EROSION AND THE SUMMIT LEVEL OF THE ALPS ii 



of a renewed upheaval which took place after a more or less complete 

 degradation of the region as prompted by an earlier upheaval. 

 Early recognition of this principle is found in the writings of Ramsay, 

 'Powell, and others. Nearly eighty years ago, Ramsay clearly ex- 

 plained the possibility that the hills of South Wales might result from 

 the incision of valleys in an uplifted plain of marine denudation, 

 which had been abraded across a mountainous area of earlier defor- 

 mation and uplift,^ and the latest physiographic studies of this region 

 give good reason for thinking that the old master was right, at the 

 same time amplifying his view by showing that the present hill-top 

 levels indicate the occurrence of two plains of marine origin; an 

 inner and higher plain partly consumed by the undercutting of 

 an outer and lower one, the two being separated by a scarp which, 

 although still recognizable, is now like the two plains well dissected.^ 

 Twenty years later Powell, when he visited the Rocky Mountains 

 of Colorado in 1867 with a party of students, came upon a similar 

 principle, namely, that the mountains of today may be in a second 

 cycle of erosion after almost complete obliteration of more primitive 

 mountains by subaerial degradation in an earlier cycle; but his 

 only mention of this discovery s in a brief statement published 

 some years later, where it did not attract attention.^ In the same 

 later volume he first announced an explanation of the Basin ranges 

 which was later very generally accepted, his idea being that before 

 the upheaval of the existing ranges their region was "a compara- 

 tively low plain, constituting a general base level of erosion to which 

 that region had been denuded in Mesozoic and early Tertiary time 

 when it was an area of dry land ; for I think that from the known 

 facts we may reasonably infer that the Basin ranges, though 

 composed of Paleozoic and Eozoic rocks, are as mountains of very 

 late upheaval."* That was a very sagacious utterance for its time. 



^ A. C. Ramsay, "Denudation of South Wales," Mem. Geol. Surv. Great Britain, 

 Vol. I (1846), p. 327. 



^ The only published statement of these later studies, made by O. T. Jones, is 

 contained in a rather inaccessible volume: Souvenir of the Aberystwyth Conference, 

 National Union of Teachers, igii. Edited by John Ballinger. Published by the 

 National Union of Teachers, Bolton House, Russell Square, London, 191 1. 



^Geology of .... the Uinta Mountains, Washington (1876), p. 27. 



4 Ibid., p. 32. 



