378 DAVIS — PLAINS OF MARINE AND SUBAERIAL DENUDATION. 



which the rival conclusions may be distinguished, the test being devel- 

 oped from a study of the natural history of rivers. 



The English School. 



Ramsay is believed to have been the first advocate of marine erosion 

 as an agency for the production of broad plains of denudation. In de- 

 scribing the action of the sea on the land he wrote : 



"The line of greatest waste on any coast is the average level of the breakers. 

 The effect of such waste is obviously to wear back the coast, the line of denudation 

 being a level corresponding to the avera^^e heij^ht of the sea. Taking unlimited 

 time into account, we can conceive that any extent of land might be so destroyed, 

 for though shingle beaches and other coast formations will api)arently for almost 

 any ordinary length of time protect the country from the further encroachments 

 of the sea, yet the protections to such beaches being at last themselves Avorn 

 away, the beaches are in the course of time destroyed, and so, unless checked by 

 elevation, the waste being carried on forever, a whole country might gradually 

 disai)p9ar. 



"If to this be added an exceedingly slow depression of the land and sea bottom, 

 the wasting process would be materially assisted by this depression, brin<2;ing the 

 land more uniformly within the reach of the sea, and enablinir the latter more rap- 

 idly to overcome obstacles to further encroachments, created by itself in the shape 

 of beaches. By further gradually increasing the depth of the surrounding water, 

 ample space would also be afforded for the outspreading of the denuded matter. 

 To such combined forces, namely, the shaving away of coasts by the sea, and the 

 spreading a])road of the material thus obtained, the gix^dt plain of shallow sound- 

 ings which generally surrounds our islands is in all i)robability attributable."^' 



At this early date Ramsay attributed not only the plains themselves, 

 but also the valleys which now interrupt ancient and uplifted plains of 

 denudation, in greatest part to marine action, and allowed but little 

 effect to subaerial denudation. On this topic he said : 



"The power of running water has also considerably modified the surface, but 

 the part it has played is trifling compared with the effects that have sometimes 

 been attributed to its agency. ... In the larger valleys, where the streams 

 are sluggish, instead of assisting in further excavations, the general tendency is 

 often rather to fill up the hollow with alluvial accumulations, and so help to 

 smooth the original irregularities of the surface." f 



Thirty years later Ramsay ascribed greater results to subaerial agents. 

 Referring to the generally even sky-line of South Wales, he wrote : 



" The inclined line that to.uches the hilltops mustliave represented a great plain 

 of marine denudation. Atmospheric degradation, aided by sea waves on the cliffs 

 by the shore, are the only powers I know of that can denude a country so as to 



* Denudation of South Wales. Mem. Geol. Surv. Great Britain, vol. 1, 184r», p. 327. 

 flbid., pp. 3:52, 333. 



