454 MR T. J. JEHU ON 



Oawlyd and Llyniau Mymbyr. In Cwellyn only do we find the greatest depth situated 

 towards the low end of the lake, and in this case the lake is very deep all along to near 

 the head. 



All the different stages in the existence of lakes are exemplified amongst those 

 found in Carnarvonshire. Idwal, Eigiau, Llyniau Mymbyr, and Dinas are gradually 

 becoming filled up, the deposition of sediment forming shallows over which vegetation 

 pushes its way from the lake-margins. The sections across certain portions of those lakes 

 show a tendency to a shallow pan-shaped structure. This structure, however, is most 

 beautifully seen in the case of Ogwen, which is in the last stage of its existence. This 

 lake is filling up very evenly, a carpet of thick mud or clay being spread over the 

 bottom. Were the water finally to disappear, a flat meadow-like expanse would be 

 left. 



Traces of old lakes which have finally disappeared are abundant in this district. 

 Their place is now often marked by a turbary or peat-moss, as may be seen in Nant- 

 y-Benglog, between Ogwen and Capel Curig. Alluvial flats are also common, marking 

 the sites of old lakes which have become drained at the exit. Some flats occur along 

 the Vale of Gwynant and at Beddgelert. But the most remarkable is that which 

 occurs on the bottom of Nant Francon between Ogwen bank and the falls of Ogwen 

 (Plate II. fig. 2). The valley, for a distance of almost three miles, presents a remark- 

 ably flat bottom, with steeply-sloping sides, find this flatness is maintained right up 

 to the head, where the river Ogwen descends over a steep rocky escarpment from the 

 lake at the head of Nant Francon. The photograph shows how the river meanders 

 through the flat meadows which now mark the site of the old lake. The lake must 

 have been nearly equal in extent to the double lake of Llanberis ; but, judging from 

 the shape of the ground, it was probably deeper. The depression in which the lake 

 rested had here also the form of a simple trough, with steeply-sloping walls, and a 

 nearly flat floor. 



Comparing the lakes of Snowdonia and of eastern Carnarvonshire with the English 

 lakes described by Dr Mill, we note that the latter are all valley-lakes, occurring at a 

 comparatively low level, only one of which, namely, Haweswater, has its surface at an 

 elevation of over 500 feet. The English lakes are generally much larger in area than 

 are the Welsh lakes. No large lakes comparable in extent to Windermere and Ulles- 

 water exist in Caernarvonshire. The only lake which exceeds two miles in length is 

 that of Llanberis. But though the Welsh lakes are so much smaller in size, some of 

 them rival in depth even the largest of the English lakes ; and when considered in 

 proportion to their extent, their depth is far more striking. 



In some of the English lakes a portion of the bottom lies below sea-level : in none 

 of the Welsh lakes is the depth sufficiently great to bring any part of their bottom 

 below the level of the sea. But the deeper Welsh lakes are, as a rule, situated at much 

 higher levels than are the English lakes described by Dr Mill. 



