OWING TO THE EXCAVATION OF THE VALLEY. 303 



would accumulate on the high grounds in the form of ice and 

 snow, and would every spring produce floods much greater 

 than any which now occur. 



We now come to the light- colored sandy marl (fig. 138 c). 

 It is described by Mr. Prestwich as follows : " White siliceous 

 sand and light- colored marl, mixed with fine chalk grit, a 

 few large sub-angular flints, and an occasional sandstone 

 block, irregular patches of flint gravel, bedding waved and 

 contorted, here and there layers with diagonal seams, a few 

 O3hreous bands, portions concreted. Sand and freshwater 

 shells common, some mammalian remains." 



In the pits at Amiens this bed is generally distinct from 

 the underlying gravels, owing perhaps to the upper portion 

 of the gravel having been removed ; but in several places 

 (Precy, Ivry, Bicetre, etc.) this section is complete, the 

 coarser gravel below becoming finer and finer, and at length 

 passing above into siliceous sand. These sections evidently 

 indicate a gradual loss of power in the water at these parti- 

 cular spots ; rapid enough at first to bring down large pebbles, 

 its force became less and less until at length it was only able 

 to deposit fine sand. This, therefore, appears to indicate a 

 slight change in the course of the river, and gradual exca- 

 vation of the valley, which, by supplying the floods with a 

 lower bed, left the waters at this height with a gradually 

 diminishing force and velocity. 



The upper part of the section at St. Acheul consists of 

 brick earth (fig. 131 a), passing below into angular gravel, 

 while between this and the underlying sandy marl is some- 

 times a small layer of darker brick earth. These beds, how- 

 ever, vary much even in adjoining sections. Taken as a 

 whole, they are regarded by Mr. Prestwich as the represen- 

 tatives of that remarkable loamy deposit which is found 

 overlying the gravels in all these valleys of Northern France, 

 and which, as the celebrated " loess " of the Rhine, attains a 



