CHAKLKhWOuTii — Glacial Geology of North-West of Ireland. 2ij7 



eiu-rent of local ice flowed slowly N.-N.-E. along Gleiigesh, or that tlie valley 

 was occupied by a mass of more or less stagnant ice, unable to emerge from 

 its mouth because of the great pressure of the ice at this point. The former 

 seems the more probable. 



4. The Dcrryveaglb and Gleiidoivan ^fountains. 



The Derry veagh and Glendowan Mountains with those of the Errigal- 

 Muckish range lying to the north form a nucleus of hills excelling in altitude 

 and extent any other knot of mountains in Donegal. In early glacial times 

 they doubtless gave rise to radiating glaciers. 



During the maximum phase, as no escape was possible to the south 

 because of the great pressure of ice from this direction, the excess of snowfall 

 over the hills was compelled to seek relief by moving off to the north and 

 north-west. The Derryveagh hills, therefore, behaved in almost all respects 

 like the smaller Glengesh plateau just described ; both acted as sub-centres, 

 especially in the earlier stages, and though able, because of their bulk and 

 capping of ice to deflect the powerful glaciers proceeding from the main 

 Donegal ice-siied, causing a cleavage on their southern and eastern faces 

 respectively, they were only partially successful in their attempts to ward off 

 the invader. 



That no ice streamed south-east from the Glendowan Mountains, the more 

 easterly of the two granite ranges, is proved by the drifts of the bleak and 

 desolate moorland drained by the Cummerk river and its tributaries. This 

 critical area is composed of different members of the metamorphic rocks; and 

 the drift, exposed in good sections in all the streams, is seen to consist only 

 of schist, diorite, quartz, and quartzite ; for though a careful search was made, 

 no granite pebbles or boulders were discovered. Had glaciers flowed over 

 this area from the granite hills, boulders of this rock would occur in great 

 profusion. 



The Derryveagh Mountains are covered with moutonn^e surfaces, though 

 these are sometimes hidden beneatli a turf accumulation, while countless 

 boulders of perched blocks of all shapes and sizes (up to 20 feet in length) 

 everywhere dot the granite surface. Slieve Snaght, the highest summit of 

 these granite hills, together with the adjacent mountains, was overriden 

 from the south. This ice excavated several rock basins — e.g.,. L. Atirrive — 

 and moulded the valleys. 



The walls of the TJ valley of tlie Poisoned Glen are covered with count- 

 less moiotonnde surfaces, frequently striated and grooved. Glaciers apparently 

 poured into the glen from tlie Ballaghgeeha Gap at its head, and flowed 

 towards Dunlewy. This plunging ice doubtless scooped out a hollow in the 



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