WALTON-COMMON" SECTION", 



451 



Reputed Middle Bagshots 



Lower Bagshots 



London Clay 



Total above O.D 



230 



This calculation brings the base of the Middle Bagshots in St. 

 Anne's Hill to 170 ft. above O.D. — a level almost identical with that 

 on St. George's Hill. Making due allowance for errors and mis- 

 calculations, we may safely say that the levels are within 15 ft. of 

 each other. 



A glance at the diagrammatic section (fig. 3) at once makes us 

 inquire why, if the basal beds of the Middle Bagshots lie so high 

 up on St. George's Hill and on St. Anne's Hill, they should have 

 descended so low in Woburn Hill, which lies between the two. In 

 other words, is the mapping of Woburn Hill as Middle Bagshot 

 justified? 



The chief point for our consideration is whether there exists any 

 evidence of a trough between St. George's Hill and St. Anne's Hill, 

 produced either by an ordinary syncline or by erosion of a pre-Bag- 

 shot surface. If there is no evidence of such a trough, then, on 

 stratigraphical grounds, the Hatch brick-earth must be a member of 

 the Lower Bagshots. I am quite prepared to admit the existence 

 of some peculiarity hereabouts, because the Thames suddenly 

 changes its mean direction just opposite Woburn Hill, recovers its 

 former course for a few hundred yards, and then suffers final deflec- 

 tion to the E.N.E. The confluence of the Wey and the Thames 

 marks the most southerly point attained by the principal river ; 

 and no doubt the causes which induced and afterwards arrested the 

 southerly course of the Thames are to be sought in the nature and 

 disposition of the beds in this immediate neighbourhood. The 

 Thames was probably drawn southwards by the fall in the London- 

 Clay surface, since it was much easier to eat away the loose sands 

 of the Lower Bagshots than the strong blue clay on which they 

 rest. Hence the most southerly point of the Thames valley probably 

 coincided with the maximum depression of the London-Clay surface 

 on the flanks of the main valley. But the cap of clay on what is 

 now Woburn Hill helped to keep together the incoherent sands 

 beneath ; so that we perceive St. Anne's Hill, Woburn Hill, and St. 

 George's Hill are primarily caused by a capping of tenacious 

 material which is of an entirely different nature in each case. 



We are quite prepared for a synclinal, then, if it can be proved 

 to exist ; but at present I have not been able to obtain evidence that 

 such is the case, at least not to the extent necessary to bring the 

 Woburn-Hill beds into alignment with the basal beds of the Middle 

 Bagshots. At the same time, there can be no doubt that the fall 

 of the Bagshot base between Walton Common and the River Wey 

 is considerable. Thus at the eastern extremity of the formation in 

 this district, 658 yards west of Walton Station, the Bagshot base is 

 85 ft. above O.D., whilst near the railway-bridge over the Wey it 



