| OCR Text |
Show ARIDITY AND EROSION. 171 terminates present characteristics peculiar to themselves. Below, we have rounded buttresses, and mounds and hills of sand, and piles of great, angular bbcks; above, the walls are of columnar structure, and sometimes great columns, seen from . a distance, appear as if they were elaborately fluted. The brink of this escarpment is a well uefined edge. But if these formations extended over the underlying beds at one time, and if they have buen carried away by rains and rivers, why has not the country between been left comparatively level, or embossed with hills separated by valleys' It is easy to see that a river may cut a channel, and leave its banks steep walls of rocks; but that rains, which are evenly distributed over a clistrict, should dig it out in great terraces, is not so easy to perceive. The climate is exceedingly arid, and the scant vegetation furnishes no protecting covering against the beating storms. Bnt though little rain falls, that which does is employed in erosion to an extent difficult to appreciate by one who has only studied the action of water in degrading the land in a region where grasses, shrubs, and trees bear the brunt of the ~:;torm. A little shower falls, and the water gathers rapidly into streams, and plunges headlong down the steep slopes, bearing with it loads of sand, and for a few minutes, or a few hours, the district is traversed by brooks and creeks and rivers of mud. A clear stream is never seen without going up to a moister region on some high mountain, and no permanent stream is found, unless it has its source in such a mountain. In a country well supplied with rains, so that there is an abundance of vegetation, the water slowly penetrates the loose soil, and gradually disintegrates the underlying solid rock, quite as fast as, or even faster than it is carried away by the wash of the rains, and the indurated rock has no greater endurance than the more friable shales and sandstones; but in a dry climate, the softer rocks m·e soon carried away, ~hiie the hardm· rocks are washed naked, and the rains make but slow progress in tearing them to pieces. vVhen a great fold emerges from the sea, or rises above its base level of erosion,* the axis appears above the wnter (or base level) first, and is immediately attacked by the rains, and its sands are borde off to form new deposits. It has before been explained that the em~rgence of the fold is bnt • For oxplanntion of this term, "base Jovol of orot~ion.'' soe Chapter XlL |
