390 Mackintosh— On the Cliffs and Valleys of Wales. 



furrow the south-eastern side of the hilly desert of Central Wales, 

 which at one time must have been a comparatively level table-land. 

 To assert, however, that the same agency could have levelled down 

 the plain would be to attribute to rivers a power of performing the 

 most contradictory feats. To be consistent, subaerialists must admit 

 that there is a tendency in brooks and rivers to persist in the channels 

 they have selected, and to wear these channels dowmoard as long as 

 there is a sufficient inclination to give excavating power. A river 

 when it enters a plain may wander over its surface to any extent ; 

 but a plain even approximately level, or a wide smooth valley or 

 basin, must be the result of a wide-spread denuding action. That 

 the sea, under certain conditions, planes down the land all must ad- 

 mit ; but it is likewise, under other conditions, capable of producing 

 inequalities. Eivers (apart from deposition during floods) can only 

 produce the latter. 



The Great Bemidation Puzzle. — One of the best districts for study- 

 ing denudation in South Britain may be found between Newbridge 

 and Marteg Bridge. On following the course of the Wye from New- 

 bridge northwards, you pass through a narrow gorge, and soon find 

 yourself in the open valley of Doldowlod. You see on your left a 

 transverse gorge running into the bosom of the hills and suddenly 

 terminated by a cliff. You can scarcely resist the idea that this is 

 an old inlet of the sea scooped out backwards, and not excavated by 

 any descending stream. Still following the Wye, you again pass 

 through a narrow gorge, and suddenly arrive in the irregular plain 

 of Ehayader. Within the space traversed you have had a sufficient 

 example of what may be called the Great Denudation Puzzle — the 

 problem to be grappled with before any real progress can be made 

 in determining the relative claims of the sea and rivers, namely, the 

 cause of the narrow gorges which connect comparatively wide and 

 level areas — ■gorges which cut through ridges, escarpments, and 

 sometimes table-lands. Two theories have been advanced by the 

 subaerial school of geologists to account for these gorges. 



Professor Bamsay's Theory. — The following explanation is given 

 by that formerly able advocate of marine denudation. Professor 

 Ramsay : — " It is a trick that rivers have ; they will cut through 

 escarpments in what seems an unnatural fashion " (Physical Geology 

 of Great Britain, p. 145).^ But the very abrupt commencement of the 

 gorges under consideration would seem to be inconsistent with the 

 idea of their having been worn through by any obliquely-directed 

 aqueous action, or an action which must have had a vent along 

 the base of the escarpment while the cutting-through process was 

 going on.^ 



Professor Juices^ Theory. — Professor Jukes has lately advocated the 



1 Professor Ramsay says of the estuary of the Humber (op. cit. p. 147), " The sea 

 ^ected a breach in the rocks. Then suppose these lands to have been heaved up, 

 .... the river then ran through it." Prof. Eamsay says much more in proof that 

 he does not deny the efficacy of marine denudation, but we have not space to quote it 

 here. — Edit. 



* Many of the gorges are so narrow that a river could not have found room to bend 

 TOTind and return during the progress of the supposed excavation. 



