434 MR T. J. JEHU ON 



cause the lake to broaden in its upper half, and then approaching nearer the opposite 

 side, and causing the lake to narrow in its lower half, and especially near its termina- 

 tion. The hillsides are well wooded in places along the lake-margins. The ground rises 

 steeply from the right bank to a height of over 1000 feet, the contour-lines following 

 each other rapidly, but the interval between the 300 feet and 400 feet lines widens so 

 as to form a shelf running above the lake. The opposite coast is flatter for some dis- 

 tance from the lake-margin, after which the hills again rise steeply to heights of over 

 1000 feet. Towards the lower end the hills close in from both sides, and from a distance 

 appear to shut in the lake completely. An exit is found, however, from the narrow 

 square-cut end, and the river goes on its way down a lovely glen to reach Llyn Dinas 

 lower down the valley. 



The work of sounding was done in July 1900 by Mr Philip Lake and myself. 

 Finding a convenient bench-mark near the lake-margin, we used an Abne)^ level to 

 determine the altitude of the water-surface. We found it to be 215*5 feet. This is a 

 little less than that found by the Ordnance Survey in February 1886, which is given on 

 the map as 21 6 "9 feet; the water no doubt standing a little higher in winter than in 

 summer. 



The total number of soundings taken was 105. The greatest depth found was 54 

 feet. The mean depth is 18 "6 feet, being 36 per cent, of the maximum depth. The 

 volume of water contained in the lake is estimated to be approximately 91 million cubic 

 feet. The deepest water lies in an irregular narrow depression defined by the 50 feet 

 line. This area is found about halfway down the lake, but close to the right margin, 

 where the rocks slope at a high angle into the water. All the contour-lines follow each 

 other closely off the right bank, but they widen out in all other directions. The areas 

 enclosed within the 40 feet, 30 feet, and 20 feet contour-lines narrow towards then- 

 upper ends and broaden out towards their lower ends, so as to approach nearer to the left 

 bank where the lake begins to narrow. Except at this place, the loch is shallow on the 

 left side, and it is also shallow at the head and for a long distance from its lower end. 

 Over 40 per cent, of the area of the lake has a depth of under 10 feet, and only 13 '6 per 

 cent, of the area has a depth of over 40 feet. 



Three sections are shown with the map (Plates V. and VIII.). The longitudinal 

 section A-B shows that the basin is deepest about the middle in that direction, and 

 that it shallows rapidly towards the lower end, and more gently towards the upper end. 

 The cross-section E-F brings out very clearly the fact that the deeper water lies close 

 under the right bank, and that the lake shallows rapidly towards the left bank. C-D 

 is a section taken across the lake where it begins to narrow, and shows that its slope 

 becomes comparatively steep here from the left side also. An isolated sounding of over 

 30 feet was obtained a short distance from the left bank, along the line C-D, causing 

 the irregularity shown in the section. 



Llyn Dinas. — This lake, little less beautiful than Gwynant, lies a mile and a half 

 farther down the valley, and its main axis runs in the same direction — from north-east 





