Geology of the Grand Canyon: Chapter Iv. The Great Denudation.
CHAPTER IV. THE GREAT DENUDATION.
Before leaving the terraces we may with advantage pause to contemplate the great lesson in geology which they lay open to us. The subject of the lesson in Erosion . In a preliminary way we examined the type of it in the San Rafael district, which was briefly treated of in the first chapter. The same fact confronts us again in the Grand Cañon district. Here, however, the attended facts are more complex, more difficult to grasp, and less easy to summarize. And vast as the erosion has been in the San Rafael it has been many times greater in the Grand Cañon district. In this discussion three classes of facts will utilized: 1st, the stratification; 2d, the faults and flexures, or vertical displacements; 3d, the drainage. Each class furnishes its quota of evidenced. Yet, so intimately are the several threads of argument interwoven, that it is almost impossible to separate them and view each independently of the others. Hence if the argument skips about from on to another before the one is fully developed, it is because no other method of treatment seems practicable.
The geologist seeing the series of Mesozoic and Eocene strata suddenly terminating in the terraces in the faces of the cliffs, would at once say that these strata formerly extend further southward. For he is ever mindful of the fact that in the lapse of long periods the rocks decay, and the rains and rills gather up the débris and carry it away. He also has had impressed upon his mind the general fact that the most rapid waste takes place on the edges of the strata exposed in vertical wall-faces. Every year the rains wash away something from the mural fronts. In a single year it may be a mere film, but in the lapse of thousands of centuries the amount whittled off becomes a vast aggregate. Like the motion of the fixed stars the change is not perceptible to a generation; but a million years would change the aspect of a denuding country as profoundly as they would change the aspect of the heavens. How long in terms of years this “Recession of the Cliffs“ has been going on, the geologist does not know, though he presumes the period to have been certainly hundreds of thousands of years and very probably some millions. Feeling assured, then, that the terraces once projected further south, the inquiry arises, how far? Let me answer at once. They extended southward over the entire Grand Cañon district, into central Arizona, where they ended along the shore of the ancient mainland from which their materials were in part derived. The distance of that shore-line, from the summit of the Pink Cliffs, is from 130 to 180 miles, and the width of the denuded district is from 120 to 140 miles. From the base of the Vermillion Cliffs the distance is 25 to 30 miles less. The area of maximum denudation is from 13,000 to 15,000 square miles, and the average thickness of the strata removed from it was about 10,000 feet.
The general reader will no doubt feel a strong aversion to such prodigious figures on their first presentation, and even the geologist whose credulity has been shocked so often that he has gotten used to it may wince once more. It is not from a love of the marvelous or dramatic; it is not without a full sense of the oppression of unaccustomed magnitudes that these assertions are made. They are made because they follow inexorably from the facts, and because they are necessary conclusions from clear premises. But, in order that the reader may not be obliged to carry a heavy burden of prejudice as he follows the various steps of the argument, it is well to anticipate some part of the discussion, and thus relieve him of a great part of the load at the outset, for it can be shown that the figures, while they are certainly very large, are in no respect abnormal, and in only one respect are they at all unusual.
