440 MR T. J. JEHU ON 



The work on Ogwen was carried out in June 1901, and the weather being beauti- 

 fully calm, a very complete survey was taken. Ten sections were run, including 102 

 soundings. The lake is remarkable for its extreme shallowness, the greatest depth 

 registered being only 10 feet. The total bulk of water comes to about 28 million cubic 

 feet, and the mean depth is 6 "8 feet, this being 68 per cent, of the maximum depth. A 

 glance at the map (Plate VI.) will show that practically the whole lake is under 10 feet, 

 that depth only being obtained in two small and isolated areas. Contour-lines have 

 been put in to mark depths of 8 feet and 5 feet. The configuration of the lake bottom 

 is extremely simple; 47'6 per cent, of the total superficial area is enclosed within the 

 8 feet line. The area attaining this depth lies mostly in the eastern half of the lake, 

 and is broadest near the head, narrows as traced westwards, and terminates about a 

 third of the way from the foot of the lake. Except along the margins and opposite the 

 points where the streams flow in at the head, depths of five feet are everjnvhere attained. 



A thick yellow mud extends like a carpet from near the foot of the lake to near 

 the upper or eastern end, where the mud is replaced by very fine sand, and afterwards, 

 as the mouth of the river Dena is approached, by coarser sand and gravel. Under the 

 water along the margins lies a row of boulders, and at a few places rushes and other 

 water-plants extend into the lake. 



The longitudinal and cross-sections given (Plate VIII.) bring out clearly the remark- 

 able flatness of the lake bottom. It has the form of a shallow pan. Llyn Ogwen is 

 evidently becoming filled up gradually and evenly. Were the lake to disappear entirely, 

 a flat meadow-like expanse would be left with the stream meandering through it. 



12. Llyniau Mymbyr. — These two lakes are situated at the lower end of Nant-y- 

 Gwrhyd, a bare and open valley extending from Penygwrhyd, near the base of Snowdon, 

 eastwards for a distance of four miles to Capel Curig. To the north lie the slopes of 

 Y Glyder-fach, and the vale is bounded on the south by the base of Moel Siabod. The 

 bottom of the valley is largely covered with drift, which extends upwards on the hill- 

 slopes for some distance. At Capel Curig, Nant-y-Gwrhyd is met by Nant-y-Benglog 

 coming from the direction of Llyn Ogwen in the north, and the two join to form 

 Dyffryn Llugwy, or the Vale of the Llugwy, which runs down to Bettws-y-Coed. The 

 lakes stretch upwards from the lower end of Nant-y-Gwrhyd for a mile up the valley. 

 The many peaks of Snowdon rise grandly in the distance, and the view of that moun- 

 tain obtained from these lakes is unsurpassed. The lakes are now separated by a tongue 

 of land, covered with grass, which seems to be made up of boulders and rock-rubbish. 

 This is not a stream delta, but appears to have been formed by the slipping down of rock 

 material from the hill-sides to the left. The stream joining the two lakes runs to one 

 side just under the southern margin. 



' The river Gwrhyd flows into the upper lake on the left side near the upper end at 

 the apex of a delta. Another streamlet from the south joins the lake at its head. 

 There are no other tributary streams of any importance, but a few tiny rills run in from 

 the left side, and one coming down from Moel Siabod enters the lower lake near its 



