Charlkswokth — Glacial Geology of North- West of Ireland. 297 



. embouchure of the stream where it left its gorge and entered the lake.^ About 

 one-and-a-half miles farther north another stream (entering Lough Foyle at 

 Sand Point) shows a section in a similar accumulation exhibiting current- 

 bedded and water-worn sand and gravel. 



During tlie formation of the huge terrace skirting the southern shores 

 of Lough Foyle and of the discontinuous strips of shelf fringing the Inishowen 

 shores of the estuary, it seems necessary to suppose the existence of a great 

 lake held up by an ice-barrier blocking the mouth of the lough. 



The outlet of this lake is to be sought in the valley running north-west 

 from Londonderry to Burnfoot and to Lough Swilly, and traversed by the 

 Londonderry and Lough Swilly Eailway. Throughout the greater part of its 

 length it is quite streamless, yet it possesses the form of a large river valley, 

 and was clearly produced by a stream of considerable magnitude which flowed 

 in wide-swinging curves towards the Swilly. The highest point in its floor 

 is 50 feet, O.D. This depression may be regarded as the overflow channel 

 which carried off the waters of the huge lake impounded in Lough Foyle, 

 and which has been deserted save, perhaps, by the sea of raised beach times, 

 since the final retreat of the Scottish Ice.^ 



The latter, proceeding westwards off the north of Ireland, east of the 

 mouth of Lough Foyle, hugged the Antrim coast, spreading on to the land 

 only where this was of small elevation ; the great dome-shaped mass of 

 Knocklayd, south of Ballycastle, the towering bastion of Benevenagh and 

 the heights of Inishowen Head obstructed its landward progress. East of 

 Knocklayd, as Dr. A, E. Dwerryhouse has shown, the ice pressed southwards, 

 causing the drainage to pass behind this mountain, while over the extensive 

 moorland, stretching southward between Knocklayd and Benevenagh, was 

 thrust a great lobe of Scottish Ice, which threw down at its edge the 

 Ballymoney-Capecastle moraine, charged with the Ailsa Craig paisanite and 

 Scottish rocks. Sweeping round the mass of Benevenagh, the Scottish Ice 

 advanced across the wide mouth of Lough Foyle. Pressing as a great ice-lobe 

 up the slowly-shelving shore of this estuary, it ponded back the normal 

 drainage of the country, converting the estuary into a lake — " Lake Foyle." 

 The ice did not apparently extend as far westward as the Culdaff depression 

 where the drumlins trend south-north. Tlie direction of the ice thrusts, the 



'This depo.sit is described as "River Gravels" in the Londonderry Memoir, 

 p. 31. It would seem to be impossible to produce this terrace with the present relative 

 position of the stream and sea level. 



- The wide depx'ession — the '"' Pennyburn depression" — in which this smaller valley 

 lies, is obviously pre-glacial ; the floor is formed of boulder clay, both Scottish and 

 Ponegal, and of gravels and sands. 