Erosion, viewed in one way, is the supplement of the process by which strata are accumulated. The materials which constitute the stratified rocks were derived from the degradation of the land. This proposition is fundamental in geology-nay, it is the broadest and most comprehensive proposition with which that science deals. It is to geology what the law of gravitation is to astronomy. We can conceive no other origin for the materials of the strata, and no other is needed, for this one is sufficient and its verity a thousand times proven. Erosion and “sedimentation“ are the two half phases of one cycle of causation-the debit and credit sides of one system of transactions. The quantity of material which the agents of erosion deal with is in the long run exactly the same as the quantity dealt with by the agencies of deposition; or, rather, the materials thus spoken of are one and the same. If, then, we would know how great have been the quantities of material removed in any given geological age from the land by erosion, we have only to estimate the mass of the strata deposited in that age. Constrained by this reasoning, the mind has no escape from the conclusion that the effects of erosion have indeed been vast. If, then, these operations have achieved such results, our wonder is transferred to the immensity of the periods of time required to accomplish them; for the processes are so slow that the span of a life-time seems too small to render those results directly visible. As we stand before the terrace cliffs and try to conceive of them receding scores of miles by secular waste, we find the endeavor quite useless. There is, however, one error against which we must guard ourselves. We must not conceive of erosion as merely sapping the face of a straight serried wall a hundred miles long; the locus of the wall receding parallel to its former position at the rate of a foot or a few feet in a thousand years; the terrace back of its crest line remaining solid and uncut; the beds thus dissolving edgewise until after the laps of millions of centuries their terminal cliffs stand a hundred miles or more back of their initial positions. The true story is told by the Triassic terrace ending in the Vermilion Cliffs. This terrace is literally sawed to pieces with cañons. There are dozens of these chasms opening at intervals of two or three miles along the front of the escarpment and setting far back into its mass. Every one of them ramifies again and again until they become an intricate net-work, like the fibers of a leaf. Every cañon wall, throughout its trunk, branches, and twigs, and every alcove and niche, becomes a dissolving face. Thus the lines and area of attack are enormously multiplied. The front wall of the terrace is cut into promontories and bays. The interlacing of branch cañons back of the wall cuts off the promontories into detached buttes, and the buttes, attacked on all sides, molder away. The rate of recession therefore is correspondingly accelerated in its total effect.
The largeness of the area presents really no difficulty. The forces which break up the rocks are of meteoric origin. The agency which carries off the débris is the water running in the drainage channels. Surely the meteoric forces which ravage the rocks of a township may ravage equally the rocks of the county or state, provided only the conditions are uniform over the larger and smaller areas. And what is the limit to the length of a stream, the number of its branches and rills, and to the quantity of water it may carry? It is not area, them, which oppresses us by magnitude, but the vertical factor-the thickness of the mass removed. But upon closer inspection the aspect of this factor also will cease to be forbidding.
For if the rate of recession of a wall fifty feet high is one of a given number of years, what will be (ceteris paribus) the rate of recession in a wall a thousand feet high? Very plainly the rate will be the same.* If we suppose two walls of equal length, composed of the same kind of rocks, and situated under the same climate, but one of them much higher than the other, it is obvious that the wall-face will be proportional to their altitudes. In order the rates of recession may be equal, the amount of material removed from the higher one must be double that removed from the other, and since the forces operating on the higher one have twice the area of attack, they ought to remove from it a double quantity, thus making rates of recession equal. In the same way it may be shown that the rate of recession is substantially independent of the magnitude of the cliff, whatever its altitude. Here a momentary digression is necessary.
We have hitherto spoken of the recession of cliffs as if it comprised the whole process of erosion, and have hardly alluded to the possible degradation of the flat surface of plateaus, terraces, and plains. Is it meant that there is no degradation of the horizontal surfaces, and that the waste of the land is wholly wrought by the decay of cliffs? Approximately that is the meaning, but some greater precision may be given to the statement.
Erosion is the result of two complex groups of processes. The first group comprises those which accomplish the disintegration of the rocks, reducing them to fragments, pebbles, sand, and clay. The second comprises those processes which remove the débris and carry it away to another part of the world. The first is called disintegration; the second, transportation. We need not attempt to study these processes in all their scope and relations, but we may advert only to those considerations which are of immediate concern. When the débris produced by the disintegration of rocks is left to accumulate upon a flat surface it forms a protecting mantle to the rocks beneath, and the disintegration is greatly retarded, or even wholly stopped. In order that disintegration may go on rapidly the débris must be carried away as rapidly as it forms. But the efficiency of transportation depends upon the declivity. The greater the slope the greater the power of water to transport. When the slope is greater than 30° to 33° (“the angle of repose“) loose matter cannot lie upon the rocks, and shoots down until it finds a resting place. Hence the greater the slope the more fully are the rocks exposed to the disintegrating forces, and the more rapidly do they decay. This relation is universal, applying to all countries, and explains how it comes about that the attack of erosion is highly effective against the cliffs and steep slopes, and has but trifling effect upon flat surfaces.
