It) Proceedings of t lie lioijal Irish Acudem/j. 



ture and abaiidoued hollow, known -ak the Glen, was acconijjanied, even in its 

 early stages of development, )jy the present ravine of the Lifley, and the 

 intervening part of the barrier, on which the hotel now stands, formed an 

 elongated island in the stream. The north-west branch of the river, probably 

 in consequence of its somewhat shorter course,' undercut the south-east 

 branch, and drew off the full flow of the stream. At present we may ascend 

 tlie dry ravine (Plate III, fig. 7), where the vertical rock-walls remain in 

 places unobscured, and emerge at its head on a grassy upland at about 620 ft. 

 above the sea. From this to the Liffey at the ruined Horsepass Bridge 

 (550 ft.) there is a fall of 70 ft. (21"3 m.) ; this measures the amount of 

 glacial drift which formerly supported the stream, and which has been 

 swept away from above the barrier. Looking south from above Horsepass 

 Bridge, where the old coach-road crossed the river, it is interesting to observe 

 the head of the south-eastern notch on the left of tlie view, high above the 

 tioor of the present gorge (Plate III, fig. 8). 



We cannot point to any difference in resistance to eiosion between the 

 rocks forming the Pollaphuca barrier and those immediately above it, when 

 we seek to account for the immense contrast in the form of the valley in the 

 ravine region on the one hand, and near Horsepass Bridge upon the other. 

 The Pollaphuca gorge is essentially a young feature, due either to the general 

 rejuvenation of the district, or to the fact that the river has only recently 

 begun to flow in this direction. Both causes may have operated together ; 

 but I Ijclieve that the latter is by far the more important. 



The Basin of Blessington. 



The drift-tilled basin above the Pollaphuca barrier (Plate III, fig. 9) camiot 

 have been excavated by a stream flowing south across the barrier. Nor can 

 it have been formed by a stream flowing northward, since such exposures of 

 bare rock as we possess provide good evidence that the slope of its floor is in 

 the opposite direction. We are slow to invoke earth-movements or faulting 

 to account for such a depression. It has all the aspect of a hollow of erosion, 

 subsequently filled and overfilled by glacial drift. Considering that it has 

 been developed in schists and slates, which go to pieces so readily under 

 glacial conditions, its excavation may be most reasonably attributed to 

 prolonged glacial scour.' 



' Compare B. Dietiicli, " Movphologie des Moselgebietes, " Veihaudl. naturhist. Vereins der 

 preuss. Rheiiilande, 1911 (for 1910), p. 159. 



'^ On rock-barriers in the course of glaciers and the formation of basins bcbiud tbem, see A. Penek, 

 " Die Alpen im Eiszeitaltcr " (1902), pp. 143 and 254. 



