63 



For a long period the Medway has been encroaching 

 on and sapping the basin of the Darenth, it is doing so 

 now, and probably its small Wealden basin is being 

 robbed much faster than the retreat of the chalk crest is 

 increasing its area. 



The dry valley which lies in the chalk between Mers- 

 tham and Croydon represents a river course which has 

 received a surprising curtailment from the South and from 

 the North ; the system of branches on either the East or 

 the West of it is similar in most respects to other streams, 

 such as the Darenth and Medway in a similar situation, 

 and offers apparently no obstacles to the belief that once 

 this valley called Smitham bottom carried an open stream. 

 As, however, the denudation from the South advanced, the 

 Wealden drainage was curtailed, and the valley was not 

 able to keep up the excavation of the channel, and when 

 at last the greensand escarpment was undermined, the 

 stream became fitful, an ailbourne, in fact, until the denu- 

 dation reached a point at which even that ceased. The 

 curtailment from the South was mainly accomplished by 

 the enlargement of the basin of the Mole, assisted by the 

 Old Darenth, both at the expense of the Old Wandle. 

 Some of the tributaries of the present Mole whether run- 

 ning North or South still indicate somewhat the lines of 

 the Old Wandle, and the probable sites of its breaches in 

 the greensand crest. 



At a later period the Medway by its branch, the 

 Eden, cut back and usurped that part of the Darenth which 

 had formerly occupied the area of drainage of the Caterham 

 branch of the Old Wandle. The Mole which disputes with 

 the Medway at the present day the old drainage area ot 

 the extinct stream, drains a considerable area to the South 

 and Eastward of its gorge in the chalk, after passing 

 which it inclines Westward towards the Wey. 



The Wey now occupies as its area of drainage a very 

 large part of the North-Eastern portion of the Weald, 

 almost from the gap made by the Mole in the chalk crest 



