1865.] FOSTEE AND TOPLEY DENTJDATIOjST OE THE AYEALD. 473, 



In the case of the "Weald we have a long escarpment formed by 

 the Chalk, and another by the Lower Greensand. We have already 

 spoken of the many objections to their marine origin. There re- 

 mains then only pure atmospheric denudation to account for these 

 escarpments ; and as we have what we consider proof that the 

 Medway has deepened its valley 300 feet, we are not afraid o£ 

 ascribing great effects to such a cause as atmospheric denudation. 

 It must not be inferred, however, that we consider the escarpments 

 to be river-cliffs. The longitudinal streams, though running pa- 

 rallel to these escarpments, do not run directly below them, but 

 often, as with the Medway itself, at a considerable distance. No 

 river-gravel in this area is ever found on the face of the escarpment ; 

 nor can we discover thereon any traces whatever of river-action. 

 We have no reason then to ascribe them to the immediate action of 

 the streams. 



The manner in which we consider the denudation of the Weald to 

 have taken place is as follows. After a large portion of the Terti- 

 ary and Upper Cretaceous strata, with some of the Lower Cretaceous 

 beds, had been removed by marine denudation*, a comparatively plane 

 surface was formed, which gradually appeared above water ; probably 

 the centre of the Wealden area rose out first, forming an island, and 

 then as the land rose a spread of country was formed sloping down 

 to the north and south from an east and west ridge. The central 

 ridge determined the flow of the water that fell upon the area, 

 streams began to flow to the north and to the south, and in this 

 manner the transverse valleys of the Wey, Mole, Darent, Stour, 

 Cuckmere, Ouse, Arun, and Arun were first started. At the same 

 time the longitudinal valleys along the strike were formed, on ac- 

 count of the difference in hardness between the various rocks. The 

 moderately liard porous Chalk has suffered less than the soft imper- 

 vious Gault, and the hard porous Lower Greensand has been less 

 denuded than the soft impervious Weald Clay. As we are dealing 

 with limestone beds, we must take into consideration the chemical 

 action of the rain charged with carbonic acid. The top of the Chalk 

 and Kentish Rag certainly suffer from this action, and their general 

 level is gradually being lowered. The mechanical atmospheric 

 denudation, however, exceeds the chemical denudation, and, in 

 spite of the general lowering of the Chalk and Kentish Eag, they 

 still form escarpments. 



CoNCLtrsioN. 



In conclusion, we wiU revert to the main points discussed in this 

 paper. 



After describing the gravel of the Medway valley, we have endea- 

 voured to prove that an old river-gravel of the Medway occurs 300 

 feet above its present level. We have then shown that, if this 

 fact be admitted, it follows that so large a denudation has been 

 effected by rain and rivers that there can be but little difficulty in 



* The term "plane of mai'ine denudation" was first used by Prof. Kamsay 

 (see Brit. Ass. Eep. 1847, Trans. Sects, p. 66). 



