170 



PKOF. J. PRESTWICH ON A SOUTHERN DRIFT IN THE THAMES 



the soft unprotected Tertiary strata on either side have been removed 

 by denudation . That the gravel is of great age is indicated by the 

 extreme v^eathering of the flints, and by the alteration and dis- 

 coloration of their substance ; while although flint-pebbles from 

 the Tertiary are abundant, debris from the Lower Greensand is 

 extremely scarce, as though the denuding agencies had at that time 

 scarcely reached down to the Lower Greensand, a state of things 

 that may be represented by the following diagrams (figs. 6, 7) : — 



Pig. 6. — Theoretical longitudinal Section alone/ the summit of Well 

 Hill (ante p. 157) to theJlanJcs of the Wealden Range. 



Well Hill. 

 600 ft. 



Slope of the Wealden The range of the 

 anticlinal. Lower Grreensand. 



The Chalk 



escarpment. 



700 ft. 



- ~^^^-^Xx^^ 





3. Chalk and Gault. 



4. Lower Grreensand. 



5. Wealden strata. 



1. Well-Hill gravel. 



'2. Lower Tertiary strata. 



The dotted lines show the prolongation of the beds before denudation. 



The transverse section shows the position of the gravel in rela- 

 tion to the Tertiary strata formerly on either side. 



Fig. 7. — Theoretical Section across Well Hill and the old 



Gravel-stream. 



w. 



Valley of E. 



the Darent. 



The upper dotted line shows the position of the removed Tertiary strata. 



The conjecture that at this time much of the Wealden and Lower 

 Greensand were still hidden under a mass of Chalk and Tertiary 

 strata, finds corroboration in the analogous condition of the gravel 

 on some of the other higher and earlier drift-covered hills, where 

 flints from the Chalk, or flint-pebbles from the Tertiary strata, con- 

 stitute the whole or nearly the whole of the mass of gravel, and 

 Lower-Greensand debris is either absent or very scarce. Thus, for 

 example, on Hungary Hill there are but few traces of Lower-Green- 

 sand debris, and in the flint-gravel of the hills near Canterbury it 

 is absent or nearly so. 



