592 DENUDATION AND SCENERY. 



rents, joints, and fissures in the rocks no doubt gave a first direction 

 to many of the valleys, which have been enlarged by rain, and 

 streams, and rivers, and even by glaciers. So that really the main 

 features of Wales, as we now see them, are due chiefly to fresh- 

 water denudation, although there is every reason to suppose that 

 the sea exercised some modifying influence on the land during the 

 long course of ages and the many changes that have occurred since 

 later Palaeozoic times. 



In attempting to explain the origin of the main features in our 

 country, by a study of the agents now at work, we must remember 

 that all the features along the coast have not been produced by the 

 sea, nor have all the features inland been sculptured simply by rain 

 and rivers or other agents of what is called Subaerial denudation.' 

 We must be prepared, in fact, to acknowledge help on all sides. 

 Subaiirial agents inland may be at work in modifying features that 

 were broadly sketched out by the sea, while the sea is evidently 

 in many places altering or destroying the features that have in past 

 times been produced by rain and rivers. Rain, frost, and springs 

 greatly assist the sea in wasting the cliffs ; the former produce the 

 landslips, the sea removes the fallen material. A sun-dried crack 

 allows water to penetrate ; frost, by its expanding action, severs 

 the bed : even lightning may exercise some influence in shivering 

 masses of rock. The influence of glaciers, which are well known 

 to have existed on the high grounds of Wales and the north 

 of England, has been spoken of, and some of the valleys have been 

 considerably modified, if not to a great extent formed, by them. 



In late years much discussion has arisen respecting the relative 

 influence of marine and subaerial agents in modifying the surface 

 of the land. The majority of the formations indeed were accumu- 

 lated on the sea-bottom : though we do here and there, and 

 particularly in the Coal-measures, in the Wealden beds, and in the 

 Glacial Drift, find evidence of terrestrial and fluviatile deposits. 

 But the amount of sediment and material in solution brought down 

 by rivers far exceeds that worn away from the land by the sea, 

 and we must remember that so many changes have taken place, 

 that the old soils, river-sediments and lake-deposits have been, 

 except in a few instances, obliterated, for the land has been con- 

 tinually destroyed to form new strata on the ocean-bed. 



Cliffs formed by the sea are cut indifferently through many 

 formations in succession ; its tendency is to form long and com- 

 paratively straight lines of cliff in homogeneous strata ; neither 

 valleys nor narrow and winding inlets are produced by the sea, 

 although it sometimes occupies and enlarges them, while small 

 valleys and combes are frequently cut off abruptly by the sea-cliffs.- 



The shape of the British Isles is, in a great measure, although 



^ The term Meteoric Abrasion, used by Mr. G. P. Scrope, has been adopted by 

 Mr. G. H. Kinahan. 



^ O. Fisher, Q. J. xvii. 2 ; W. Whitaker, On Subaerial Denudation, G. Mag. 

 1867; Jukes, Ibid. 1866, p. 233. 



