THE LAKES OF SNOWDONIA AND EASTERN CARNARVONSHIRE. 461 



Many examples of such U-shaped valleys occur also further south in Merionethshire, 

 one of the most remarkable of which is the Vale of Festiniog. 



The Vale of Conway, which marks the eastern border of Carnarvonshire, presents 

 similar features for a long distance below Bettws-y-Coed. This valley lies along a line 

 of fault, but it has all the appearance of having been over-deepened by moving ice. 

 The side-walls are steep, being remarkably precipitous on the left or Carnarvonshire 

 side. Here the tributary valleys join the main valley at a considerable height above 

 its present floor, and we see, too, a series of streams — Afon Crafnant, Afou Ddu, Afon 

 Porthulwyd and Afon Dulyn, cascading down the precipitous walls to join the river 

 Conway below. 



If glaciers have thus in Wales, also, eroded the channels along which they flowed, 

 the excavation of rock-basins below the general level of the valley floor at certain places 

 where the conditions were especially favourable, need no longer excite surprise or be looked 

 upon as anything more than subordinate incidents in the general history of ice-erosion. 

 Turning to the lakes themselves, we shall consider first those lying at low levels in the 

 valleys which encircle the foot of Snowdon. The lakes occupy in their respective 

 valleys just those positions in which the glaciers might be expected to have carried on 

 most actively the work of erosion. 



The Lakes of Llanberis. — These lakes lie at the distal end of the steep and narrow 

 Pass of Llanberis, occupying the lower reaches of the valley for a distance of over three 

 miles. We may consider the lakes as forming one sheet of water, and originally this 

 was of greater extent, as shown by the alluvial tract stretching up from the head of 

 Llyn Peris to Gwastad-Nant. From the top of the Pass down to this point the incline 

 is very great, but below Gwastad-Nant the valley loses its steepness. Looking down 

 from the summit of one of the spurs in the Pass, one can see the alluvial stretch, and 

 the surface of the lakes extending as a straight flat plain as far as the lower end of Llyn 

 Padarn (Plate II. fig. 1). The valley is held in on either side by steep rocky hills, and it 

 narrows towards the foot of the lake. Nowhere in North Wales are the signs of glacial 

 action so striking as on the rocky slopes which border the lakes. The rocks are smoothed, 

 rounded, and striated from a height of several hundred feet above the lakes right down 

 to the water's edge, and the strise all run north-westerly — in the direction of the valley. 

 Here, if anywhere, we have all the conditions requisite for the hollowing-out of a rock- 

 basin. The glacier, as it emerged from the steeper part of the valley and attained the 

 more gently inclined reaches below Gwastad-Nant, would exert greater pressure upon 

 its rocky bed, and as the ice would be thus retarded in its flow, this pressure would be 

 still further increased by the heaping up of the ice to form a mass of greater thickness. 

 Mr Marr has suggested that the strip of low ground occupied by alluvium, which leaves 

 the lake on the western side near the lower end, may mark the site of a drift-filled 

 depression. But there is no evidence to show that the drift occurs here to any great 

 thickness. The form of the hollow in which the rock lies, favours the view that we 

 have here a rock-basin. The contoured maps and sections show that Padarn and Peris 



