458 MR. J. V. ELSDEET ON THE [Aug. I9O9, 



The relief -model illustrating this paper brings out very clearly 

 certain points not so readily realized, either from a study of the 

 ground, or from an examination of contoured maps, owing to the 

 difficulty in each case of taking a comprehensive view of a 

 sufficiently large area. 



The first point to be noticed is the marked symmetry of these 

 depressions. Starting at the water-partings as dip-valleys, they 

 gradually wind round into strike-valleys, which direction is main- 

 tained until they debouch into the main river-valley. This would 

 be expected to be the normal course of tributary valleys formed by 

 surface-drainage. Another striking feature is their symmetrical 

 transverse outline. Each one presents a gradual slope in the 

 direction of the dip, followed by a steep rise in the opposite 

 direction. This is also characteristic of valleys of surface- erosion. 

 It has lately been suggested that such valleys in Chalk areas are 

 solution-subsidences along the course of master-joints in the Chalk, 1 

 and it is true that the Chalk in this area does occasionally show 

 prominent dip and strike-joints. Thus the slips on the steep face 

 of Hindover often follow a marked zigzag course, following in 

 turn each of these sets of joint-planes, w r hile in Denton Chalkpit 

 pronounced vertical dip-joints are visible. The stratigraphy of the 

 beds, however, at once disproves the theory of a .sagging of the 

 strata along the course of swallow-holes following joint-planes in 

 the Chalk ; for, if this had been the case, the outcrops would show 

 no relation to the surface-contours. I have carefully examined the 

 stratigraphical features of all the valleys intersecting the Chalk in 

 this area within the limits of the three upper zones, and there 

 is clear evidence that no such surface-subsidence has taken place. 

 The successive outcrops invariably maintain their correct relations 

 to the surface-contours. 



The same conclusion is confirmed by the fact that the dry 

 valleys have been swept remarkably free from superficial deposits. 

 There are terrace-gravels along the rivers, and there are remnants 

 of high-level gravels, and debris of Tertiary beds on the high ground; 

 but, with the exception of a thin rainwash, the valleys themselves 

 are free from these deposits. At Friston, for example, there is a 

 deposit of 10 feet of coarse gravel at an altitude of 350 feet, which 

 is sharply truncated by the Jevington dry valley. On the hill 

 above Hobb's Hawth, there is a similar sudden termination to the 

 thick covering of debris overlying the zone of Actinocamax quadratics, 

 as soon as the Hobb's Hawth valley is entered. The solution- 

 theory would require that such insoluble deposits should, above all 

 others, be let down into these valleys in their full thickness. 



Both from the symmetry and from the stratigraphy of these 

 valleys, therefore, they must be regarded as true valleys of erosion. 

 Mr. Clement Eeid's ingenious theory of their origin, 2 to which I 



i Eev. E. C. Spicer, Quart. Journ. Geol. Soc. vol. hriv (1908) pp. 335-42. 

 2 Quart. Journ. Geol. Soc. vol. xliii (1887) p. 364. 



