310 RECAPITULATION. 



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 several 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 recrosses the line of 

 descent, and gradually hollows out another curve lower down 

 in the opposite 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 de- 

 posited. 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 disappeared ; as, for instance, on 

 the right side of the valley between Amiens and Pont 

 Remy, where hardly a trace of the high-level gravels is to 

 be seen. 



The neighbouring shores of England and France show 

 various traces of a slight and recent elevation of the land. 

 Raised beaches have been observed at an elevation of from 

 five to ten feet at various points along the coasts of Sussex 

 and the Pas de Calais. Marine shells also occur at Abbeville 

 * Lyell, Principles, p. 206. 



