Perhaps the greatest erosion is in the passes through which the winds rush into the desert. Here they not only eat into the ledges and cut away the rock faces, but they make great wash-outs in the desert itself. These trenches look in every respect as though caused by water. In fact the effects of wind and water are often so inextricably mixed that not even an expert geologist would be able to say where the one leaves off and the other begins. The shallow caves of the mountains—too high up for any wave action from sea or lake, and too deep to be reached by rains—have all the rounded appearance of water-worn receptacles. One can almost see the water-lines upon the walls. But the sand-heaped floor suggests that the agent of erosion was the wind.
Cloud-bursts.
Canyon streams.
Yes; there is some water on the deserts, some rainfall each year. Even Sahara gets its occasional showers, and the Colorado and the Mojave show many traces of the cloud-burst. The dark thunder-clouds that occasionally gather over the desert seem at times to reserve all their stores of rain for one place. The fall is usually short-lived but violent; and its greatest force is always on the mountains. There is no sod, no moss, to check or retard the flood; and the result is a great rush of water to the low places. In the canyons the swollen streams roll down bowlders that weigh tons, and in the ravines many a huge barranca is formed in a single hour by these rushing waters. On the lomas and sloping valleys they are not less destructive, running in swift streams down the hollows, and whirling stones, sand, and torn bushes into the old river-beds.
Desert floods.
Power of water.
In a very short time there is a great torrent pouring down the valley—a torrent composed of water, sand, and gravel in about equal parts. It is a yellow, thick stream that has nothing but disaster for the man or beast that seeks to swim it. Many a life has been lost there. The great onset of the water destroys anything like buoyancy, and the tendency is to drag down and roll the swimmer like a bowlder. Even the enormous strength of the grizzly bear has been known to fail him in these desert rivers. They boil and seethe as though they were hot; and they rush on against banks, ripping out the long roots of mesquite, and swirling away tons of undermined gravel as though it were only so much snow. At last after miles of this mill-racing the force begins to diminish, the streams reach the flat lake-beds and spread into broad, thin sheets; and soon they have totally vanished, leaving scarce a rack behind.
Water-pockets.
No running streams.
The desert rainfall comes quickly and goes quickly. The sands drink it up, and it sinks to the rock strata, where, following the ledges, it is finally shelved into some gravel-bed. There, perhaps a hundred feet under the sand, it slowly oozes away to the river or the Gulf. There is none of it remains upon the surface except perhaps a pool caught in a clay basin, or a catch of water in a rocky bowl of some canyon. Occasionally one meets with a little stream where a fissure in the rock and a pressure from below forces up some of the water; but these springs are of very rare occurrence. And they always seem a little strange. A brook that ran on the top of the ground would be an anomaly here; and after one lives many months on the desert and returns to a well-watered country, the last thing he becomes accustomed to is the sight of running water.