THE bed of Hermit or Shinumo Creek, with its tributaries, is more or less typical of every canyon in the Plateau Country. The creeks are fed by the small arroyos, and the arroyos, in turn, by side swales and washes. At its head the arroyo derives from a watershed and it is only after it has cut down its bed that the side-washes are formed as feeders and drainage streams.
This process carries on regardless of whether the area is one of loose gravel or hard rock. If you will follow to its source the large arroyo back of the Hermit Camp you will find that it is fed by a wet-weather stream pouring down from the top of the
Red WaII. The upper stream is brought together on the slopes of the Supai above the Red Wall and, pursuing a channel of its own cutting, pours over the edge in a waterfall. The fallen water gathers together once more on the lower Tonto platform, in which it has also cut a channel, and goes down to join Hermit Creek, and thus to the Colorado. Where it pours over the Red Wall you will find a lip, like that of a pitcher, worn in the rock, and under it a staining of the rock with black and gray lichens for several hundred feet. This lip is the beginning of a cut-back, a fluting, an arena.
You will see the lip perhaps oftener and with better effect on the walls of the higher stratathe Coconino and the Kaibab. For the processes of stream-formation, rock-cutting, arena-carving can be traced without the least confusion up through the various slopes and strata to the Rim of the Canyon (Plates 11, 12). Every terrace or slope, however steep, is guttered and depressed in places, and it is in these depressions that the rain gathers, forms into a stream, and runs down over the wall, falling on the platforms below. Water is drained from roof to roof, from the highest to the lowestthe roofs flattening and spreading out in area and in watershed as they descend. The steep walls of the Kaibab and the Coconino pour water upon the slopes of the Supai, the Supai sends its gatherings over the Red Wall upon the Tonto platform, which in turn empties its streams into creeks that cut through the Archaean walls and finally reach the Colorado.
Now it must have been noticed by the most casual of observers that every stream coming down a slope or over a wall, by its own wear keeps cutting back into the slope or wall, grooving or notching the rim or edge over which it runs. In the course of time, and with the deepening of the cut, the sides of it begin to break away and widen through lateral cuts that develop secondary grooves and notches. Where the rock is soft, the cutting and the widening go on together with swift pace; where the rock is hard, the cutting is narrower, deeper, producing rather abrupt walls., Thus the lateral cut-backs through the Archaean and Tapeats walls down at the River are generally sharp, rough defiles, narrow paths with vertical sides that one cannot go up or down without a rope (Plate 20). Above these walls the green shales of the Tonto Group are a much softer formation, and the streams easily wear them down and back. But not so with the next stratumthe Red Wall. Here we have a stubborn limestone that does not easily disintegrate. The gathered rains go over it in cataracts or waving waterfalls, and the upper edge of it is usually marked by a lip or narrow trough that pours the water down in a compact stream.
If you locate this lip or trough accurately you will find that it is usually in the central depression of a quarter-circle. No portion of the Red Wall runs on for any distance in a straight line. Every-where it bends in and out, is serpentine in projections and recessions. The arenas or quarter-circles in it were all started by the widening of the lip at the back, and each arena in the course of time continues to widen through smaller lateral cuts along its rim. A very large quarter-circle in the Red Wall will not only have its large trough and stream-bed at the extreme back but have also several lateral lips and beds formed, or forming, on its sides. Thus the arena is broadened and enlarged, often to great size, by the cut-backs of the half-dozen or more streamlets that pour over its edge.
Higher up on the walls there are waves and crescents cut in the Supai formation, above the Red Wall; but the sandstone and shale there is softer than the limestone, and the arena forms in it are not so large. They are too easily broken down to endure long in quarter-circle form. The appearance then on the Supai slopes is that of smaller serpentine windingswindings of the walls and ins and outs of the various layers. The same kind of erosion goes on there as with the Red Wall below, but the cutting is faster and more uniform, since the tendency of the water is to come down a series of steps as well as in a series of t oughs or depressions in the slopes. It is the broad wash from these red Supai steps that pours down the face of the Red Wall, staining it and making it appear as a red wall, although in reality it is a blue-gray lime-stone.
Above the Supai shales come the hard, abrupt walls of the Coconino sandstone and the Kaibab limestone, the latter being under your feet as you stand on the Rim. The water-wear on these upper walls is similar to that upon the Red Wall belowthat is, the wear is in the most recedent portion of the Rim where the rain gathers and pours over the edge from a lip or trough. The tendency here as elsewhere is to form the arena, the half or quarter circle. Look along the Rim from where you stand and you will discover that it runs in flutings like a Doric column. Sometimes these flutings or arenas are not fifty feet across. They are, in fact, of all sizes. A small one shows directly in front of the hotel where you go to see the Canyon for the first time. On the way from the hotel to the head of Bright Angel Trail is a Iarger one that looks like an irregular crescent cut in the Rim. The middle of itthe point farthest back in the Rimis just in front of the Bright Angel Camp. A stream runs there in wet weather and keeps cutting back, deepening the crescent. Around the edge of the crescent you can see on rainy days other little streams running together and finding ways by tiny lips over the edge and down the wall. These again are the beginnings of lateral cut-backs.
