Geology of the Grand Canyon: Corrasion.
Mr. G. K. Gilbert has embodied in his admirable monograph on the Henry Mountains, a chapter on Land Sculpture, which sets forth in most logical and condensed from the mechanical principles which enter into the problems of erosion. In his analysis my be found a discussion of the conditions under which the sculpturing forces and processes achieve such peculiar results as we observe in the Plateau country.
The perusal of that chapter will to the geologist's comprehension of the subject a most delightful definiteness and precision, and the reader, however learned he may be, will take great satisfaction in finding a subject so complex made so intelligible. The principles laid down by Mr. Gilbert will be adopted here and applied. For that purpose I quote from the chapter referred to such statements as are of immediate service.
The mechanical wear of streams is performed by the aid of hard mineral fragments carried along by the current. The effective force is that of the current; the tools are mud, sand, and bowlders. The most important of then is sand; it is chiefly by the impact and friction of grains of sand that the rocky beds of streams are disintegrated.
Where a stream has all the load of a given degree of comminution which it is capable of carrying, the entire energy of the descending water and load is consumed in the translation of the water and load, and there is none applied to corrasion. If it has an excess of load, its vclocity is thereby diminihsed so as to lessen its competence and a portion is dropped. If it has less than a full load, it is in condition to receive more, and it corrades its bottom. A fully-loaded stream is on the verge between corrosion and deposition. * * * The work of transportation may thus monopolize a stream to the exclusion of corrasion, or the two works may be carried forward at the same time.
The rapidity if mechanical corrasion depends on the harduess, size, and number of the transient fragments, on the hardbess of the rock-bed, and on the velocity of the stream. * * * The element of velocity is of double importance, since it determines not only the speed, but, to a great extent, the size of the pestles which grind the rocks. The co-efficients upon which it [velocity] in turn depends, namely, declivity and quantity of water, have the same importance in corrasion that they have in transportation.
Let us suppose that a stream endowed with a constant volume of water is at some point continuously supplied with as great a load as it is capable of carrying. For so great a distance as its velocity remains the same, it will neither corrade nor deposit, but will leave the declivity of its bed unchanged. But if in its progress it reaches a place where a less declivity of bed gives a diminished velocity, its capacity for transportation will become less thaan the load, and a part of the load will be deposited. Or if in its progress it reaches a place where a greater declivity of bed gives an increased velocity, the capacity for transportaion will become greater thatn the load and there will be corrasion of the bed. In this way a stream which has a supply of débris equal to its capacity tends to build up the gentler slopes of its bed and to cut away the steeper. It tends to establish a single uniform grade.
Let us now suppose that the stream, after having obliterated all of the inequlaties of the grade of its bed, loses nearly the whole of its load. Its velocity is at once acceleraated and vertical corrasion begins through its whole lenth. Since the stream has the same declivity, and consequently the same velocity, at all points, its capacity for corrasion is every where the same. Its rate of corrasion, however, will depend upon the character of its bed. Where the rock is hard, corrasion will be less rapid than where it is soft, and there will result inequalities of grade. But so soon as there is inequality of grade there is inequality of velocity and inequality of capacity for corrasion; and where hard rocks have produced declivities, there the capacity for corrosion will be increased. The differentiation will proceed until the capacity for corrasion is everywhere proportional to the resistance to be encountered; that is, until there is equilibrium of action.
In general we may say that a stream tends to equalize its work in all parts of its course. Its power inheres in its fall, and each foot of fall has the same power. When its work is to corrade and the resistance unequal, it concentrates its energy where the resistance is great by crowding many feet of descent into a small space, and diffuses it where the resistance is small by using but a small fall in a long distance. When its work is to transport, the resistance is constant and the fall is evenly distributed by a uniform grade. When its work includes both transportation and corrasion, as in the usual case, its grades are somewhat unequal and the inequality is greatest when the load is least.
The foregoing analysis is applicable to the Colorado. It is, in respect to corrasion, an exceptional river. Nearly all the large rivers of the world along their lower and middle courses have either reached, or closely approximated to, that condition of equilibrium which Mr. Gilbert speaks of, in which the transporting power is nearly adjusted without excess to the load to be carried. They have little or no surplus energy to spare for corrasion, and therefore neither corrade nor deposit. But the Colorado is corrading rapidly, and has doubtless done so with little interruption throughout the entire period of its existence. The cause may be discerned in one important fact already brought out. The region it traverses has been throughout Tertiary time steadily rising, and the total elevation has been enormous. This progressive elevation has antagonized the tendency of the river to reach the adjustment of its energy to the work of transportation alone, and has kept alive its corrasive power. There have been probably some limited periods in the history or the river in which, for the time being, it had sunk its channel until it reached a “base-level“-a grade below which it could not corrade. But this state of affairs was afterwards subverted by a further elevation which increased the declivities of the channel, restoring the corrasive power. The last great upheaval, exceeding in amount 3,000 feet, was of comparatively recent occurrence, and the river has not yet reached the new equilibrium of action and the new adjustment of its energy to the work of simple transportation.
