1894. THE EVOLUTION OF THE THAMES. 103 



have split into two tongues, one of which kept to the south of the 

 Chalk ridge, as far as St. Albans, Hendon, and Finchley, while the 

 other flowed along the foot of the Chalk escarpment past Hitchin and 

 Dunstable, whence it trended off to the north-west towards Leckhamp- 

 stead. The Chalk hills of this district thus rose above the level of 

 the ice which lay upon their lower flanks. ^ This tongue of the glacier 

 seems to have had but little erosive power and to have flowed only along 

 lines of low land previously in existence. Thus in the valleys of the 

 Brent and the Lea it simply occupied pre-existing valleys without 

 eroding fresh ones. As, moreover, it was apparently unable to sur- 

 mount the Chalk ridge, it does not appear likely that this thin lobe of 

 ice eroded the valley it occupied on the north side of the escarpment. 

 It is much more probable that it there extended so far south owing to 

 the former occurrence of a valley along which it flowed. That is to 

 say that the valley at the foot of the Chalk escarpment, and the escarp- 

 ment itself, are both older than the Boulder Clay. 



We have next to consider the question whether the Thames ever 

 flowed up the valley of the Thame, across the watershed, and down 

 the Ouse into the Wash. Conclusive evidence against this can only 

 be obtained by a detailed study of the distribution of the gravels upon 

 the watershed between the Thame and the Ouse. Until such evidence 

 is forthcoming we are thrown back on two general considerations : in 

 the first place, the great irregularity and sinuous course of this watershed 

 renders it very improbable that it could have been formed by a line 

 of elevation which broke across the former course of a river. Further, 

 the only way in which the Thame could have surmounted the barrier 

 would have been by its waters having stood at a higher level; there is 

 no evidence of any great lake which discharged to the north or of 

 movements of the country which have since lowered the valley of the 

 Thame. Apparently Professor Prestwich's only reason for suggesting 

 this connection between the Thames and the Wash was the difficulty 

 of explaining the formation of the gorge through the escarpment 

 between Moulsford and Pangbourne. But this gorge is simply the 

 southernmost of the series of parallel valleys through the Chilterns. 

 The level of the floor of the gorge is lower than that of the passes 

 through the escarpment further to the north-east, because the height 

 both of the ridge and the passes descend as the former is followed to 

 the south-west. The Thames gorge is so closely analogous to the 

 series of valleys parallel to it, that it appears most probable that it 

 was formed at the same time — viz., before the period of the Boulder 

 Clay. We may summarise, then, the sequence in the Chilterns as 

 follows : — 



2 Mr. Worthington Smith has recently described some drift deposits on the 

 higher parts of the Downs near Dunstable as a "Boulder Clay." This has not 

 been generally accepted as such ; and even if it be glacial in origin, as it contains no 

 northern boulders, and is exclusively composed of local materials, it does not prove 

 that the northern ice-sheet ever occurred on the higher parts of the Downs. 



