On the Weald. 69 
or where, being bent into the form of a trough, streams were 
enabled to flow through the trough, and so get over it. 
Another effect of the formation of these hills would be, that 
the water, being uanble to get through them, the streams 
would be diverted eastward or westward, in which directions 
they would flow until they were able to join some stream 
which had been able to get through the Lower Greensand and 
then, by adding to its strength and therefore to its cutting 
power, they would materially assist it in its struggle with the 
other streams. The denudation of the Weald Clay therefore 
must in this way have been fatal to many of the lateral 
branches of the streams, and its effect upon the others would 
be that some would be considerably diminished in size by the 
stoppage of part of their supply of water, whilst the water of 
which they were thus deprived would go to swell the volume, 
and would therefore increase the strength of rival streams. 
A very similar state of things would be repeated in the case 
of the Gault. Its denudation has converted the space between 
the Chalk and the Lower Greensand into the valley which I 
have already described. The Gault being softer than Chalk, and 
therefore more easily worn down, would soon be reduced below 
the level of some of the valleys in the Chalk through which 
the streams or their lateral branches flowed, and as this took 
place the several branches or streams, some of which would 
be already much weakened by the effects already described of 
the denudation of the Weald Clay, would be cut off entirely 
from the Weald, and their valleys would be necessarily con- 
verted into dry valleys.* At the same time the water which 
formerly flowed through them would be diverted eastward 
or westward down the valley, and would go to swell and 
strengthen rival streams. The competition, however, would 
still continue between these, and the very existence of each 
of them would come to depend entirely upon its ability to keep 
the area which it drained and its channel through the Lower 
Greensand and Chalk below the level of the area and channel 
of its rival neighbour. As each failed to do this it would be 
cut off from the source from which its water was derived, its 
valley would be converted into a dry valley, and its water 
would be swallowed up by its victorious rival. In this very 
simple way the whole of the streams, with all their lateral 
branches, which formerly flowed from the Weald northward 
through the chalk hills between the Mole and the Medway, a 
distance of 35 miles, seem to have disappeared from the scene, 
* It is probable that many of these valleys later on were much 
deepened and enlarged by the action of periodical Bourne streams, like the 
one so frequently seen in the Caterham Valley. These Bourne streams 
would naturally be much more numerous in past times than they now are. 
