Chaulksworth — Glacial Geology of North-West of Ireland. 197 



this part of the ice-shed lay tlie summits uf Croaghnameal and Aidmore 

 Hill. 



The accuracy with wliich this line can be fixed varies over different parts 

 of its length ; in some places, as in the valley of the Barnes river, it can he 

 laid down within a few yards ; in others a strip of indeterminate country 

 exists within which its position cannot he definitely located. As over the 

 whole length the rocks on either side of the ice-shed are of one kind, either 

 granite, as in the central part, or schist, as in the terminal portions, erratics 

 of these are quite useless for fixing its position. The roches moutonnees, 

 however, serve as a very clear and infallible guide.' 



In places the ice- shed would appear to have been eccentric and to have 

 lain somewhat to the north, though only slightly, of the present watershed. 

 This eccentricity was probably due to the more favourable conditions on the 

 southern side for the ready descent to the sea, the distance being much 

 shorter and obstructing hills absent, while the resistance of the Scottish ice 

 to the east and north-east may have been a contributory cause. 



Lough Derg, situated astride the watershed towards one extremity of this 

 ice-shed, was the site of a pool of ice on the line of parting, from which 

 glaciers moved away to the north-east and to the south-west. From this 

 great gathering-ground, located chiefly in the granite hills of Croaghgorm, 

 Bluestack, and Barnesmore, the ice proceeded irresistibly outward and for- 

 ward, occasionally ignoring mountains and ridges in its course, yet in general 

 conforming with the valley-lines. It overrode the hills of Inishoweu ou the 

 north-east, the Sperriu Mountains on the east, and the Fintona and Slieve 

 Beagh hills ou the south-east. Only to tlie w^est was the advance of the ice 

 unimpeded by any great hilly barrier, so that here it was able to proceed 

 unhindered far west over the low-lying land of south Donegal and over 

 Donegal Bay to the open Atlantic and the 100-fathom line. 



As there can be little doubt that some relationship exists between the 

 height and size of a mountainous mass acting as a radiation centre and the 

 extent oi the country covered by the glacial streams proceeding from it, it is 

 not surprising to find that the Donegal hills, though much smaller than the 

 congeries of mountains which formed the glacial centres of the Scottish 

 Highlands and the territory covered by their glaciers, should have given rise 

 to an ice-sheet which buried a very large region, whose great extent, prior to 



' As observed in the Geological Survej- Memoir (Sheet 24, p. 38), ' ' flatly worn pieces of 

 mica-schists " are found occasionally on the highest parts of the Croaghbarnes hills, at a 

 level of about 1600 feet. Though some of these are most probably not erratics, as is there 

 suggested, but residual pieces of the probably once continuous roof of schist which covered 

 over the granite dome, the majority are doubtless piuuf of eccentricity of the ice-shed. 



