ATfD OF THE KEIGHBOUEHOOD OF -WATFOED. 93 



a somewhat limited extension, for we find them apparently de- 

 posited in an area scooped out of the Chalk. They are of marine 

 origin. They extend from a little westward of London to 

 the Isle of Thanet, where they are largely developed; but 

 they do not terminate there, for we find in the old Tertiary 

 times, when the Straits of Dover did not exist, that the seas 

 which deposited the sands in the Kentish and Surrey area ex- 

 tended over the Paris Basin and over part of Belgium. Similar 

 strata, or strata of the same age, occur both in the Belgian and 

 Parisian areas, and are known by difi^erent names according to the 

 district in which they are found. In Belgium they are termed 

 the Systeme Landenien, and in the Paris Basin are represented by 

 the sands of Bracheux and Abbecourt. 



These Thanet Sands thicken eastwards, and thin out west- 

 wards, and partly extend under London, but do not reach this 

 district, although found north of the Thames at Purileet and 

 Grays, in Essex. They are (or were) the means by which London 

 is partly supplied with deep and pure well water from what are 

 known as artesian wells. It is the permeable nature of the 

 sands, allowing them at their outcrop to receive the rain, kept in 

 by the impervious nature of the London Clay above, and the 

 satm'ated Chalk below, which causes them to form part of the 

 stores of the deep well supply of the London area.* 



The Thanet Sands indicate moderate climatal conditions, and, 

 according to Mr. Prestwich, were formed in a sea of a somewhat 

 temperate climate, inasmuch as the nature of the shells found in 

 them fAstarte, Cyprina, TrophonJ would scarcely indicate a sea 

 of a tropical character.! 



* " The Chalk is the chief source of water-supply for the deep wells in the 

 London district. IV ot very many years ago the overlying Thanet Sand was the 

 great water-bearing bed, but its limited outcrop and small thickness soon caused 

 it to be unable to bear the greatly increasing drain on it, and most London 

 wells were then deepened, new wells being almost universally carried into the 

 Chalk." — Whitaker, ' Guide to the Geology of London,' p. 20. 



t " It was, probably, on the shores of the dry land of this period (the Thanet 

 Sands), that the innumerable flint pebbles so perfectly rounded — a process 

 indicating a vast lapse of time — which we find in higher portions of the 

 Lower Tertiary strata, as at Blackheath and Addington, were formed; these 

 accumulations of shore-pebbles having been spread out over the Thanet Sands 

 at that next succeeding Lower Tertiary period. After the period of the Thanet 

 Sands a further subsidence of the northern part of the southern continental 

 area took place, and the sea then spread itself over the greater part of the Isle 

 of Wight and Paris Tertiary districts, leaving some of the higher lands as 

 islands. One of these islands occupied probably the area now forming the 

 Weald of Kent and Surrey, then not denuded of all its chalk dome. It was 

 during this second period that the strata of the Woolwich and Reading series 

 were formed." — Prestwich, ' The Ground Beneath Us,' p. 73 ; see also ' Quart. 

 Jom-n. Geol. Soc.,' vol. x. p. 135. 



M. Elie de Beaumont, also, suggests that probably, at the Lower Tertiary 

 period, an island extended somewhere in the present position of the Wealden 

 and part of the north of France, and a smaller one of the elevated land, the 

 Pays de Brny. See his memoir " Sur I'Etendue du Systeme Tertiaire inferieur," 

 in the ' Mem. Soc. Ge'ol. de France,' ser. 1, vol. i. pp". Ill, 112, and pi. 7, fig. 5. 



