42 LONDON DISTRICT. 



It is hardly possible that the chert and the quartz pebbles 

 should have reached their present position from opposite direc- 

 tions. It has, however, been mentioned that the Barton Pebble 

 Bed contains chert and that it transgresses the under-lying 

 divisions (p. 33). It seems reasonable to suppose that this 

 bed originally extended north of the Thames and, if so, contri- 

 buted at its destruction to the formation of the gravels. The 

 present limit of the chert, running through what appear to be 

 relics of a single deposit, would correspond approximately with 

 the margin of the Barton sea. The former extension of Upper 

 Eocene beds over this area is also suggested by the occurrence 

 of Venericardia planicosta, a Bracklesham fossil shelJ, in the 

 Boulder Clay at Finchley (p. 45). 



Whether these beds are of Pliocene or later age cannot be 

 definitely proved in the absence of fossils ; it may be mentioned 

 that the late Clement Reid, whose knowledge of the Pliocene 

 Beds was unrivalled, searched the district for traces of such beds 

 and failed to find them. 1 He was inclined to regard the Stanmore 

 gravels as of Bagshot age, comparing them with undoubted 

 Eocene gravels in the Hampshire Basin and with the pebble bed 

 at Highgate which forms the base of the Bagshot Sand (p. 34) 2 ; 

 but the Highgate Bed occurs also at Hampstead at the base of 

 the Bagshot, while small patches of the high level gravel (not 

 shown on the map) are found there resting on that division. 



Owing to the large percentage of Eocene flint pebbles, 

 Mr. Whitaker calls these deposits the ' Pebble Gravel ' and heads 

 his description of them ' Pre-Glacial ( ?)', 3 That they are prior 

 to the advance of an ice-sheet into our area is certain, but opinion 

 is still so much divided that we can as yet say nothing more 

 definite. In the forthcoming Memoir on the country around 

 Beaconsfield (Sheet 255) the term ' Pebble Gravel ' is used. 

 Prestwich's term ' Westleton Beds ' included such varied deposits 

 that it has been generally abandoned. 



In Essex no re-survey has as yet been made, and it is difficult 

 to distinguish the ancient gravels from those associated with the 

 Boulder Clay on the one hand and from Bagshot pebble beds on 

 the other. The latter have to some extent been re-arranged by 

 surface-slipping and no hard and fast line can be drawn between 

 those which are practically in situ and those which should be 

 regarded as a new deposit. Where pebbles of Bunter quartzite 

 and other foreign rocks have been introduced, we have good 

 reason for calling the deposit Glacial, but in many old accounts 

 the term ' quartzite ' probably implies sarsen. 



A gravel which should certainly be included in the High Level 

 Series occurs at Lambourne End, and at High Beech, where it 

 attains an altitude of 362 ft. and consists of loam with pebbles 

 mostly of flint, and a few of quartz. 



1 Quart. Journ. Geol. Soc., vol. Ixxv, 1920, p. 47. 



2 ' Summary of Progress for 1899 ' (Mem. Geol. Surv.), 1900, p. 140. 



3 Guide to the Geology of London ' (Mem. Geol. Surv.}, Ed. 6, 1901, p. 64. 



