ORIGIN OF VALLEYS. 531 
or, as happens at times in the Pacific, it may be reduced mostly to a 
single cascade of a thousand feet or more. 
The progress of this change may be better understood from the fol- 
lowing cut. 
ee 'B 
A BCD is the rock to be cut through by the stream. Suppose 
denudation to produce first the course C n’. The stream is filled, 
as is commonly the case, by lateral channels and rills down the sides 
of the gorge, as well as by the main source; and the amount or depth 
of water is thus in constant increase, as it flows onward. Denudation 
is consequently most rapid the farthest from the head, or towards 
n'; the valley, therefore, increases in depth in this part till the slope 
has become so gentle here as to counterbalance the greater amount of 
water, at which point the bottom of the valley ceases to increase in 
depth ; in this condition 7* 2° becomes the bottom of the lower valley, 
and C n° the steeper portion above it. In the same manner the valley 
bottom continues to prolong at nearly the same slope, and Cn’, Cn’, 
C n> become successively the course of the stream descending into it. 
And even Cn’, is not an exaggeration of possibilities, for many ex- 
amples of it are met with. 
But the results explained are but a part of the actual course of 
things in these regions of horizontally stratified rock. As on Oahu 
and elsewhere, when the denudation at bottom has reached its limit, 
the waters exert but little degrading power except during floods, and 
this takes. place by the sides of the overflowing stream ; at the same 
time, depositions of detritus take place along its banks. The result is 
that the rocks bounding the valley are worn away below, and are often 
undermined, as explained on page 387; the valley widens at bottom 
to a flat plain, while the enclosing wall by the process becomes nearly 
vertical. A narrow riband of land between high precipices of rock is 
therefore a necessary result of the action. 
Degradation still continues along the upper or steep part of the 
main stream, and also along the many streamlets and rills pouring 
down the valley’s sides ; and in each of these streamlets there is a ten- 
dency to produce below a flat-bottomed valley. The consequence is, 
that they increase the width and extent of the main valley-plain ; for 
