CANONS IN THE SANDSTONE. 179 
Temple. They are due to the crumbling of softer rocks, which underlie 
harder beds, the friable material being carried away by springs, or wet 
weather streams. The greater number are found at the heads of little 
gulches. 
In many places the walls of the canon are .of homogeneous sandstone, 
and where the river sweeps in a great curve at the foot of the wall, mural 
cliffs are found. 
The oak glens have been excavated by springs, and the alcoves are the 
channels of intermittent rills. 
Away from the river, on either side, there are broad stretches of naked 
sandstone, carved by the rains into gentle billows or mounds. As the rains 
gather into streams, the little valleys, or grooves, between the mounds 
become gulches, and where the smaller streams gather into larger the gulches 
become canons, often having vertical or even overhanging walls. 
When, in the progress of corrasion, these streams have cut through 
harder beds, and reach softer, the channels are seen to widen. The 
manner in which this widening occurs is curious. The streams are 
everywhere tortuous, and, as the power of the water is constantly exerted 
in corrasion, the streams are not only made deeper, but the curves are 
increased by methods well known to those who have studied the origin and 
change of river channels; so the walls are often undermined on the outer 
side of curves, and here overhanging cliffs are found. 
So these canons are not only flexuous in horizontal outline, but they 
are also flexuous in vertical outline, giving them warped or tortuous courses. 
The streams do not always cut channels with vertical walls. Occasionally, 
deep water-ways are found, with flaring walls to the very bottom. Such 
canons usually occur where the beds of streams are in rocks quite as hard, 
or even harder, than those above. A good illustration of such a channel is 
seen in Figure 48. Besides the grooves, gulches, and canons that head 
among the mounds, we have another class of water-ways, to which the former 
are sometimes tributary. Many streams come down from distant mountains, 
where they receive a more constant supply of water. They often run for 
many miles through narrow, winding canons, with walls so precipitous that 
they cannot be scaled, and they form impassable barriers to the traveler. 
