GRADUAL EXCAVATION OF THE VALLEY. 373 



water to a greater height than at present, and this also would 

 have rendered floods more frequent and more severe than at 

 present. 



Eeturning to the fig. given on p. 3*71, we now come to the 

 light-coloured marl (fig. 202 c). Mr. Prestwich describes it 

 as follows : Of white siliceous sand and light-coloured 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 ochreous 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 particular spots ; rapid 

 enough at first to bring down large pebbles, its force became 

 less and less, until at length it was only able to carry fine 

 sand. This, therefore, appears to indicate a slight change in 

 the course of the river, and gradual excavation 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. 202 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 representatives 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 Ehine, attains in some places a 



