OitAULKSWouTH — Glacial Geology of North-West of Ireland. 257 



events brouglit out iu Uie examination of this and tlie adjacent areas, that 

 they represent tlie materials swept into the quiet waters of a lake out of 

 the ice which stood, not ahoxe but hdow this point. 



An indistinct moraiuic ridge running along the eastern side of the 

 Gortnessey valley was formed at a slightly earlier stage. Formed about this 

 time, and as^the ice withdrew from the south-west flanks of Slieve Buck, are 

 the poor channels observed to the east of The Cross, falling north into the 

 Gortnessey valley. 



The Fiucarn Glen^ carried off for a short period the surplus waters of 

 " Lake Faughan," and was abandoned when the westward retreat of the 

 ice-face freed the main valley of the Faughan at Drumahoe. The small 

 gravel mounds south of this last locality and on the south side of the 

 Faughan and Burngibbagh valleys are moraines of this stage. 



The position of the ice-margin during this, the fourth phase of the retreat 

 from the Sperrin Mountains, is represented on the map (Stage 4, PI. VIII). As 

 in the previous stage, the drainage from almost all these hills, both south 

 and north of the watershed, and the melt waters proceeding from some forty 

 nriles of ice-face, were carried by a series of marginal channels and a chain of 

 lakes along the western flanks of these mountains northwards to the Foyle 

 estuary. The most important feature of this period was the " Inver 

 Channel." During all the phases of the retreat, represented on the north 

 by the complex drainage-system briefly outlined above, this great transverse 

 valley continued to convey the waters of the south side of the range to the 

 north side. It persisted right through to the Burngibbagh stage, and even 

 after that channel had ceased to operate. 



{e)— Fifth Stage. 

 The next phase of the retreat saw the recession of the ice off the hills 

 east of the Foyle and the opening of the normal drainage of this river. The 

 position of the ice-front at a late stage in the existence of " Lake Burndennet " 

 is given by the relatively small overflow channels occurring at higher levels 

 north of this lake. The first of these occurs just east and below the summit 

 of Killymallaght Hill. It intakes below 800 feet, O.D., and was fed 

 only by the waters pouring directly off the ice itself. The stream flowed 

 into a lakelet held np in the embayment north of the hill, its surplus 

 waters excavating the " Curryfree overflow "- on the western flank of the hill 

 of that name. Its intake is about 670 feet, O.D., the channel falling north. 



1 A photograph of this channel forms the frontispiece of the Londonderry Memoir 

 (1908). 



2 Reference to this overflow is made in the Londonderry Memoir, p. 82. 



