62 



The river system is composed of several streams, flowing off 

 comparatively high ground, and uniting to form a river which 

 opens on to a low flat country ; hence, as the power of a stream 

 to deepen its channel depends more on its velocity than its 

 volume, it is obvious that erosion will proceed much more 

 rapidly in the higher portions of the valley, and that the oldest 

 gravels in these localities will be left at a much higher level 

 above the river than those of the same age lower down the 

 valley. Moreover, if the river lose so much of its velocity before 

 reaching the sea as to deposit material instead of deepening its 

 channel, the positions and ages of the gravels will be reversed, 

 the lowest beds in the delta and the highest beds on the steeper 

 slopes being respectively the oldest, while between the two 

 there will be a space where the valley opens out, and vv^here 

 there is little difference of level between the oldest and newest 

 deposits. 



It might therefore be expected that the terraces of gravel 

 would be found more widely separated, and more distinctly 

 marked, as the sources of. the river were approached ; this is 

 indeed the case as far as they are preserved, but it must be 

 remembered that the atmospheric agencies of denudation take 

 much greater effect on the steeper slopes, so that the oldest 

 terraces in the higher parts of the tributary valleys will often 

 have been entirely destroyed and washed down into the more 

 recent gravels. 



Consequently it is in the lower parts of these valleys and in 

 the higher portion of the resultant main valley, where the 

 erosive power of the stream is still considerable, but the lateral 

 denudation is not so great, that we should expect to find these 

 terraces best preserved and at the same time tolerably distinct 

 from one another. 



This is the case in the valley of the Cam ; and for some 

 distance south of Cambridge three sets of gravels can be clearly 

 distinguished, but north of the town they spread out over a 

 wide area, and there is not so much difference in level between 

 the highest and lowest of the terraces, so that it becomes more 

 difficult to separate them one from another. 



Leaving these lower plains for the present, I will first 

 describe the river gravels above Cambridge, taking the several 



