. The Quarterly journal of the Geological Society of London. 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 (seefig. 2), where, the drainage immediately percolating the loose piles ofstones, no pent-up water is found within the barrier. Fig. 2.—Cwm Graianog, Nant Francon, having a terminal moraineof loose stones which admits of the drainage of the But these moraines are often at a lower level than much of
. The Quarterly journal of the Geological Society of London. 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 (seefig. 2), where, the drainage immediately percolating the loose piles ofstones, no pent-up water is found within the barrier. Fig. 2.—Cwm Graianog, Nant Francon, having a terminal moraineof loose stones which admits of the drainage of the But these moraines are often at a lower level than much of thePleistocene drift, as in the case of Llyn Idwal, where the level of thelake is about 1000 feet beneath the level of the ordinary drift on thesummit of the cliffs, which I have already stated is in a high isolatedvalley, open at both ends (see p. 372). It is without tributary valleys,and the small rivulet which gradually gathers in its hollow, and flowsinto the Pass of Llanberis, has rarely succeeded in cutting throughthe drift of the higher ground to the solid rock beneath. It is im- 376 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. portant here to recall the circumstance that the drift of this valleyattains an elevation about equal to that of similar deposits, which maybe traced without a break from the seaward flanks of CarneddLlewelyn to the shores of the Menai. If, therefore, the loose moraine-heaps of Cwm Idwal had beenformed during the great glacier period already mentioned, it wouldeither in all probability have been destro
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Keywords: ., bookcentury1800, bookdecade1840, booksubjectgeology, bookyear1845
