THE LAKES OF SNOWDONIA AND EASTERN CARNARVONSHIRE. 427 



The altitude of the water-surface, as determined by the Ordnance Survey, was 

 14157 feet above sea-level. The lake has a total length of 1950 yards, with a maximum 

 width of 460 yards. The mean breadth is 301 yards, being 15 per cent, of the length. 

 The area covered by the waters of the lake is approximately 587,200 square yards. In 

 all, twenty-six sections were taken, the total number of soundings being 292. The 

 greatest depth recorded was 190 feet, and this was found near the upper end of the lake. 

 The mean depth is estimated to be about 77*4 feet, and this is 41 per cent, of the 

 maximum depth. The bulk of water contained in the lake is approximately 409 million 

 cubic feet. 



The general configuration of the subaqueous scenery can be gathered from an 

 examination of the contoured map of Llydaw with the accompanying sections (Plates 

 IV. and VIII. ). The lake is very deep in its upper reaches and shallows as traced 

 downwards. The area within each contour-line is broadest at its upper end, and tapers 

 rapidly as traced downwards. The 60 feet contour-line ends just opposite the point of 

 exit of the river Glaslyn. Beyond this point no depths of over 60 feet were registered. 

 The lake is shallowest on either side of the causeway and for some distance out from 

 this. Beyond the causeway, towards the eastern end of the lake, we met with a con- 

 siderable area over 40 feet, the greatest depth recorded within this area being 55 feet. 



We may divide the lake roughly into two portions, the one lying to the south-west 

 of the place where the river Glaslyn emerges from the lake, and the other lying to the 

 north-east of this. The first half forms a deep basin and the second is comparatively 

 shallow. It is quite evident, from the amount of earthy material, stones, and blocks of 

 all sizes, which cumber the ground, and can be seen at places below the surface of the 

 water, that the portion of the lake immediately opposite the point of exit of Glaslyn and 

 that at the north-east of this owes its shallowness largely to morainic material scattered 

 along the lake bottom. On the shores bordering that portion of the lake we meet with 

 the remains of the huge moraine mentioned above. When we get clear of the remains 

 of the big moraine, the lake deepens again, as shown on the map, in that portion of the 

 lake lying to the east of the causeway. So that were it not for this morainic material, 

 it is probable that the 40 feet contour-line would extend all the way to the eastern end 

 of the lake, and that the 60 feet contour-line would extend a considerable way in that 

 direction also, and the 80 feet line to a less extent. This would make the configuration 

 of the lake basin comparatively simple. The deepest hollow lies in the upper third, and 

 the lake shallows gradually towards the lower end. These points are seen clearly in the 

 sections, which show that Llydaw lies in a deep, somewhat trough-shaped basin. Its 

 gradual shallowing, as the region of the moraine is approached, is well brought out 

 in the longitudinal section. As much as 40 per cent, of the total superficial area is 

 covered by water attaining depths of over 100 feet, and 64 per cent, of the total area is 

 enclosed within the 40 feet subaqueous line. Only 18"8 per cent, of the total superficial 

 area corresponds to depths below 20 feet. 



In the upper and deeper part of the lake the isobaths are seen to cling closely to the 



TRANS. ROY. SOC. EDIN., VOL. XL. PART II. (NO. 20). 3 t 



