48 THE HIGH-LEVEL SAXDS AND GRAVELS [vol. lxxv, 



W} r combe and Amersham, they rest on Micraster-corangiiinam 

 Chalk. Still farther north, there is evidence that the Reading Beds 

 rest on even lower horizons ; for example, near Markgatestreet 

 they seem to come down to a little above the Chalk Hock. 



The plane on which the Reading Beds rest is the same as the 

 plane regarded as of Pliocene age. This coincidence seemed to 

 the speaker to weigh heavily against Mr. Barrow's conclusion that 

 the plane was cut in Pliocene times. 



Mr. H. Dewey objected to the Authors' suggestion that the 

 plateau- gravels of West Surrey were of Pliocene age. The reasons 

 adduced in support of their contention are the presence in those 

 gravels of white quartz-pebbles and the supposed marine origin. 

 As to the mode of origin, it should be remembered that existing 

 conditions of subaerial denudation are forming two plateaux, a 

 higher and a lower one. separated the one from the other b} r sloping- 

 lands. Bagshot Beds form the surface, and are divisible into an 

 upper and a lower series of sands and an intermediate group of clays 

 and loams. The sands form wide fiat tracts, heath-covered and 

 often swampy, whereas the clays give rise to gently undulating 

 ground. Precisely similar features occur at the Chobham Ridges, 

 but these great spreads of gravel envelop the underlying solid 

 formations and mould themselves to the pre-existing topography. 

 There is, therefore, no reason to invoke marine erosion when sub- 

 aerial denudation locally produces similar peneplains. 



The Authors infer from the presence of white quartz-pebbles in 

 these, the Chobham Ridge gravels, their origin from the Lower 

 Greensand deposits lying to the north of the Chiltern escarpment; 

 but a more likely source of these quartz-pebbles is surely to be found 

 in the local pebble-beds. Such are found at many localities at the 

 base of the Barton Sands and in the Bagshot Sand, and contain 

 many white quartz-pebbles, although fewer than do the gravels 

 described by the Authors. 



When mapping this district for the Geological Survey the 

 speaker came to the conclusion, after weighing all the available 

 evidence, that these gravel-spreads are analogous to the outwash 

 fans of Glacial regions. 



He did not, however, consider that the Weald had ever been 

 glaciated, but that its heights had collected on them many years of 

 snowfall, which on thawing liberated torrential floods carrying the 

 frost-splintered rock-debris of the district to the lowlands. The 

 gravels contain up to 50 per cent, of Greensand chert, which must 

 have been brought through the present-day ; dry gap ' of the 

 Hlackwater. Many large sarsens and blocks of abraded and bat- 

 tered flint are common in the gravels, while the upper beds show 

 contortions and gnarled structures. 



Nothing that the speaker had heard that evening had modified 

 the opinions which he had formed and afterwards published in the 

 Windsor <fe Chertsey Geological Survey Memoir. 



Mr. Ll. Treacher suggested that the small quartz-pebbles 

 described by Mr. Barrow, although doubtless ultimately derived 



