RAMSAY DRIFT AND GLACIERS OF N. WALES. 



375 



Fig. 1 . — Diagram illustrative of the damming up of lakes by means 

 of moraines, as in the case of Llyn Idwal, and of the relation 

 of the Drift to the subsequent Glacier-conditions. 



•^S 



Silurian Rocks. 



1. Moraine forming the dam of the lake. 



2. Grooved and polished surfaces formed by more ancient glaciers. 



3. Pleistocene " drift" at an elevation of 2300 feet. 



There are other cases, as in Cwm Graianog, Nant Francon (see 

 fig. 2), where, the drainage immediately percolating the loose piles of 

 stones, no pent-up water is found within the barrier. 



Fig. 2. — Cwm Graianog, Nant Francon, having a terminal moraine 

 of loose stones which admits of the drainage of the valley. 



But these moraines are often at a lower level than much of the 

 Pleistocene "drift," as in the case of Llyn Idwal, where the level of the 

 lake is about 1000 feet beneath the level of the ordinary "drift" on the 

 summit of the cliffs, which I have already stated is in a high isolated 

 valley, open at both ends (see p. 372). It is without tributary valleys, 

 and the small rivulet which gradually gathers in its hollow, and flows 

 into the Pass of Llanberis, has rarely succeeded in cutting through 

 the drift of the higher ground to the solid rock beneath. It is im- 



