ST. ACHEUL. 287 



rivers, that along the sides of their valleys are patches of old 

 gravels left by the stream at various heights, before they had 

 excavated the channels to their present depth. Mr. Prest- 

 wich considers that the beds of sand and gravel can generally 

 be divided into two more or less distinct series, one continuous 

 along "the bottom of the valleys and rising little above the 

 water level these he calls the low level gravels ; the other, 

 which he terms the upper or high level gravels, occurring 

 in detached masses at an elevation of from fifty to two hun- 

 dred feet above the valley. They seem to me to be only the 

 two extremes of a single series, once continuous, but now 

 generally presenting some interruption. A more magnified 

 view of the strata at St. Acheul, near Amiens, is shown in 



FIG. 140. 



Section at St. Acheul. 



fig. 140. The upper layer of vegetable soil having been 

 removed, we have 



1. A bed of brick earth (a) from four to five feet in thick- 

 ness, and containing a few angular flints. 



2. Below this is a thin layer of angular gravel (5), one to 

 two feet in thickness. 



3. Still lower is a bed of sandy marl (c), five to six feet 

 thick, with land and fresh water shells, which, though very 

 delicate, are in most cases perfect. 



4. At the bottom of all these, and immediately overlying 

 the chalk, is the bed of partially rounded gravel (d) in which 



