464 



MR T. J. JEHU ON 



surface being 1415 feet above sea-level. This lake is generally admitted to be in part 

 dammed up by glacial deposits ; but while Mr Mark thinks that it is probably altogether 

 held up by such deposits, Mr "Watts and Mr Dakyns believe that it lies in a rock-basin. 

 The soundings show clearly that there is a deep corrie or cirque-like basin at the upper 

 end. The shallowing lower down is largely due to the morainic material which is 

 scattered about. Mr Watts found live rock at a depth of 40 or 50 feet below the level 

 of the lake in the stream which issues from it. Mr Marr traced another depression by 

 which the lake may formerly have been drained, but in this depression Mr Dakyns found 

 solid rock 40 feet below the level of the lake. As Llydaw has been found to be nearly 

 200 feet deep, we can only conclude that while its level may be raised by a barrier of 

 morainic material, the water is, for the most part, resting in a rock-basin. The fact 

 that the greatest depth occurs quite at the head of the lake is not in consonance with 

 the view that we have here an upland valley which has been drowned by means of a 

 barrier of drift keeping back the water which drains it. 



Other examples of cwm or corrie lakes dealt with in this memoir are Llyn Idwal and 

 Llyn Dulyn. 



Llyn Idwal lies in Cwm Idwal, above the head of Nant Francon, at an elevation of 

 1223 feet. It is very shallow, the greatest depth obtained being only 36 feet. The floor of 

 the lake is very irregular, small rocky knobs rising up here and there, and much morainic 

 debris and boulders lying scattered about. It was probably at one time much deeper 

 but has been gradually getting filled up by rock-falls from the neighbouring heights, 

 and by the washing in of morainic detritus and the deposition of sediment. Ramsay, 

 not knowing the depth or the form of the lake-bottom, was of opinion that the lake was 

 partly formed by a dam of terminal moraine, and was partly retained in a rock-basin. But 

 the mass of drift crossing the valley at the foot of the lake seems to be of sufficient depth 

 to account for its formation. The configuration of the lake bottom, also, as shown in the 

 contoured maps and sections, supports this view. There is no deep cup-shaped depression 

 such as is found in Grlaslyn, but an irregular floor with rocky knobs jutting up here 

 and there. Therefore, although it was once probably deeper, the evidence seems to 

 show that we have here not a rock-basin, but a barrier-basin, the floor of which may 

 have been modified to some extent by glacial erosion. 



Llyn Dulyn, situated at the southern base of Y Foel Fras, has the surface of its. 

 waters at an elevation of 1747 feet. It occupies a deep basin-shaped hollow at the foot 

 of a mural precipice. Rock can be traced all round the lake-margin, except on the 

 eastern side. Here the ground, rising steeply from the water, is covered with drift. 

 There is no reason to think that the drift is of any great thickness. At the south 

 corner the outflowing stream has cut through it, and runs out over solid rock. Dulyn 

 is excessively deep, especially when the depth is considered in proportion to its extent. 

 A sounding of 189 feet was obtained in one place. The map and sections show that 

 the form of the hollow is that of a deep basin or cup, such as a glacier would excavate on 

 the floor of a cirque. This water, therefore, in all probability, occupies a rock-basin. 