Now the whole Rim of the Canyon, on both sides of the River, is fluted and indented, notched with crescents of more or less pronounced character.
The arena formation is, in fact, characteristic of every wall hereabouts. The one to the left of the hotel to which attention has been called is only a small one. It is an arena within an arena. You do not perhaps see the larger one because of its bulk. It runs from Hopi Point to Yavapai Point a distance of several miles. The hotel buildings stand in the recess of it. Where is the stream-bed that originally cut it back ? Why, just to the west of the head of Bright Angel Trail. The trail lower ‘ down follows the stream-bed all the way to the River. It was this stream that not only created the larger arena but was responsible for the lateral canyon down which the trail runs.
Every one of the side-canyons here was started as a lip in the rock, became a crescent, and to this day many of them keep the semblance of the crescent form. That is to say, they are great indentations between outstanding points. The stage of beginning with a half or quarter circle is apparent every-where. As for the water-wear, that, too, is so apparent, so obvious, that one asks naturally enough: Why is there so much more of it here than else-where? Why the torn arroyo, the slashed canyon, the cut and carved strata?
It is not difficult to answer those questions. The Plateau Country through which the Canyon runs is largely desert in character. The infrequent rain, the thin soil, the high altitude all combine against any pronounced growth of vegetation, and there are great areas where practically nothing at all grows. Pines, pinyons, and junipers make something of a show along the Rim, but the trees are wide apart and rather stunted in growth; the under-brush is scattered, and as for the mosses and grasses, they appear only in small clumps and beds.
The rainfall here is not nearly so great in the aggregate as along the Atlantic Coast; but what rain there is, usually falls in heavy showers, often in what are called “cloudbursts.” It descends in torrents for perhaps an hour, and then stops. Falling upon a rocky bed, or one with only a few inches of soil-covering, there is no chance for it to sink in anywhere. Neither is there sufficient grass, moss, or undergrowth to check its run-off. Immediately it begins to gather in countless little streams. And each stream as it runs carries with it sand and gravel which it empties into a larger stream. The force is cumulative, and the descent down the slopes to the secondary and lateral canyons and thus to the River is very swift. Each stream be-comes a Colorado in miniature, battering and sawing its way along its bed, carrying what it cuts and loosens down to the greater River. No wonder the Plateau landscape is guttered and cross-guttered with arroyos and barrancascanyons in little.
The Grand Canyon is the direct result of an erosion that has been going on for hundreds of centuries. Water is sufficient in itself to account for the great bulk of the destruction here apparent. And yet there are other causes that help on the general drag-down of the walls and add to the tale of ground rock that is carried each year from the Plateau to the sea.
The driving rains that beat directly against the faces of the Kaibab, the Coconino, and the Red Wall seem very futile in their fury. The walls throw them off easily enough, shunt them into the streams. But there is always a certain amount of damage done. In summer the rains beat into the seams and cracks of the rock and dissolve some of the cementing material that binds the grains together. Disintegration sets in. Certain particles are carried off by the drip of water; other particles are loosened and fall down as sand. Again, rain running down a face-wall follows the wall into hollows and caves, creeping and seeping along the ceiling, and perhaps finally dropping far within the cave. This once more produces a solution and a falling of rock particles from the ceiling as sand. Almost every shallow cave that one enters in this Canyon region will show a floor covered with sand-sometimes several feet of it.
Another process of destruction goes on in winter with the freezing and thawing of rain and snow lodged in the crevices of the rock strata. Layers and sections of the wall are thus pried away from the main face and, in time, fall as blocks upon the taluses below. Eventually these blocks push down into the valley, become smaller, flatten out on the slopes or are ground to sand in the swift-running streams. Not only the walls but the buttes, the pinnacles, the ledges, the platforms, all suffer from frost. It is true that Nature tries to mitigate the damage and hold off destruction by growing wherever she can bushes, grasses, flowers, mosses, lichens that act as protectors of the stone. It is astonishing the places she chooses to grow them. What food there may be for plant life in a fissure of rock one hardly knows, but one finds flowers and grasses growing there almost as a rule rather than as an exception. Destruction to the rock is thus for a time stayed. But eventually the crumbling and falling process carries on.
Still another process of disintegration follows the rasp and cut of the uneasy winds. Always they are eddying and circling about the walls, the buttes, the spines, the towers, the ridges. They creep in and out of crevices and hollows and rush around arenas and amphitheatres often with much force. Almost everywhere they move, they carry or drive with them particles of sand. These are flung against the walls or driven in an eddy about a shallow recess, or hurled with fury around the base of pillars. They cut like a miniature sand-blast. The result is more destruction. It is greater from the action of wind than is generally supposed, be-cause it is incessant and wide-spread. No rock face escapes the blast. Whether gentle or fierce, it rubs and wears away.
And these rocks at the Canyon are peculiarly susceptible to the wear of wind and water. For one thing, they lie in horizontal beds, which is not unfavorable to erosive processes. Again, they are all of them, except those of the Inner Canyon at the River, made up of sedimentary deposits and lack the consistency of metamorphic and igneous rocks. Sandstones and limestones have not the resistant powers of schists and gneisses.
But the rock strata at the Canyon make up such an extraordinary story that they require a chapter by themselves.