Charleswoktk — Glacial Geolocjy of North-West of Ireland. 247 



trleiigomna outlet out of action and iuitiated tlie carving out of the magni- 

 ficent Butteiiope Glen (B.L). This glen falls rapidly from its intake at 780 

 feet, O.D., to about 630 feet, O.D. Here it now falls over a step into the wide 

 valley of the Inver Burn, but was doubtless originally continuous with a tor- 

 tuous channel, the " Balix " overflow, which runs with but little fall along the 

 side of the Inver valley, and whose intake is about 600 feet, O.D., i.e. some 

 30 feet below the exit of the Butterlope Glen. These two channels were the 

 product of the marginal streams deflected into the rock by the ice which stood 

 in the depression of the Inver Burn. A still further withdrawal to the west 

 threw out of action the Butterlope and Balix Glens and opened the Inver valley, 

 which then carried off llie waters of the lake to the south. This line of 

 drainage, initiated by the retreat from the hills to the north of Plumb Bridge, 

 persisted for a considerable period and excavated a channel, second in 

 importance to none in the Sperrin Mountains, and in size inferior only to the 

 Burngibbagh valley to be described later. It intakes at about 580 feet, O.D. 

 The winding of this glacial river has separated the Balix and Butterlope 

 valleys, which, as stated above, were originally continuous. 



"Lake Glenelly," the waters of which escaped by this " Inver Channel," was at 

 this stage held up by ice, which rested on the southern shoulders of Crockaclady 

 and swung across to Slievebeg, just west of Plumb Bridge, the ice-front standing 

 not very distant from the slopes of Sturrakeen to the north of this village. This 

 proximity is confirmed by the deep Glengoppogagh (Gg.) which is cut into the 

 '' solid " rock on the south-west flanks of this hill. It intakes at about 750 

 feet, O.D. , and was obviously produced as the ice retreated off this shoulder 

 as an intermediate step from the Butterlope Glen to the Inver Channel. The 

 period of the occupation of Glenoppogagh was very brief, as only a slight 

 recession of the ice-front would throw it out of action. The surplus waters 

 of " Lake Gortin " were discharged by the Barnes Gap overflow into " Lake 

 Glenelly " when the height of the latter was determined by the Inver channel. 

 The 600-foot terrace formed at the entrance into the lake clearly proves this. 

 During the Butterlope Glen stage of " Lake Glenelly " this lake, the level of 

 which stood at about 780 feet, O.D., must have been some 13 miles long and 

 not less than 400 feet deep, with finger-like ramifications up all the tributary 

 valleys. 



The Inver overfiow continued in operation for a very considerable period, 

 as gauged by the great size of the valley, the huge terraces produced in the 

 lake into which it poured its waters, and by the fact that not until a further 

 retreat of the ice-front some six or seven miles to the westward had taken 

 place, was a lower and alternative means of escape possible. 



The drainage of the deep and large valley to the west of Crockaclady Hill 

 was simultaneously impounded, forming a lake independent of " Lake 



