Charleswokth — Glacial Geology of North- West of Ireland. 311 



stuetchiiig in a south-easterly direction to Lough Derg, though extending 

 also to the north-west, and at a somewhat later stage linking up witli the 

 glaciers of tlie Sligo hills and forming a huge pool of ice over the intervening 

 depression of Lough Erne. Its lines of ice-flow were ronghly coincident with 

 the trend of the major valleys. Its confluent glaciers extended to the 

 Atlantic and Donegal Bay on the west, suS'ering deflection by the Glengesh 

 Plateau and the Derryveagh Mountains. Tliey spread over the drainage 

 basins of the Finn and Foyle, and down Lough Foyle and Lough Swilly ; over 

 Inishowen, the Sperriu Mountains, and Slieve Gallion, and down the valley 

 of the Bann ; across the plains of Tyrone to Lough Neagh and beyond ; over 

 the Fintona Plain, the Cloglier valley, and Slieve Beagh, and iip the valley of 

 the Erne to tlie Central Plain. 



Tliere was no axis of dispersion along the line running north-east from 

 tlie Erne valley to Lough Neagh and the Antrim coast as suggested by 

 E. Hull, though tlie Sligo hills to the west most clearly acted as a radiation 

 point. 



Boulders of the Donegal, Barnesmore and Tyrone granites serve as tlie 

 chief indicators of the lines of ice-flow from the Donegal centre. Those of 

 the Barnesmore granite occur across north-west Ireland from sea to sea, from 

 the southern extremity of the St. John's Peninsula in Donegal Bay to 

 Inishowen Head, at the mouth of Lough Foyle, a distance of 77 miles ; they 

 cover an area of over 1,200 square miles. (Fig. -1, p. 219.) 



The retreat of the Donegal ice is most clearly marked by moraines and 

 marginal drainage channels ; these connected with great numbers of e.\tra- 

 glacial lakes ranging up to some thirty-three square miles in surface extent 

 and fifteen miles in lenoth. 



O 



The wide valleys of the Sperriu Mountains were the sites of large glacier 

 lakes, which draiued by overflow valleys cut across spurs and cols, and which 

 were successively thrown out of action by the continuous retreat of the ice. 

 The withdrawal (PI. YIII) was affected by a shrinkage off the highest peaks 

 and the main watershed, an occasional extra-glacial lake being formed 

 continuous over the cols. More rapid retreat on the northern flanks caused a 

 drainage, northward across the cols, of lakes impounded in the deep recesses 

 on the southern flanks. The waters from both the northern and southern sides 

 of these mountains were carried into a lake, held up by ice standing along the 

 Foyle side of the great basalt escarpment stretching south from Benevenagh 

 to Benbradagh, the drainage finally escaping along the face of the former 

 mountain. (Stage 1, PI. Aail.) 



Further retreat produced a series of large lakes standing at a lower level. 

 Those on the southern side of the watarshed, e.g., " Lake Glenelly," discharged 



