Cole — The Problem of the Liffe/i Vallcn. 17 



Reference has already been made to the ice-tongue that pushed through 

 the Brittas valleys from the north, while the ice of the Irish plain overtopped 

 the Slievefchoul ridge, and pressed down against that of the main chain. The 

 residtant movement led to a considerable flow southward across Britonstown, 

 even when natural watersheds and snowsheds were obliterated during the 

 maximum of the Ice Age. If, as has been suggested, the floor of the Lifi'ey 

 and Kings Eiver valley then sloped northward, the Britonstown barrier formed 

 a col at the head of a tributary stream, and the ice-flow was forced to ride 

 up over it. When the local ice was free to assert itself during the second 

 glacialion, its denuding action was most powerful as it emerged on the main 

 north-and-south \'alley across the junction of the granite and the schist, 

 where the geological structure and the resulting form of the pre-Grlacial 

 valley-side promoted a steep downward plunge. During both these glacial 

 stages, it is probable that considerable overdeepening of the main valley 

 about Blessington took place. Farther north, near Brittas, where no large 

 valleys opened on the main one, the only serious agent was the ice-tongue from 

 the north. The main scouring therefore occurred near Blessington, and the 

 quantity of limestone dx'ift remaining in the basin shows that the hollow was 

 eroded almost entirely during the earlier and more important glacial stage. 

 The products of local erosion were carried out over the Britonstown col, no 

 doubt abrading and lowering it, but not reducing it to the level of the region 

 to the north, where sub-glacial plucking was most intense. When the ice- 

 movement ceased, a hollow had been formed, by local overdeepening, some 

 250 ft. below the pre-Grlacial valley-floor. Into this, the gravel of limestone 

 and other stones, imported from the north and north-west, sank as the ice- 

 mass stagnated in the valley. This limestone drift, though it was doubtless 

 to some extent incorporated in the later and more local glaciers, fills much 

 of the basin of Blessington at the present day (Plate III, flg. 10). When the 

 ice-shrinkage allowed of the reappearance of ordinary streams, these began 

 to flow on the plateau of glacial drift. If the PoUaphuca barrier had not 

 been by this time sufficiently lowered, the accumulation of drift against it 

 might well have maintained the flow of the Kings Eiver and the Liffey north- 

 ward. As it was, the general slope of the infilling of drift was southward, and 

 the drainage of the district began to notch out the twin ravines of Britons- 

 town. In proportion as these ravines were deepened, the basin above became 

 cleared of some of its burden of glacial drift. The meanders above Horsepass 

 bridge have not yet found the old ice-deepened floor. It evidently lies here 

 somewhere near 550 ft. (168 m.) above the sea, and the surface of the drift- 

 plateau rises 100 ft. above it. 



