220 A. C. RAMSAY ON THE PHYSICAL 



it is preglacial. The broad features of the history of this part of 

 the valley are sufficiently obvious ; but from Erbistock to the source 

 of the river in Bala Lake, the case is different ; and the physical 

 history of that long and often steep-sided river-valley has, as far 

 as I know, not been analyzed. 



At Holt, seven miles south of Chester, the river is about 100 feet 

 above the level of the sea; near Erbistock, eight miles further 

 south, where the valley enters the Permian strata, about 225 feet ; 

 in the vale of Llangollen, four miles west of the town, about 475 

 feet; near Corwen, about 500 feet; while the level of the lake where 

 the Dee rises is only 600 feet above the level of the sea. The fall, 

 therefore, from Erbistock to Chester, where the river is tidal, is 

 about 225 feet in a north and south distance of 15 miles as the crow 

 flies, and from Bala Lake to Erbistock about 375 feet on a direct 

 line nearly east and west ; so that the average slope is nearly the 

 same the whole way, or about 15 feet per mile, taking no account 

 of the numerous minor curvatures of the river. 



I have said in previous memoirs that the main great contours of 

 "Wales were approximately the same as now, before the beginning of 

 the Glacial epoch, and that all the glacial striations of the mountain 

 regions of North Wales correspond to the trend of the greater 

 valleys. The chief effect of the glaciers was therefore to grind and 

 smooth the rocks into moutonnee forms in the bottoms of the valleys 

 and on the sides of the mountains, and in doing this more or less to 

 deepen and widen these valleys, and here and there to scoop out 

 those rock-bound lake-basins so common in all old intensely gla- 

 ciated regions, of which Bala Lake is one. The proof of its being a 

 true rock-basin is simple. 



At present the outflow of the lake is through an alluvial flat, the 

 western end of which helps to dam up the water. But this allu- 

 vium, as already shown, is the gift of the tributary rivers that here 

 enter the Dec. In older times the lake was more than double its 

 present length ; for while a mile and a half of delta at its south- 

 western end, where the river Wnion enters, marks its ancient extent 

 in that direction, the sheet of water also once extended 2% miles 

 east of Bala over the area now occupied by alluvium, giving the 

 ancient lake a curved form, before torrents and rivers from the north 

 and south filled that space with detritus borne down valleys that 

 penetrate far in among the hills. The effect of this is, that the 

 lake now is little more that 3-i- miles long, while formerly it was 

 8 miles in length. 



Bala Lake being 600 feet above the sea, and about 130 feet deep, 

 its bottom is 470 feet above the sea-level : and probably its rock- 

 bottom may be deeper still ; for two large and three small streams 

 enter the lake on either side as it now stands, in addition to the 

 main rivers of Afon Llyn, and Dwfrdwy, which have deposited the 

 delta at its south-western end; and doubtless the bottom of the 

 lake, by means of these rivers, may be covered with detrital matter. 



But even assuming that the present depth of the lake represents 

 its rocky bottom, the fall of the river is only a few feet in the whole 



