452 



ME. W. H. HUDLESTOIST ON THE 



cannot well exceed 30 ft. above O.D., and may be less. We have, 

 then, a fall of say 55 ft. in 2^ miles in a direction from E.N.E. to 

 W.S.W., which serves to show that the London-Clay surface falls at 

 the rate of 1 in 216 throughout the portion traversed by the London 

 and South-Western Railway, between the above-named points. 

 There is also additional evidence that the London-Clay surface 

 continues to fall very slightly still further down the line, since the 

 Bagshot base at Brookwood is 21\ ft. below O.D.*. But the above 

 considerations will only slightly affect a line drawn from St. George's 

 plateau to St. Anne's Hill (fig. 3), which is nearly at right angles to 

 the direction of the main L. & S.W. Railway. 



Hence, we are not entitled to assume the existence of a marked 

 synclinal in the position occupied by Woburn Hill on the evidence 

 above stated ; whilst, on the other hand, if there be no synclinal, 

 the ascertained position of the recognized basal beds of the Middle 

 Bagshots in the district is very much against the notion that the 

 Woburn-Hill Clay (Hatch brick-earth) should be on their horizon. 

 At the same time I am ready to admit that the base of the Ongar- 

 Hill brick- earth, about two miles on the other side of Addlestone, 

 which apparently must be correlated with the recognized basal beds 

 of the Middle Bagshots, cannot be much more than 100 ft. above 

 O.D. ; but even this is fully 40 ft. higher than the base of the 

 Hatch brick-earth, and more in the direction where the Bagshot 

 base is undoubtedly falling f . Altogether, it must be considered 

 that if the Hatch brick-earth belongs to the Middle Bagshots, as 

 indicated on the Survey Maps, there must be an exceptional condi- 

 tion of the local stratigraphy. In point of fact we are faced by the 

 following difficulty : — Seeing that the base of this brick-earth is 

 about 60 feet above O.D., and assuming that the Lower Bagshots 

 retain their average thickness of 120 ft., we should have the London- 

 Clay surface 60 ft. below O.D. at this point. It is for those who 

 maintain that this brick-earth belongs to the Middle Bagshots to 

 bring proof of this. 



Even in the actual valley of the Thames there is no instance 

 that I know of, on the Surrey side, where the London -Clay surface 

 sinks below O.D. At Chertsey, in the heart of the Thames-valley 

 " shingle," the London-Clay surface does not fall below Ordnance 

 Datum, though it approaches very near it. At the brewery, where 

 the well-mouth is probably 44 ft. above O.D., this happens to be 

 the exact thickness of the superficial beds, thus : — 



ft. 



Surface-mould and loamy clay 5 



Gravel and sand 35 



Dark sand 4 



Total of beds above London Clay 44 J 



* W. Whitaker, ' On some Surrey Wells and their Teachings,' p. 47. 



t The clay beds at Ongar Hill and the adjacent Eow Hill afford a remark- 

 able instance of the development of argillaceous beds in the Bagshot system, 

 which seems to point to considerable variation in their volume and importance- 

 even on a well-recognized horizon. J Whitaker, op. cit. p. 49. 



