THE LAKES OF SNOWDONIA AND EASTERN CARNARVONSHIRE. 437 



The area enclosed within the 100 feet contour line corresponds to as much as 35 per 

 cent, of the total superficial area of the lake. The area with depths of over 60 feet is 

 estimated at about 64*5 per cent, of the total area. The area between the shore-line and 

 the 20 feet contour-line amounts to only 12*7 of the entire superficial area of the lake. 



Fig. 2. — Sections of Cwellyn. The black portion shows the true slopes ; the outline shows the slopes 

 exaggerated ten times. A-B longitudinal ; C-D cross section. 



A comparison of the map with the sections (fig. 2 and Plate VIII.) makes it clear 

 that the lake has the form of a simple trough, with steejDly sloping walls and a nearly 

 flat floor. For the greater part of the way the slope is steeper on the Mynydd Mawr 

 side, but towards the lower end of the lake, where the section C-D is taken, the slope 

 is somewhat steeper on the Snowdon side. The floor slopes up steeply at both ends, as 

 shown in the longitudinal section A-B, and the deeper water is seen to lie towards the 

 foot of the lake. C-D is a section taken across the deepest part, and shows how the 

 bottom of the lake forms a nearly flat plain. E-F is a section across the middle of the 

 lake, where the slope is steeper on its left side than it is on the right. The section G-H 

 is taken obliquely, and the flatness of the bottom is not so well shown. Maps and 

 sections bring out clearly the fact that this lake-basin has a pronounced chaldron structure- 

 In this it resembles the lakes of Llanberis, which lie on the other side of Snowdon. 



10. Llyn ldwal. — This gloomy lake lies in the great hollow of Cwm Idwal, a high 

 mountain valley at the head of Nant Francon, and penetrating into the mountains on 

 the south-west. It is backed by the steep sides of Y Glyder Fawr and Y Garn, and the 

 rocks towering upwards at the far end of the cwm are cleft into a great chasm known as 

 Y Twll-Du, or the Devil's Kitchen, which is famous for its Alpine plants. Below the 

 Devil's Kitchen, in the bottom of the cwm, lies Llyn Idwal, a small lake nearly half a 

 mile in length, the main axis of which runs north and south. The main features of 

 this valley were described over half a century ago by Darwin, and again more fully by 

 Ramsay nearly twenty years later. Evidences of the former existence of glaciers are 

 to be seen on all sides. 



A huge terminal moraine dams back the water at the lower end of the lake. 

 •Long green mounds, arranged one within the other, skirt the western side, and are 



