Charlesworth— Glacial Geology of North- West of Ireland. 205 



consequence present convex slopes to the north, and' concave slopes to the 

 south. This direction is confirmed by the striations at Clogher bearing 

 S. 30° W. and by the great blocks of schist scattered hereabouts, and derived 

 from the hills immediately to the north-east of this locality. 



At Gortalia, the magnificent Carboniferous conglomerates reposing against 

 the schist are covered with fine striations, which vary from E.-W. to W. 30° S. 

 They were produced by ice passing westwards along Donegal Bay and pressing 

 in on to the land. Here also occur large boulders of schist derived from the 

 hills to the north-east. 



On the summit of the cliif at Malin Beg, striae going N. 30° W. were 

 observed by the Geological Survey. This direction, as the map represents, is 

 at variance with the other striae directions recorded from this region. Since 

 it is very improbable that this striated surface was produced by grounding 

 .bergs, it would seem that its formation must be referred to late glacial times 

 wlien, the ice having retreated from tlie Glengesh Plateau, a great press of ice 

 still moved westwards and seawards down Donegal Bay from the main ice- 

 shed. At Malin Beg, being no longer restrained between the walls of the 

 bounding hills, it would appear to have spread out fan-shaped. This 

 suggestion of a fanning at the mouth of Donegal Bay by ice which further 

 eastwards was hemmed in to the south of tl:e Glengesh plateau would seem 

 to -find support in the occurrence of large boulders of Carboniferous 

 conglomerate which were observed at Malin Beg, and which were derived 

 probably from Muckros or Gortalia on the coast to the east. 



Tlie absence of shells in the drift at Malin Beg is probably due to the fact 

 that the shelly floor deposits of the bay had already been cleared away by the 

 earlier ice. 



There is ample evidence to show that at some period during the 

 glaciation, the chief ice-movement in Donegal Bay on the south, and in 

 Loiighros Bay on the north, was along the coast of the Glengesh Plateau, 

 from east to west, i.e. right across the mouths of the major valleys, while the 

 abundant striated surfaces high up on the edge of the plateau, hundreds of 

 feet above the level of the sea beneath, prove the direction of ice-flow in this 

 upper region to have been roughly at right angles to this course. The 

 glaciers of the plateau appear therefore to have suffered no deflection at the 

 very margin of the plateau by the main ice passing westward along the bays. 

 This may mean that the ice streaming along the northern and southern foot 

 of the plateau was less than the height of the edge of the plateau in thick- 

 ness, so that the plateau-iee passed unobstructed on to the surface of the main 

 sheet proceeding westward. This would give a thickness of the latter ice of 

 approximately 1,000 feet, as the depth of the sea, especially to the north of the 

 plateau, is negligible. 



R.I. A. PBOC, VOL. XXXVI, SECT. B. [2 i*] 



