iO proceedings of the Royal Irish Academy. 



valley, liowever, starting at 780 feet (238 m.j above the sea, was aliaiuloiied by 

 its original stream before it liad attained full maturity. We must remember 

 that the upland ice that once lay Ijeyond its southern end gave rise to 

 copious streams on melting, and some of this water may have run north- 

 ward through the Slade. But the steep valley is of pre-Glacial origin, 

 since it was entered by liie northern ice, which deposited an infilling of 

 limestone gravel on its floor, in places more than fifty feet in depth. The 

 head of the Slade served for the passage of an ice-tongue southward, 

 which helped to fill with exotic gra\'el the great hollow stretching down 

 tu Pollaphuca. 



To illustrate the features of the Slade of Saggart, we must look beyon 

 ihe Liffey basin. Throughout the counties of Dublin and Wicklow we find 

 valleys overdeepened and new ravines carved out, and the present streams still 

 actively engaged in these erosive enterprises. Yet the valleys in which they 

 run are often cumbered by glacial drift, and we cannot attrilnite a Glacial or 

 post-Glacial date to the whole of this series of ravines. 



Mr. G. W. Lamplugh,' in connexion with his masterlj' explanation of the 

 Scalp, agrees with Mr. Maxwell H. Close' in assigning a late Glacial 

 origin to the far larger notch of the Glen of the Downs ; and 

 rock-ravines certainly occur, like the Gap of Dunran near Killiskey, 

 and St. Kevin's Gap near Hollywood, carved in anomalous positions on the 

 hillsides of Wicklow, and attesting the vigoiur of the rivers that accompanied 

 the shrinkage of the ice. Immatiu-e features, however, that are strikingly 

 contrasted with the forms of the older valleys of the Leinster Chain, occur on 

 a large scale at various levels along the lower slopes of the chain, and notably 

 therefore among the Ordovician and Cambrian foothills. It is difficult to 

 regard the Devil's Glen near Ash ford, by which the Vartry River descends 

 from the plateau of Eoundwood, as the result of fluvio-glacial flooding. 

 Together with the Dargle ra\ine at the foot of the mature valley of Glencree, 

 it appears to have originated in pre-Glacial, if late Cainozoic times. At the 

 head of the Devil's Glen, the Vartry notches the plateau at a height of 

 500 feet (150 m.) above the sea, and its old meanders are now entrenched in 

 the Cambrian slates for a quarter of a mile further on the level land. This 

 latter feature in itself suggests a recent uplift of the eountiy. The Dargle 

 ravine miist be regarded in connexion with the hollow of Glencree, rather 

 than with the cirque in the Powersconrt Deerpnrk, over the cliff of which 



'" Geology of the country around Dublin," Mem. Gaol. Survey (1903), p. 50. 

 ^ M. H. Close, " Notes on the General Glaciafion of the Rocks in the Neighhourbood of Dublin," 

 Journ. R. Geol. Soc. Dublin, toI. i (1864), p. 12. 



