CONTINUAL CHANGES OF RIVER COURSES. 379 



dotted, while the gravels are represented as rudely stratified. 

 In this case I suppose the river to have run originally on the 

 level (1), and to have deposited the gravel (a) and the loess 

 (a) ; after a certain amount of erosion, which would reduce 

 the level to (2), the gravel would be spread out at (6), and 

 loess at (&'). Similarly the loess (c) would be contemporaneous 

 with the gravel (c). 



Thus, while in each section the lower beds would of course 

 be the oldest, still the upper-level gravels as a whole would 

 be the most ancient, and the beds lying in the lower parts of 

 the valley the most modern. 



For convenience, I have represented the sides of the valley 

 as forming a series of terraces ; and though this is not actually 

 the case, there are places in which such terraces do occur. 



It is, however, well known that rivers continually tend to 

 shift their courses ; nor is the Somme any exception to the 

 rule; the valley itself indeed is comparatively straight, but 

 within it the river winds considerably, and when in one of its 

 curves the current crosses " its general line of descent, it eats 

 out a curve in the opposite bank, or in the side of the hills 

 bounding the valley, from which curve it is turned back again 

 at an equal angle, so that it re-crosses the line of descent, and 

 gradually hollows out another curve lower down in the oppo- 

 site bank," till the whole sides of the valley, or river-bed, 

 " present a succession of salient and retiring angles."* During 

 these wanderings from one side of the valley to the other, the 

 river continually undermines and removes the gravels which 

 at an earlier period it had deposited. Thus the upper-level 

 gravels are now only to be found here and there, as it were, 

 in patches, while in many parts they have altogether disap- 

 peared ; as, for instance, on the right side of the valley between 

 Amiens and Pont Eemy, where hardly a trace of the high- 

 level gravels is to be seen. 



* LyelTs Principles, p. 206. 



