CHAHLKSWOitTH — Glacial Geology of North- West of Ireland. 283 



and drained by the unmistakable overflow valley which falls north-v?est, 

 and which is situated rather more than one mile S.-W. of Derryreel- Lough- 

 These waters probably escaped by the valley falling northward to Blackburrow. 

 The morainic mounds' encountered on the Dunfanaghy-Falcarragh road, 

 about one and-a-half- miles K.-E. of Itay, mark a slightly earlier position of 

 the ice-front. 



With the warring of the ice-sheet, the snow-clad peaks of the Errigal- 

 Muckish ridge would project above tlie surrounding ice, and the glaciers from 

 the Derryveagh Hills have access to the lowland plain of the west by the 

 large transverse valleys of Alton Lough and Muckish Gap, and unite to form 

 a large Piedmont glacier west of the quartzite ridge, 'i'he retreat of this ice 

 is especially well seen in the valley of the Eay Eiver. The earliest recognis- 

 able moraine of this glacier was noted at Falcarragh Station. This is, 

 however, greatly exceeded iu size by the large terminal moraine which spans 

 this valley obliquely ■ some two miles west of Muckish Gap. Just within 

 the Gap is tunrultuously heaped up a series of large morainic mounds, 

 consisting very largely of the local quartzite, and doubtless derived from the 

 widening of the Gap. 



The shrunken remnant of the Sheephaven Glacier formed the glacier in 

 occupatioir of Glenveagh. One of its recessional moraines is thrown across 

 the valley just below Glenveagh Bridge; a second, iircluding an outwash fan, 

 spans the valley just below the entrance of the Ashelleen Burn and above 

 the head of Lough Veagh. A lateral moraine skirts the east side of the glen 

 below Glenveagh Cottage. 



The Glenna Glacier parted to the north of Errigal from the Alton Lough 

 glacier, its neighbour on the east, and withdrew southward between that 

 mountain and the hill-mass of Tievealehid towards the Dunlewy valley. Its 

 retreat is marked by two doubtful " dry " valleys : the one south of Gashel 

 Hill, the other taken by the railway, and containing Lough Trusk. 



The steep and precipitous scree-covered faces of Muckish, Aghla More 

 and Aghla Beg are probably partly due to the frost action of late-glacial times, 

 which appears to have told more severely on the angularly-jointed quartzite 

 than on the other rocks of this region. There is also a suggestion of snow 

 screes below the summit of Aghla More and Aghla Beg, those of the latter 

 forming a fairly considerable feature ; this may, indeed, be morainic. 



' They contain well stratified sand and gravel, derived chiefly from the ]3reak-down 

 of quartzites (probably Muckish), though they also include pebbles of diorite, granite and 

 schist. Drumlesk L. rests iu a slight depression in outwash sands and gravels. 



- It has driven the river across to the side of the valley and into the quartzite. The 

 lake, which clearly existed originally above this morainic dam after the disappearance of 

 the ice, was subsequently drained by the trenching of the barrier. 



