Geology. 15 



beds rest on the bottom-bed of the Woolwich loams, which are 

 barely 10 feet thick, while in the opposite brickfield, 200 yards 

 away, at least 40 feet of Woolwich Beds underlie the Blackheath 

 pebbles. 



In thickness the Blackheath Beds are the least considerable of 

 the local Tertiaries, yet their highly permeable nature has enabled 

 them to resist denudation to such an extent that they cover fully 

 two-thirds of the area and impart to the scenery and vegetation a 

 characteristic aspect. The dry, pebbly heaths and commons of 

 E'lumstead, Chiselhurst, Keston, Hayes, Bostal Heath, and Black- 

 heath derive no small part of their beauty from the silver birches, 

 Scotch firs, heather, gorse, and broom which thrive upon the Black- 

 heath pebble-beds. 



In addition to the sections already noticed, Blackheath Beds are 

 well displayed at the following places : 



Plumstead Common (N.E. corner) : about 20 feet of light-coloured 

 sands, capped by a thick pebble-bed, rest on a denuded surface of 

 Woolwich clays. A few shells occur, mostly broken. Many of the 

 pebbles, as was first noticed by Mr. Chandler, are dented by mutual 

 pressure ; some are even crushed, others show white spots at the 

 points of contact. The pittings and white spots may be due to 

 abrasion during slow movements caused by expansion and con- 

 traction of the beds under changes of temperature and of saturation ; 

 but this hardly seems to explain the crushing of tough flint- pebbles. 



Bostal Hill (roadside-section) : sands and pebbles. 



Belvedere : section of the pebble-bed. 



Keston : several good sections on the Common. 



Probably the finest and most interesting section was that shown 

 during the construction of a new road from Well Hall into Eltham 

 in June, 1905. Only the decalcified shell-bed can now be seen, and 

 this is rapidly becoming overgrown. The details given below were 

 noted when the section was open to the bottom of the sewer- 

 trenches. 



Soil passing into brownish-red sand with pebbles, 13 feet. 



Irregular bed of coarse pebbly sand with crushed shelly material, 

 all showing signs of movement after partial decalcification. The 

 lower part consists of a band of OSTBEA (chiefly O. TENERA), 2 4 

 feet. 



Pebbly sand with OSTREA, CYRENA, etc., 2 feet. 



Shelly sand showing false-bedding, 2 feet. 



Strongly marked conglomerate of pebbles and OSTREA, 2 feet. 



Light-coloured shelly sands, false-bedded, 3 feet. 



Very hard conglomerate with OSTREA BELLOVACINA, 2 feet. 



