452 MR T. J. JEHU ON 



In Llyniau Mymbyr (Capel Curig), which lie at an elevation of over 500 feet, and 

 originally formed one sheet of water, we meet an example of a lake occurring at the 

 lower end of a mountain valley (Nant-y-Gwrhyd), just above its junction with another 

 mountain valley (Nant-y-Benglog). Below the lake these two mountain valleys unite 

 to form the Vale of Llugwy. 



Llyn Ogwen, lying at an elevation of nearly 1000 feet, is an example of a mountain 

 valley lake situated right at the head of the valley. 



Of the upland valley-lakes found in the wild mountain tract to the north-east of 

 Capel Curig, Crafnant and Geirionydd have their surfaces at an altitude of only a little 

 over 600 feet ; but Cawlyd and Eigiau are at much higher levels, their surfaces standing 

 at an elevation of over 1000 feet above the sea. All these lie in long, narrow valleys : 

 Cawlyd high up in its valley, and extending almost right up to the head ; Eigiau at a 

 considerable distance below the head of a cwm of the same name ; and Crafnant and 

 Geirionydd, about midway between the heads of their respective upland valleys and the 

 place where these unite lower down to form one valley. 



Cirque, or corrie, or cwm lakes are common in the district, and three, or perhaps 

 four, of these are dealt with in this paper. Idwal has its surface at an elevation of over 

 1200 feet, while Glaslyn and Dulyn are at heights not far short of 2000 feet. Llyn 

 Llydaw may be looked upon as partly a cwm or corrie lake and partly an upland- 

 valley lake. It lies at an elevation of over 1400 feet. . 



The lakes of Caernarvonshire are all comparatively small, the great majority of 

 them bein^ under a mile in length. The largest of them are those dealt with in this 

 memoir. The double lake of Llanberis, Padarn and Peris, has, if we regard both as 

 one sheet of water, a total length of over three miles. Of the others, only three, 

 namely, Cawlyd, Cwellyn and Llydaw, exceed a mile in length. The widest of all 

 the lakes is Cwellyn, with a maximum breadth of 665 yards and a mean breadth of 

 505 yards. The lakes may be arranged according to their depths into three main 

 divisions — the shallow, the intermediate and the deep. 



The Shalloiv Lakes are six in number, including Ogwen, Llyniau Mymbyr (2), 

 Dinas, Eigiau, and Idwal. In all these the average or mean depth is under 15 feet, 

 and in none of them does the maximum depth reach 40 feet. 



The Lakes of Intermediate Depths include but three — Gwynant, Geirionydd and 

 Crafnant ; they have a mean depth of over 15 feet but under 35 feet, and a maximum 

 depth of between 40 and 75 feet. 



In all the lakes belonging to the above two divisions (except Ogwen, the bottom of 

 which has been converted into a flat plain by the deposition of sediment), the ratio 

 of the average to the maximum depth is considerably under 50 per cent. 



The Deep Lakes comprise all the others, seven in all, namely, Padarn, Peris, 

 Cwellyn, Glaslyn, Llydaw, Dulyn and Cawlyd. These have a maximum depth over 75 

 feet (all except Padarn are over 100 feet), and a mean or average depth exceeding 50 

 feet. In this division the mean depth approaches, indeed in most of the lakes exceeds, 



