488 Whitaker — On Subaerial Denudation. 



the northern side of the London basin than on the southern, so that 

 the beds have a greater chance of spreading over a "wider tract. 



Of course delays in denudation raay be owing also to change of 

 condition, climatal or otherwise. 



6. — Chalk and Tertiary Cliffs. 



It is usual to talk of cliffs as the work of the sea alone ; and 

 those who say that subaerial actions are too weak to do the work of 

 denudation in forming hills and valleys are wont to point to what is 

 now going on along our shores as evidence that the sea and the sea 

 only is nature's great tool for making ridges. I am willing how- 

 ever to meet them on their own groimd, thinking that if it can be 

 shown that the sea alone does not make the cliffs, but is very largely 

 helped by those atmospheric actions which they despise, their state- 

 ments as to the powerlessness of those actions will have all foundation 

 destroyed, and will therefore fall to the ground, carrying with them 

 the theories which they support. 



Let us examine the Chalk-coast of Kent. The cliffs are for the 

 most part nearly vertical ; indeed I can call to mind but one place 

 where this is not the case, the well-known Shakspeare's Cliff, the 

 higher part of which is a sharp slope, whilst near the bottom it is 

 slightly overhanging (on account of a hard bed which stands out). 

 Sometimes they are quite vertical ; hardly ever are they undermined. 

 Now if made by the sea alone, which can act only at their base, 

 surely they should mostly overhang ; but, in fact, they often project 

 slightly at the bottom by a series of small steps. It is clear there- 

 fore that the upper part wears away as quickly as the lower, and as 

 the sea can hardly attack the top of the cliff, one hundred feet or 

 more high, one must look about for some other wearing power that 

 can. 



For that purpose let us go to the cliff-top and see what is going 

 on there. We shall find that the action of the weather is nearly 

 everywhere separating masses of Chalk, some of which, from the 

 slow dissolving away of the surrounding rock, stand out for many 

 years as pinnacles or needles, whilst others are soon hurled to the 

 bottom. Where the Chalk is most jointed there of course the power 

 of frost has most chance of showing itself: where too there are 

 large pipes of sand and clay in the Chalk small needles are common 

 along the top of the cliff, as in parts of the coast of Normandy. 



When the softer and more yielding beds below the Chalk crop out 

 near the base for some distance, the fall of the cliff sometimes takes 

 place on a very large scale, and "undercliffs" are formed. Thus at 

 Tolkestone the porous yielding Upper Greensand has given way to 

 the influence of springs and to the pressure of the great overlying 

 mass of rock, which has in consequence slid down over the moist 

 slippery surface of the Gault. The undercliff of the Isle of Wight 

 is far longer and broader, and the nearly vertical cliff of hard Upper 

 Greensand, which has resulted from its formation, is at a great 

 height above the sea and often a third of a mile distant therefrom, 

 so that no one can well call it a sea-cliff. 



