380 ELEVATION OF THE LAND. 



The neighbouring shores of England and France show various 



O O O 



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

 beaches have been observed at an elevation of from five to ten 

 feet at various points along the coast of Sussex and the Pas 

 de Calais. Marine shells also occur at Abbeville about twenty- 

 five feet above the sea-level,* and no doubt this change of 

 level has had an important bearing on the excavation of the 

 valley. 



Mr. A. Tyler/)- in a recent memoir, agrees with me that the 

 upper-level and lower-level gravels are merely the extremes 

 of a series, seldom complete, but generally imperfect, some- 

 times in one part, sometimes in another. But he also main- 

 tains that the surface of the chalk in the valley of the Somme 

 had assumed its present form prior to the deposition of any 

 of the gravel or loess now existing in it. As, however, he 

 admits that the materials forming this gravel and loess are 

 derived exclusively from the area drained by the Somme and 

 its tributaries, he involves himself in a double difficulty. In 

 the first place he maintains that the materials, by the removal 

 of which the valley was formed, were swept completely out 

 of the valley, which, considering its length, depth and narrow- 

 ness, appears to be impossible ; and in the second place, the 

 admission that the gravel and sand consist of flint debris 

 brought down by the Somme and its tributaries is fatal to his 

 argument, since you cannot remove matter from one place to 

 another without affecting the configuration of the surface in 

 both. In admitting, then, that " the gravel in the valley of 

 the Somme at Amiens is partly derived from debris brought 

 down by the river Somme, and by the two rivers, the Celle 

 and the Arve, and partly consists of material from the adjoin- 



* The higher -level gravels in to an encroachment of the sea on 

 some places fringe the (toast at an the land, and the consequent inter- 

 elevation of as much as one hundred section of the old river-beds at a 

 feet ; this phenomenon, however, I higher level, 

 should be disposed to refer partly f Geol. Journal, vol. xxiv. p. 105. 



