60 J. R. Daki/iis — Some Snowdon Tarns. 



landlord of Pen y Gwiyd, was present at the search ; and he told 

 a friend of mine that in one phxce no bottom was reached at the 

 depth of 200 feet. Unless, then, this estimate was grossly erroneous 

 Llydaw must lie in a rock basin. 



There is not much to be said about Llyn dur Arddu. It lies at 

 the head of Cwm Brwinog under the precipice called Clogwyu dur 

 Arddu, which forms the south side of the Cwm, and rock is seen 

 at the outlet on the south side of the stream ; but no rock is to be 

 seen on the north side of the lake, nor, as far as I know, for a long 

 distance down-stream ; in fact, on that side nothing is to be seen 

 but a steep bank strewn with large blocks of stone, so that we 

 cannot tell whether the tarn lies in a rock basin or not. Although 

 nothing but the rock-strewn bank is to be seen on the north side of 

 the Cwm, yet the unsophisticated intellect conceives that the Cwm 

 is bounded on that side, too, by solid rock, hidden, however, by 

 superficial detritus. There is absolutely no evidence to support the 

 assertion of Messrs. Marr and Adie that the lake once drained more 

 northward down a valley now buried under moraine. Not only is 

 there no evidence in favour of such a view, but the chances are all 

 the other way, for it presupposes a far greater thickness of drift 

 than is known to occur in any part of the district. There are many 

 places in the Snowdon region where the thickness of the glacial 

 deposits can be measured. As far as I know, these deposits are 

 thickest under Yr Aran, on the left bank of the Colwyn, about 

 a mile above Pont Caer-gors, and here the thickness is less than 

 a hundred feet. In most cases it is much less than this. Thus it 

 is most improbable that there should be in Cwm Brwinog so great 

 a thickness of drift, and that for the distance of a mile or more, as 

 must be the case if there is such a buried valley as Messrs. Marr 

 and Adie assume. 



The tarns in Cwm Glas are so shallow that they are of little 

 importance, but the lower one, Llyn Glas, most certainly lies in 

 a rock basin, for, without going below the level of the tarn, one can 

 walk all round it without ever treading on anything but solid rock. 



I shall not at present say more about Cwm Glas, nor about Cwm 

 Brwinog, as I hope on a future occasion to deal fully with the 

 geology of these corries. Nor shall I now treat of the Snowdonian 

 lakes as distinct from the tarns ; but as Messrs. Marr and Adie, in 

 the part of their paper on the Snowdon lakes dealing with Llyn 

 Cwellyn, have indulged in some speculations and statements about 

 a part of the country with which I am intimately acquainted, I must 

 point out some mistakes into which they have fallen. 



On p. 55 of their paper they say "a drift-filled depression is 

 traceable up a tributary joining it" (the main stream) "a few yards 

 south-east of Cwellyn State Quarry." This is a mere assertion : no 

 proof is offered of the existence of drift in this depression ; it is 

 simply assumed, and in fact no proof can be offered, for along the 

 course of the little valley there are no sections in superficial material 

 save one near Ffridd quarry, where a pit has been dug for walling- 

 stones in some loose stuff which may be morainic. Throughout the 



