450 Maw—On Subaerial and Marine Denudation, 



consider himself at issue with subaerialists in attributing to the sea 

 the planing down of flat table lands. The discussion commenced 

 " On the Formation of Hills and Valleys," and there has been no 

 point more strongly insisted on by those who attribute the sculpturing 

 of the land to subaerial agency, than the contrast between the work 

 of the sea, as expressed in the level surfaces of flat-topped ranges and 

 elevated plains, and the work of rains and rivers in excavating the 

 intersecting valleys. Professor Eamsay, in the concluding paragraph 

 (p. 237) of his '' G-eology of North Wales," says — " To the eye of on© 

 who appreciates the physical features of a country there is, indeed, 

 on ascending a height, nothing more striking than the average 

 flatness of the tops of many of the hills, especially when the rocks 

 composing them are of tolei-ably uniform texture — a flatness, be it 

 remembered, not connected with anything like a horizontal position 

 of the beds, for everywhere they are contorted and often stand on 

 end. All Wales shows this feature from the Towey to the slaty 

 hills that flank Cader Idris and the Arans on the south and east, and 

 even in the mountain land, from Cader Idris to the Menai Straits, 

 traces of a similar approximate uniformity in height are plain to the 

 experienced eye, — showing the relics of an old form of ground in 

 which deep valleys have been not rent but scooped out. In lower 

 ground, the features of Denbighshire, east of the vale of Clwyd, in 

 a remarkable manner agree with these principles. There, in an 

 average table land, tlie result of quiet marine denudation of disturbed 

 strata, innumerable valleys have been cut out of the solid mass, the 

 accmnulated draining of which forms streams of tolerable size, 

 which, from higher to lower levels, have gradually cut through an 

 unfaulted escarpment of Carboniferous Limestone on their way to 

 join the Clwyd. Such principles, I am convinced, give, when well 

 considered, the true key to the meaning of the present outlines of the 

 country ; and few subjects in physical geology woidd promise greater 

 interest than a complete account of the denudations by which, after 

 the disturbance of the strata, Wales assumed its present form." 



Striking examples of these level platforms also occur in the uni- 

 form outliue of the surface of the Mountain Limestone, Durdliam 

 Down, near Bristol, at an average height of 280 feet above the sea, 

 and the level top of the headland of Devonian Limestone bounding 

 Babbicombe Bay, east of Torquay, given by Mr. Chambers in his 

 "Ancient Sea Margins" (page 246) as 278 feet high. Of another 

 similar platform' on the south-west side of Tor Bay, Mr. Chambers 

 remarks : " The one which forms the south side of Tor Bay dividing 

 it from the valley of the Dart, has a remarkable ajjpearance from 

 Hope's-nose, the opposite promontory, for its flatness is like that 

 which we should make m a drawing with a ruler, and it perseveres 

 for many miles inland." The absolute identity of height between the 

 Bristol platform, and that at Babbicombe Bay is remarkable ; Berry 

 Head, though supposed to be less by Mr. Chambers, is, I believe, 

 identical in height, and it seems highly probable that these uniformly 

 flat surfaces were all contemporaneously ploughed down by the sea. 



The whole tendency of the sea appears to be to work on as straight 



