52 J. E, Marr and R. H. Adie — The Lakes of Snowdon. 



to this. Amongst other features parts of the coastlines of Anglesey 

 are determined by these planes of weakness. The Post-Carboniferous 

 planes run approximately north-and-south and east-and-west. The 

 Glaslyn below Beddgelert runs generally along a north-and-south 

 plane and the Capel Curig Valley along one extending in an east-and- 

 west direction. 



The Snowdon mass, with its northern prolongation forming the 

 Moel Eilio range, is of a rectangular shape. It is about ten miles 

 long, and has an average width of about four miles. The ridge runs 

 in a general north-west and south-east direction, whilst the ends are 

 at right angles to this, for the Snowdon mass is bordered by 

 depressions coinciding with planes of weakness produced during 

 the Post-Silurian period of earth-movement. On the north-east side 

 the mass is bounded by the upper portion of the Seiont Yalley, 

 containing the two lakes of Llanberis ; on the south-west side by the 

 upper part of the Gwrfai Valley, holding the lakes of Cwellyn and 

 Llyn-y-gader, and the head of Nant Colwyn ; the south-eastern 

 boundary is formed by the Vale of Gwynant, with Llyn Gwynant 

 and Llyn-y-ddinas ; and on the north-west is a portion of the Seiont, 

 east of Carnarvon, which has worn its bed along the soft Arenig 

 shales. Of ridges determined by the Post-Silurian changes, the pre- 

 vailing one is that which runs north-west and south-east from Moel 

 Eilio, through Moel Gooh and Moel Cynghorion, over the summit 

 of Snowdon, and is continued to the south-west as the buttress of 

 Lliwedd. At right angles to this is the ridge of Crib-y-ddysgl, and 

 also the ridge running from Snowdon on the Beddgelert side known 

 as Llechog, part of which, however, runs parallel to the north-west 

 and south-east system, as does also the ridge which culminates in 

 the peak of Yr Aran. Of the ridges determined by the north-and- 

 south and the east-and-west planes of weakness, the most important 

 are that of Crib Goch, which runs east and west, and that extending 

 between Snowdon and Y Geuallt, which is at right angles to this. 

 Bounded by these ridges and others having the same general directions, 

 lie the six beautiful cwms of Snowdon, four of which contain one 

 or more lakelets. We have laid stress upon the planes of weakness, 

 because thej' contribute some information concerning the origin of 

 the lakelets. The south-eastern and south-western shores of Llyn 

 Llydaw are defined by two Post-Silurian planes of weakness. In 

 the cwm on the west side of Snowdon, the three upper tarns, Llyn 

 Glas, Llyn Coch, and Llyn-y-nadroedd, occur on a north-east and 

 south-west line ; the stream from the central one, Llyn Coch, runs 

 at first along a north-west and south-east line, and this, if continued, 

 runs along the long axis of Llyn Ffynnon-y-gwas. 



The principal precipices of Snowdon occur on the north-east side, 

 and the same feature is seen in the case of the Glyder range ; and 

 in the Lake District the east side of the Helvellyn and High Street 

 ranges is the precipitous one. Minor examples may readil}' be 

 called to mind showing a precipitous eastern slope and gentle 

 western one, and the cases are too frequent to be merely accidental. 

 It is possibly due to the rainfall from the south-west, and the south- 



