306 Proceedings of the Royal Irish Academy. 



the rejuvenated rivers, e.g., G-weedore, Eske, ]<lriie, etc., have cut deep gorges. 

 With this coastal peneplain — probably because of tlie steepness in places of its 

 inner margin to be regarded in part as a plain of marine denudation — were 

 graded the floors of the deep U-vallej's. Tliese have profoundly dissected an 

 older surface several hundreds of feet higher tlian their floors and the level of 

 the coastal plain. To this older rejuvenation is in tlie main to be attributed 

 the size and to a less extent the form of these striking valleys. 



The largely non-glacial origin of these features is also clearly to be seen in 

 the case of the Glengesh Plateau. Here there occiu's at least one great U- 

 valley, that of the Owenwee Eiver, cut into the northern face, which lay 

 obliquely, roughly 70° or 80°, to the direction of ice-flow. As a great press 

 of ice was passing wesLward along the face of the Plateau and across the mouth 

 of the valley, Utile ice motion can have taken place in the deeper layers along 

 the length of the valley. Though indistinguishable in form from the other U- 

 valleys of Donegal, this depression cannot in consequence have been serioiTsly 

 modified by glacial erosion. 



Tet were the U-valleys now perfectly graded to the coastal plain, some 

 glacial erosion may nevertheless have taken place, for this plain has itself been 

 lowered, though by how much is indeterminate ; in view of the soft (e.g., schist 

 and Carboniferous rocks) or greatly jointed (e.g., granite) chai-acter of its rocks, 

 it may be not inconsiderable. 



The floors of the U-valleys are, however, no longer graded to this plain ; 

 Glenveagh clearly illustrates thi.s. Lough Veagh is a rock basin carved out 

 of the floor of the valley to a depth of 139 feet;^ the islands at its lower end 

 are roches moutonnees, while granite can be traced completely across the 

 valley of the Owencarrow Eiver. In this glen, glacial over-deepening of 

 some 200 feet would appear to have taken place, though the amount of fresh 

 and hard rock, un weakened by pre-glacial disintegration, may be somewhat 

 less. The otlier large lakes of this highland region are similar rock basins, 

 ■ pi'oving glacial erosion in these valleys of approximately the same extent. 

 They are largely confined to the valleys of the hilly tract, where the erosive 

 power of the ice was greatest ; where the valleys open out, the erosion 

 diminished, and the over-deepening was lessened in amount. 



It is significant that in the valley of the Gweebarra Eiver, where free 

 outward motion of the ice was, in general, imj)0ssible, on account of tlie 

 glaciers streaming from the Barnesmore Hills, no rock basin has been pro- 

 duced; the shallow Lough Barra is held up by moraiuic accumulations. 



Hence, apart from the excavation of these basins, comparatively little 



' Mem. Geol. Survey, Sheets 3, etc., \u 10. 



