466 MR T. J. JEHU ON 



irregular outline, and do not conform to the trend of the lake margins or to the land 

 contour-lines above. The snows gathering in the cirque-like head of the valley would 

 give birth to a glacier whose motion would be so much impeded at the narrow gorge that 

 the ice would heap up to a great thickness, and exert a greatly increased pressure on 

 the valley floor, and so excavate a rock-basin. 



Llyn Geirionydd is possibly in part drift-dammed, for though live rock can be 

 seen in the stream at some distance below the exit, a considerable portion of the valley 

 floor near the sides is covered with drift. The lake proved to be shallow, the greatest 

 depth being only 48 feet. This occurs near the head of the lake. There is an irregularity 

 on the western margin, but the 20 feet contour-line is not affected by it. When the 

 water is low, rocky knobs can be seen near the foot of the lake. These are severely 

 glaciated on their upper sides. The configuration of the valley would favour the 

 work of ice-erosion, and so it is not unlikely that its waters are partly retained in a 

 rock-basin. 



Llyn Cawlyd lies at an elevation of 1164 feet. It occupies the higher reaches of the 

 valley, and is held in at the sides by high mountains. The watershed at the head of 

 the lake is low, and some of the ice moving down in the direction of Nant-y-Benglog 

 was probably diverted into this valley, where the lake lies. The valley is very narrow 

 above, but lower down it widens, and so the ice-stream which was confined within a 

 narrow space above would spread out here, and its erosive power would be much 

 diminished. Below the lake the valley slopes gently eastwards, and is covered with 

 drift. The thickness of the drift is not known, but at a level of about 100 feet below 

 the lake, masses of rock appear in the valley floor. The lake attains a depth of 222 

 feet, being the deepest in the district. Its floor is very regular, and has the form of a 

 long trough, deepest in the middle, and basin-shaped in cross-section. The probability 

 is that the valley is partly blocked by drift, but that this is not of sufficient thickness 

 to account for the great depth of the lake. Ice-erosion will also have played an 

 important part in the formation of the lake-basin. 



Llyn Eigiau is situated at an altitude of 1217 feet in the lower reaches of Cwm 

 Eigiau, just where the valley widens. It is very shallow, the greatest depth obtained 

 being only 32 feet. Drift and alluvium cover the valley floor at the foot of the lake 

 and along its eastern shore. This drift is probably sufficient to account for the present 

 depth of the lake. 



When the lakes are thus studied in their relation to the glaciation of the district, 

 and in the light of recent observations on the eroding power of glaciers, we are driven 

 to conclude that whilst some may be partly rock-basins and partly barrier- basins, and a 

 few simply barrier-basins, most of them lie in rock-basins. 



(This memoir embodies the results of research work carried on by the author as 

 He riot Fellow of the University of Edinburgh.) 



