Charles WORT II — Glacial Geology of North-XVest of Ireland. 245 



the small chaimel west of this, cut into the terracic and luorainic deposits of 

 " Lake Genedra." 



During the stage represented by these channels, the ice swung clear of 

 the basalt escarpment ; the normal drainage from the latter and from the 

 ice-free portion of the Sperrin Mounlains, together with the melt waters 

 from the ice-front, was now able to escape northward by the Eiver Eoe. 



Hence, as the map (Stage 2, PI. VIlI)i represents, the drainage flowing 

 northward and southward from the main watershed- of the Sperrin Mountains 

 was, at this stage of the ice-retreat, completely distinct, the surplus waters 

 of the one series of lakes flowing by the Banagher channel into the Eiver 

 Eoe, those of the extensive " Lake Glenelly " and its tributary, " Lake 

 Glenlark," into the Moyola River. At no subsequent period of the retreat 

 did this complete separation of these drainages obtain, as they were linked 

 up by the large connecting- through valley of the Inver Burn. 



About the time represented by this chain of lakes, the higher parts, 

 probably above the 1,250 contour of Mullaghcarn, began to project as a 

 nunatak above the ice-surface ; the line of junction between the ice-free 

 summit and the ice covering the lower flanks of the hill probably sloped 

 somewhat to the north-east, as the maximum pressure was from the south- 

 west. 



(c) — Third Stage. 



The highest lake in the series, formed during the next phase (Stage 3, 

 PL VIII), was situated in the angular recess west of Mullaghcarn. The position 

 at one stage of the ice-front, wliich impounded the waters of this " Lake 

 GuUion," is marked by a belt of irregular mounds and hummocky ridges, 

 extending from Cullion Bridge to Turcur. The escape of the waters of this 

 deep (about 350 feet) lake into the vast " Lake Gortin " was by the Altaravan 

 Glen (A.), in the very angle of the recess. This valley intakes at 848 feet, 

 O.D., and falls to 800 feet, O.D. The irregular, moundy terrace,^ which at 

 this height runs so conspicuously along this part of the hillside, is its delta. 

 This feature has been profoundly modified by the later glacial streams, much 

 of the lower terrace having been derived from its destruction.' 



The western wall of ice, which impounded " Lake Gortin," swung from 

 the northern flanks of the Bally natubbrit Mountain, across the big Gortin 



' This line represents a somewhat later stage in tlie south than in the north, when 

 the Altnaheglish and Cloghornagh overflows had been thrown out of action by a further 

 retreat of the ice. 



- These liuge mounds, as seen from Slievebog, the hill to the N. of the valley, are 

 seen to approximate to one level and to be terracic in form, with flat top and steep face. 



^ The small lakes and tarns on this hillside, e.g., Black Lough and Oak Lough, are 

 impounded by this deltaic material. 



