Chari.eswoeth — Glacial Geology of Nor th-West of Ireland. 259 



At an early stage this river did not escape by the lower part of the valley 

 of the Faiiglian, but by the valley running northwards, east of Kilnappy 

 Hill. A slight retreat, however, opened the normal drainage valley of the 

 Faughan. 



The deposits of sand and gravel which occur near Gortniessan Church 

 were formed fiuvio-glacially as the ice retreated from the hill-flanks some 

 two miles west of Dunamana Station. The mounds near Taninabraddy 

 and W. of Clampernow, north of this, are most probably morainic, and of 

 slightly earlier date.' 



The ice retreating off the western flanks of the Sperrin Mountains opened 

 up the normal drainage lines successively from north to south. Hence, while 

 the Foyle, north of Strabane, was ice-free, a lake was held up in the recess 

 S.-E. of this town. As the ice retreated off Knockavoe, the hill to the west 

 of Foul Glen (F.G.), it formed two very small channels: the one, east of 

 Calheme, intaking at about 600 feet, O.lJ., tlie other, about one mile south of 

 Jjallee. These carried the drainage northwards along the ice-margin into 

 "Lake Gleumornan." The largest of these channels, however, is the Strabane 

 Glen, just E. of the town of Strabane. It intakes at about 310 feet, O.D. 



The ice in its withdrawal from the hill-slopes east of Sion Mills formed 

 along the hillside a sandy moraine. Corresponding with this position of ihe 

 ice-margin is the valley east of Edymore, falling northwards. A later and 

 even more clearly defined stage is marked by the deep " Sion Mills overflow," 

 which intakes at about 170 feet. The hummocky strip of moniiuic country 

 exiending from The Loop, \ia Drunmahoe. Lisnafin, almost to Glenknock 

 Cottage (about four miles in length), marks the position of the ice to the south 

 of this channel. 



The winding valley, W. of Bogside, was initiated as the recession of the 

 ice carried the ice-front west of Sion Mills. The mounds of local material at 

 Three Mile Hill- and extending southward to Knockboro are the morainic 



[Biu-ngibbaglij valley may possibly have been foinied by the Buradeiuiet stream, Ihc 

 course of which was probably deflected near Duiuiaiuauagh station into tlie Foyle by the 

 large deposits of glacial gravels formed there." The present Burudennet stream is 

 totally' inadequate to produce the large valley of the Burngibbagli, or the extraordiuury 

 large accumulation of sand and gravel below the outlet of the Burngibbagh, to which 

 reference -will be made later. On the contrary, the Burndennet stream without doubt 

 flowed westwards to the Foyle valley, so soon as the constraining influence exerted by 

 the ice was removed. 



' The small channels running parallel with the hillside, just E. of the Foyle valley, 

 e.g., the one about half a mile E. of Gortmessan Church, and the other just above 

 Londonderry coming into the main road to Claudy, probably indicate marginal drainage 

 along the hill-flank and independent of the great drainage lines further E. The valley 

 running N. to .Meeuaghhill may represent a slightly later stage. 



- Situated two miles S. of Sion Mills on the right bank of the Mourue River. 



