18 LONDON DISTRICT. 



to Purfleet. South and east of this line there are extensive 

 outliers, north and west an outcrop along the river cliff from 

 Lewisham to Erith and inliers near Bromley. Underground 

 the beds thin- out to the north-west, the limit running approxi- 

 mately through Hendon, Baling, Sunbury and Wey bridge. 



The Thanet Beds consist mainly of fine sand, passing down 

 into a silt, and attain a thickness of nearly 70 feet in this district ; 

 in the most westerly outcrop near Ewell they do not exceed 

 1 5 feet. In colour they are buff or pale greenish-grey and speckly, 

 but often they appear very pale and, at a distance, almost as 

 white as chalk. 



At the base there is a layer of green loam with green-coated 

 flints known as the ' Bull Head Bed ' ; though worn or corroded 

 flints occur here and there the majority are unworn. Where 

 the Bull Head Bed happens to coincide with a layer of flints in 

 the Chalk, the nodules may be green-coated above and unaltered 

 below; at Stone, near Dartford, some of them appear to have 

 been shattered and re-cemented by secondary flint. 1 The green 

 colour is due to silicate of iron. The green sand at the base 

 consists of quartz 45 per cent., flint 20, glauconite 15, together 

 with felspar, magnetite, spinel, zircon, rutile, tourmaline, etc. 

 In the higher beds the percentage of flint falls to about 5. 2 



The junction with the Chalk is usually even and seemingly 

 conformable ; occasionally the Thanet Beds rest in widely 

 scooped hollows in the Chalk, 3 and near the present margin of 

 the deposit may descend into ' pipes ' ; both phenomena are, 

 however, due to solution of the Chalk in post-Eocene times. 



The Thanet Sands are mainly of marine origin, though it is 

 possible that the distinction between the fluviatile, estuarine 

 and marine phases of the Reading and the Woolwich Beds (see 

 below) is foreshadowed in the upper part of the sands. 4 



Few fossils have been found in these strata in the London 

 District, and they are poorly preserved, being mostly in the 

 form of casts. They include remains of fish, the bivalve shells 

 Cyprina and Pholadomya and the univalve Pyrula. The beds 

 can be assigned to the zones of Pholadomya konincki and 

 Cyprina morrisi in the Lower Landenian of continental authors. 5 



The Thanet Sands are exposed in road -cuttings and pits at 

 Beddington, in railway -cuttings between St. Mary's Cray and 

 Farningham Road, at Dartford, and in extensive pits at Cray- 

 ford, Erith and Charlton ; they stand firmly in almost vertical 

 faces. The beds are well seen at Purfleet, and in neighbouring 

 parts of Essex; the sand is occasionally indurated into masses 

 like grey wether or sarsen stone. 6 



1 Priest, S., Proc. Geol. Assoc., vol. xxvi, 1915, pp. 81-85. 



'- Gardiner, Miss M. I., * The Greensand Bed at the Base of the Thanet 

 Sand,' Quart. Journ. Geol. Soc., vol. xliv, 1888, pp. 755-760. 



3 Stamp, L. D., and S. Priest, ' Geology of the Swanscombe Outlier,' Proc 

 Geol. Assoc., vol. xxxi, 1920, p*. 188. 



* Baker ; H. A., Geol. Mag., 1920, p. 420. 



6 Stamp, L. D., op. tit., p. 190. 



6 Hinton, M. A. C., and A. S. Kennard, Essex Nat., vol. xi, 1901, p. 342. 



