HISTORY OF THE DEE, WALES. 225 



In the most elevated parts of North Wales, the height of the 

 highest mountains as they now stand between Cader Idris, Aran 

 Mowddwy, and Snowdonia varies from about 2900 to 3571 feet. 

 During the time of submergence, when the ground further east was 

 buried under Carboniferous strata, the western mountain region 

 stood well above the level of the sea, and higher relatively to the 

 plain of denudation than it does now ; for long before the Carboni- 

 ferous epoch, and ever since that time, it has suffered from un- 

 ceasing subaerial degradation, and long-lost hills and valleys must 

 have prevailed in that old land, the shape of which it is now vain to 

 speculate about. 



Outside of this region, after the partial removal of the Carboni- 

 ferous rocks, there lay, as already said, a wide tableland, extending 

 far to the south, and also to the east and north-east. In the latter 

 district this tableland sloped gently north-easterly and easterly; 

 judging by those elevated parts of it that still remain more or less 

 entire, in a distance of about 30 miles between Aran Mowddwy and 

 the highest part of the Carboniferous escarpment, not far north of 

 the Dee, the slope probably fell in height 700 or 800 feet, at an 

 angle of about 14°, equal to 23 feet in a mile. 



When by the drainage of this old land, the Dee began to flow in 

 its earliest channel, induced by minor undulations of the ground, it 

 is clear that its present source, Bala Lake, had no existence ; for 

 whereas the river at that time must have flowed on a surface of 

 land not less high than that on either side of the present valley near 

 Corwen and Llangollen (now, in places, from 1600 to 1800 feet high), 

 the surface of Bala Lake is only GOO feet above the level of the sea, 

 while the neighbouring watershed between the lake and Dolgelli is 

 only 200 feet higher. As the river could not flow up hill, it is clear 

 that in that early stage of its history the valley of the Dee about 

 Bala must have been at least from 1300 to 1400 feet higher than it 

 is now, and consisted of a mass of Silurian rocks, great part of 

 which has since been removed by denudation. 



As that country, according to the view already stated, has in the 

 main been land ever since the Permian epoch, it seems to me that 

 this waste of rocks was essentially produced by watery erosion, aided 

 in a much less degree at a late epoch by glacier ice; and thus 

 it happened that the Dee, a river of very ancient date, wandering 

 hither and thither, by degrees deepened its channel in the same 

 manner that the Rhine and the tortuous Moselle have cut out theirs, 

 as described in my memoir " On the Physical History of the Valley 

 of the Rhine"*. 



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 ; for when the Dee began to 

 run, the Carboniferous escarpment had no existence, and the strata 

 of these formations stretched further to the west, ending along some 

 line now unknown in a sort of feather edge, and forming part of the 



* Quart. Journ. Geol. Soc. vol. xxx. (1874), p. 81. 



