148 Henry Dewey- — Land-forTns in Caernarvonshire. 



Ffynnon Lloer, from which a small stream, the Afon Lloer, flows 

 down to Llyn Ogwen. To the south it is flanked by a series of 

 magnificent precipices terminated by the serrated edges of Tryfaen 

 and Glyder Fach. There are many fine cwms along this ridge, all 

 facing north-east, and in one lies Llyn Bochlwyd. 



The lower slopes of Tryfaen and Carnedd Dafydd both bear record 

 of the thickness of the former ice-sheet in their roches moutonnees 

 and striee which extend far up their slopes, and also to the action of 

 frost and ice in their cwms, llyns, and moraines, but all these 

 features are still more perfectly preserved at the western end of 

 Llyn Ogwen. 



The Holyhead road follows the side of the lake for a distance of 

 over a mile and at approximately the same level the whole way, 

 namely, a thousand feet above the level of the sea. A spur of 

 Carnedd Dafydd bounds the western end. of the lake and is rounded 

 into smooth mammillations and roches moutonnees. It is seen in 

 PI. VII, Fig. 1. The waters of the lake, however, flow across this 

 smooth rock-barrier in a low gorge and then suddenly plunge down 

 into a deep chasm. This feature will be described later in relation 

 to a similar one connected with it. 



Llyn Ogwen lies on an upland plain ; arising steeply from this 

 plain is a rock-step, deeply scored with glacial striae, through which 

 a mountain torrent, the Afon Idwal, has ripped a gorge ; if this 

 torrent be followed, a second plateau is soon reached. It spreads 

 out in front of a ring of magnificent precipices that form the base of 

 Olyder Fawr and Y Garn and embrace the gloomy Llyn Idwal. 

 This plateau rests at a height of about 1,250 feet above sea-level and 

 is largely covered with strewn blocks derived from the almost 

 vertical walls of the precipices, and in part arranged as moraines. 

 Ramsay notes that moraines now skirt Llyn Idwal, the progressive 

 retreat of the glacier being marked on the western side of the lake 

 by four moraines arranged concentrically one within another. On 

 the south and on the east of the lake there are patches of moraine 

 matter, and other moraines dam back the waters at its northern end. 

 Other glacial features, such as blocs perches, roches moutonnees, and 

 glacial strige, are conspicuous, the striae all being directed towards 

 Nant Ff ran con. 



Professor Jehu ^ investigated the lake and its surroundings, and 

 notes that it is broadest at its lower end, whence the River Idwal 

 issues. The length of the lake is 846 yards, maximum width 

 340 yards, area of water 159,300 square yards, mean breadth 

 188 yards or 22 per cent of its length. He took eighty-one 

 soundings, which prove the bottom to be very irregular, in places 

 muddy; but over a large part boulders of all sizes seemed to be 

 scattered about and interfered with the soundings. The greatest 

 depth registered was 36 feet in two places, the mean depth was 

 1 1 feet, while the greater part of the lake was found to be extremely 

 shallow, 57 per cent of the total area corresponding to depths under 

 10 feet. The deepest part of the lake lies close to its western shore. 

 Professor Jehu considers that Llyn Idwal was probably at one time 



1 Op. cit. 



