Cole — The Problem of the Liffey Valley. 13 



the south coast of the county of Wexford, and they appear to come direct from 

 chalk concealed beneath the sea. We have, then, to reckon with the probability 

 that, even in late Cainozoic times, chalk remained as a cover to many of the 

 slopes now familiar to us. Such a covering would maintain large areas as 

 land during a general subsidence, and would for a long time protect the older 

 rocks from denudation. 



The mature features, then, developed on the uplands may have been once 

 continued across lower grounds or foothills of Cretaceous strata. When these 

 rocks were removed, the trenching of the older surfaces was resumed, and the 

 steeper slopes of these surfaces allowed the streams to cut ravines. Protection 

 being aflbrded at some places longer than at others, the features that are still 

 comparatively immature have no general level along the Leinster foothills. 



After this discussion, which offers two causes for the relative immaturity 

 of the Slade of Saggart, both of wliich have probably been in operation, we 

 may return to connect the Slade with the Liffey basin to the south. 



Suggestion of a Foilmer Flow of the Liffey System Xorthward. 



If the Slade of Saggart was at one time occupied by a large stream, this 

 stream was generated on the uplands to the south. At present, however, these 

 uplands are drained by the Liffey system in an opposite direction. The Slade 

 of Saggart is connected with this system by the through valley of Brittas. 

 As we have seen, a merely slight modification of the floor of the Liffey at 

 Kilbride would suffice to carry its waters through the Brittas valley to the 

 Slade of Saggart. The lower part of the Liffey at the present day is largely 

 fed by the Kings Eiver, coming down from Wicklow Gap. We may ask 

 ourselves if it is possible to bring this river into harmony with the course 

 suggested for the pre-Glacial Liffey, and to picture the joint waters flowing- 

 northward through the mature Brittas valleys, and developing features of 

 gorge and waterfall in the Slade of Saggart, as the rocks there became exposed 

 to river action. 



The Upland of the Liffey in G-lacial Tdies. 



Before we consider the cause of the reversal of drainage which must in 

 that case have taken place, leading to the present Liffey system, it is well to 

 recall the conditions that prevailed in the district during Glacial times. The 

 oncoming of the epoch of refrigeration cheeked the process of river-erosion, 

 since the land became covered by unmelting snow. Dry arctic conditions set 

 in, which abolished the head-waters of the streams. As glaciers developed 



