138 PEOF. PRESTWICn ON THE AGE, FORMATIOX, AND 



then likewise effective agents in the denudation of this adjacent 

 district. Before, therefore, the deposition of this river-drift, a valley 

 of considerable width and 200 to 300 feet deep had been excavated 

 between the Lower-Greensand hills and the adjacent Chalk plateau, 

 by which the future Chalk escarpment was first brought into relief. 

 This channel (which is on the line of the Gault) was subsequently 

 worn deeper, and the escarpment loomed higher when, in later Glacial 

 and post-Glacial times, river- and flood-action played a more promi- 

 nent part. 



But the present valley-channels at the base of the Chalk escarp- 

 ment are not continuous along the whole length of the escarpment. 

 They each have relation to the several rivers which drain the 

 Wealden area, and each has its culminating point or watershed 

 intermediate between these lines of escape. Thus the Darent, 

 which drains into the Thames, is separated from the adjacent valleys 

 draining into the Med way by narrow watersheds projecting as low 

 ridges between the Chalk escarpment and the Lower-Greensand 

 hills. This is a feature common throughout the great primary valley, 

 which extends all along the base of both the North and South 

 Downs, and is one of the many conclusive arguments against the 

 marine origin of the escarpment, as that would necessitate a uniform 

 level line for its base, whereas the line forms a succession of rises and 

 falls (see line mn^ fig. 3, PI. VI.). Amongst the most remarkable 

 of these watersheds or cols is the one which intervenes between the 

 Darent and the Oxted stream — a small tributary of the Med way — 

 at Limpsfield. Whilst in general these passes rarely rise to the 

 height of more than 200 or 300 feet, in the Limpsfield case the 

 summit of the pass which connects the Chalk range with that of the 

 Lower Greensand attains a height of rather more than 500 feet 

 above O.D. The relation of the pass to the adjacent ranges and 

 river-basins is shown in the sections on the next page, figs. 5 and 6. 



The position of the gravel on the watershed is so equally balanced 

 between the Darent and the Oxted Valleys that, independently of other 

 evidence, it would be difficult to decide to which of the two it belonged. 

 But, as I shall have occasion to explain farther on, it shows so close 

 a relation with other beds of gravel lower down the Darent Valley 

 that I quite concur with Mr. Topley * in placing it in that valley- 

 system, and we can only suppose that the original ridge separating 

 the two valleys, of which Westheath Hill, w^hich is 516 feet high 

 and bare of drift, may be a remaining portion, has been removed by 

 denudation subsequentl)' to the deposition of the gravel. 



The character of the Limpsfield gravel is very distinct. It con- 

 sists altogether of debris from the Tertiary strata and the Clialk, 

 with the exception of a small portion derived from the substratum 

 of Lower Greensand. A fine section of it is exposed in the old pit 

 on the north side of Limpsfield Common. It is there from 8 to 10 

 feet thick, and is composed of the following materials — given in 

 the order of their relative abundance — embedded in loam and clay 



* Topley, ' Geology of the Weald,' pp. 193 and 289. 



