CHAP. VII. GEOLOGICAL DESCRIPTION OF THE SOMME VALLEY. 107 



tertiary strata, of which the hxrger part is missing, because 

 their denudation has contributed largely to furnish the mate- 

 rials of gravels in which the tlint implements and bones of 

 extinct mammalia are entombed. From this source have 

 been derived not only the regular-formed egg-shaped peb- 

 bles, so common in the old fluviatile alluvium at all levels, 

 but those huge masses of hard sandstone, sevei'al feet in 

 diameter, to wdiich I shall allude in the sequel. The upland 

 loam also (No. 4) has often, in no slight degree, been formed 

 at the expense of the same tertiary sands and clays, as is 

 attested by its becoming more or less sandy or argillaceous, 

 according to the nature of the nearest eocene outlier in the 

 neighborhood. 



Fig. 7. 



Section across the Valley of the Somme in Picardy. 



1 Peat, twenty to thirty feet thick, resting on gravel, o. 



2 Lower level gravel with elejihants' bones and flint tools, covered with 



iiuviatile loam, twenty to forty feet thick. 



3 Upper level gravel with similar fossils, and with overlying loam, in 



all thirty feet thick. 



4 Upland loam without shells [Limon des ^jZ((#ea«w;), five or six feet 



thick. 



5 Eocene tertiary strata, resting on the chalk in patches. 



The average width of the Valley of the Somme between 

 Amiens and Abbeville is one mile. The height, therefore, of 

 the hills, in relation to the river-plain, could not be correctly 

 i-epresented in the annexed diagram (fig. 7), the hills having 

 been reduced to one-fourth of their altitude. It would other- 

 wise have been necessary to make the space between c and b 

 four times as great. The dimensions also of the masses of 

 drift or alluvium, 2 and 3, have been exaggerated, in order to 

 render them sufficiently conspicuous; for, all-important as we 

 shall find them to be as geological monuments of the post- 

 pliocene period, they form a truly insignificant feature in the 



