RATE OF WEAR OF THE CLIFFS 71 



The extent of denudation that the " Head," where 

 it forms a cliff, has undergone by the action of the 

 sea, offers a better chronometrical scale. Though 

 here again exact measures are yet wanting, we can 

 form some idea of what may be the limits it indicates. 

 The old (raised) line of coast, as it existed before the 

 spread -. *tje Bubble-drift, differs but little from that 

 of the existing line, so that, if the extent of the 

 retrocession of the coast-line since the Submergence 

 which originated the " Head " could be determined, 

 we might, knowing the present rate of wear of the 

 cliffs, arrive at some approximate idea of the time 

 that has elapsed since the erosion of the existing 

 line of cliffs commenced. Judging from an estimate 

 I made, 1 the loss of land on the south coast in the 

 districts occupied by soft Cretaceous, Oolitic, and 

 Liassic strata, since the date of the Bubble-drift, 

 amounts, at the extreme, to about four miles ; but 

 generally it is limited to a breadth of one to two 

 miles or less. Taking the present known rate of the 

 wearing back of the cliffs at from one to three feet 

 annually, this would bring the total loss roughly 

 within the limits of 6,000 to 12,000 years. (See 

 Fig. 1, p. 22.) 



On the south-west coast composed of hard 

 Palaeozoic rocks, the extent of wear of the land has 

 been comparatively trifling. No exact measure has 

 yet been made of what the rate of wear is at present. 

 That it is exceedingly slow is evident from the cir- 



1 Quart. Journ. Geol. Soc., Vol. xlviii., p. 264, and PI. viii. 



