THE LAKES OF SNOWDONIA AND EASTERN CARNARVONSHIRE. 463 



Two of the upland valley lakes of Snowdon were sounded, namely, Glaslyn and 

 Llydaw, both of which lie in Cwm Dyli. The depths obtained in the two lakes were 

 remarkably great. 



Glaslyn, situated at an altitude of 1970 feet above sea-level, occupies a cup-shaped 

 depression just under the great precipice of Y Wyddfa, and is a typical corrie lake 

 With regard to the lake Mr Watts says : " Glaslyn is bounded on all sides by live rock, 

 except at and near its outlet. This exit is over moraine, which, however, is evidently not 

 very deep, for rock makes its appearance just below, and in such a way as to almost compel 

 belief in a complete rock bar. Beside the present course of the effluent stream is a 

 parallel strip of moraine running down towards Llyn Llydaw, but living rock soon 

 makes its appearance in this in such a way as to show that if there is any old channel 

 in this direction, it must be exceedingly narrow and tortuous. Thus, if this lake is not 

 contained in a true rock-basin it must be very shallow, or else must have found exit by 

 a gorge quite as narrow as those found at the end of some of the Swiss glaciers." Mr 

 Marr, visiting the lake later, fastened on the " parallel strip of moraine running down 

 towards Llyn Llydaw " as possibly marking the site of a gorge now filled up with drift. 

 Some time afterwards Mr Dakyns pointed out, in his paper on " Some Snowdon Tarns," 

 that solid rock can be seen in the bed of the stream a little below the lake, and " at the 

 old mill, seventy yards from the lake, solid rock extends right across the stream from 

 the Gribbin on one side to the cliffy ground on the other, along which the road from the 

 mines runs." The writer can confirm this statement, at any rate so far as to say that 

 the interval where solid rock cannot be seen is so small that it can easily be stepped 

 over. This point is 40 feet below the level of the lake. The soundings give a greatest 

 depth of 127 feet, and this proves pretty conclusively that the water is held in a rock 

 basin. The contoured map and sections show that the basin has steeply sloping sides, 

 and that the deep water does not extend into the bay on the north side. On each side 

 of the entrance into the small bay are seen sections of what looks like a morainic mound. 

 The cirque or cwm in which such a lake lies is not the work of glaciers, but it is the cup- 

 shaped depression lying at the bottom of such a cirque that has been ground out by the ice. 



Such corrie-lakes or tarns are often very deep in proportion to their extent, and such 

 is the case with Glaslyn. Many niche-like cirques occur which have not a basin or cup- 

 shaped depression in their floor ; such a niche is seen to the north above Glaslyn, and is 

 shown in the photograph of Glaslyn appended (Plate I. fig. 1). It is during the early 

 and late stages of the Glacial Epoch that the small local glaciers in the cwms would 

 hollow out their floors. Owing to the slope of the cirques, the ice would be able to 

 concentrate its poWer upon a small area, and this accounts for the great depths obtained 

 in some of these cirque-basins. That the cirque in which Glaslyn lies underwent severe 

 glaciation is shown by the rounded and smoothed faces of the rocks forming the spur of 

 Y Gribbin at a considerable height above the lake on the south side, and also by similar 

 features seen on the rocks on the opposite side of the lake above the outlet. 



Llyn Llydaw lies in Cwm Dyli, at a lower level than Glaslyn, the altitude of its 



