290 Proceedings of the Royal Irish Academy. 



from north of Ardstvaw across tlie Derg Eiver, which breaches the moraine 

 just above its confluence with the Toyle, and continues by Mullaghamley to 

 Wood Hill and the slopes of Bessy Bell at Harry Avery's Castle. It is most 

 magnificently displayed at Wood Hill, where it rises roughly 100 feet above 

 the adjacent country. It contains great quantities of sand, and exhibits other 

 features indicative of water action. 



The ice in its further withdrawal shrank into the valleys to form a couple 

 of glaciers, the one retreating up the valley of the Mourne Beg Eiver, the 

 other along that of the Derg. Evidence of the former of these two glaciers 

 was not obtained till it had withdrawn almost to its place of divergence from 

 the glacier streaming to Stranorlar; unmistakable witness of the latter is 

 borne by the " dry valley " at Aghalunny, about one-and-a half miles west of 

 Killeter.' It intakes at about 370 feet, O.D., and carried off the surplus waters 

 of a lake held up in the recess now drained by the Glasgagh Burn.'' This 

 channel, situated on the south side of the Derg valley, and the " Fyfin- 

 Victoria Bridge channel " on the north side, prove most conclusively the 

 existence of a Derg glacier. 



The great mass of ice standing on the site of Lough Erne, and which at 

 an earlier stage had thrust large lobes into the Clogher valley, the Fintona 

 Plain and along the Erne depression, above Enniskillen, shrank behind the 

 watershed and down the hill-slopes north-east of Kesh, forming a few extra- 

 glacial lakes. The highest of these discharged by Killen Gap, wliile the wide 

 valley north of Tappaghan Mountain, falling east, drained a lake held up in 

 the recess to the west. A later phase of this retreat is marked by the valley 

 falling east from the village of Lack. jSTo further indications of the mode of 

 retreat of the ice covering this great depression were observed. 



The retreat of the northern edge of the "Great Finn Glacier" may now 

 be briefly sketched. The mounds near Church Town in the valley of the 

 Foyle are morainic features. Of similar origin are the mounds, aboiit two miles 

 S.-W. of Lifibrd, the ridges at Clady and C^loghfin (the current bedding in 

 the mounds of the latter, as seen in the railway cutting, indicating ice to the 

 west), the mounds near Prospect Hill, and the low ridge of gravel and sand 

 half a mile long observed S.-W. of Carrigans.' These moraines were produced 

 by the ice receding southward along the valley of the Foyle into that of the 

 Finn. 



1 The maiu road from Castle Derg to Pettigo runs along the valley. 



- The channels draining into this lake and situated farther south, though recognizable, 

 are not well developed. 



' These were regarded as eskers in the Londonderry Memoir, p. 39. Some of the 

 sands and gravels at Carrigans probably represent outwash material. 



