572 Geological /Society. 



plateau. This plateau is dissected by deep gorges, with their walls 

 scarred by potholes through which the rivers flow in a series of 

 waterfalls, cascades, and rapids. 



This plateau is terminated inland by degraded cliffs rising 

 abruptly from 400 feet above sea-level, while the plain slopes 

 gently to the recent sea-cliffs, mostly over 300 feet high. The 

 plateau has been cut across rocks of different degrees of hardness, 

 and is overlain by deposits of detritus and peat. Wherever the 

 plain occurs, the scenery is featureless, and the land boggy and 

 waterlogged. 



The widespread occurrence of this plain over Cornwall and Devon 

 at a uniform height suggests that in its final stages it was a plain 

 of marine erosion. The author accepts Mr. Clement Reid's con- 

 clusion that its date is not later than Pliocene. Its uplift in post- 

 Pliocene times led to rejuvenescence of the rivers, initiation of 

 coastal cascades, and the production of gorges aided by the 

 formation of potholes. 



At Lydford, on the western flank of Dartmoor, the uplift led to 

 the diversion of the Lyd by a stream that breached the valley- 

 side and tapped the head-waters of the river. 



A small stream, the Burn, riow flows past Was Tor and Brentor, 

 through the valley formerly occupied by the Lyd. The shortened 

 journey to the lowlands bestowed such enhanced cutting-power 

 upon the river that it quickly incised a chasm through which 

 it now flows more than 200 feet below the base of its former 

 valley ; while a tributary enters as a waterfall from a hanging 

 valley near Lydford Junction. 



The elevation of the land also led to formation of gorges of 

 similar character in other upland plateaux. These plateaux have 

 been described by Mr. Barrow in the Quarterly Journal of the 

 Geological Societ}^ and reference is made to them by the author 

 in connexion with the effects upon them of the uplift. 



There are thus in Cornwall and Devon two characteristic types 

 of scenery, to which in great part these counties owe their charm. 

 Wide featureless plains covered with heath and marshland and 

 dominated by tors and crags, on which the drainage is sluggish 

 and vague, alternate with deeply- incised rocky ravines where rivers 

 flow as rapids and cascades. These two types mark successive 

 periods of erosion. Post-Pliocene uplift gave such increased 

 cutting-power to the rivers that they quickly incised chasms in 

 their former valleys, employing while so doing the activity of 

 waterfalls and rapids. 



