THE LAKES OF SNOWDONIA AND EASTERN CARNARVONSHIRE. 453 



half of the extreme depth. In L]ydaw, it is true that for the lake as a whole the 

 ratio of the mean to the maximum depth falls to 41 per cent., but this is due to the 

 shallowing caused by morainic material towards the lower end ; in the upper deep basin 

 the ratio is much greater than this. Penck and Mill have pointed out that where, 

 as in the lakes of this division, the ratio of the average to the minimum depth is 

 great, the hollow presents a pronounced chaldron structure. All the deep lakes lie 

 in depressions which have steeply sloping walls, as is well brought out in Table III., 

 which shows the large percentage of areas at great depths ; this is also illustrated in the 

 sections which are appended with the maps. 



It is interesting to note that the deepest lakes often occur at very high elevations. 

 One of the most remarkable facts brought to light is the great depth of some of the cwm 

 or corrie lakes. They usually lie in crater-like hollows with somewhat circular outlines. 

 Glaslyn, with a total length of 535 yards and a mean breadth of 197 yards, has a 

 maximum depth of 127 feet and a mean depth of 62 '6 feet, which is nearly half of the 

 maximum depth. Dulyn is still more striking ; with a total length of 560 yards, and a 

 mean width of 297 yards, it has a maximum depth of 189 feet, and a mean depth of 

 104 feet, this being 55 per cent, of the maximum depth. Thus it is seen that these 

 cwm-lakes are remarkably deep in proportion to their extent, much more so than is the 

 case with the valley-lakes at low levels. Idwal is shallow, and does not show the 

 characteristics of the other cwm lakes ; it lies at a much lower level, and its shallow- 

 ness is partly due to the great amount of morainic de'bris scattered over its floor, and 

 partly to the sil ting-up process which has been going on. Rock-falls are continually 

 taking place at the head of the cwm, and the detritus is gradually carried into the 

 lake. Llydaw lies in a cirque-valley, the upper end of the lake resting in a corrie, and 

 it is here that the basin is deepest, a depth of 190 feet being obtained, and formerly, 

 before the artificial lowering of the lake took place, it exceeded 200 feet. 



Of the valley-lakes the most striking is Llyn Cawlyd, which lies at a high level. It 

 has a maximum depth of over 200 feet, and a mean depth of over 100 feet. It is 

 deeper than any of the other lakes sounded. The only other lakes which exceed 100 

 feet in depth are Peris and Cwellyn, which lie at the base of Snowdon. 



Padarn, Peris and Cwellyn lie in hollows which are trough-like with steeply slop- 

 ing sides and a somewhat flat floor. 



The other valley-lakes present more basin-shaped cross-sections. The deep cwm- 

 lakes lie in deep basin-shaped hollows with steep sides. Ogwen shows a pan-shaped 

 structure on cross-section, and this is characteristic of a lake which is in process of 

 being silted up evenly. 



The deep cwm-lakes are deepest approximately in the centre. But it is noteworthy 

 that in the valley-lakes the deepest part of the basin often lies in the upper portion towards 

 the head. This is the case in Llydaw, Eigiau and Geirionydd. Of the Llanberis lakes, 

 too, the higher one is the deeper, and in Padarn (the lower lake) we find the deeper area 

 near the head. The deepest part is approximately central in Gwynant, Dinas, Crafnant, 



