IBO ME. W. H. HUDLESTOI^ ON A SECTIOlf THROUGH 



In block C the contrast of colouring is very effective. The dark 

 blues of the moist clays, and the reds, chocolates, yellows, and 

 whites of the sands produce a pleasing and striking effect, which 

 prolonged exposure to the atmosphere will doubtless tone down. 

 At first sight there is a superficial resemblance between the London 

 Clay of the section and No. 2 of the Bagshots, but the former is 

 more homogeneous, and not laminated, and the thin ribands of 

 intercalated sand, which it contains at rare intervals, are regular in 

 their occurrence and deposition. 



The entire section is strongly suggestive of unconformity between 

 the London Clay and the Lower Bagshots at this point. It would 

 be impossible to find a sharper lithological contrast than is pre- 

 sented by the London Clay and the yellow sands of No. 1, the lowest 

 Bagshot bed. At the same time the stratigraphical evidence seems 

 also to point in the same direction. There is no conformability be- 

 tween the London-Clay surface and the undulations of No. 1. On 

 the contrary, where the surface of the London Clay presents a slight 

 depression this bed not seldom shows a protuberance. To me it seems 

 probable that these loosely aggregated and current-bedded sands were 

 deposited against the eroded slope of the London Clay, just as we 

 now see them. An important feature in the case is that the London- 

 Clay surface rises at least 4 feet higher on the east than any part of 

 the lowest sand-series (see %. 1). Doubtless both the blue beds 

 (No. 2) and the buff laminated sands (No. 3) overlapped, and have 

 been removed by denudation ; but this does not alter the fact that 

 the lowest bed of the Bagshots is seen to abut against a surface of 

 London Clay, and not to conform to it. Moreover, the westward 

 slope of the London Clay visible in section, which falls from 13 feet 

 to zero in about 100 yards, has not by any means the appearance of 

 a true dip-slope. On the contrary, the undulations of its surface 

 seemingly point to erosion previous to the deposition of any of the 

 Bagshot beds*. 



As before observed, the very marked change in the lithology also 

 points in the same direction. There are, of course, numerous 

 instances where a clay series gradually becomes a sandy series, and 

 yet where the change of colour is often rather sharp, although the 

 lithological differences at the point of contact of the two colours may 

 not be very strong. But in this case the contrast between the grey 

 London Clay, sticky and dark coloured, and the bright-yellow sand 

 of No. 1, incoherent and full of false-bedding, is fairly borne out by 

 an examination of their intimate composition. A brief notice of the 

 lithology of the London Clay at this place has already been given, 



'^ It is much to be regretted that the works of the bridge prevent a thorough 

 understanding of this, the most critical part of the whole section. The mean 

 slope of the London-Clay surface at this place may be taken at 1 in 23, which 

 gives an angle of about 2i° to the westward, or not very much in excess of the 

 observed dip of the London Clay of the central portions of block B. Unfor- 

 tunately that portion of the London Clay which underlies the Bagshots is too 

 much "muddled" for the bedding to be made out. If we could prove with 

 certainty that the ends of the beds were truncated by the slope, the evidence of 

 unconformity would be complete. 



