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, angu 
lar blocks; 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 defined edge. But if these 
formations extended over the underlying beds at one time, and if they have 
been earned 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 district, 
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. But 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 storm. 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 are soon carried away, 
while the harder rocks are washed naked, and the rains make but slow 
progress in tearing them to pieces. 
When a great fold emerges from the sea, or rises above its base level 
of erosion,* the axis appears above the water (or base level) first, and is 
immediately attacked by the rains, and its sands are borne off to form new 
deposits. It has before been explained that the emergence of the fold is but 
* For explanation of this term, " base level of erosion," see Chapter XII. 
