172 PROF. J. PRESTAVICH ON A SOUTH.EEN" DRIFT IN THE THAMES 



had been removed, and when the valley was prolonged in a southern 

 direction into the Wealden area, and the drainage brought down the 

 usual complement of Lower-Greensand debris from the hills beyond 

 the Chalk-escarpment. 



At this stage, some disturbance connected with the Central 

 Wealden area diverted the drainage in another direction, cutting off 

 the upper part of the valley, denuding the face of the Chalk-escarp- 

 ment, and leaving the truncated end of the valley at a height of 174 

 feet above the stream at the foot of the escarpment (h, fig. 9) which 

 now drains into the Mole. The bed of the valley at the point where 

 it is cut off is 438 feet above O.D., while the hills on either side 

 rise nearly 200 feet higher, showing the valley to have been in an 

 advanced stage, but still 200 to 300 feet short of the depth subse- 

 quently acquired by that of the Medway or of the Darent. 



The date of this old transverse valley is, however, far removed 

 from that which has left its traces on the summit of Well Hill, inas- 

 much as at the latter date the Tertiary strata had not been denuded 

 from the surface of the Chalk floor, whereas at this time not only 

 had the Tertiary strata been denuded from the Chalk-plateau, but 

 the Chalk itself had become channelled to a considerable depth. The 

 position of the valley in its longitudinal course at the time of its 

 arrested growth, when it had its origin in the Wealden area, is 

 shown in the following diagram : — 



Fig. S. — Course of the Valley of Smitham Bottom^ restored. 



^' 



The W^ealden Range of the 



anticlinal. Lower G-reensand. The North Downs. 



a. Level of the A-alley of Smitbam Bottom through the Cbalk-hills. 



a'. The same proloDged beyond the chalk -hills southward before the 



denudation of the area. 

 3. Chalk. 4. Lower Greensand, 5. Wealden. 



The section transverse to the above showing the intersection of the 

 valley in the face of the chalk escarpment is as under (fig. U). 



The gravel at Croydon, which belongs to the earlier stages of the 

 valley, is not that on the lower levels, which is of Post-Glacial age, 

 but that which forms the higher banks on Duppa's Hill on the west, 

 and Park Hill on the east of the town. 



We will now revert to the earlier streams of Lower-Greensand 

 Drift, described in pages 156-162, and connected with the pri- 

 mary streams or torrents flowing from off the slopes of the Wealden 

 highlands and over the Chalk-plateau. At this period the Wealden 



