CHAPTER YI. 



VALLEY GRAVELS OF THE PRESENT RIVER SYSTEM. 



The gravels which occur in the present valley of the Cam, and 

 along the banks of its tributaries, offer rather a complicated 

 subject for study, inasmuch as they have been deposited at 

 different times and at different levels, according to the changes 

 in the courses of the streams and the depths to which they had 

 successively excavated their channels. 



Indeed it would not be possible to unravel the relations of 

 all the ridges, terraces and patches of gravel which occur at 

 various heights above the present streams, without carrying out 

 a far more detailed survey of the whole valley than has yet been 

 made, and accurately determining the height of every such 

 terrace and patch above ordnance datum. The following pao-es, 

 therefore, must be regarded as containing only a first attempt at 

 giving a connected historical account of the river-gravels of this 

 valley. 



By Prof. Sedgwick these deposits were all classed together 

 under the name of the Fine Gravel of the Plains, and all such 

 beds were at that time considered to be of marine origin. 

 Prof Seeley perpetuated this idea, and was even bold enough to 

 correlate them with the Upper Boulder Clay of Norfolk (see 

 ante, p. 20), but their fluviatile origin has since been acknow- 

 ledged by Prof Bonney and other observers. 



Before proceeding to describe the several terraces of gravel, 

 I will offer a few remarks upon their general mode of occurrence, 

 and upon the extent to which they are likely to be preserved in 

 different parts of a valley such as that of the Cam. 



