WOOLWICH AND READING BEDS. 21 



obscured. The freshwater bed has been found in the bed of 

 the Thames at Limehouse Reach, at Dulwich, Peckham, the 

 north end of Elmstead Tunnel, Beckenham, Beddington and 

 Brockley. At the last named locality a seam crowded with 

 Viviparus, Unio, etc., is still (1921) visible in a brickyard beside 

 the Brighton Railway 1 mile south of New Cross Station. The 

 distribution of the plant beds is similar. 



A number of sections at and near East Wickham show how 

 the Blackheath pebble-beds cut into the Woolwich Beds in 

 scoops of varying depths; sometimes the upper sandy beds of 

 the latter are present up to a thickness of 15 feet, while in a 

 neighbouring section both these and the shell-beds are cut out, 

 leaving only a varying amount of the Bottom Bed present. 1 

 The same phenomenon is well seen in the Erith sand pits, where 

 14 feet of shell-beds found at the south-east are completely cut 

 out at the west end ; 2 in some parts of the pit the strata below 

 the shell-bed consist of sand which can only be distinguished 

 from Thanet by the presence of the Bottom Bed between the 

 two. 



On the eastern margin of the area, between Greenhithe and 

 Green Street Green the Bottom Bed is ferruginous, and appears 

 to have been worked as an iron-ore in Lord's Wood. The 

 shell-bed is sometimes very prolific and besides the usual forms 

 yields the freshwater univalve Neritina, some specimens retaining 

 the original colour-stripes. 



Under London, on the northern side of the Thames, the 

 strata belong for the most part to the Reading type, and, as 

 noted by Mr. Barrow, they consist in the upper part of mottled 

 clay occasionally divided by a band of loam and sand, the 

 united thickness in one place between Euston and Hampstead 

 Road being as much as 58 ft. ; underneath these occur about 

 20 ft. of green sands with pebbles and clays and then 18 ft. of 

 sand ; the lower 12 \ ft. are referred to Thanet Sand. 3 It is difficult 

 to decide from records of shafts and borings where the junction 

 of Reading and Thanet Beds should be taken ; nor is it always 

 possible to decide in open sections, as at Ewell and Erith. 



The green sands and clays in the Rotherhithe Tunnel included 

 at the base a bed with Ostrea tenera, 4 ft. thick, while the shell-bed 

 contained a seam of limestone and a lenticle of peat. 



Mr. Barrow observed that the junction with the London 

 Clay in the Tube Railways north of the Thames was clear and 

 sharp ; but occasionally the beds are faulted (see p. 39). 



In certain areas the Woolwich and Reading Beds immediately 

 underlie the river deposits. At Deptford the Chalk reaches 



1 Leach, A. L. and B. C. Polkinghorne, ' Excursion to East Wickham and 

 Bostall Heath,' Proc. Geol. Assoc., vol. xix, 1906, pp. 341-347. 



2 Chandler, R. H., and A. L. Leach, ' Excursion to Erith,' Proc. Geol. Assoc. 

 vol. xxiii, 1912, pp. 183-185. 



3 * Summary of Progress for 1905 ' (Mem. Geol. Surv.), 1906, pp. 168-170. 



