ASPECTS OF DENUDATION 145 



of Britain is its extraordinary inequality. In one region 

 the framework of the land has been cut down into 

 the very Archasan core, while in the immediate vicinity 

 there may be many thousands of feet of younger strata 

 which have not been removed. This inequality must 

 result from difference in total amount of upheaval above 

 the base-line of denudation, combined with difference 

 in the length of exposure to denudation. As a rule 

 the highest and oldest tracts will be most deeply 

 eroded. Much of the denudation of Britain appears 

 to have been effected in the interval between the 

 close of the Carboniferous and end of the Triassic 

 period. 1 This was a remarkable terrestrial interval, 

 during part of which the climate was so arid that salt 

 lakes were formed over the centre of England. Yet 

 the denudation ultimately accomplished was enormous, 

 thousands of feet of Carboniferous rock being entirely 

 removed from certain areas, such as the site of the 

 present Bristol Channel. An interesting analogy to 

 this condition of things is presented by the Great 

 Basin and adjoining tracts of Western America, where 

 at the present time marked aridity and extensive 

 salt-lakes are accompanied by great erosion. 



The deeply-eroded post-Carboniferous land of Britain 

 was eventually screened from further degradation, either 

 by being reduced through denudation to a base-level or 



1 Other periods of prolonged denudation might be mentioned. 

 Thus in pre-Cambrian times the Lewisian Gneiss was stupendously 

 eroded before the deposition of the Torridon Sandstone, which in 

 turn was enormously worn down before the Cambrian sediments 

 were spread unconformably over it. The deposition of the Old Red 

 Sandstone was preceded and accompanied by a vast degradation 

 of the pre-existing rocks. 



