﻿74 MB. H. DEWEY ON THE ORIGIN OF SOME [vol. lxxii, 



Plate VI. 



St. Nectan's Kieve, the Rocky Valley, near Tintagel. This view shows the 

 waterfall cutting a gorge by means of potholes. The ' kieve ' or bowl is 

 behind the hole through which the water issues to form the second fall into 

 the large pothole in the foreground, whence it emerges as a cascade into the 

 valley. The steepness of the walls of the gorge is notable. (See p. 68.) 



Plate VII. 



Map illustrating the present and the former course of the River Lyd, on the 

 scale of 3 inches to the mile, or 1 : 21,120. 



Discussion. 



Mr. G. B arrow drew attention to the special interest that the 

 phenomena described by the Author had for those who were well 

 acquainted with areas that had been glaciated. In Devon and 

 Cornwall there had been no glaciation in the ordinary sense of the 

 term, yet many of the valleys showed phenomena often held to be 

 distinctive of glaciated areas. The phenomena seen in the South- 

 West of England could be clearly traced to an uplift taking place 

 more rapidly than the rivers could cut back, deepen, and widen 

 their valleys. Prof. Garwood, in a paper read before the Society, 

 had claimed for certain hanging valleys in the Alps that they are 

 not necessarily connected with glaciation, and with his view the 

 speaker agreed. The truth is that a rapid uplift produces con- 

 ditions similar to some of those that result from the covering of an 

 area with an ice-sheet. Besides producing hanging valleys, the 

 rapid uplift often results in a total change of course of some of 

 the rivers. This is best shown by the Fowey, the oldest course of 

 which was seaward in an easterly direction : later on it flowed west 

 past Gossruoor, to the sea at what is now Newquay Bay ; its 

 modern course is southward. The changes took place during the 

 rapid uplift that followed the 750-foot and the 430-foot platforms. 



An interesting feature of the whole area is the clear evidence of 

 a pluvial period, or one of especially heavy rainfall, practically 

 contemporaneous with the Glacial Epoch, strongly supporting the 

 modern view that the ice-sheet of the northern areas was largely 

 due to increased precipitation. The Fowey, again, gives clear 

 evidence of this : where it flowed along a specially flat section of 

 the valley, above the 750-foot platform, at the time of the pluvial 

 period, everything but gravel was swept away, and the tin and 

 wolfram-ores concentrated. At the present day, the river is unable 

 to keep the base of the valley open, and fine sediments have 

 accumulated above the gravel to a thickness of over 20 feet. 



Dr. R. L. Sherlock remarked that he had mapped one part of 

 the region dealt with. He believed the explanation given by the 

 Author of the present course of the Lyd was correct, and that 

 the river formerly flowed along the line of fracture now occupied 

 by the River Biu'n, a tributary of the Tavy. He was doubtful 

 whether the whole of the 430-foot platform was eroded by the sea, 



