Cole — The Prollem of the Lifet/ Vulle//. 15 



seat of an ice-fall, and ultimately of a waterfall. In the cases of Glendasan 

 and Glendalongh on the east side of the chain, this step-feature remains 

 conspicuoua. In the Liffey upland, the great Kings River basin opens on the 

 main valley at Baltyboys House, through a comparatively narrow hollow. 

 The rock-step has been here worn down, and the only indication of its former 

 presence is the narrowness of the valley in the schist region between Lackan 

 and Burgage Bridge. Drift that was piled across the outlet, rather than any 

 residual roek-barrier, no doubt closed the basin as the Ice Age waned, and 

 was responsible for the lake which is traceable in the fiat floor of the Lackan 

 hollow. 



When the ice melted away, the Slade of Saggart must have run almost 

 dry. The post-Glacial stream has never been a strong one, and it has done 

 little to remove the mass of drift deposited in the valley head. 



The Pollaphuca Barrier (Britonstown). 



The gorge in which the waterfalls of Ballymore Eustace and Pollaphuca 

 (Plate II, fig. 2) occur has been excavated in a barrier of Ordovician slate and 

 volcanic rocks. The north-east side of this barrier runs at right angles to 

 the strike of the strata, so that it is not due to the outcrop of any rock of 

 special hardness. Whatever the direction was in which the pre-Glacial Liffey 

 escaped from its somewhat singular imprisonment among the foothills under the 

 strike-ridge of Slievethoid, the hollow north of the Pollaphuca barrier requires 

 explanation. The barrier connects the drift-covered plateau of Bishops- 

 land (600 ft.; 183 m.) with a height of 1079 ft. (329 m.) in Lugnagroagh 

 upon the granite. Looking south-east along it from a height on the main 

 road of 622 ft. (190 m.), where we stand upon the bare edges of the slates, 

 we realize how sharply it has been cut through by the Liffey, which runs 

 90 ft. (27'4 m.) below us (PI. II, figs. 5 and 6). The local ice, in its final 

 extension, after the continental type of ice-sheet had withdrawn, spread across 

 this barrier, and carried granite boulders over it, as it did against the east 

 side of the Ordovician ridge above Tinode. The Liffey gorge, above and below 

 Pollaphuca, has been eroded through this covering of late Glacial drift, and is, 

 I believe, entirely post-GIacial. The river, when it reached the solid rock, 

 began by carving out a ravine which is the main feature of the townland of 

 Britonstown ; this is now dry in all but its very lowest portion, where it 

 emerges steeply on the present Liffey gorge below Pollaphuca.' This imma- 



• This viuiiie was described as an eailier course of tlie Liffey by Wm. Fitton, "Notes on the 

 Mineralogy oi Part of the Vicinity of Dublin" (1812), p. 39. The hollow above the barrier was 

 regarded by Fitton as the basin of a lake. 



B.l.A. PROC, VOL. XXX., SECT. B. [V] 



