ESSAYS 313 



and escarpments in the Chalk and Upper Greensand ; and it is easy to see that 

 the effect of planation is to bring the successive formations to the surface in a 

 series of roughly parallel or concentric belts. 



As the main axis lies approximately east and west, the earliest rivers ran down 

 the slopes to the north and south ; and when elevation was complete, they would 

 evidently pass over each of the strata in turn. But of course the uplift was really 

 gradual, and not sudden and abrupt, so that the streams actually began on the 

 oldest (or central) beds, and only reached the others as they successively appeared 

 above the sea. But the different strata offer widely different resistance to the 

 action of rain and rivers, hardness being perhaps the main factor, but porosity 

 being also important ; and the result of this is that the less resistant beds (clays 

 and some sands) are quickly removed and worn into valleys, while the more 

 resistant (Chalk, and chert beds of the Lower Greensand) stand up as ridges ; 

 and since, as we have seen, the strata are arranged in roughly parallel belts, 

 running mainly east and west, it follows that the main trend of hills and valleys is 

 also in this direction. It is in these "longitudinal" valleys that most of the 

 principal rivers run, but practically all of them sooner or later join one or other 

 of the original transverse rivers, and these, continuing to deepen their beds as the 

 land rose higher above the sea, now run, in narrow steep-sided valleys, right 

 through the longitudinal hill-ranges, which owe their very existence to the 

 excavation of the valleys on either side. 1 The process of wearing away of 



*IG. I.— Diagrammatic section across the Weald, showing (1) former continuation of 

 strata; (2) plain of marine denudation; (j) present surface of the ground, with 

 escarpments in the Chalk and Upper Greensand. 



the softer beds may then continue, as it actually has done in most parts of this 

 region, until the land in the centre, where the transverse rivers rise, is actually 

 lower than the peripheral hills which they afterwards traverse. 



A word about the escarpments. The Chalk, though highly resistant to direct 

 assault (by rain from above), is much more vulnerable to flank attack. Not only 

 is the Lower Chalk usually softer than the Upper, but rain and rivers, by eating 

 away the soft Selbornian beds, tend to undermine it, and it is to this that the 

 formation of the escarpments is due. The process is in many ways comparable 

 to that by which a sea-cliff is undermined by the waves, only it is so infinitely 

 slower that the Chalk, instead of standing vertically, has time to become dis- 

 integrated by atmospheric action and crumbles away to a steep slope instead. 



Like sea-cliffs, then, the escarpments (in Chalk, Greensand, or other formation) 

 are receding ; and it is this recession which forms the key-note of the first, or 

 S! Chalk Dome Hypothesis," which supposes that the summit of the arch of Chalk 

 has, except perhaps for some pre-Eocene denudation, been entirely removed by 



1 The modern theory of river-evolution is admirably explained by Prof. 

 W. M. Davis {Geog. Journ. vol. v. 1895, PP- I2 7 et se 9-\ an d more briefly by 

 Mr. Makinder {Britain and the British Seas, pp. 144-5), an d Lord Avebury 

 {Scenery of England, 1902, pp. 364-5). 



