210 EXPLORATION OP THE CANONS OF THE COLORADO. 
evidences that the lines of cliffs themselves have been carried back for great 
distances as cliffs by undermining, which is a process carried on only in an 
arid region. 
The evidence is of this character. I have stated that the drainage of 
the inclined plateaus is usually from the brink of the cliffs backward; i. e., 
the water falling on the plateau does not find its way immediately over the 
cliffs, but runs from the very brink or edge of the plateau back toward the 
middle or farther side, which is usually found against the foot of another line 
of cliffs, and here the waters are turned toward some greater channel, which 
runs against the dip and cuts through the cliffs. Now the water-ways at the 
heads of these streams that have their sources near the brink of the cliffs 
would always be small, shallow, and ramifying into many minute branches 
if the line of cliffs were a fixed or immovable line, but we often find that the 
cliffs have been carried back by the undermining process until all these mi 
nute ramifications have been cut off; and we find canons opening on the 
faces of the cliffs, the waters of which run backward as above described. 
Let us suppose that we have a line of cliffs with an escarpment facing 
the south. The rain, falling on the escarpment and in the region south of 
the cliffs, would run toward the south or along the foot of the cliffs until it 
reached some more important water channel ; the rain falling on the plateau, 
from the brink of the cliffs backward, would run toward the north, and the 
waters falling on this upper region would excavate channels for themselves, 
and, under proper conditions, canons would be cut. As the cliffs are under 
mined and this line carried back into^the plateau, the area with a southern 
drainage would be increased, the area with a northern drainage correspond 
ingly diminished, and, when the process had continued for a sufficient length 
of time, we would find the southern edge of the plateau carried away by this 
undermining process, until all the heads of the streams were cut off and until 
the line had reached the canons. 
Gradually, during the progress of erosion, the excavation of the bottom 
of the canons would cease, as the supply of water running through them 
would be cut off, and such caiions would have to be considered as compara 
tively ancient. Such facts are frequently observed in this caiion and cliff 
country. 
From such considerations, it seems that we may safely conclude that 
