256 Proceedings of the Royal Irish Aeademjj. 



off the hill just north of the Oaks, and uncovered the col at Gortnessey, were 

 these overflows thrown out of action. Prior to this lowering of the lake the 

 water surface covered some 22 sq.'nnles.^ 



The depth and size of the " Gortnessey Channel " (G.C.) are doubtless 

 partly- to be ascribed to the erosion exercised by tlie large stream, which, as 

 a glance at the map (PL YIIl) will show, carried off at one period almost the 

 whole of the drainage of the Spei'rin Mountains. 



The Castle Eiver valley probably conveyed the drainage, as the ice, at a 

 slightly earlier stage, fell away from the hillsides to the east of this. 'J'lie 

 Sheskin River also served for a shore time as a glacial overflow. 



The " Gortnessey Channel" (G.C.) drained a lake — "Lake Paughan," the 

 successor of " Lake Claudy " — extending up the Fauglian as far as Teenaght, 

 where the 300-foot terrace begins ; indeed all this upper part of the lake 

 would appear to have been completely infilled by sediments brought down by 

 the greatly swollen Faughan. The water- worn sands and gravels, which are 

 heaped up in the form of mounds, and fan out to north and north-west, 

 represent materials swept into the lake from the ice-face. The finest parts 

 of the oOO-foot terrace, however, are those occurring near the entrance of 

 Bond's Glen into the Paughan valley.' They were clearly the deltas of the 

 large stream pouring out of the higher "Lake Burndennet." 



The position of tlie ice-front at this stage, as it curved across the Paughan 

 valley from the Gortica Hill, is clearly marked by the morainic ridge near 

 this place, described, though not recognized as a moraine, in the " Londonderry 

 Memoir " (p. 58), and by the very fine mounds developed at The Cross.^ 

 Hereabouts they cover an area of considerably over one square mile, and consist 

 of immense deposits (sections show over fifty feet of the material) of sand and 

 gravel. They were regarded by J. E. Kilroe ^ as the result of the " prolonged 

 discharge " of " glacier waters when tlie Paughan valley above this point was 

 still occupied by ice." There can, however, be no doubt, from the sequence of 



' The sands and gravels of Toneduff and the terraces in the depression Is. of 

 Dunnamanagh. e.g. W. of Gortuasky and N. of Lower Town, at about 420 feet, probably 

 indicate the extension of this lake over the gi'ouud now occupied by Bond's Glen into the 

 area of " Lake Burndennet," and that the latter overflow was initiated as soon as the 

 waters in the Paughan Valley were lowered and permitted of drainage across the 

 connecting neck. 



- They are possibly in part the efi'ect of rejuvenation. . 



' They occur, for example, just N.-W. and S.-W. of Toneduff Bridge, extending as a 

 distinctly recognizable teri'ace for almost a mile below this bridge and along the west bank 

 of the Faughan. The best view of these terraces is obtained from the main road 

 (Londonderry to Claudy) just W. of Claudy. 



* A map of these mounds is given in the Londonderry Memou- (fig. 8, p. 59). 



^ Ibid., p. 50. 



