CharlesWorth — Glacial Geology of Morth-West of Ireland. 243 



roughly 12 miles, aiul a depth of over 500 feet ; its surface covered nearly 

 15 square miles. The surplus waters of this "Lake Glenelly " were dis- 

 charged by the valley of the Glengomua Water (G. W.) into the Moyola Eiver, 

 its detritus probably contributing iu no small way to the building up of the 

 delta of this river separating Ijough Beg from the main water of Lough 

 Neagh. That this outlet was free indicates a great retreat of the ice south- 

 Wards along the Bann valley, a retreat vastly greater in amount than the 

 corresponding shrinkage to the north of the Sperrin v/atershed. The glacier 

 flowing down the Bann obtained its fresh accretions by a very circuitous 

 route, via the " Omagh-Draperstown corridor," and would doubtless retreat 

 earlier than the more readily and directly-fed glaciers flowing along the 

 northern face of the Sperrin Mountains. 



The intake of the Glengonnia overflow is about 850 feet, O.D.^ Glenelly> 

 west of the outlet, is choked with stratified sands and gravels thrown down into 

 the lake. They are exposed in the fine stream sections at Crocknakiu and just 

 below Granagh. Much of this material, as of that which swathes the sides 

 of this huge valley, was doubtless swept out of the ice and into the lake as 

 the ice-front receded westwards. The moundy ridge running along the 

 valley side, just east of the Glenlark overflow (G.G.), at an altitude of 1,000 

 feet O.D.,^ represents an earlier phase in the retreat when the ice was 

 thrust into Glenelly from the east, and the drainage was over the main 

 Sperrin watershed, while the smaller mounds, south-west of Crocknakin, near 

 the intake, and down the valley of the Glengomna Water, indicate later halts. 

 The pre-glacial rock-floor of the Glenelly and its tributaries is, in consequence 

 of the extraordinary thickness of the lacustrine deposits, very seldom seen.' 



With the westward retreat down Glenelly, the ice gradually and simulta- 

 neously withdrew from the hill- range to the south of the valley impounding the 

 waters of the Glenlark Eiver. These drained northwards by an overflow (G.G.) 

 at the head of the glen (intake about 1,050 feet, G.D.) into " Lake Glenelly," 

 ceasing at the altitude of "its Glengomna outlet. The sands and gravels occur- 

 ring in the upper part of Glenlark represent the lake deposits formed at this 

 period. A lake was similarly held up in the upper part of the Coneyglen, 

 K-W. of Mullaghturk, and drained by the Altaturk Glen (A.G.) into the 

 Moyola Pdver. The summits of the Craignamaddy range running west from 

 Barnes Gap (B.G.) were in all probability ice-free at this stage, the moundy 



' The Glengomna Water now enters the main valley of the Glenelly over a corrom. 



- The position of these mounds is indicated very well on the one-inch O.S. map. Refer- 

 ence is made to them in the Geol. Survey Memoir, Sheet 26, p. 21. 



3 In one case only, that of the Garvagh Burn, does the tributary enter by a rock 

 gorge. Here the stream has obviously failed to find its old course, and had to carve out 

 a new one through the scliist. 



