SUBCUTANEOUS EROSION 



33 



the presence of clay marked, the number of springs greater, the marls 

 nearer to the surface, in immediate contact moreover with the super- 

 posed limestones. These different conditions produce different effects. 

 At first, however, soakage passes down the slope exactly as in the two 

 instances already given. As before, the percolated water banks up at 

 the base of the succeeding western cliff; as before, it soaks and oozes 



O ' ' 



away until an incipient water-channel is established ; as before, this 



Slope towards creek. 



" Sag deepened from a U to a V a normal valley formed save for the impossibility 



of lateral expansion'' 1 



water-channel deepens until a scour is created ; as before, this scour 

 begins to eat out the topmost constituents of the hidden interstice ; as 

 before, a sag which deepens into a lap appears ; as before, the lap 

 develops into a shallow U. Now, however, occurs a change. The base 

 of the U rests not on pervious sands, as on our conjectural blocks of the 

 centre and west, but on a stiffer material, marl. Rain passing off be- 

 tween a top spit of fibre and grit and an impervious subsoil, forms a 



c 



