Rivers of Wales. 523 



the rivers of Wales, whether flowing through Silurian, 

 Old Eed, or Carboniferous rocks, have been busy scoop^ 

 ing out their valleys ever since the close of that great 

 continental epoch that ended with the influx of the 

 Ehsetic and Liassic sea across the Triassic salt lakes, 

 and though these valleys were modified by ice, and 

 partially filled with detritus, during a short episode 

 of submergence in glacial times, the rivers re-asserted 

 their rights to their old channels when emergence took 

 place. All the important rivers, therefore, that flow 

 east and west and north and south through the Silurian 

 rocks of Wales, are in their origin approximately of the 

 same age, and from Cader Idris to Pembrokeshire they 

 have all cut their way through a tableland with minor 

 undulations, while here and there remains a higher 

 hill, the rocks of which were unusually hard. This 

 old upland was indeed of great extent, and its relics 

 stretch far and wide into the northern part of Denbigh- 

 shire, and into Montgomeryshire and South Wales. 

 As already stated, standing on the summit of Cader 

 Idris or of Aran Mowddwy, 2,960 feet high, and looking 

 east and south, the eye, as far as it can reach, ranges 

 across a vast extent of old tableland, the plane surface 

 of which near the Arans is about 1,900 feet above 

 the level of the sea, or more than 1,000 feet below the 

 summits of the neighbouring mountains. All inter- 

 sected by unnumbered valleys, to the ordinary observer 

 it is merely a hilly country, while an eye versed in 

 physical geology at once recognises that all the 

 diversities of feature are due to fluviatile erosions that 

 have scooped out the valleys. 



For this reason it also happens that the Dee now 

 cuts right across the Carboniferous escarpment west 

 of Erbistock and the lower area of the Permian strata ; 



