Charleswouth — Glacial Geology of North- West of Irelctnd. 307 



deepening of tlie valleys has taken place. They have been merely 

 moulded, the gradient modified, the spurs truncated or blunted — though 

 those in the narrow glens of the granite region can never have been very 

 pronounced — and the sides straightened and somewliat over-steepened. This 

 view of the comparatively slight glacial erosion of the U-valleys is in general 

 agreement with that of Professor J. W. Gregory, which he based on observa- 

 tions on the valleys of Arran and North Wales.^ 



Lough Eske is likewise a rock basin, for Carboniferous rocks can be traced 

 all round its shores. Its maximum depth, which I have ascertained by a 

 careful sounding of the lake, is lOlJ feet; its surface is just 100 feet above 

 sea-level. Its existence is to be chiefly ascribed to ice erosion. 



Lough Erne also owes its existence to glacial scooping which, neglecting 

 late and post-glacial infillings, has carried the floor to a maximum depth of 

 77 feet below sea-level. 



Though many of the smaller Donegal lakes are due to the impounding of 

 drainage by drift accumulations, more especially in the schist coirntry and 

 the peripheral area of deposition, some being certainly so held up entirely, 

 others only partially so, and though the origin of many others, as Lough 

 Gartou in the Crohy peninsula and to some extent Lough Salt, may be 

 attributed to solution of soluble limestone, either metamorphic or carboniferous, 

 great numbers are doubtless true rock basins. 



Though the excavation of some of these hollows is indubitably due to 

 grinding, others are as certainly the result of plucking, e.g., the lakes of 

 the Bosses. The result of this operation is very well seen in the case of 

 Lough Magrath Beg (S. of the Owen River). Here the schist beds, dipping at 

 high angles (about 70°) to the north-east, were ripped off by the Barnesmore 

 ice flowing roughly at right angles to the line of strike, the successive plucking 

 causing the continued eastward retreat of the eastern end of the hollow in 

 which the lake now lies. In similar fashion, great masses of the steeply- 

 dipping quartzites of Errigal have been carried away and the southern and 

 western flanks considerably over-steepened. 



This erosion has been greatly faeilitated by the contact of rocks possessing 

 different powers of resistance to erosion. Many junction lakes have so 

 arisen; of these, the finest is, perhaps, that of Lough Nabrackbaddy, where 

 schist and diorite are in contact, the former being situated on the lee-side. 

 The glaciers have readily eroded the relatively softer schist, the harder diorite, 

 on the contrary, persisting as a pronounced ridge overlooking the lake. 



'The Pre-glacial Valleys of Arrau and Siiowdon. Geol. M.ag., vol. Ivii (1920), 

 pp. 145-161. 



