168 PKOF. J. PEESTWlCn ON A SOUTHEEN DRIFT IN THE THAMES 



Yig, 5. — Section of the Clialh-escaiyments above Lenham^ sliowimj 

 the former extension of the Levham SanrJs. 



1. Lenham Eeds (early Pliocene). 



2. Chalk. 3. Gaiilt. 

 4. Lower Greensand. 



a. Sandpipes in the Chalk. 



a'. The doited lines represent the former extension southward of the Lenham 

 Beds oyer the Chalk and Wealden. 



the Wealden area and over a part of ^""orth- western France and of 

 South-western Belgium, before the elevation and denudation of 

 those areas and the excavation of the Straits of Dover. 



As neither the White nor the Red Crag of Suffolk extend over 

 the Lenham Sands, while the sand-pipes could only have been 

 formed after the emergence of the latter and a lengthened exposure 

 to atmospheric agencies, it is evident that the east and west elevation 

 of the Wealden area must have taken place at or soon after the 

 White-Crag epoch, possibly at the insetting of the Red-Crag epoch. 

 The effect of this emergence was to bar the Crag sea of the Eastern 

 Counties on the south, and to establish a land-connexion with the 

 continental area along the line of the Wealden and Boulonnais 

 anticlinal, which then, with that of the Ardennes, probably formed 

 a continuous range. 



The final elevation of the broad anticlinal dome of the Weald was 

 therefore subsequent to the deposition of the Lenham Sands, and 

 it must have been after that time that the excavation of the valleys 

 V, fig. 5, commenced. Hence we may conclude that the denudation 

 of the Weald, as generally understood, is of a date subsequent to the 

 early Pliocene period, that is to say, after the emergence of the 

 Lenham or Diestian Beds, just as we have shown that the exca- 

 vation of the valleys of the Thames district took place after the 

 elevation of the Westleton Shingle, or subsequent to the early 

 Pleistocene period. 



5. Origin of the Sovthern Drift. 



The foregoing conclusion respecting a Wealden barrier in Pre- 

 Glacial and Glacial times is confirmed by the circumstance that the 

 component materials of the Southern Drift consist exclusively of 

 debris derived from rocks in the Wealden area, or from those which 

 formerly extended over it. The deep valley (v, fig. 5) now separating: 

 the range of the Chalk-Downs from the range of the Lower Greensand 

 could, for a long time, have had no existence, for otherwise the 

 debris of chert and ragstone of the Lower Greensand could not 



