438 MR T. J. JEHU ON 



probably the remains of lateral moraines. Remnants of smaller terminal moraines also 

 occur at the upper end of the lake, projecting into the water where the lake narrows. 

 On the east lies a slope of drift, through which the bare rock is often seen rising. The- 

 lake is broadest at the lower end, where it has a fairly even outline, but as followed 

 upwards, it narrows and becomes more irregular until near the head it is quite con- 

 stricted. Several small streamlets trickle down the face of the cliffs at the head of 

 the cwm, and find their way into the lake. After heavy rains the volume of water 

 coming down is largely increased. A few others run into the lake from the slopes 

 on the east and west sides. The river Idwal flows out at a point on the western 

 side, a short distance from the lower end of the lake, and runs north-eastwards 

 towards the head of Nant Francon. 



The lake lies at a high altitude, the elevation of the water-surface above sea- 

 level being, according to the Ordnance Survey Map, 1223'4 feet. When I visited the 

 lake in July 1900, the water was abnormally low, as shown by the markings on the 

 rocks. This was confirmed by the boatman, who knew Idwal well. The length of 

 the lake is 846 yards, and it has a maximum width of 340 yards. The area covered 

 by the water is about 159,300 square yards, and the mean breadth is estimated at 

 188 yards, being 22 per cent, of the length. The number of soundings taken was 81. 



The bottom proved to be very irregular ; in places it was very muddy, but over a 

 large part of it boulders of all sizes seemed to be scattered about, and this must have 

 interfered to some extent with the soundings, for the lead, coming in contact with a 

 huge boulder, would at that point indicate less than the real depth. 



The greatest depth registered was 36 feet, and this was obtained in two places. 

 The total volume of water is calculated to be 16 million cubic feet, and the mean depth 

 11 feet, being 31 per cent, of the maximum depth. The greater part of the lake 

 is extremely shallow, 5 7 '3 per cent, of the total area corresponding to depths under 

 10 feet. A glance at the map (Plate V.) will show that the deepest part lies close to 

 the west shore, a little below the middle of the lake, and from this shore, opposite the 

 depression, a rocky knob dips steepty into the water. The areas enclosed within the 

 20 feet and 10 feet contour lines lie in the lower half of the lake ; but a small depres- 

 sion, over 10 feet in depth, lies in the narrow part, near the head of the lake. 



A longitudinal section is given (Plate VIII.) along the axis of maximum depth, and 

 shows clearly that the hollow is deepest in its lower half. In the neck, which separates 

 off the narrow part at the head of the lake, the depth is nowhere more than 3 feet. 



Two cross-sections are given, C-D at the lower end of the lake, and E-F at its 

 middle. At the lower end, the sides slope gently from both banks to a nearly flat 

 bottom. The configuration of the lake bottom, in the deeper area near the middle, 

 shows more irregularity. Section E-F shows an abrupt descent from the western 

 shore into deep water ; that from the eastern shore is more irregular, a small rocky 

 islet lying at a short distance off the lake-margin, and a ridge rising from the deeper 

 water, a little more than midway across the lake. The irregularities found in the 





