﻿part 1] RIVER- GORGES IN CORNWALL AND DEVON. 65 



were not traced by the Geological Surveyors. The deposits on 

 St. Austell Moor have been dredged and turned over in search of 

 stream-tin and wolfram, and are everywhere detritus composed of 

 granite, quartz-sand, and clay, but no shells are recorded. The 

 deposits, however, represent a long-continued process of surface- 

 erosion, partly in Pliocene and Glacial, and to some extent in post- 

 Glacial, times. 



The evidences of marine erosion are therefore well-marked : 

 namely, the similarity of height of the plain above the sea over 

 wide areas, the degraded sea-cliff, and the beds of pebbles. 



This planation repeated a process which had affected the topo- 

 graphy of Cornwall and Devon on at least two previous occa- 

 sions ; but the two earlier plains have already been described by 

 Mr. G. Barrow, 1 and reference will only be made to them when 

 necessary. 



There are thus two principal types of scenery in Cornwall sharply 

 contrasted one with the other : namely, the upland moors and the 

 rocky gorges — one brought about by prolonged denudation, and 

 the other by acceleration of river-erosion resulting from sudden 

 uplift of the land. 



The gorges, however, show in plan sinuous or meandering courses, 

 and thus indicate that, according to current views, the land was 

 not uplifted by a slow continuous process, but rather in two 

 stages. The Tamar exemplifies these two stages when its course 

 is viewed on a map. It flows in a series of loops, often nearly 

 circular, in a deep gorge. The loops were initiated when the 

 river flowed sluggishly across a flat, but on uplift of the land it 

 commenced to dig itself in, and has since continued in its early 

 channel ; or else it has followed lines of structural weakness, such 

 as are known to occur in Cornwall and Devon. 3 



Both the planation and the uplift have had important effects 

 upon the drainage of Dartmoor. At the period of emergence 

 Cornwall and Devon probably consisted of a step-like series of wide 

 flats bearing rocky tors, separated one from the other by steep 

 slopes, and drained by sluggish streams. As elevation ensued, the 

 rivers gained acceleration, and cut back into the flats to form 

 gorges. These features are met with repeatedly in Cornwall and 

 Devon, and are especially conspicuous on Dartmoor in the valleys 

 of the Tavy, Lyd, Dart, and Teign. 



The Gorges. 



Where the plain was undercut by the sea it was bounded by 

 cliffs, and the rivers formed waterfalls when they reached the edge 

 of the cliffs. These falls immediately commenced sawing down- 

 wards and backwards into the land, so rapidly in some cases that 

 they led to the formation of gorges. 



1 Q. J. G. S. vol. lxiv (1908) pp. 384-400. 



2 See ' The Geology of Dartmoor ' Mem. Geol. Surv. 1912, p. 64. 

 Q. J. G. S. No. 285. f 



