236 Proceedings of the Royal Irish -Academy. 



is further complicated by "desk-structure."^ In several localities in Donegal, 

 ridges occur which strongly resemble in external form true morainic accumu- 

 lations, but which, ou examination, prove to be bosses or ridges of some hard 

 rock, with just tlie merest cover of drift. 



In the peripheral areas, however, a distinct change takes place. Here 

 the morainic accumulations are extremely well developed; the contrast 

 between the great morainic development of the plains and valleys of Fer- 

 managh and Tyrone, and the occasional and isolated occurrence of a morainic 

 mound in the valley of Donegal is indeed most striking. This difference may 

 be attributed in part, though not entirely, to the respective rates of waning 

 of the ice-sheet. In the earlier stages of the recession, now represented by 

 the moraines of the peripheral belt, the retreat was comparatively slow, with 

 frequent pauses; in the later stages, when the glaciers had shrunk back into 

 the hills, it was rapid and practically continuous. This deduction is in agree- 

 ment with that drawn from the corries. The moraines occur frequently as a 

 series of confusedly arranged wavy mounds, and where breached by river 

 erosion, are frequently seen to consist of sand and gravel with included blocks 

 and boulders. 



In the absence of moraines in the central highland region, it is somewhat 

 difficult to separate the glacier stage from the preceding ice-sheet phase. 

 This separation is rendered all the more difficult by the coincidence, in 

 general, of the directions of the ice-motion during the two stages, which is a 

 consequence of the radial dispersion of the gla,ciers along the major valleys. 

 In these cases, though only small streams are involved, the marginal drainage 

 phenomena have proved invaluable. 



In the description of the retreat it is proposed to discuss the areas and 

 the glaciers in the following order : — 



1. Sperriu Mountains. 



2. Tyrone glacier. 



o. Slieve Beagh and Clogher Valley. 



•4. Fintona Hills. 



5. Fintona and Erne glaciers. 



6. Foyle and Swilly glaciers. 



7. jMulroy, Sheephaven, and Glenna glaciers. 



8. Eosses and Dunlewy glaciers. 



9. Glengesh Plateau. 



10. Great Finn glacier. 



11. Barnesmore Hills. 



' E.g., the ridges running east-wesf ^s foof-hills ulong the northern slope of Muckish. 

 These are really due to the gentle dip of the tjuartzites to the south, though the rocks are 

 to aoiue extent masked by drift. 



