301 Proceedings of the Royal Irish Academy. 



formation of talus slopes, the deposition of deltaic fans, and tlie erosion of 

 small gorges in " solid " and in drift materials. 



The greatest modification o'f the scenery has been the production of land- 

 slips, the finest of which tower before Benevenagh. They were produced in 

 post-glacial times. Their pre-glacial predecessors, which probably rose up the 

 cliff almost to its summit, were doubtless summarily swept away by the great 

 ice masses which streamed northward out of the Foyle. These, by the erosion 

 of the soft Triassie and Liassic strata, over-steepened the cliff face, so that 

 almost immediately after the removal of the pressure of the ice, consequent 

 upon the retreat, the insufficiently-supported, unstable, and heavy super- 

 structure of basalt was precipitated into the valley.^ 



The slips resting against the face of Shean North (locally known as Magho), 

 rising from the shores of Lough Erne, are similarly the product of post-glacial 

 erosion, caused by the removal of the pre-glacial material, the glacial 

 steepening of the sides, and the erosion of Lough Erne. 



X. — Some General Considerations. 



Nowhere in Donegal or in the north-west of Ireland were signs of the 

 melting in situ of stagnant ice observed ; on the contrary, as the marginal 

 drainage phenomena and the arrangement of the moraines most conclusively 

 show, with the single exception of the Lough Erne area, where the manner of 

 disappearance is somewhat obscure, evidence of a constant recession of moving 

 glaciers in active continuity with the great centre of dispersal was everywhere 

 obtained. 



The severe glaciation to which the north-west of Ireland has been subject 

 has produced by erosion and deposition profound modifications in the 

 topography and physical relief. Over the greater part of the area the ice 

 drowning the hills from top to bottom was powerful and capable of free 

 outward movement. The mountain tops and ridge summits are characterized 

 by beautifully-rounded outlines, e.g., the Barnesmore and Derryveagh 

 mountains; crags and projecting bosses have had their asperities removed, 

 while the flanks and summits have been dressed and rounded. By ice- 

 moulding the valleys of the upland region have been simplified, widened and 

 deepened, their profiles reduced or modified, and the ends of their projecting 

 spurs destroyed by lateral erosion with the evolution of straightened and 

 steep-sided valleys. These are prevalently trough-shaped, especially in the 

 granite and mountainous region, or wide and open in the country composed 

 of schist. 



1 Thi.s slipping was regarded by J. R. Kilroe as intermediate in age between his second 

 and third glaciations {op. cit., B.N.F.C., p. 652). 



