Chaelesworth — Glacial Geology of Norlh-West of Ireland. 313 



The ice shrank off Tnishowen to form two powerful glaciers — the one in 

 occupation of Lough Foyle, the other of 1 ,ough Swilly ; the withdrawal of 

 these glaciers is indicated by marginal channels and morainic mounds, the 

 largest lake— "Ijake Mintiagh " — being held up in the great recess to the 

 north-east of Buncrana. A similar withdrawal uncovered the smaller 

 peninsulas farther west, and caused the formation of a series of ^'.'glaciers 

 in occupation of the larger depressions, e.g., those of Mulroy Bay and 

 Sheephaven ; the Glenveagh glacier was the shrunken remnant of the 

 latter. 



Tiie ice wliich buried the extreme north-west retreated southward along 

 the Rosses, one of its principal feeders parting from it to the west of Gweedore 

 to form the Dunlewy glacier, which shrank eastward to Dunlewy and the 

 Poisoned Glen. 



The great masses of ice, covering the Derg, Finn and Foyle country, split 

 into a couple of large . glaciers ; the southern one, in occupation of the Dero- 

 valley, withdrew westward up the valley, passed Castlederg and Killeter to 

 Lough Derg; the larger lobe shrank into the valley of the Finn, in its earlier 

 retreat impounding iu the Deele valley a large lake, the history of which 

 has been clearly traced. (Fig. 3.) 



Moraines in the Barnesmore Hills and Derryveagh Mountains represent 

 the last stages in the waning of the Donegal ice-sheet. 



While the Donegal Tee still lingered over the country south of London- 

 derry City, and skirted the western foot of the Sperrin Mountains, and while 

 the "Burngibbagh channel" was still in operation, the Scottish Ice 

 re-advanced along the northern coast. It thrust a large lobe into the mouth 

 of the Foyle estuary and converted this into a lake, which drained by the 

 valley running north-westward from Londonderry to Lough Swilly. The 

 great gravel-spread skirting the southern shores of the estuary is, in part, the 

 terrace of this large "Lake Foyle." 



The effect of the glaciation was in the main to soften the relief, though 

 in places it emphasized the surface features. Hock basins abound, especially 

 in the large valleys of the Donegal hills, e.g., Lough Veagh, Lough Glen and 

 Alton Lough. Lough Eske and Lough Erne "are hollows scooped out of a 

 Tertiary coastal peneplain. 



The U-valleys of the Donegal Highlands owe their origin to a Tertiary 

 uplift and rejuvenation. Glacial erosion has somewhat modified their form, 

 liaving altered the gradients, widened, over-deepened and over-steepened the 

 valleys, and blunted or truncated their spurs. The corries are likewise 

 pre-glacial in date, their outlines having been merely modified by glacial 

 erosion, possibly by early glacial corrie glaciers, more profoundly by the 



