WALTON-COMMOJST section. 



447 



settled up to a certain point. Henceforth, if we would further 

 endeavour to study the development of the Lower Bagshots of this 

 part of West Surrey, it will be necessary to take a leap more or less 

 in the dark. We leave the positive for the inferential, and, what 

 is more, we find ourselves in collision with the Geological Survey as 

 represented in their mapping of the ridge known as Woburn Hill 

 between Addlestone and Chertsey (sheet 8 of the solid geology and 

 surface geology of London and environs). This "Woburn Hill, or 

 Woburn Park, as it is sometimes called, constitutes a promontory of 

 Bagshot Beds, about 90 ft. above O.D., projecting into the great 

 mass of alluvium and valley-drift at the junction of the Thames and 

 Wey, which takes place about 30 ft. above O.D. The ridge is about 

 two thirds of a mile in length from E.N.E. to W.S.W., and the 

 upper portion is composed of a very stiff clay, which has been worked 

 for a considerable period towards its western extremity at Hatch 

 Farm. This forms a portion of the " clays most extensively deve- 

 loped round Addlestone and Chertsey, where they attain a thickness 

 of 10 to 20 feet," referred to by Prof. Prestwich * as constituting 

 part of his Middle Bagshot beds. It is not to be denied therefore that 

 the mapping of the Survey in respect to Woburn Hill has the sanction 

 of the great pioneer of Tertiary geology in the London basin. 

 Before proceeding to express my doubts on this point, and also before 

 proceeding in the attempt to define what should be regarded as 

 Middle Bagshots in this part of West Surrey, I propose to give a 

 description of the 



Hatcli-farm Clay -pit (fig. 2, p. 448). — The pit is practically a trans- 

 verse section of the west end of the top of Woburn Hill. We perceive 

 at once that the brick-earth does not occur as an ordinary seam of clay 

 parallel to the underlying sand, but that it occupies a basin-shaped 

 hollow in that sand. There is, in fact, every reason to suppose, from 

 the upward curve of the underlying series, that we simply see the 

 transverse section of a lenticular mass of clay, which on the north 

 is truncated by the escarpment, but towards the south has the appear- 

 ance of going out altogether. Another feature in the brick-earth is 

 the remarkable amount of current-bedding with a prevailing southerly 

 dip, towards the centre of the hollow, in fact ; whilst further south, 

 where the hollow in the sand is less pronounced, the laminations 

 are almost horizontal, 



The clay of this pit is pretty strong, being used for making red 

 bricks and stock bricks, whilst the presumably Middle-Bagshot clays 

 of Ongar Hill, worked by the same proprietor, are also largely used 

 for pipes and tiles. The blue portions of the clay are full of flattened 

 pyritous lumps of a different shape from those previously noticed in 

 No. 4 of the Oatlands-Park cutting, and there is an abundance of 

 microscopic crystalline aggregates of pyrites and some very fine 

 quartz-grains. - Carbonaceous matter is plentiful throughout, but 

 glauconitic granules are scarce in all examples examined by me. 

 The hill itself forms one of the stiffest clay soils known in this part 

 of the country. 



* Q. J. G. S. vol. hi. p. 383. 



2h2 



