Geology. 13 



The thinly bedded orange-coloured, brown and yellow sands 

 (b and c), with alternating clays, make a very striking section in the 

 upper part of the pit. Further notes on the East Wickham Valley 

 sections are given in Proc. Geol. Assoc., vol. xix., part 9, 1906. 



TJwnet Sand. 



The lowest division of the local Tertiaries is well exposed at 

 Charlton, Wickham Lane, Erith, Chiselhurst, and in many of the 

 pits near Crayford. Wherever it is seen in section with the Chalk 

 a very marked feature is the constant occurrence, at its base, of 

 a layer of green-coated flints, often very large, embedded in a dark 

 sandy and ferruginous clay, which rests immediately on an almost 

 even surface of chalk. The flints in this base-bed, or " Bull- 

 head," are very rarely abraded. A very few ovoid pebbles 

 occur, but the bulk of the flints retain the characteristic outlines 

 of fresh chalk-flints The formation of this bed of green-coated flints 

 at the base of the Thanet Sand is now generally admitted to be due 

 to the solvent action of percolating water charged with carbonic 

 acid. The chalk has passed away in solution, leaving, as an 

 insoluble residue, its flints, together with the ferruginous and 

 clayey matter, which forms a considerable part of the Bull- 

 head. 



For a few feet above its base the Thanet Sand is usually tinged 

 green by glauconitic matter. The bulk of the deposit is a fine- 

 grained, light-coloured, clean quartz-sand, so firmly bedded as to 

 stand upright in section even when exposed to the weather for 

 years. In thickness it vares from 40 feet at Charlton to 5(3 at 

 the sand-pit in King's Highway. Although a marine sand, it rarely 

 yields fossils in this locality. At Charlton PHOLADOMYA FICOIDEA 

 and PHOLADOMYA MARGARITACEA occur in the form of internal 

 sand-casts from which every trace of the shell has disappeared. At 

 Plumstead Station, in 1887, Mr. J. G. Goodchild found CARDIUM, 

 CYPRINA and other shell? in a hardened mass of sand (8). 



An analysis of Thanet Sand will be found in the " Geology of 

 London." It is very largely used in glass and pottery manufacture 

 and as a moulding-sand. Woolwich Arsenal is said to owe its 

 establishment to the presence of this sand, which was found highly 

 suitable for iron-moulding. 



In addition to the exposures already mentioned, good sections 

 may be seen in the numerous pits bordering Wickham Lane, and at 

 several places on the western slope of the Cray Valley. The outcrop 

 of the Thanet Sand is usually too narrow to exhibit any special 

 scenic features, but where the roads from Chiselhurst to the Cray 



(8) See Geology of London, Vol. I., p. 108. 



