Our little span of human life is far too narrow for us to be a witness of any of the great earth changes. These changes are so slow,—oh, so slow,—and human history is so brief. So far as we are concerned, the gods of the earth sit in council behind closed doors. All the profound, formative, world-shaping forces of nature go on in a realm that we can reach only through our imaginations. They so far transcend our human experiences that it requires an act of faith to apprehend them. The repose of the hills and the mountains, how profound! yet they may be rising or sinking before our very eyes, and we detect no sign. Only on exceptional occasions, during earthquakes or volcanic eruptions, is their dreamless slumber rudely disturbed.
Geologists tell us that from the great plateau in which the Grand Canon is cut, layers of rock many thousands of feet thick were cut away before the canon was begun.
Starting from the high plateau of Utah, and going south toward the canon, we descend a grand geologic stairway, every shelf or tread of which consists of different formations fifty or more miles broad, from the Eocene, at an altitude of over ten thousand feet at the start, across the Cretaceous, the Jurassic, the Triassic, the Permian, to the Carboniferous, which is the bottom or landing of the Grand Canon plateau at an altitude of about five thousand feet. Each step terminates more or less abruptly, the first by a drop of eight hundred feet, ornamented by rows of square obelisks and pilasters of uniform pattern and dimension, "giving the effect," says Major Dutton, "of a gigantic colonnade from which the entablature has been removed or has fallen in ruins."
The next step, or platform, the Cretaceous, slopes down gradually or dies out on the step beneath it; then comes the Jurassic, which ends in white sandstone cliffs several hundred feet high; then the Triassic, which ends in the famous vermilion cliffs thousands of feet high, most striking in color and in form; then the Permian tread, which also ends in striking cliffs, with their own style of color and architecture; and, lastly, the great Carboniferous platform in which the canon itself is carved. Now, all these various strata above the canon, making at one time a thickness of over a mile, were worn away in Pliocene times, before the cutting of the Grand Canon began. Had they remained, and been cut through, we should have had a chasm two miles deep instead of one mile.
The cutting power of a large, rapid volume of water, like the Colorado, charged with sand and gravel, is very great. According to Major Dutton, in the hydraulic mines of California, the escaping water has been known to cut a chasm from twelve to twenty feet deep in hard basaltic rock, in a single year. This is, of course, exceptional, but there have, no doubt, been times when the Colorado cut downward very rapidly. The enormous weathering of its side walls is to me the more wonderful, probably because the forces that have achieved this task are silent and invisible, and, so far as our experience goes, so infinitely slow in their action. The river is a tremendous machine for grinding and sawing and transporting, but the rains and the frost and the air and the sunbeams smite the rocks as with weapons of down, and one is naturally incredulous as to their destructive effects.
Some of the smaller rivers in the plateau region flow in very deep but very narrow canons. The rocks being harder and more homogeneous, the weathering has been slight. The meteoric forces have not taken a hand in the game. Thus the Parunuweap Canon is only twenty to thirty feet wide, but from six hundred to fifteen hundred feet deep.
I suppose the slow, inappreciable erosion to which the old guide alluded would have cut the canon since Middle Tertiary times. The river, eating downward at the rate of one sixteenth of an inch a year, would do it in about one million years. At half that rate it would do it in double that time. In the earlier part of its history, when the rainfall was doubtless greater, and the river fuller, the erosion must have been much more rapid than it is at present. The widening of the canon was doubtless a slower process than the downward cutting. But, as I have said, the downward cutting would tend to check itself from age to age, while the widening process would go steadily forward. Hence, when we look into the great abyss, we have only to remember the enormous length of time that the aerial and subaerial forces have been at work to account for it.
Two forces, or kinds of forces, have worked together in excavating the canon: the river, which is the primary factor, and the meteoric forces, which may be called the secondary, as they follow in the wake of the former. The river starts the gash downward, then the aerial forces begin to eat into the sides. Acting alone, the river would cut a trench its own width, and were the rocks through which it saws one homogeneous mass, or of uniform texture and hardness, the width of the trench would probably have been very uniform and much less than it is now. The condition that has contributed to its great width is the heterogeneity of the different formations—some hard and some soft. The softer bands, of course, introduce the element of weakness. They decay and crumble the more rapidly, and thus undermine the harder bands overlying them, which, by reason of their vertical fractures, break off and fall to the bottom, where they are exposed to the action of floods and are sooner or later ground up in the river's powerful maw. Hence the recession of the banks of the canon has gone steadily on with the downward cutting of the river. Where the rock is homogeneous, as it is in the inner chasm of the dark gneiss, the widening process seems to have gone on much more slowly. Geologists account for the great width of the main chasm when compared with the depth, on the theory that the forces that work laterally have been more continuously active than has the force that cuts downward. There is convincing evidence that the whole region has been many times lifted up since the cutting began, so that the river has had its active and passive stages. As its channel approached the sea level, its current would be much less rapid, and the downward cutting would practically cease, till the section was elevated again. But all the time the forces working laterally would be at work without interruption, and would thus gain on their checked brethren of the river bottom.
There is probably another explanation of what we see here. Apart from the mechanical weathering of the rocks as a result of the arid climate, wherein rapid and often extreme changes of temperature take place, causing the surface of the rocks to flake or scale off, there has doubtless been unusual chemical weathering, and this has been largely brought about by the element of iron that all these rocks possess. Their many brilliant colors are imparted to them by the various compounds of iron which enter into their composition. And iron, though the symbol of hardness and strength, is an element of weakness in rocks, as it causes them to oxidize or disintegrate more rapidly. In the marble canon, where apparently the rock contains no iron, the lateral erosion has been very little, though the river has cut a trench as deep as it has in other parts of its course.
How often I thought during those days at the canon of the geology of my native hills amid the Catskills, which show the effects of denudation as much older than that shown here as this is older than the washout in the road by this morning's shower! The old red sandstone in which I hoed corn as a farm-boy dates back to Middle Palaeozoic time, or to the spring of the great geologic year, while the canon is of the late autumn. Could my native hills have replied to my mute questionings, they would have said: "We were old, old, and had passed through the canon stage long before the Grand Canon was born. We have had all that experience, and have forgotten it ages ago. No vestiges of our canons remain. They have all been worn down and obliterated by the strokes of a hand as gentle as that of a passing cloud. Where they were, are now broad, fertile valleys, with rounded knolls and gentle slopes, and the sound of peaceful husbandry. The great ice sheet rubbed us and ploughed us, but our contours were gentle and rounded aeons before that event. When the Grand Canon is as old as we are, all its superb architectural features will have long since disappeared, its gigantic walls will have crumbled, and rolling plains and gentle valleys will have taken its place." All of which seems quite probable. With time enough, the gentle forces of air and water will surely change the whole aspect of this tremendous chasm.