﻿662 MR. H. BTTRY ON THE DENUDATION OF [Nov. I9IO, 



shed dividing the northern from the southern rivers seems always 

 to have run through or close to Blackdown, and the Lower Greensand 

 ridge between Hindhead and Hascombe is notched by several valleys 

 which formerly extended farther south. Hindhead itself , however, 

 shows no trace of such notches, and the inference seems to be that 

 the consequent streams which now run down its northern slope 

 must have been cut off from Blackdown by the Haslemere Valley 

 at quite an early stage. Martin (12, pp. 46 & 197) asserted that 

 this valley marked the central anticline of the Weal den area, but, 

 so far as I can ascertain, it coincides with a syncline ; and the 

 question arises whether its stream is to be regarded as a subsequent 

 or as a longitudinal consequent. In the former case we may reason 

 as follows : although the Hythe Beds were exposed by marine 

 planation at Hindhead, yet owing to the dip of the strata, the soft 

 Folkestone Beds must have appeared immediately to the west, and 

 the syncline would cause a tongue of them to stretch eastwards 

 between Hindhead and Blackdown. Owing to their softness, they 

 would be lowered more rapidly than the Hythe Beds, and thus a 

 subsequent valley would very quickly arise. If, on the other hand, 

 we regard the stream as a longitudinal consequent, we must, on 

 the hypothesis of marine planation, conclude that the syncline in 

 which it runs was formed at the time of upheaval. There is nothing 

 impossible in this ; but, having regard to the rarity of any rivers 

 which can be regarded as longitudinal consequents, I prefer, on the 

 whole, the first hypothesis. The earliest drifts in the region 

 attributed to the Blackwater are those of the Farnham Plateau, 

 which have been fully described by Prestwich (18, p. 161), Monck- 

 ton (13, p. 30), and others. Usually they do not exceed 10 or 12 

 feet in thickness, but it may be worth while to record that I have 

 measured a depth of 27 feet on Beacon Hill, and also immediately 

 south of the main road, half a mile west of Lawday House (the 

 'Wilderness' on the 6-inch map reprinted in 1904). If we may judge 

 by the gradients already given, the gravels of Easthampstead Plain 

 •cannot be of much later date : due allowance must of course be 

 made for the curve of the ' thalweg' ; but the more distant gravels 

 give evidence of so much transporting power that we must not 

 make this curve too flat. There is, however, a marked difference 

 in composition between the two drifts. On the Farnham Plateau 

 Prestwich gives the percentage of Lower Greensand fragments at 

 6 percent. Mr. Monckton reduces this to 5 percent., but even that 

 seems to me rather a high estimate ; Hythe Bed fragments (chert), 

 at any rate, are so scarce on most parts of the plateau that a pro- 

 longed search is necessary to find any at all, and even then the 

 pebbles are extremely small, seldom exceeding 1 inch in diameter. 

 In the Easthampstead gravels, on the other hand, although twice the 

 •distance from the nearest Hythe Beds (Hindhead), chert is exceed- 

 ingly common, being estimated by Mr. Monckton at 4 per cent, of the 

 large, and 9 per cent, of the smaller material. Prestwich accounts 

 for the scarcity of chert on the Farnham Plateau by supposing that, 

 at the time when the Drift was deposited, the Hythe Beds were 



