172 EXPLORATION OF THE CANONS OF THE COLORADO. 
little faster than the degradation of its surface, but, as it comes up, the 
wearing away is extended still farther out on the flanks, and the same beds 
are attacked in the new land which have already been earned away nearer 
the center of the fold. In this way the action of erosion is continued on 
the same bed from the up-turned axis toward the down-turned axis, and it 
may and does often happen that any particular bed may be entirely carried 
away, with many underlying rocks, near the former line, before it is attacked 
near the latter. Now, as the beds are of heterogeneous structures, some 
hard and others soft, the harder beds withstand the action of the storms, 
while the softer beds are rapidly carried away. 
The manner in which these beds are degraded is very different. The 
softer are washed from the top, but the harder are little affected by the 
direct action of the waters they are torn down by another process. As 
the softer beds disappear, the harder are undermined, and are constantly 
breaking down; are crushed, more or less, by the fall, and scattered over, 
and mingled with the softer beds, and are carried away with them. But the 
progress of this undermining and digging down of the cliff is parallel with 
the upturned axis of the fold, so that the cliffs face such an axis. 
When the fold is abrupt, so that the rocks on either side are made to 
incline at a great angle, ridges are formed, and this topographic structure of 
a country may be found even in a land of rains, though the ridges will 
usually be low, rounded, and more or less irregular, while in a dry climate 
they will be steep and regular, and will usually culminate above in a sharp 
edge; but where the rocks are slightly inclined, terraces will be formed, with 
well defined escarpments. 
It is interesting to note the manner in which the textures of these hard 
capping rocks affect the contours of the cliffs. When the hard rocks are 
separated into well defined layers, or beds, the cliffs will be more or less 
terraced, as the strata vary in hardness. This is well seen in the Brown 
Cliffs and the upper portion of the Book Cliffs. In the last mentioned escarp 
ment the harder beds are underlaid by soft, bluish shales, which appear 
below in the beautifully carved buttresses. 
In the Orange Cliffs there are a thousand feet of homogeneous light 
red sandstone, and this is underlaid by beds of darker red, chocolate, and 
