1852.] PRESTWICH ON THE THANET SANDS. 241 



about 50 to 60 feet. In some parts of the neighbourhood of Can- 

 terbury they cannot be much less than 80 to 90 feet thick. They 

 then apparently maintain a tolerably uniform thickness of from 60 to 

 70 feet, as far as Chatham, Upnor (Sect. No. 4, PI. XV. ), andGravesend. 

 At Bexley Heath they have been ascertained to vary in thickness 

 from 45 to 55 feet, and at Woolwich I find that they are 60 feet thick. 

 Beneath London their thickness averages from 30 to 40 feet. They 

 then become more rapidly thinner as they trend underground further 

 westward, being only 20 feet thick at Wandsworth, 1 7 feet at Isleworth, 

 7 feet at Twickenham, and 3 feet at Chobham, beyond which they 

 thin out, although I believe that originally they probably had a range 

 westward co-extensive in some measure with the green-coated flints 

 overlying the chalk. Along their south line of outcrop westward of 

 London, they are exposed at Croydon and Carshalton, and disappear, I 

 think, somewhere about Ewell or Epsom *, but the sections are too 

 few and imperfect to determine this point. To the north of London 

 the Thanet Sands do not range so far as Hertford, for there, and also 

 at Northaw, as I shall afterwards show, the sands and conglomerates 

 which repose immediately upon the chalk belong to the middle di- 

 vision of the Lower Tertiaries. (See figs. 1, 2, & 3.) 



In North Essex the zone of outcrop of these sands usually occurs 

 on the slope of hills, and they therefore form a very narrow belt, 

 which is further frequently so obscured by drift, that they do not 

 constitute any marked feature in this district. Owing to this cause 

 and the want of sections, their structure there remains uncertain f . 

 Their thickness may be from 30 to 50 feet. In South Essex, how- 

 ever, they are well exhibited in the line of country between Purfleet, 

 Grays, and East Tilbury ; and are as fully and similarly developed 

 there as in the opposite part of Kent. 



[Besides the localities mentioned elsewhere in the text, the following are some 

 other places at which sections of the Thanet Sands are exposed — Harbledown and 

 Whitehall westward of Canterbury ; around Boughton near Faversham ; on the 

 N.W. of Shottenden Hill, four miles S.E. from Faversham ; and again in the lanes 

 traversing Bysing Wood, one mile W. from Faversham ; the hill between Key Street 

 and Newington Street, near Sittingbourne ; Ottersham Quay, near Rainham ; and 

 in the lane leading from Lower Rainham to Rainham, and that between Tweedale 

 and Gillingham ; road between Stroud and Gad's Hill, and west slope of Gad's 

 Hill ; lane leading from the high-road to Shorne Ridgeway, and also the one leading 



* In the brick-field at Nonsuch Park, near Ewell, the " Mottled Clays" are well 

 exhibited, and evidently descend very near to the chalk, but the section is im- 

 perfect. In the railway- cutting at Epsom I found the '* Thanet Sands " reduced 

 to a thickness of 14 feet. At Headley-on-the-Hill, the mottled clays with an 

 underlying band of Ostrea Bellovacina repose immediately on the chalk, without 

 the intervention of the Thanet sands. At Fetcham, a few miles west from Epsom, 

 a bed of sand 25 to 30 feet thick overhes the chalk, but I believe it to belong to 

 the •' Mottled Clay " series, as it is underlaid by the bed of Ostrea Bellovacina. 



f The country is so obscure on the borders of Herts and Essex, that I cannot 

 well determine where the ** Thanet Sands " end and are succeeded by the " Mot- 

 tled Clay" group. This latter, however, is in full force at Ware, and again near 

 Bishop Stortford, but its structure is here very variable, frequently passing into 

 sands difficult, without better sections, to distinguish from the " Thanet Sands," 

 which, judging from several small sections, probably commence somewhere near 

 this latter point. 



