278 PHYSIOGRAPHY. [CH. xvii. 



that below those deposits of soil, which show, by the charac- 

 ter of their embedded remains, that they are of Roman date, 

 there is first a thia layer of clay, and then a deposit of gravel, 

 F. These lower beds did not yield, at this locality, any 

 relics of human workmanship ; and, indeed, the explorer 

 Avho examines them, soon finds that he is dealing with 

 deposits in which neither Englishman, Roman, nor Briton 

 has left any mark. 



The gravel exposed in this section forms part of a wide- 

 spread sheet, which covers a large portion of the valley of 

 the lower Thames. Its range in the neighbourhood of tlie 

 metropolis is shown on Fig. 8i ; which is a map, giving the 

 area of the superficial deposits, or " drifts," as they are 

 sometimes called,' in the Thames valley between Kingston 

 and Wool»vich. The dotted part shows the gravel as 

 exposed at the surface. This gravel consists, principally, of 

 rounded and subangular pieces of flint, derived from the 

 chalk, the spaces between the stones being mostly filled with 

 sand. I'he origin of so vast a deposit of gravel is by no 

 means clear _; but there is reason to believe, that much of it 

 is an old river-gravel, formed by the Thames when the river 

 flowed at a greater height ; and, probably, with much larger 

 volume, than at the present day. 



Along the banks of the Thames, and of most other rivers, 

 it is not uncommon to find successive terraces of gravel, 

 which mark the height at which the river flowed at diff'erent 

 periods. Thus in Fig. 82 (which is a section from Wimble- 

 don to Wandsworth Common across the river Wandle, 

 similar to that given on p. 139) it is believed that the river 

 (R) ran at one period, over the higher terrace, No. i, and 



^ So called because it was formerly believed that such deposits had 

 been driven over the surface by great floods. The word is conveni- 

 ently retained without any reference to the origin of such deposits. 



