238 Proceedings of the Royal Irish Academy. 



vast aiiiouut of terracic material ; and this area perliaps more than any other 

 raises the question of the source of so much gravel and sand. The rapid 

 waste of drift in the higher parts of the valleys and on the steeper slopes of 

 the ice-free hills, the excavation of the large glacial drainage channels, and 

 the sheets of sub- and en-glacial material swept from the ice itself seem, 

 however, cjuite adequate to account for their prodigious development. 



In the " Londonderry Memoir" (p. 78), " the terraced structure of the sides 

 of the Faughan valley," e.g., in the neighbourhood of the " Oaks," and at 

 Burntollet Bridge, is ascribed to erosion by " streams flowing in late-glacial 

 and post-glacial times." The terraces are, however, unmistakably accumula- 

 tion features ; the coincidence in height of their upper surface with the 

 intake of glacial overflow valleys draining marginal lakes, which covered the 

 areas in which they occur, their cliaracteristic location at the debouchure of 

 large glacial streams draining higher extra-glacial lakes, their magnificent 

 delta structure, and the water-worn nature of the fine sand and gravel of 

 which they are composed, prove most conclusively their accumulative and 



not erosive origin. 



(a) — First Stage. 



In early glacial times the Sperrin Mountains formed, as has been 

 shown, the centre of an independent ice-radiation. In later times, as the 

 marginal drainage phenomena most strikingly prove, the ice completely 

 covering these hills gradually diminished in thickness until the backbone of 

 the range was exposed. From this main watershed the ice slowly retreated, 

 falling away simultaneously to north and south, and uncovering some of the 

 deep recesses on either side. In the earliest of these retreat phases lakes 

 were continuous over the cols of the main watershed. This is most clearly 

 seen in the ease of the col to the east of Sawel Mountain (2,240). The 

 well-marked notch, cutting across the spur to the south-east of Barnes Top 

 (1,506), with an intake just Ijelow 1,250 feet, O.D., could not have been 

 formed unless the col, some 150 feet lower, were covered with water held 

 up by ice in the south. 



This lake — "Lake Cloghornagh" — over 150 feet deep on the col, at the 

 initiation of the valley, was held up to north and south by barriers of ice, 

 which had already shrunk from the main watershed, and whose melt waters 

 could only escape by the channel to the south of Barnes Top. 



The recessed character of the hills to the south of tlie main Sperrin 

 watershed brought about, as the ice-front still further reti'eated, the 

 formation of a series of marginal lakes, each of which had its own separate 

 overflow channel cutting across the main watershed into the temporary lakes 

 held up in the corresponding recesses in the northern flanks. These overflows. 



