94 



Proceedings of the Royal Irish Academy. 



themselves. It is also shown that sub-glacial rivers are very effective 

 erosive agents ; and M. J. Vallot, from observations beneath the 

 Mont Blanc glaciers, denies the great erosive power attributed to the 

 ice. Tourists may remember the wall or bank of solid strata which 

 crosses the Ehone Valley between the Dent du Midi and the Bent de 

 Morele, near St. Maurice ; this, and a corresponding wall of lime- 

 stone, across the Aar Valley, near Meiringen, are referred to by 

 Brunhes and Martel, as considerable difficulties in the way of 

 unquestioned acceptance of valley-formation by glaciers. 



The facts above recorded impose upon us a measure of reserve in 

 admitting all that we are asked to believe concerning the exclusive 

 efficacy of glacier-erosion. "While we have abundant proof that this- 

 agency has operated in Ireland, there are reasons for regarding the 

 operations as limited, if not of comparatively small amount. 



Of the fact that a glacier passed through the Killaloe gorge, and 

 considerably affected the river-bed, there can, I think, be no doubt ; 

 the hollowing of the gorge and the moraines at its opening are 

 sufficient evidence of it. 



The question, however, presents itself : Why did ice, moving from 

 the north, flow through the gorge, and not through the Scarriff and 

 jS'enagh valleys ? jN"o doubt, at an early stage of Irish giaciation, the 

 overwhelming southerly ice-flow sent lobes through these valleys, 

 where we now find drifts ; but the Clare accumulation, which sent an 

 ice-sheet across the Cratloe hills, as before mentioned — the latest of 

 which we have indications of, in the form of striae — would have 

 blocked the way for an ice-flow by the Scarriff valley ; and the ice 

 descending from the Devil's Bit, and Keeper Hill range, and from 

 Slieve Arra, would, similarly, have blocked the way along the l^enagh 

 valley. The ice from the north, therefore, moving along the Shannon 

 basin, and swelled by accessions from the neighbouring groups, forced 

 itself through the gorge with great erosive power, especially where it 

 worked its way around the corner of Slieve Arra towards the south, 

 the deepest part of the present lake. The way in which the deepest 

 l)art of Lake Iseo correspondingly hugs the prominence around 

 which it turns westward has above been noted. 



iN"© geologist can contemplate the prodigious effects attributed by 

 masters of this branch of the subject on the Continent to ice-action, 

 without realising that in this agency exists a doughty rival to the 

 combination of forces productive of sub-aerial denudation. Without 

 insisting upon any special operations of glaciers in moulding, for 

 example, the features already pointed out in Killaloe gorge, it seem^ 



