﻿70 



MB. H. DEWEY ON THE ORIGIN OF SOME 



[vol. lxxii, 



country appearing from a distance as an undissected plain. Be- 

 yond the limits of the river's action the plain is covered with stony 

 detritus, which often supports peaty marshes, and contains, locally, 

 pebbles of cassiterite and wolfram. These valuable ores were 

 derived from the kaolinized granite, and are still found in the form 

 of stringers and stockworks threaded through the granite when the 

 china-clay is washed off by hydraulic mining. 



These deposits are the residue of a prolonged denudation of the 

 granite, which must have been very active when the tin was 

 rounded into pebbles and swept forward over the plateau, but 

 diminished later and permitted the accumulation of the ' head ' 

 and peat. 



The rejuvenating effects of the uplift upon the power of the 

 rivers is well represented in the neighbourhood, where some head- 

 waters of the Fowey have cut V-shaped troughs in the wide valleys 

 of earlier streams. 



The plain is further traceable over much of the country between 

 Lostwithiel and Launceston, indicating the former extension of the 

 sea far inland. Near Lawhitton and Lifton many of the hills are 

 truncated, and are merely remnants of the plateau ; but much 

 higher ground bounds them on the east, and sweeps upwards to 

 the high tors of Dartmoor. 



Lydford Gorge. 



The importance of the extent of the plateau along the Tamar 

 Valley is best realized by consideration of the great alteration that 

 it has produced upon the drainage-system of Westejya Dartmoor, 

 including the incision of the deepest gorge in the whole of the 

 West of England, namely, Lydford Gorge. 



The gorge has been described by many writers, including 

 Dr. R. L. Sherlock, 1 who called attention to the different re- 

 sistant powers of metamorphosed and unaltered rocks Avhen 

 attacked by the river, and ascribed to this agency the production 

 of the gorge itself. It is another example of a sunken ravine, 

 which is unseen from the neighbouring country and its existence 

 unsuspected until it is almost entered. 



The gorge arose as a result of the diversion of the Lyd to a 

 shorter course and steeper fall, 3 brought about by the breaching of 

 the side of its original valley by another river. Previous to its 

 present course, the Lyd flowed south past Was Tor (as indicated 

 by a broken line on PL VII) and along the deep wide valley now 

 drained by an insignificant stream known as the Burn. The 

 water-divide between the Lyd and the Burn is near Lydford 

 Junction, and since its diversion the Lyd has cut at this locality 



1 ' The Geology of Dartmoor' Mem. Geol. Surv. 1912, pp. 58-59. 



2 The gradient of the Lyd previous to its diversion was 1 foot in 1 70 ; 

 whereas afterwards it became 1 foot in 39, as measured from fixed points 

 marked by remnants of the Pliocene plateau near Tavistock and Coryton 

 respectively, and the breached valley near Was Tor. 



