THANET BEDS. 17 



the higher divisions of the Eocene, and by a great change in the 

 organic remains. 



It was held by Prestwich as far back as 1852 that the uplift 

 of the Wealden anticline commenced in early Eocene times, and 

 flint -pebbles were formed on the shores during the earliest stages 

 of the period, and further formed or redistributed at successive 

 stages. Other tracts of Chalk must similarly have been upraised, 

 and although it is difficult to decide how much was eroded by 

 marine action during the Eocene period, it is certain that con- 

 siderable thicknesses of Chalk were removed from the London 

 District before the deposition of the Eocene strata, probably 

 in part by subaerial agencies. In 1911 Mr. Dewey discovered 

 pebbles and fragments of Lower Greensand Chert in the pebble- 

 bed which marks the base of the Barton Sands at several localities 

 a few miles to the north of Woking (beyond our western limit). 1 

 This material has not been found in any of the older Eocene 

 pebble beds ; it may therefore be concluded that the Lower 

 Greensand was first exposed to denudation in the Weald during 

 Upper Eocene times. 



The unconformity between the two systems is to some extent 

 indicated by the varying thickness of the Chalk beneath the 

 Eocene covering, and has been clearly demonstrated by detailed 

 studies of the zones of the Chalk. 2 Thus the Marsupites zone 

 which extends over a considerable area in the south-eastern 

 part of our district is not known in the northern part. Moreover, 

 there is no evidence in the district of the Zones of Belemnitella 

 mucronata and Ostrea lunata elsewhere present in England. Still 

 higher zones of Chalk occur in Denmark, Holland and Belgium. 

 Further, it is clear that, before the plain of marine erosion on 

 which the Tertiary strata rest had been cut, the Chalk had been 

 thrown into gentle folds ; in the western part of the area these 

 folds appear to run approximately east and west, 3 but may 

 perhaps swing round to the north under Central London. 4 



The term ' Lower London Tertiaries ' has been used by 

 Prestwich and Mr. Whitaker for the Thanet Sand, Woolwich 

 and Reading Beds, and Blackheath Beds. 



THANET BEDS. 



The Thanet Beds, the oldest of our Eocene strata, are exposed 

 only along the southern and eastern sides of the London District. 

 On the four sheets of the London Map they are coloured pale 

 brown with a yellow stipple, but on the newer maps (Sheets 270 

 and 271) a slate-blue tint, stipples being reserved for pebble- 

 beds. The main outcrop runs from Ewell to Croydon, Farn- 

 borough, along the Cray Valley to Erith, and across the Thames 



1 ' Geology of Windsor and Chertsey ' (Mem. Geol. Surv.), 1915, p. 53. 



2 ' Cretaceous Rocks of Britain ' (Mem. Geol.. Surv.), vol. iii, 1904, p. 172, 

 and references in previous chapter. 



3 ' Geology of Windsor and Chertsey ' (Mem. Geol. Surv.), 1915, p. 14. 



4 Baker, H. A , Geol. Mag., 1918, pp. 296-305. 



