Chaulkswoutii — Glacial Geology of North- West of Ireland. 21 1 



tlie erratics would appe.ar to have been derived from Mognmna and other hills 

 rising south and south-east of Ardara. 



]ce passed over the summit of Aghk Mountain (J, 961) in a direction 

 slightly north of west as shown by striae on the very summit of the 

 mountain, while a few others possessing the same general direction have been 

 met witii on its Hanks. A large glacier also flowed along the length of the 

 depression in which lies Lough Muck, which seems in some measure to have 

 guided the flow of the ice in this region. 



Ice swept from the heights of Aghla Mountain, Scraigs, and Clogherdoo 

 down the steep face which rises abruptly from the southern shores of J,ough 

 Linn, and passed obliquely across the depression and over Kuockrawer and 

 the neighbouring hills. 



Continuing in this westerly direction, tlie ice overrode the summits of the 

 hills rising to the west of the Shallogan valley, flowing ca. W. 20° IST. towards 

 the Gweebarra liiver. The drift is chiefly granitic, being the greater part of 

 the material eroded from the granite hills lying to the south of the Gweebarra 

 depression, though quartzites from the south are not lacking. 



On Croaghleconnell, which lies to the west of the Gweebarra valley, a 

 crossing of striae is extremely common, and these scratches can be resolved 

 into two distinct sets, one bearing N.-W., the other 10° S. of W. They are 

 rnost distinctly marked and occur on the same or on contiguous surfaces. 

 Shnilar conflicting systems of striae occur on Jkleenabollagan (311), in this 

 case trending N. 30° W. and N. 60° and 70° W. These clearly cannot be due 

 to any constraining influence of the local configuration of the country, as 

 they occur on the very summit as well as on the flanks of the hill, nor can 

 they be ascribed to the small variations in direction of striae, inevitable 

 beneath any ice-sheet composed of constituent streams of varying strength, as 

 the two sjstems are too constant in their respective directions, are separated 

 by too wide a gap, and intermediate values are absent. 



These cross striations were noted by J. F. Campbell in liis paper on 

 the " Glaciation of Ireland. "^ He accounted for them by supposing the 

 descent of the glacier down the Gweebarra valley, antl on its complete 

 retreat its succession on the ground by a smaller mountain-glacier, flowing 

 from the north-west, and descending from the top of Slieve Snaght. There 

 can, however, be little doubt that the ice producing this latter set of striae 

 proceeded from the contrary direction, i.e. from the south-east. 



In the Geological Survey Memoir on the north-west of Donegal, already 

 referred to, the system going something south of west is ascribed to " the 



■• Q. J.G.S., vol. xxix (1873), p. 208. 



