18 Pfocerd/nfff: of the lioi/al Irish Academi/. 



Conclusion. 



The main points of the foregoing considerations are as follows : — Both the 

 Kings Eiver and the Liftey, running at first north-west and north, are drawn 

 o£P to the south-west along angular bends. This suggests an interception of 

 their original courses by a stream working back from the south-west. Before 

 we accept a theory involving a reversal of the drainage-system of this part of 

 the foothills of the Leinster chain, we should be prepared to indicate a 

 previous outlet for the rivers of this system. The large mature valleys near 

 Brittas, now occupied by insignificant streamlets, may at one time have held 

 the Lifiey, which reached the plain by a rapid fall in tlie Slade of Saggart. 

 This steep valley is considered in relation to others on the margins of the 

 Leinster chain, and the conclusions arrived at are, it may be remarked, 

 independent of the question of its former occupation by the Liffey. When we 

 examine the possible cause of the reversal of drainage, it is found in a 

 considerable scouring and overdeepening of the valley-floor near Blessington 

 by glacial erosion. Even when the basin so formed had become choked by 

 glacial drift, its surface was lower at Britonstown than at the junction of the 

 Kings Eiver and the Liftey, which now began to flow again after the Ice Age. 

 The point of jvmction, moreover, was lower than that of the entry of the 

 post- Glacial Liffey on the main valley near Kilbride. Hence the new 

 flow was directed southward, over the barrier that remained under the 

 drift at Britonstown. The PoUaphuca gorge results from the post-G-lacial 

 trenching of this barrier, and the river is now removing the infilling of 

 drift in the glacially eroded hollow around Blessington. The floor of this, 

 hollow represents a local overdeepening of about 250 feet (77 m.) in the 

 easily eroded SUurian slates, and this, by its excess over the small over- 

 deepening near Kilbride, suifices to draw the Kings River and the Liffey 

 southward. The Liffey at Kilbride has consequently been able to lower 

 its bed as the PoUaphuca outlet deepened, and the small local drainage from 

 the gap at Brittas runs down into it along large valleys once excavated by 

 the river in its northward course. 



There is much in the foregoing pages that must be regarded as suggestive,, 

 rather than capable of proof. I have attempted, however, to state the. 

 problems that are raised by the phenomena in the field. It is hoped that "the: 

 general considerations involved may aid in the more extended study of the: 

 surface-features of south-eastern Ireland. 



