150 



PKOF. J. PRESTWICH ON THE RELATION OF THE 



Pig. 14. — Biaf/ram-sectioii of the Valley of the Lea at Ware 



T i 



^^^:^r^_^ ■'"'^y 



- ~i_^_-^ 







ch if 





'IS 



i 



b" 



c. 



Glaciai :,:ii\Ufi and gravel with 

 Boukler-clav at*. 

 Westleton Shingle. 



T. Lower Tertiaries and London 



Clay. 

 Ch. Chalk. 



A B. Denudation during the Glacial Period. 

 C. Denudation during the Post-Glacial Period. 



the emergence of the Westleton floor and is due to early Glacial 

 action. To what extent the subsequent denudation (B) is due to con- 

 tinued glacial action it is difficult to say ; but as the gravel-bed 

 jS^. 2 (fig. 13) belongs to one of the earlier stages of the Glacial 

 period, and Post-Glacial river-action (C) is limited to narrow bounds, 

 there is reason to infer that the valley to about the base of B is 

 due to erosion during the Glacial period. 



The position of the Westleton Shingle enables us also to suggest 

 a solution of another question, namely, that relating to the age of 

 the great escarpments of the Chalk and Oolites which run through 

 the centre of England. The fact that the Westleton Shingle caps 

 various Tertiary hills on the Chalk-plateau, which Tertiary strata 

 extend to the edge of the Chalk-escarpment, renders it probable 

 that this Shingle had, before the denudation of the Tertiary strata, 

 a range to a certain extent coextensive with that of the Tertiary 

 strata, and extending further northward than at present. This 

 is manifested by the presence of the Shingle on an outlier so near the 

 escarpment as Nettlebed Hill, aud still more by the outlier on the 

 very edge of the escarpment at "Woodcote. Consequently, if, as 

 this would show, the Westleton Beds had a range over and beyond 

 the escarpment, the origin of that escarpment must be of a date 

 subsequent to that of the Westleton Beds (PL VIl. fig. 1). 



Another argument is, I think, conclusive. The terraces of 

 Glacial gravel (No. 2, fig, 13) which flank and originally spread 

 across the gorge of the Thames at Goring at the height of 440 feet 

 are evidently connected with the outliers capping the Coralline 

 Oolite at Poxcombe and Wytham Hills near Oxford, at a height of 

 about 500 feet, and which, again, cap the Porest Marble and Great 

 Oolite on the higher hills of Wychwood Porest, near Witney. At 

 the time that this spread of gravel took i)lace, the intc-vening 

 valleys must have been bridged over by a flooring of Cretaceous 

 and Jurassic strata, which have been since removed and denuded 

 to the depth of from 300 to 350 feet, as shown in the following 

 diagram (tig. 15). 



If, therefore, the channel throngh which the Glacial Drift 

 I'.assed from tlie north into the Thames Valley was formed, as 

 we suppose, subsequently to the spread of the Westleton Shingle, 

 but before the deeper late (jlaeial erosion, then it is evident that 

 as the valley at the foot of and giving rise to the escarpment could 



