THANET BEDS. 429 



The following subdivisions in the Thanet Beds have been noted 

 by Mr. Whitaker : ^— 



(e) Fine sharp light-grey sand, slightly greenish, often iron-shot, with layers 

 of calcareous sandstone here and there ; the fossils sometimes silicified ; 

 thins eastwards, and is almost confined to East Kent, where it attains 

 al^out 40 feet in thickness, and passes down into — 



{(f) Bluish-grey sandy marl, weathering to a pale yellowish-gi'ey, often rather 

 hard, with green grains and fossils, more sandy at top ; thins westward, 

 and is almost confined to East Kent, where it is the thickest member of 

 the series near and beyond Canterbury. 



(r) Fine light-buff sand, mostly soft, with few fossils (only some very obscure 

 remains have been found) ; it is thickest in West Kent (up to 60 feet or 

 more), where for the most part it forms nearly the whole of the Thanet 

 Beds, thinning out westward in Surrey, and eastwards in Kent. 



(d) Alternations of brown clay and loam, without fossils, thin and local (in 

 part of East Kent). 



(a) The 'base-bed,' clayey greensand, with unworn green-coated flints, 

 resting on the Chalk, thin (rarely over 5 feet) but constant. This is 

 called the ' Bull's Head bed ' by workmen. 



In every large section the junction between the Chalk and Thanet Beds is even ; 

 where, however, there is but a thin capping of the latter it often fills ' pipes,' 

 irregular-shaped hollows, that have been formed since the deposition of the beds, 

 by the infiltration of water holding carbonic acid in solution. The layer of green- 

 coated flints at the base is considered to have been formed after the deposition of 

 the Thanet Sand by the decomposition of the top of the Chalk.- 



In the neighbourhood of the Bedwins (in Wiltshire), near Savernake Forest, 

 and also in Berkshire, Mr. Whitaker has noticed in many places, close by the 

 junction of the Reading Beds and the Chalk, a bed of reconstructed Chalk.* 

 This bed, which in places may be 20 feet in thickness, contains blocks of chalk, 

 scattered lines of flint, and much rubbly chalk ; and it may possibly have been 

 formed at the period when the Thanet Sands were elsewhere deposited. 

 A similar bed occurs in a pit north-east of Northaw, in Hertfordshire. 



The Thanet Beds are a marine deposit, formed probably in a 

 shallow sea open to the north.-* They yield remains of Mollusca, 

 Polyzoa, Crustacea, Entomostraca, Echinodermata, Sponges, Fora- 

 minifera, and Plants. Among the more prominent species are the 

 Mollusca Cyprina Morrisii, Pholadomya cuneata, P. Koninckii, 

 Cytherea orbicularis, Aslarte tenera, Nucula Bowerbankii, Panopcva 

 gramilata, Thracia oblata, Ostrea bellovacina, Corbula Regulbiemis 

 (of which a band occurs near the junction with the Woolwich Beds 

 at Heme Bay), Scalaria Bozverbafikii, Dentalium nitcns, Natica 

 subdepressa, and Aporrhais Soiverbyi. The fern Osmunda {Osmundiies) 

 Dowkeri has been obtained at Heme Bay. 



The beds are well exposed in the cliffs of Pegwell Bay, and 

 those east of Heme Bay,^ also in pits near Upnor and Erith, 

 at Charlton near Woolwich, by the S.E. Railway near Westcombe 

 Park, Greenwich (see Fig. 74), at Loam Pit Hill, Lewisham 



^ Geol. London Basin, Mem. Geol. Survey, iv. 56. 



2 Whitaker, Q. J. xxii. 404 ; T. McK. Hughes, Ibid. 402 ; see also G. Dowker, 

 G. iMag. 1866, p. 210. 



3 Q. J. xvii. 527. 



* Prestwich, "The Ground Beneath Us," 1857, p, 71 ; see also J. S. Gardner, 

 Q J. xxxix. 203. 



* Prestwich, Q. J. viii. Plate xv. 



