92 PROF. MOEEIS — GEOLOGY OF THE LONDON BASIN 



deposition of the Cretaceous and certain overlying strata, the beds 

 underwent plications and foldings, in consequence of subterra- 

 nean movements, by which they partly received their present 

 configuration ; since, however, considerably modified by denuding 

 agency, which has swept olf the beds from the anticlinal folds, 

 and allowed their edges to be exposed at the margin of the basin, 

 the older strata occurring at the margin, and the younger succes- 

 sively appearing towards the centre. 



The Tertiary strata of the London Basin are but partially repre- 

 sented in the vicinity of Watford. Aiter the close of the Chalk 

 period a limited area was probably submerged beneath the waters 

 of the ocean. Not improbably this district of Hertfordshire was not 

 so much submerged as to be covered by the earliest Tertiary stratum, 

 or the Chalk was not subjected to much erosion and destruction; 

 for, as shown by Prestwich and others, by comparing the thick- 

 ness of the Chalk strata in Kent and Surrey with that of 

 Hertfordshire, we find that the Chalk in the latter district attains 

 a far greater thickness — about 1000 feet — than in the area to the 

 south of London, where it is in some places but 300 or 400 feet 

 thick.* Hence, therefore, it is not improbable that, through the 

 physical changes which caused so marked a feature between the 

 Secondary and Tertiary periods, there was at the commencement 

 of the Tertiary period a movement of the water which scooped out 

 and destroyed the Chalk bed in one district more than in another. 



Upon the Chalk was deposited the first of the series of the 

 Tertiary strata.f These are called by Mr. Prestwich the Thanet 

 Sands, from their constituting an important feature in the physical 

 geology of the Isle of Thanet and its neighbourhood. They have 



beds, and it is considered to be a continuation of tbe axis of the Pays de Bray. 

 But, as M. Barrois remarks, it is very difficult to follow these lines of elevation 

 with exactitude, as the forces which produced them have acted with var)'ing 

 intensity from one point to another, and with different effects. — See also papers 

 by MM. Hebert and Mercey, in the 'Bull. Geol. Soc. France,' ser. 2, tome xx. 

 pp. 615, 643. 



* " But in Suffolk and Xorfolk, the upper part of the Chalk is, like the lower 

 part, nearly destitute of Hints. This portion of it I believe to have been denuded 

 at London, and cousequently the thickn(\ss of the mass has been much diminished. 

 Thie may have arisen from a greater ril;iti\ e tlcvatiou of the bed of the sea to the 

 south at the conclusion of the Cr^■t;l^(l.ll^ |i( riod, in conseciuence of which the 

 earlier Tertiary seas planed down its \mn-v exposed and elevated surface; and 

 thus the upper beds of the Chalk, whicli to the northward, owing to their greater 

 depth from the surface, escaped tliis denudation, may have been removed by it in 

 proportion as they trended southward. Hence there would result a gradual 

 decrease in the thickness of the chalk as it ranges from north to south." — 

 Prestwich, ' The Water-bearing Strata,' 1851, p. 139. See also ' Quart. Journ. 

 Geol. Soc.,' vol. viii. p. 257. 



t Overlying the Chalk, however, and separating it from the Thanet Sand 

 above, there occurs a bed of green-coated flints, known as the BuH's-head Bed; 

 these unrolled and uiuuoved flints formed one of the original zones of tlint in 

 the Chalk, from whicli the latter has been removed by chemical action either 

 before, but more jjrobably suhsecpiently to, the deposition of the Thanet Sand, 

 during which they also ac(iuired tlie green coating of silicate of iron. This bed 

 is well seen at Grays, Eritn, Charlton, and Croydon. 



