﻿72 



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



of this Pliocene plateau and the rejuvenescence of the rivers 

 brought about by its uplift. The higher plateaux also form con- 

 spicuous Hats of marshy land where no rock is seen at the surface, 

 but between these several plateaux the rivers have cut deep gorges, 

 the gorges gradually biting back into the flats and exposing bare 

 and fresh rock on their valley- slopes. 



The effect of this rapid erosion is important to the petrologist, 

 for without it he could not have obtained material unaffected by 

 weathering. Now the rivers have ripped through the zone of 

 weathered rock, marked in the mining regions by the ' gossans,' 

 and exposed unweathered rock in which the constituent minerals 

 can be recognized ; whereas specimens collected from near the 

 surface only provide material for the study of rock-decomposition. 



At the same time, these plateaux are interesting as instances of 

 land that has not suffered glaciation, and is still in the condition 

 of the rest of England in pre- Glacial times. It is covered with 

 debris of a weathered country and graded to a featureless plain. 



The Dart. 



In the neighbourhood of Ashburton the Dart has cut a deep 

 gorge partly in granite, but for the most part in metamorphosed 

 killas of Carboniferous age. Its present course is remarkable, and 

 results from a series of diversions, with w^ch I do not propose 

 to deal now ; but there is sufficient evidence to indicate that its 

 former course was nearly due west from Holne Park, past Ash- 

 burton, and thence along the direction and much in the position 

 now followed by the Teign. Reference may be made to the fact 

 that, before entering the sea near Dartmouth, the Dart traverses 

 a wide ridge of land well over 500 feet high, a fact which 

 indicates that in the time when the 430-foot plateau was cut 

 this ridge barred access to the south, and its subsequent breaching 

 must have been effected by a river which afterwards diverted the 

 Dart to its present course. Two of its tributaries : 1 namely, the 

 Eastern and Western Webburn, have cut deep gorges, and are 

 rapidly sawing back into the plateau at 750 feet ; and their re- 

 juvenescence is also due to the uplift after the 430-foot plateau 

 was formed. 



The Becka Brook. 



This river is similar to the others already described, in that it 

 flows sluggishly through peat-flats until it reaches the margin of 

 the 750-foot plateau, down which it forms' a magnificent waterfall 

 (the Becka Falls), and flows thence through a deep gorge by 

 Houndtor Wood, to join the Bovey. This river also flows through 

 a gorge, the noted ' Lustleigh Cleave,' and cuts a sharp trench- 

 like valley in the 430-foot plateau, near Pullabrook. 



The plateau is well preserved on both sides of the Bovey at this 



1 See also 1 The Geology of Dartmoor ' Mem. Geol. Surv. 1912, pp. 69-71. 



