedingly slow. Their rate of decay by solution, if applied as a factor to the recession of the walls of the Grand Cañon, would give a period of time so vast that the mind would promptly reject it because of its very enormity. But we shall find that the recession of those walls goes on, slowly indeed, but at a rate very much greater than would be inferred from an inspection and comparison of the works of human antiquity.
It is at once obvious that the building-stones are not a fair criterion. They are selected for their durability, and of all rocks they represent those which offer the greatest possible resistance to weathering. Taking the common rocks only-those of frequent and world -wide occurrence-there is reason to believe that their rates of decay under equal conditions vary among themselves enormously. Leaving out of the account the unconsolidated or loosely consolidated strata, it cannot be doubted that some indurated rocks decay fifty times faster than others, the conditions being identical as to climate and exposure. We have, it is true, no experimental or laboratory data upon which this assertion can be based, but it is, I am confident, quite defensible, and will appear to be so when we examine the results of weathering in the rocks in place. For there is another consideration which is not apparent in the decay of building-stones. The strata are disintegrated by a process which includes something far mor efficient than mere solution or chemical decomposition.
At the base of every cliff in the Plateau country we find a large talus consisting of fragments fallen from the rock faces above. The fragments vary in size from bowlders of many tons' weight down to the finest gravel, sand, and clay. Here is proof at once that the decay of cliffs goes on chiefly through the breaking off of fragments. It soon appears that the amount of material remove from the wall in solution is but a very trifling fraction of the quantity which has spalled off from the face of the wall. As the large fragments fall off from the vertical front they are dashed to pieces below. In this fragmental condition they expose a much greater surfaceto weathering and are dissipated with correspondingly increased rapidity. And now we come to the key of the problem. The explanation of those persistent profiles of the Grand Cañon is found when we analyze the formation and decay of talus. It is one of the most charming studies in the whole range of physical geology.
In the Carboniferous strata of the Grand Cañon we have a mass of rocks widely varying in litholofical characters, but which on the average are just about as obdurate to weathering as the average of rocks found in other regions. So far as can be seen or inferred, in this respect they differ not at all from the strata of other regions. Some of them weather easily, some are very obdurate. Perhaps the only qualification to this comparative statement is that there are no extremely perishable strata in the whole series. The softest beds are still firm and perfectly indurated. The degrees of obduracy, however, appear to vary greatly in the series.
The upper stratum in the cañon wall is a cherty limestone, which is harder* than the average, though not extreme in tha respect. It forms usually a precipitous face, though it is frequently breached and broken down. It is out of this series that the rows of pinnaches in the crest of the cañon are carved.â Beneath it is another series of limestones of less than average hardness. They are sometimes a little cherty [U. S. GEOLOGICAL SURVEY. THE MARBLE CAíON. ANNUAL REPORT 1881. PL. XXXV.] but far less so than the overlying beds. They break down always into a steep slope, covered with a talus partly of their own débris and partly of the cherty nodules weathered out from above. Beneath the limestones lies the cross-bedded sandstone, one of the most conspicuous members of the cañon. Of all the strata it is the hardest; in truth, is about as adamantine as any rock to be found in the world. It forms everywhere the vertical frieze of the upper wall and is very seldom broken down into a slope. Underneath it comes the great series of Lower Aubrey sandstones, a thousand feet thick, made up of very many individual beds. They are similar in character, and all of them weather rapidly. We have, then, in the Upper and Lower Aubrey Groups, which form the outer chasm wall at the Toroweap (and which are almost exactly the same elsewhere throughout the Grand Cañ), four groups of strata which are alternately hard and soft (Fig. 16), (1) a hard cherty limestone, (2) a softer limestone, (3) an extremely hard sandstone, (4) a great thickness of much softer sandstones.
[Fig. 16.-Development of cliff profiles by recession in the upper wall of the chasm.]
It has already been explained that the attack of erosion is made chiefly upon the scarp walls and steep slopes of a country and only feebly upon level surfaces. Imagine, now, a cut made by a corrading stream into such a series of strata as that which has just been described. It will soon appear that it is quite immaterial whether the cut be made very gradually or instantly,-by a miracle, as we may suppose. Weathering at once attacks the face of the wall. The softer beds yielding much more rapidly, gradually undermine the harder ones above, and the latter cleave off by their joints and great fragments fall down. If we suppose the corraded cut to have been made instantly and the river to be flowing in it, the fragments would at first fall into the stream and be devoured by corrasion as fast as they fall. But after a time the widening of the cut so produced would leave a platform on the margin of the stream where the fragments would begin to form a talus. As the recession by waste goes on, the talus grows larger and larger, and gradually mounts up on the breast of the lower wall. Now the effect of a talus is to protect the beds upon whose edges it lies, and to retard their rate of decay by virtue of that protection. At first, then, the talus causes the lowest beds to lag behind in the recession. As it mounts up the wall, higher and higher beds gain protection, and they, too, begin to lag behind, until at last the talus mounts up very nearly to the base of the extremely hard, crossbedded sandstone. Thus the entire lower series of soft strata becomes converted into a slope covered with talus, and at this stage all the beds above stand as a single vertical face. But immediately a second cliff and talus begin to form above the hard sandstone; for since the lower soft beds are protected and their rate of recession reduced by the talus, the upper soft beds, being naked, must recede at a more rapid rate, until they, too, become a slope and receive the protection of a talus from the hard limestone at the summit.
It appears, then, that the recession of the hard beds is accelerated by undermining, while the recession of the soft beds is retarded by the protection of the talus. The result is the final establishment of a definite profile, which thereafter remains very nearly constant as the cliff continues to recede. Thus the talus is the regulator of the cliff profile. There are many minor features which may be explained as satisfactory, and one of them is the curvature of the Lower Aubrey profile.
Throughout the greater part of the chasm the slope of the Lower Aubrey is a very graceful curve, but in the Kaibab division it is usually straight and descends at an inclination of about 30 degrees, the angle of repose, or very nearly so, for the debris which occurs here. Taking first the Toroweap section, we remarks that at the base of the main palisade is the broad esplanade or plain which forms the floor of the upper chasm. It is from a mile to three miles in width. In a great talus the fragments are slowly and continually creeping down by the action of rain and frost. The plain at the base acts as a check to the descent. Nowhere except, perhaps, at a notable distance away from the base or in the very lowest part of the stratigraphic series are beds wholly buried in talus. Considerable areas of rocks surface project through the covering. The tendency of the descent of talus under the conditions here considered is to give more protection to the lower beds than to the higher. The check given to the descent of the talus by te level plain is felt more strongly at the base of the slope than higher up. Moreover, the finer débris is more readily washed down a slope of given declivity than the coarse,and thus the bébris at the base of the talus is finer than that above; and fine débris is a more efficient protection than coarse. In consequence of this greater protection, the recession of the lower beds is less rapid than that of the higher ones, and in general terms the protection of any given bed in the slope is inversely proportional to the square of some other complex function of its height above the base. The curved profile at once follows, and it is demonstrably of the hyperbolic class.
In the kaibab the case is different. Here the mighty plinth of the Red Wall limestone cuts off the foot of the Lower Aubrey slope, giving a free discharge to the fragments into the depth below. There is no check to the descent of the talus; the amount of protection given by it to all the beds of the Lower Aubrey is very nearly uniform, and the slope becomes straight. But whenever, as sometimes happens, the top of the Red Wall precipice stands at an unusual distance from the Lower Aubrey, the curvature of the profile of the latter appears, and its emphasis is proportional to the distance which separates the vertical planes of the Red Wall and of the cross-bedded sandstone.
Many details of repetitive or systematic sculpture are presented in the great chasm, and they may be explained as readily as the profiles. Only one other feature can be alluded to here, and the allusion will be brief. It concerns the plan or horizontal projections of the component features of the Kaibab division, the blocking out of the cloister buttes and the temples, and their reduction to their present forms. In a general way it is apparent that these have been originated by the profound corrasion of short lateral tributaries of the Colorado and the subsequent widening of the cuts into the present amphitheaters and alcoves; the buttes and temples being the residual masses between them. But the contours of the latter are striking and peculiar in the extreme. They are explained by observing that wherever recession of the cliffs takes place it proceeds with great uniformity along the entire front. It starts along the line of a stream which is tortuous, but as it proceeds it carries back the cliff in a succession of curves, and in process of time minor inequalities are obliterated. Each larger bend of the stream gives rise to its own curve in the trend of the wall, and where successive curves interest they form very sharp cusps. Everything here depends upon uniformity in the rate of the recession of all parts of the cliff. Where the outward spreading circles of erosion two distinct alcoves or amphitheaters meet by recession in opposite directions, a butte is cut off and a saddle or “col“ is formed. The cusps between two interesting circles are exceedingly sharp and well formed, and three circle generally give rise to a fine gable.
The peculiar cliff-forms of the Plateau country would hardly be possible in any other, for no other presents those conditions which are necessary for them. These conditions may be summarized as follows: (1.) The great elevation of the region. (2.) The horizontality of the strata. (3.) A series of strata containing very massive beds which differ greatly among themselves in respect to hardness, but each member being very homogeneous in all its horizontal extent; in a word, heterogeneity in vertical range and homogeneity in horizontal range. (4.) An arid climate. The great elevation is essential to high reliefs in the topography. Only in a high country can the streams corrade deeply, and it is by corrasion of streams that the features are originated and blocked out. The effect of horizontality of the strata is self-evident. With regard to vertical heterogeneity, it is apparent that it is essential to give diversity to profiles. If the rocks were homogeneous in vertical range, the cliffs would all be like the Jurassic sandstone, stolid and formless. Horizontal homogeneity is essential to that rigorous uniformity in the rate of weathering which gives to the cliffs their systematic character. The effect of the arid climate of the region cannot be explained without a preliminary statement in great detail of the fundamental principles and laws of erosion. These are highly complex, and require an analysis which is more suitable to a monograph than to the present essay. Such an analysis will be attempted in my work upon the geology of the Grand Cañon District.
No doubt the question will often be asked, how long has been the time occupied in the excavation of the Grand Cañon? Unfortunately there is no mystery more inscrutable than the duration of geological time. On this point geologists have obtained no satisfactory results in any part of the world. Whatever periods may have been assigned to the antiquity of past events have been assigned provisionally only, and the inferences are almost purely hypothetical. In the Plateau country, Nature has, in some respects, been far more communicative than in other regions, and has answered many questions far more fully and graciously. But here, as elsewhere, whenever we interrogate her about time other than relative, her lips are sternly closed, and her face becomes as the face of the Sphinx.