The reader who for the first time is brought to consider the enormous depth of the gash which the Colorado has cut would naturally turn to the rivers with which he is familiar to inquire whether they disclosed evidence of similar and commensurate action. He would rarely find any such evidence. It is only by examining the physical conditions of the Colorado and comparing them with other rivers in the light of such principles as Mr. Gilbert has laid down that the facts become intelligible. The first and most important factor to be considered is its declivity. The fall of the Colorado and its principal fork, the Green River, from Green River Station, on the Pacific Railway, to the end of the Grand Cañon, a [U. S. GEOLOGICAL SURVEY. VIEW FROM THE EASTER BRINK OF THE KAIBAB-OVERLOOKING THE MARBLE CAñON PLATFORM. ANNUAL REPORT 1881. PL. XXXV.] distance, as the river runs, of about 1,050 miles, is about 5,150 feet, or very nearly five feet to the mile. The fall through the Grand Cañon is on an average 7.56 feet to the mile. Taking the several divisions of the Grand Cañon, the declivity may thus be tabulated:
|Distance in miles.|
|Fall in feet.|
|Fall in feet per mile.|
|From Little Colorado to Kaibab division|
The Marble Cañon, with a length of 65.2 miles, has a descent of 510 feet, or an average fall of 7.82 feet per mile. When compared with the declivities in the middle and lower courses of other large rivers, that of the Colorado in the cañons is seen to be very excessive. It falls about as many feet as the others fall in inches. The flow of other large rivers which are usually considered swift is calm and easy in comparison with the rush of the waters of the Colorado.
There is another factor which would be fatal to corrasion in other rivers, but which in this one greatly augments its corrosive power. Not only are few rivers so swift, but fewer still are so continuously turbid and so heavily charged with sediment. Rarely is the river clean, even in the droughts of midsummer. Immense quantities of sand and clay are swept along at all parts of the year. Ordinary rivers, and even most of the exceptional ones, would be gorged with such quantities of sand, and instead of corrading would have their energies fully taxed in carrying the load which the Colorado bears easily. This sand is the tool which it employs for its work, and it uses it with great effect. Though the river is heavily loaded, it is still underloaded, and has great power to corrade.*
To show how efficient the corrasive action may become under extremely favorable circumstances, we may cite the case of some of the great hydraulic mines in California. In these mines powerful streams of water are discharged against the gravel banks, and the spent water is gathered into a brook which finds its way over the “bed-rock“ into a tunnel, and finally escapes into some deep natural gorge below the level of the workings. As the water flows away it carries with it all the débris washed from the banks, whether coarse or fine. In the well-known Bloomfield mine I saw a gash in the solid basaltic bed-rock 12 feet in depth, which I was assured had been cut by the escaping water and gravel in a period of about sixteen months. The actual running time of the water, however, had been equivalent to about 145 days of twenty-four hours each. This case is indeed a most extreme one, and no natural river can show any such rapid corrasion of any considerable length of its bed. It is not cited to support an inference of phenomenal rapidity in the corrasion of the Grand Cañon, but rather to illustrate the efficiency of corrasive action when all the attendant conditions are extremely favorable and no countervailing condition is present. But although the Colorado is far from being such an extreme case as the one just mentioned, it is still a very strong one. Yet there are some stretches in the river where the corrasion must be proceeding at a very rapid rate-at a rate not very many times slower than in the Bloomfield mine. These portions are in the hardest rocks, and they illustrate well the law which Mr. Gilbert has so clearly enunciated (p. 157, line 44).
The course of the Colorado in the Grand Cañon is a succession of headlong rapids or cataracts and of smooth but swiftly-flowing reaches. In the Kaibab division the rapids are very numerous, very long, and very frequent, while the still reaches are short. In the Kanab division the rapids are fewer and less formidable, while the still reaches are longer. In the Sheavwits, the condition is intermediate between those of the Kaibab and Kanab divisions. The rapids, however, are of two kinds and are the results of two wholly independent causes. (1.) When the stream lies in the hard rocks, the declivity is much greater, and the rapid is then due to the greater slope of the bed. (2.) At the opening of every side-gorge, a pile of large bowlders and rubble is pushed out into the stream. Most of the side-gorges are dry throughout the greater part of the year. But when the rains do come, their effects are prodigious.* In the vast amphitheaters the water is quickly shot down into the channel and rushes with frightful velocity along the bed, which has a slope of 200 feet or more to the mile. Nothing which is loose and which lies in the way of it can resist its terrible rush. Bowlders of many tons' weight are swept along like chaff, and go thunding down the side-gorges into the main river. When the torrents reach the river the large fragments are dropped; for the maximum slope of the main stream (reckoned throughout any stretch exceeding four of five miles) never exceeds 25 feet to the mile; and the water, though great in volume at flood-time, has much less velocity than the torrents of the side chasms. The river has, however, abundant power to sweep along fragments of considerable size, which are ground up as they move onward. The coarse material, the large bowlders and rubble washed out of a lateral chasm, form a dam where the river becomes a cataract. They are also strung out for considerable distances below the dam, and thus the tendency is to build up and increase the grade of the river just beyond the rapid. But this tendency is quickly checked and brought to a stop by the increased power of the current due to the increased slope. The body of fragments brought into the river laterally is vast in amount. But on the whole it is insufficient at the present epoch to prevent the river from corrading its channel, though corrasion is greatly retarded by it. There are many stretches where there is an equilibrium between the tendency to cut deeper and the tendency to build up the bottom by the accumulation of débris; where the whole energy of the river is consumed in dissipating the fragments brought into it. But there are other portions where the river bed is in the bare rock of Palí¦ozoic and Archí¦an strata, and wherever it is so corrasion is proceeding rapidly.
|1||* The details of corrasion in the cañon will be much more fully discussed in the monograph on the Grand Cañon District.|