14 Proceedings or the Royal Irish Academy. 



from the snowfields, some of them doubtless proceeded to overdeepen the 

 mature or immature valleys of Pliocene times. The A'ale of 0\oca may ha\^e 

 been influenced by tliis process: but tjie ice occupying the Irish Sea rose up 

 against the eastern foothills, and they were again protected from local agents 

 of denudation. Narrow valleys like the Devil's Glen were choked by ice, and 

 the main glacier currents moved across them. When the ice-constituents of the 

 invadino- o-laciers melted, the stones, sand, and mud that formed a large part of 

 the lower layers remained behind. A return of cold conditions, liowever. 

 developed the local ice to such an extent that it carried boulders of granite 

 from the central chain of Leiuster across the Ordovician and Cambrian foot- 

 hills, and down upon the plateaus of older drift. These boulders are now 

 found widely scattered, though valleys as large as that of Ovoca intervene 

 between them and their source. The Liffey basin was, of course, affected by 

 all these conditions. 



The Slade of Saggart, which was being actively excavated about the epoch 

 of the pre-G-lacial beach, became choked by the ice-front that was thi-ust 

 against it.' The valleys to the south were occupied by in\adLug ice, and were 

 ultimately crossed by glaciers from the Leinster Chain, which ha\e deposited 

 numerous granite boulders on the east side of the Slievethoul range, at heights 

 of .500 ft. (150m.) above the main valley-floor. 



Long-continued glacial conditions con\erted the uplands of the granite 

 chain into a " karling "'" of the Alpine type. The sterner features that were 

 temporarily produced on the high levels by the sapping action of the ice, and 

 by frost-nibbling following upon occasional sunny hours, have long since been 

 modified by the spread of taluses and the growth of soils. Yet the shaded 

 quarter-inch map (1 : 253,440) of the Ordnance Siu-vey sufficiently reveals the 

 numerous broad glacier-basins, which became worked back at their heads 

 towards the central axis of the chain. Commonly, these basins join the lower 

 ground by narrow valleys, down which the ice descended as glacier-tongues. 

 These outlets were eroded in the first instance by streams rmming from the 

 snow-patches that gathered in the incipient cirques.^ They were occupied, 

 as the cirques grew in importance, by the glaciei-s generated in the basins. 

 As the valley below became filled with ice, the upland glaciers opened on the 

 larger ice-stream, and many of them were overridden and obliterated. When 

 conditions grew milder, the step at the mouth of the cirque again became the 



' Compare W. B. Wright, '■ Some results of Glacial drainage round Montpelier Hill, Co. Dublin,'' 

 Sei. Proe. S. Dublin Soc, vol. is, (1902,1, p. oSl. 



- .\. Penck, '■ Die Alpen im Eiszeitalter," (1902), p. 284. 



^ For modern examples, see (j. A. J. Cole, ■' Glacial features in Spitsbergen in relation to Irish 

 geology," Proe. fi. Irish Acad., vol. xxix B. (1911), PlateXI. 



