206 Proceedings of the Royal Irish Academy. 



Only where the vallej's open out to the sea, i.e., iu the lower levels of the 

 ice, is a deflection discernible. This is perhaps best illustrated by the low 

 striated ridge which occurs near the mouth of Glengesh. Here the wheeling 

 of the ice out of the valley is clearly traceable. The N. 10° W. bearing, which 

 is the prevalent trend on the plateau just above, is turned into N. 40 and 

 50' W. — a deviation to be attributed to the pressure of tlie ice passing across 

 tlie mouth of tlie glen from Ardara into the Loughros bays. Moveover. large 

 boulders of a coarse dioritic rock, greatly resembling the rock in situ on 

 Mogumna Hill to the east, lie scattered over the ridge just north of the 

 Glengesh and Bracky hills, while a boulder of granite was also observed on the 

 former liill. 



The Glengesh Piateau had a very pronounced effect on the great glaciers 

 proceeding westward from the main ice-shed of Donegal, causing the splitting 

 of these streams on the eastern flanks and a deflection to the north and south. 

 This cleavage is very well shown by the striae and the form of the ground, 

 and is brought out on tlie map (W. IX). Only in the very centre of this 

 wall was the pressure behind the impinging and extraneous ice so great that 

 tliis was able to scale the face and surmount the edge of the plateau. This 

 ascent of the ice from the low country to the east is magnificently revealed 

 by the abundant striated surfaces on the hill-sides leading up to Crocknapeast 

 and Balbane. 



Over these summits fhe ice flowed iu a westerly direction, pouring over 

 the col which lies west of Eoechrow and over that lying north of Croaghacullin, 

 and sweeping down into Glenaddragh, passed out to sea near Muckros Head. 

 It also ascended the southern slope of the spur, running out north-east from 

 hill 1652, which is beautifully rounded (an occasional granite botdder is to be 

 found on its slopes), and then plunged down into the huge corrie below ; 

 striae going N. 2U° W. were observed in several places on the rounded diorite 

 rim overlooking the corrie. 



Glaciers streamed down Glengesh from Croaghnapeast, one of the highest 

 points on the eastern edge of the plateau. At the mouth of this valley 

 striated surfaces show directions of X. 20° W., also faint traces of a N.-E. flow 

 — i.e., parallel with the axis of the valley. The latter were probably due to 

 local glaciers occupying the valley in earlj- glacial times, while the X. 20° W. 

 striations are clearly memorials of the great ice masses that, more or less 

 ignoring the configuration of the country, passed obliquely across the valley 

 at the period of maximum glaciation. These later striae, which have almost 

 obliterated the earlier, harmonize most thoroughly with the direction of the 

 ice-movement in the adjacent areas and with the gradual swerving of tiie ice- 

 sheet into the great plain of Ardara. It is possible, however, that an under- 



