284 Proceedings of the Royal Irish Academy. 



Of the eoiuparatively early emergence or Errigal as nunatak above the 

 incr-de-gleiee there can be little doubt. At its northern foot are heaped up 

 ridges and irregular mounds, burying almost all the floor of the recess 

 between this mountain and Maekoght. These accumulations probably 

 represent snow talus, yet may possibly be the moi-aine of a small corrie 

 glacier. The mateiial consists almost exclusively of quartzite. The forma- 

 tion of the angular block-debris covering the summits of EiTigal and 

 Mackisli is probably to be ascribed to disintegration by frost, acting in 

 late-glacial times. 



8. Tlie Rosses, Dmdewy mid Gwtdmrra Glaciers. 



The Eosses and Glenna glaciers parted fiom each other in the lee of the 

 Tievealehid hiH The earliest halt in the retreat of the former is represented 

 by the "line of erratic boulders" of the Gheological Survey map (one-inch. 

 Sheet 9). ITiis wall-like ridge is, without doubt, a block moi-aine foimed 

 along the edge of the ice standing at the foot of Bloody Foreland, and 

 indicates a very early halt in the retreat* A smaller ridge, farther north, 

 and of practically the same composition, though possessing a greater pro- 

 portion of quartzite and diorite to granite, represents a sUghtly earlier 

 pause. With these features are probably to be correlated the sea of huge 

 granite bouldei-s strewn along the western flank of Bloody Foreland at an 

 altitude of about 300 feet, O.D.= 



The east and west direction of this morainic ridge chai-acterizes the drift 

 underlying- the moorland extending south-eastward from Bloody Foreland 

 These ridges^ may denote halts in the southward retreat of the ice. In 

 correspondence with one or more of these is the very fine overflow channel at 

 the head of Glenhoola, which di-ained an extra-glacial lake held up im- 

 mediately to the west. It intakes at 293 feet, O.D. The gravels exposed 

 beneath the peats in the area south-west of this valley doubtless represent 

 the water-washed material carried into this temporary lake. 



No further indication of the retreat of this glacier was noted north of the 

 latitude of the Gweedore valley. Beyond the mouth of the latter, the ice 

 which was retreating southwai-d along the Bosses, parted from that which 

 issued from the Derry^'eagh Mountains at the head of the Poisoned Glen and 

 streamed along the Dunlewy and Gweedore valleys to form the large "Dunlewy 



' In the Geological Survey Memoirs (Sheets 3, 4, 5, 9, 10, 11, 15 and 16, page 110) 

 this line of granite blocks is regarded as marking '"' the northern margin of the 

 ice-sheet." 



- L. Tirrim would appear to owe its origin to this accumulation of boidders. 



- L. Doo, L. Teigha and other lakes in this moorland owe their existence to these 

 drift irre^olaricies. 



