HLSTOEY OF THE DEE, WALES. 221 



of the alluvial tract at its eastern end. Beyond this the Dee runs 

 through a narrow valley, with solid rocks down, or almost down, 

 to its banks, where in older times the original outflow of the 

 Lake was; and it may be considered certain that the old point 

 of outflow is a true rock barrier. If, however, there should still 

 be any doubt about this gorge having formed the original rocky 

 dam of Bala Lake, there remains other evidence on the question of 

 its being a true rock-basin; for, about two miles further on towards 

 Llandrillo, the rocky bottom of the river is from 500 to 510 feet 

 above the sea, while the bottom of the lake is only 470 feet above 

 that level ; and even beyond this, at the ferry halfway between 

 Corwen and Llangollen, 14 miles in a straight line E.^.E. of the 

 lake, the rocky bottom of the river is about 475 feet above the sea, 

 or 5 feet above the bottom of Bala Lake. As there is not, and never 

 could have been, any other possible outlet for the water of the lake 

 than the channel of the Dee, it is quite certain that it lies in a rock- 

 bound basin ; and indeed it seems to me more than likely that the 

 large alluvial flat more than six miles in length between Llandrillo 

 and Corwen may conceal one or more lake-basins, now filled with 

 detritus brought down to the hollow by the numerous streams, large 

 and small, that, over this space, now form tributaries of the Dee*. 

 Similar alluvial flats, once lake-basins, are common in many old 

 glacier-valleys in Switzerland, Cumberland, Scotland, and elsewhere. 



The glaciers that did this work of erosion came chiefly from both 

 sides of Aran Mowddwy, down Cwm Croes and tfre valley of 

 Dwfrdwy; and these converging near where Llanuwehllyn now 

 stands, and aided by other tributary ice streams, scooped out the 

 basin of Bala Lake, and went far beyond its ancient boundary, in a 

 valley older than the beginning of the Glacial epoch, like all the other 

 great valleys of AValcs. 



Such being certainly the case with this upper part of the valley of 

 the Dee, it is natural, and indeed inevitable, to suppose that the 

 further continuation of the valley towards and beyond Llangollen is 

 also of preglacial origin; and this, in my opinion, seems to be 

 strengthened by the erratic blocks and glacial striations mentioned 

 by Mr. Mackintosh in a memoir on boulders and erratics in Denbigh- 

 shire, even though the author seems doubtfully to attribute some of 

 these striations to the action of floating icef. If, for the sake of 

 argument, we assume that this part of the valley (unlike the upper 

 part) is not preglacial, then the mass of country through which it 

 runs must have formed a dam to the drainage of the waters of the 

 upper Dee, which would have had the effect, even if these waters 

 were only raised 200 feet, of filling with a lake ramifying in all di- 

 rections among the hills, vast tracts of Merionethshire. Of this 

 supposititious lake there is no sign whatever. 



* All the heights mentioned above were determined by angular measurements 

 with the theodolite by the late Professor Jukes, Mr. Aveline, and others. They 

 are therefore not to be depended on within a few feet; but any possible errors 

 of this kind are too small to affect the general argument. 



1 Quart. Journ. Geol. Soc. toI. xxx. p. 711 (1874). 



r2 



