Charleswoeth — Glacial Geology of North- West of Ireland. 203 



Boulders of the cavboniferous rocks occur plentifully on the outcrop of 

 these rocks, but were never found north of the line of junction with the 

 schist. This distribution is additional confirmation of the north-south trend, 

 indicated by tlie other erratics, by the striae, and the drumlins. 



3. The Glentjesli Plateau. 



The large, roughly oval plateau of an average altitude of ca. 1000 ft. O.D., 

 rising west of the line Ardara-Killybegs — the "Glengesh Plateau" of this 

 paper— was most probably an independent centre of radiation and dispersion 

 during the earlier phase of the glaciation. Even at the maximum stage, tliis 

 centre would seem to have in some measure persisted, though the eastern edge 

 was at times partly overridden by glaciers from the main Donegal ice-slied. 



Under the central moorland, drift and '' solid " are more or less effectually 

 buried, tliough drift is well exposed in the stream sections in the higher parts 

 of the Glen Hiver and in its larger tributaries, e.g. the Owenteskiny Eiver. 

 It is therefore in general only round the periphery of the plateau, where tlie 

 rocks emerge from beneath the quartzite deirisa,\\d. the turf cover, that tlie 

 direction of the ice-flow by striae can be ascertained. 



These and the rochcs moutounees indicate that the principal source of the 

 glacial streams lay to the east, and that the centre of radiation of the early 

 phase lay in the hilly nucleus at the head of Glengesh, which is coincident 

 with the greatest area of high elevation. Even during the later stages, when 

 extraneous ice invaded the plateau, this continued to be the point of dispersion. 



As might be expected from the relief of the region, the glaciers, except to 

 the east, passed radially outwards from the highest tract, moving dowir the 

 glens towards the coasts, though in places they swept across ridges and 

 uplands in a direction somewhat oblique to the valleys. 



On Croaghaveny in the north-east corner of the plateau, IST.S. striae occur 

 in several places, while all along the slopes of Glengesh Hill, striae going 

 steadily N. lO'' W. were observed on countless surfaces, proving that the 

 direction of movement at tliis height on the plateau was diagonally across 

 Glengesh. The hill summits along the edge of the plateau just south of 

 Loughros Beg Bay are finely striated, the westerly component of the striations 

 gradually increasing as we pass from Glengesh westward. Thus at Barkillew 

 Hill, just west of Glengesh Hill, and at Croaghataggart, the striae bear K 30" 

 W., at Lough Anaffrin, N. 40 to 50' W., at Loughenaherk, N. 60° W., and along 

 the shores of Kiltyfannad Lough, E.-W. and 10" K of "VV. 



Very little drift occurs in the northern valleys. It consists exclusively of 

 quartzite and diorite. 



The highly jointed and steeply dipping quartzites of Slievetoo have been 



