154 EARLY MAN IN BRITAIN. [CHAP, vi 



the arctic division farther down to the south as the cold 

 increased, and, as it waned, to allow of their coming 

 northwards again. 



The Late Pleistocene River-deposits. 



Superficial deposits containing the remains of the 

 late Pleistocene animals swept down by the floods are 

 present in most of the river-valleys in the eastern, 

 ^ central, and southern portions of Great Britain. They 

 consist of sand, gravel, and loam, or brick earth, and are 

 found sometimes below the level of the present streams, 

 as, for example, in the lower Thames in the neighbour- 

 hood of London, or, as is usually the case, on the sides 

 at various elevations above the water-level. They are 

 proved by the presence of shells of fresh-water mussels, 

 fresh-water snails, and others, to 'have been accumulated 

 at the bottom of rivers which have cut their way down 

 to, or below, their present level since they were formed. 

 From this we may conclude that the present system of 

 hill and valley was then sketched out, but that in some 

 places the valleys have been considerably deepened by 

 the erosive action of the stream ; while in others their 

 lower parts have been filled up in the late Pleistocene age. 



Sometimes the river flowed over what is now the top 

 of a hill or the top of a cliff, as in the case of Bemerton 

 (Fig. 36) and the fluviatile strata on the cliffs near South- 

 ampton Water, described by Mr. Codrington, and those 

 of Eeculver, described by Professor Prestwich. 1 



It must not, however, be supposed that the rivers in 

 any case occupied the whole of the valley at one time. 

 The swinging of the current from one side to the other, 



1 Quart. Journ. Geol. Lond. xvii. 364 ; xxvi. 528 et seq. 



