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tions of the old valleys which these ancient rivers had formed 

 and which were once continued across the country ; their lower 

 prolongations, however, have since been entirely destroyed, and 

 instead of valleys long ridges only now remain, cut through at 

 various points by channels of more recent origin. Such a 

 result might indeed have been anticipated, since water flowing 

 down a steep slope and cutting a deep groove for itself must 

 tend to keep approximately in the same channel for a longer 

 period of time than when it reaches lower and more level 

 ground, where very slight changes are sufficient to produce great 

 deviations in its covirse. Still it is none the less interesting to 

 find the result so clearly stereot3rped on the country as it is by 

 this series of gravels, which enables us to estimate roughly the 

 enormous amount of denudation which has taken place since 

 the time when they formed valley bottoms instead of elevated 

 ridges. 



We see therefore that while little change seems to have 

 taken place in the higher parts of the old valleys, where the 

 watercourses still preserve their original directions, and patches 

 even of the primeval mantle of Boulder Clay still remain upon 

 the bounding heights ; great changes have gone on outside the 

 hill district, resulting in the removal of much material, and 

 causing many important alterations in the direction of the main 

 rivers. 



The whole succession of post-glacial valley gravels seemg 

 indeed to have been singularly well preserved in this part of 

 England, and if the various beds could be thoroughly dis- 

 entangled, they would mark out the courses of the streams at 

 different times, and present us with a picture of the successive 

 changes which have taken place in the river system from the 

 glacial period to the present time. Without attempting to give 

 such a history of the manner in which Cambridgeshire was 

 moulded, I think the following statements regarding the relative 

 age of its principal physical features are warranted by the 

 evidence which has been brought forward, 



(1) That the great hollow of the north fens existed to a 

 great extent before the incidence of the Upper Boulder Clay ; 

 but that of the south fens between Ely, Newmarket, and Cam- 

 bridge had no such existence, having on the contrary been 

 formed subsequently to the removal of the Boulder Clay. 