Reverting to the main argument, it now appears the erosion goes on by the decay and removal of material from cliffs and slopes; that the recession of high cliffs is as rapids as the recession of low one, and that the quantity of material removed in a given time increases with the altitudes of the cliffs and slopes. In other words, the thickness of the strata removed in a given period of erosion should be proportional to the amount of relief in the profiles of the country. But in the Plateau country, and especially in the grand Cañon district, these reliefs are very great. It is a region of giant cliffs and profound cañons, and, as will ultimately appear, it has been so during a very long stretch of geological time. The thickness of the strata removed from it is only proportional to the values of those conditions which favor rapid erosion. In the foregoing discussion it may appear that the area of a denudation in the Grand Cañon district, though large, and the thickness of the strata denuded, though very great, are not so excessive as to impose such a heavy burden upon the credulity as the first announcement of the figures portended.
In drawing inferences from the stratification the geologist is obviously bound to presume that the strata cut off in the terraces extended originally without a break until they reached some locality where the conditions of deposition failed. There are two, and only two, cases to be considered. The first case is that in which the extension is towards the shore line of the sea or lake in which the strata were deposited. At the shore line the strata, of course, end. In the present case no shore line could have existed southward, between the terraces and the Aubrey Cliffs, beyond the San Francisco Mountains. This is quite certain. We know the country so well that if there had been such a shore line in this interval its traces would have been discovered. We are quite sure that no such traces exist. The second case arises when sediments gradually thin out seawards and either vanish entirely or become so thin that their bulk is only nominal. We have already noted that the strata in the terraces (p.79) grow thinner from west to east, and we know that the shore line of the marine basin, in which they were deposited, lay to the west and northwest. But here we are considering their extensions towards the south, and we already know that more than one hundred miles in that direction was another part of the shore line surrounding the basin trending north west and southeast. Supposing strata to attenuate as they recede from, and to thicken as they approach, their shore lines, the case we are considering would perhaps be about as follows. Southward as far as the Grand Cañon, i. e., half way or thereabout between the terraces and the southern shore, there might be some slight reason for inferring a very little attenuation, but beyond the Grand Cañon we might with equal reason infer a thickening. But this reasoning is obviously precarious, since the attenuation of strata as they approach or recede from shore lines does not follow any rigorous law-does not conform to any definite proportion. The best and apparently the only use we can make of it is rather of a negative character, leading us to infer merely that the stratification does not offer any reason for presuming that their original southward extensions were notably thinner than the portions preserved in the terraces. But there is another class of facts which is somewhat more to the purpose.
Of the denuded formations, some outliers are preserved at a considerable distance from the terraces. In the case of the Permian there is no doubt. The great Carboniferous platform of the Grand Cañon district is spotted in many places with Permian remnants, though rarely is the whole series preserved. One important remnant shows very nearly the whole series-at Mount Logan, in the Uinkaret Plateau, near the Grand Cañon. A conspicuous knoll, called the Red Butte, south of the Kaibab and about 30 miles from the San Francisco Mountains, also preserves a large part of the series, and innumerable patches of lower Permian beds are found on both sides of the great chasm. They show no attenuation whatever, and indeed the Mount Logan mass is one of the thickest exposures of Permian beds thus far discovered. The former extension of this series over the entire district in full volume may therefore be regarded as proven. In the case of the Trias the evidence is from this point of view not quite so clear. South of the Vermilion Cliffs two or three remnants of it have been seen. One lies in the Grand Wash, a lateral valley joining the Colorado from the north just where it issues from the lower end of the Grand Cañon. Another has been recognized by Mr. Gilbert under the protection of lavas in the gigantic pile of San Francisco Mountain. But in neither of them is the entire Triassic series represented. These may be held to prove also the extension of the Trias over the entire district, and they give no sign of any attenuation in the beds preserved. But of the Jura and Cretaceous not a solitary outlier has yet been detected at any considerable distance from their principal terraces. As to these two later formations we can only reason from general considerations. The Jura and Trias, wherever found, appear to be merely different portions of one period of deposition; the physical conditions attending the accumulation of both appear to have been almost identical. Nor have we any reason to doubt that the same considerations apply to the Cretaceous and Eocene.
