THE LAKES OF SNOWDONIA AND EASTERN CARNARVONSHIRE. 443 



the foot of the lake, but at the other eod leaves an interval of about 250 yards between 

 it and the head. It is closely followed by the 10 feet contour-line, only 22*9 per cent, 

 of the total area lying between these two lines, leaving 22 '5 per cent, of the area for 

 •depths under 10 feet, and of this the greater part lies at the head of the lake, which is 

 very shallow. Not far from the foot of the lake a ridge rises from the bottom, and, on 

 very low water, appears as a small rocky islet at the surface. 



An examination of the map and of the sections given (Plates VI. and VIII.) will 

 show that the shape of the lake bottom is somewhat complicated. The 10 feet and 20 

 feet subaqueous lines keep fairly close to the shore-line except at the upper end. The 

 isobath of 40 feet diverges considerably from the land, and its form is in no way 

 ■dependent on that of the coast-line. The 60 feet isobath is again quite peculiar in 

 outline, showing no relationship to the trend of the other subaqueous lines. 



The longitudinal section A-B shows how the accumulation of sediment has shallowed 

 the upper part of the lake as compared with the lower part. The main depression is 

 seen to lie about midway along the lake. Just where section C-D is taken, the slope is 

 a, little greater on the left side, but a little lower down it becomes much greater from 

 the right side. Opposite the middle, where the section E-F is taken, the slope from 

 both sides to the deepest part is about the same. G-H is a section taken towards the 

 lower end of the lake, showing the ridge rising nearly midway between the two sides. 



14. Llyn Geirionydd. — This is the most easterly of the long, narrow lakes situated 

 in the country lying north of Capel Curig, and, like the others, it drains into the valley 

 of the Conway. Its surface lies at an altitude of 616 # 4 feet above sea- level. The main 

 axis of the lake runs nearly north and south. It has a total length of 1348 yards, 

 and a maximum breadth of 356 yards. The mean breadth is 221 yards, and this is 

 16 per cent, of the length. The area covered by its waters is approximately 297,600 

 square yards. 



On both sides of the lake the ground rises steeply to heights of over 1000 feet, the 

 steepness being somewhat more marked on the left or western side than it is on the 

 right or eastern side. 



An alluvial flat extends a little way up from the head of the lake. The chief tribu- 

 tary stream enters the lake at its head, other smaller ones flow in a little lower down. 

 The Afon Geirionydd makes its escape from the lake at its foot. The right margin is 

 very regular along its whole length, but there is a marked irregularity on the left 

 margin in the shape of a bay, which occurs about a third of the way from the lower 

 -end. 



The work on Geirionydd was carried out in July 1901. A small punt had to be 

 brought up on a cart from Trefriw, and this proved to be not very well adapted for the 

 purpose. At the time when the soundings were taken the water was exceptionally low ; 

 small rocky islets stood well out of the water at the lower end. These were beautifully 

 glaciated on the side facing up the lake, but were rough on the side facing down lake. 



In all, nine sections were run, including 122 soundings. The greatest depth obtained 



TRANS. ROY. SOG. EDIN., VOL. XL. PART II. (NO. 20). 3 x 



