Mackintosh — On the Cliffs and Valleys of Wales. 395 



hills between Cader Idris and the river Mawddach are generally 

 very serrated (see Plate XV. Fig. 11, which represents one of these 

 ridges as seen from near Dolgelley) ; and the inequalities in detail 

 present a more or less sea- worn configuration. Before quitting the 

 Cader district, I may remark that I saw one striking instance of 

 what I have occasionally noticed elsewhere, namely, a stream flowing 

 very nearly along the summit of a rocky ridge, and apparently pos- 

 sessed of as much denuding power as streams in the neighbouring 

 hollows. It may be seen about halfway between Llyn Guernan and 

 Llyn Gafr, 



Torrent-ruts on the Kerry Hills. — The previous remarks may pre- 

 pare the tourist for understanding how the grass-covered escarp- 

 ments he may see on his right hand on travelling from Llanidloes to 

 Newtown, should present no signs of mere pluvial action. At con- 

 siderable intervals, however, the otherwise "smooth and continuous 

 outline of these escarpments is broken by temporary or permanent 

 torrent-ruts, and occasionally good-sized gulleys. Kerry Hill, to the 

 south of Newtown, presents the best example I have yet seen of the 

 relative eifects of subaerial and marine denudation. The north- 

 ern slope is wonderfully smooth and continuous. The ruts and 

 narrow gulleys caused by rain water collecting at intervals, and by a 

 few small streams, are evidently disfiguring and not forming the 

 inclined plane of marine denudation represented in Plate XV. Fig. 9. 

 Farther towards the east, in the direction of Bishop's Castle, the 

 hills are deeply indented by gorges and large Y-shaped hollows, 

 which I had not time to examine. 



Betrogressive Excavation hy Waterfalls (?). — About a mile and a 

 half to the south of Newtown there is a short deep gorge terminated 

 by a high cliff with a picturesque waterfall. But the latter cannot 

 be credited with having excavated the gorge backwards for the 

 following reasons : the breadth of the stream and of its channel 

 above is not equal to the extent of the cliff, and the continuity 

 of the face of the cliff has not been broken or indented by the 

 waterfall, but presents the appearance of the inner precipice of a 

 previously scooped out hollow. The same remark applies to many 

 of the waterfalls of Wales, which are merely falls or slides over 

 transversely-continuous cliffs. The waterfall near Aber furnishes a 

 striking instance of a stream tumbling over a long and continuous 

 chff of hard rock, which forms the inner boundary of a valley con- 

 taining marine drift. At the bottom of this valley, and distinct 

 from its general outline, the stream flows along a clearly defined 

 channel. A similar channel may be seen above the fall ; both are 

 the work of the stream, but the falling over the cliff is merely an 

 accident in its history.^ 



^ To the west of Aber, the channel escavated by the Ogwen may be traced from 

 the sea to Llyn Ogwen as a depression distinct from the outline of the wide area over 

 which it flows in the lower part of its course, and equally distinct from the great gorge 

 of Nant Francon, which, with the precipice of Ben Glog, must have existed before 

 the supplementary action of the river commenced. Fig. 10, Plate XV., is a view (from 

 a photograph) of all that a considerable-sized tributary of the Ogwen has been able 

 to effect ia modifying the outline of Nant Francon. 



