56 J. E. Marr and R. H. Adie — The Lakes of Snourlon. 



a valley was here stopped up by ice, and partly converted into a lake, 

 which became largely silted np with sublacustrine detritus. The 

 bed of the Colwyu runs over drift until within a short distance of 

 Beddgelert. It will be seen, therefore, that a continuous line of 

 drift-covered tract can be traced from Nant Mill, past Cvvellyn and 

 Llyn-y-gader, for a distance of about four miles. If the drainage 

 has been reversed between the present head of the Gwrfai and Nant 

 Mill, the curious course of the stream from Llyn-y-dywarchen will 

 be accounted for. This stream runs a little east of south into Llyn- 

 y-gader, whilst the Gwrfai issues from that lake in a general 

 northerly direction, whereas if the waters of the Cvvellyn and Llyn- 

 y-gader depression originally drained southwards, the Llyn-y- 

 dywarchen stream would then have proved a normal tributary to 

 the river once occupying that depression. 



We may now pass on to the consideration of the upland tarns and 

 lakes. The four tarns on the west side of Snovvdon may be dismissed 

 in a few words. Tlie late Professor Ramsay speaks of them as 

 follows : " The lake called Llyn Ffynnon-y-gvvas is possibly 

 dammed up by moraine matter " ; and again, " a minor moraine 

 encircles Llyn-y-nadroedd on the north and east, and another 

 beautiful small one made of angular blocks and stones, now covered 

 with vegetation, bounds Llyn-goch on the west and south-west, while 

 a third dams up Llyn-glas." None of these lakes, then, can be 

 claimed as resting in a rock-basin, nor can the drift-stopped tarn 

 below Moel Eilio (Llyn Cwm Dwythwch) be asserted to rest iu 

 a basin of that nature. The lakes lying to the north-west of the 

 main Snowdon ridge merit a fuller description. Below the precipice 

 Clogwyn du'r Arddu, lies the little Llyn du'r Arddu at a height of 

 1,900 feet above the sea. The extensive moraine which blocks it up, 

 and extends far down Cwm Brwynog in a series of concentric semi- 

 circles, is admirably described by Professor Eamsay. The exit to the 

 west is between the great drift dam and solid rock ; the latter is well 

 rounded with stri^ running parallel to the stream, and the rough 

 sides of the rocJies moutonnees face westward. On them rest sub- 

 angular perched blocks, whilst the innermost crescent of the drift- 

 dam consists of angular blocks, as though some at least of this 

 material was rather of the nature of snow-slope detritus than true 

 moraine. It is quite clear that the course of the stream before the 

 lake was formed cannot have been as it is now, otherwise no lake 

 would have been produced ; it must have run in a more northerly 

 direction, but this former valley is now completely buried beneath 

 gigantic moraine-mounds for a long distance. We call attention to 

 this, as we shall have occasion to recur to the point when describing 

 the exit of Llyn Llydaw. 



The water of Llyn du'r Arddu is of a deep indigo tint, a colour 

 not represented in Forel's scale of lake-colours. It is popularly 

 asserted that the colour is due to the presence of copper in the water ; 

 but we are not aware that any analysis has been hitherto made to test 

 this. Old copper-mines exist close to the lake, but as one of us has 

 been unable to find any trace of copper iu the lake- water, it would 



