298 Proceedings of the Royal Pliysiccd Society. 



has produced a multiplication of gorges. The streams have 

 generally strayed on to rock where the old valley was con- 

 stricted, and have touched it at many turns and elbows besides. 

 The significance of all this as bearing upon river- terracing 

 is immense. 



1. The gorges having given the rivers leisure for planation 

 in the soft glacial deposits of the basins above them, rivers 

 are in this way often enabled to form breadths of alluvium 

 too wide to be kept level. They have to work that part 

 of the valley in strips, as it were, and the strips not in 

 use are liable to be left high and dry. It will not be 

 supposed, however, that the influence of gorges is always 

 favourable to terrace-making. There are gorges so extremely 

 durable that the movements of the river in the basin above 

 them have been almost entirely lateral movements. In these 

 cases the river may only prey upon its own deposits and 

 older terraces. 



2. The modern rivers, as we have seen, have struck rock 

 at very variable depths. In hundreds of cases, after wind- 

 ing freely about, encountering only soft clays and the like, 

 and constructing terraces of various kinds, they have here 

 and there become rock-bound, and prevented from pursuing 

 their work of terrace-building after their former manner, as 

 well as from destroying the terraces they had already made. 



These facts, which are of constant recurrence, will be 

 illustrated by Fig. 16. The more recent of the terraces there 

 represented are grouped within two compartments, separated 

 by a few yards of rock, in which part of the stream has lain 

 passive, while its neighbouring curves have been actively 

 terrace-building. There was a time, however, when the rock 

 lay untouched beneath boulder-clay, now to be seen over- 

 lying it in section at the corner, and when the stream was free 

 to move over the surface of upper terraces, one of which is 

 represented by shading in the figure. To the rock we owe 

 the spit or spur of this shaded terrace that lies between the 

 two compartments. There is nothing more interesting, of the 

 kind, than to trace the history of the early movements of 

 rivers at times when they flowed unrestrainedly above the 

 rocks that now fetter their movements. 



