Ch. XIX] DENUDATION OF THE WEALD. 279 



val between two elevatory movements, the lower terrace will usually 

 be destroyed, wherever it is composed of incoherent materials ; 

 whereas the sea will not have time entirely to sweep away another 

 part of the same terrace, or lower platform, which happens to be 

 composed of rocks of a harder texture, and capable of offering a 

 firmer resistance to the erosive action of water. As the yielding 

 clay termed gault would be readily washed away, we find its out- 

 crop marked everywhere by a valley which skirts the base of the 

 chalk-hills, and which is usually bounded on the opposite side by 

 the lower greensand ; but as the upper beds of this last formation 

 are most commonly loose and incoherent, they also have usually 

 disappeared and increased the breadth of the valley. In those dis- 

 tricts, however, where chert, limestone, and other solid materials en- 

 ter largely into the composition of this formation (No. 4, map, p. 

 272), they give rise to a range of hills parallel to the chalk, which 

 sometimes rival the escarpment of the chalk itself in height, or 

 even surpass it, as in Leith Hill, near Dorking. This ridge often 

 presents a steep escarpment towards the soft t argillaceous deposit 

 called the "Weald clay (No. 5 ; see the dark tint in figure 321, 

 p. 272), which usually forms a broad valley, separating the lower 

 greensand from the Hastings sands or Forest Ridge ; but where sub- 

 ordinate beds of sandstone of a firmer texture occur, the uniformity 

 of the plain of No. 5 is broken by waving irregularities and hil- 

 locks. 



Pluvial action. — In considering, however, the comparative de- 

 structibility of the harder and softer rocks, we must not underrate 

 the power of rain. The chalk-downs, even on their summits, are 

 usually covered with unrounded chalk-flints, such as might remain 

 after masses of white chalk had been softened and removed by water. 

 This superficial accumulation of the hard or siliceous materials of 

 disintegrated strata may be due in no small degree to pluvial action 

 for during extraordinary rains a rush of water charged with calca- 

 reous matter, of a milk-white color, may be seen to descend even 

 gently sloping hills of chalk. If a layer no thicker than the tenth 

 of an inch be removed once in a century, a considerable mass may 

 in the course of indefinite ages melt away, leaving nothing save a 

 stratum of flinty nodules to attest its former existence. A bed of fine 

 clay sometimes covers the surface of slight depressions in the white 

 chalk, which may represent the aluminous residue of the rock, after 

 the pure carbonate of lime has been dissolved by rain-water, charged 

 with excess of carbonic acid derived from decayed vegetable matter. 

 The acidulous waters sometimes descend through " sand-pipes" and 

 " swallow-holes" in the chalk, so that the surface may be undermined 

 and cavities may be formed or enlarged, even by that part of the drain- 

 age which is subterranean.* 



* See above, p. 82, 83, " Sand-pipes in Chalk ;" and Prestwich, Geol. Quart, 

 Journ. voL x. p. 222. 



