496 



CHAPTER XXX. 



ORIGIN OF EIVER VALLEYS THEIR RELATION TO TABLE- 

 LANDS ESCARPMENTS CUT THROUGH BY RIVERS GEO- 

 LOGICAL DATES OF DIFFERENT RIVER-VALLEYS THE 



SEVERN, THE AVON, THE THAMES, THE FROME, AND THE 



SOLENT TRIBUTARIES OF THE WASH AND THE HUMBER 



THE EDEN AND THE WESTERN-FLOWING RIVERS 



SCOTLAND. 



IT is difficult, or almost impossible, even approxi- 

 mately to settle precisely what are the geological dates 

 of the valleys through which many rivers run ; or, in 

 other words, when they first began to be scooped out, 

 and through what various periods their excavation was 

 intermittently or continuously carried on. No one 

 has yet thoroughly analysed this subject, and only 

 of late years have I begun clearly to see my way into 

 it. Nevertheless a good deal has been done even 

 now, and a great deal more will be accomplished when, 

 with sufficient data, the whole subject may come to be 

 investigated. In Wales, for example, there are vast 

 numbers of rivers and brooks, small and large, and 

 when we examine the relation of these streams to the 

 present surface of the countty, we often find it, very 

 remarkable. Fig. 97 is a diagram representing no par- 

 ticular section, but simply the general nature of the 

 sections across the Lower Silurian strata of Cardiganshire, 

 as shown by myself in a paper given to the British 



