362 PHYSICAL GEOGRAPHY OF THE SOMME VALLEY. 



on this subject, and it appears, for instance, that, even so lately 

 as the reign of Queen Elizabeth, the town of Brighton occupied 

 the site of the present Chain Pier. 



The difference between the height of the high-level gravels 

 and the river increases from the source to the sea. For in- 

 stance, in the Seine Valley at the boundaries of La Brie and 

 Champagne it is nothing ; at Paris, 34 metres ; at the sea, 50 

 or GO.* 



FIG. 202. 



ft 





Section at St. Acheul. 



Mr. Prestwich has pointed outf that a section, similar to 

 that of the Somme, is presented by various rivers the Lark, 

 Waveney, Ouse, etc., while it is well shown also along the 

 banks of the Seine. Probably, indeed, it holds good of most of 

 our 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. 

 Prestwich 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 hundred feet above the valley. They seem to me, on the 

 contrary, only the two extremes of a single series, once con- 



* Belgrand, Bassin Parisien. p. 90. t Phil. Trans. 18G4. 