Still more forcibly is the same conclusion presented to us when we come to the study of the faults and flexures. The Grand Cañon district, the High Plateaus, and indeed the entire Plateau country, has been hoisted during Tertiary time far above the Sierra region lying west of it. At the western border of the plateaus are found gigantic faults where the strata have been sheared, and the country on the eastern side presents beds lying thousands of feet higher than the continuations of the same strata on the western side of the faults. These faults have been studied, and the amounts of the displacements are very approximately known. Owing to the remarkably clear manner in which all the facts are displayed, we are able, theoretically, to restore the country to the position and configuration existing before these beds were faulted and flexed. In this treatise, only the results can be given. The discussion and treatment of the problem is too purely technical for popular explanation. This restoration, so far as it has progressed, shows, without reasonable doubt, that throughout Mesozoic time, and very probably during a part of Tertiary time, the Carboniferous and Permian strata of the Grand Cañon district were horizontal and unbroken, the greatest possible discrepancies being very small. Thus another and important point is gained, for it supports the conclusion that the configuration of the Mesozoic sea-bottom, as well as its relations to the adjoining coasts, was, in the middle and southern portions of the Grand Cañon district, favorable to the reception of the same mass of sediments as we now find in the terraces of the High Plateaus.
The argument from the drainage system is, in principle, the same as that applied to the San Rafael Swell, though different in details. The Colorado River and its tributaries entering the Grand Cañon had their origin at the time the country emerged from the waters and became land. This was in early Tertiary time. The rivers then must have had their courses laid out in conformity with the very feeble slopes of the newly risen country and in conformity with the surfaces of the newest strata. In the progress of Tertiary time this surface, originally as level as the prairies of Illinois, or more, so, began to deform by unequal uplifting; but the rivers remained unchanged, and some of them are flowing to-day along the same routes as of old. Other have dried up, and the very strata which contained their troughs have been swept away. Those which remain occupy a very different relation to the strata from that which they held at first. The tributaries on the north side now run against the dips; those on the south run with the dips, or nearly so. But the change has been in the attitudes off the strata and not in the positions of he rivers. And if we theoretically reconstruct the attitudes of the strata to conform to the courses of the drainage channels we reach a reconstruction exactly the same as that which we deduced from a restoration of the faults and flexures.
Thus the stratification, the outliers, the faults and flexures, and the drainage all yield their quotas of testimony to the great fact of denudation, and indicate that at some initial epoch the whole Mesozoic system and the lower Eocene once extended over the entire platform of the Grand Cañon district, with a thickness varying somewhat, no doubt, but on the whole differing but little, from that which we now find in the terraces of the High Plateaus. It is to be noted that the evidence of this former extension is more complete in the older formations that in the younger ones. In the case of the Permian it is quite perfect; in the case of the Trias very nearly so; in the case of the Jura it is very little less cogent than in that of the Trias; and in the Cretaceous practical certainly is exchanged for a very high degree of probability barely distinguishable from certainly. In the case of the Eocene there still remains a strong probability, but there is room for reservation. No reason to the contrary can be shown at present, and it may be regarded as one of those cases where “the tail goes with the hide“; but we hide“; but we cannot promise that future research will not develop reason for a different conclusion. As the evidence now stands we are impelled to accept the full extension of the Eocene with some reservations, arising not from conflicting evidence, but from want of perfection in the evidence known to us.