152 Henry Dewey — Land-forms in Caernarvonshire. 



or to the north-east (PL VII, Fig. 2). First is Cwm Bochlwyd, 

 lying between spurs thrown out from Glyder Fach and Glyder 

 Fawr, and in which lie the sombre waters of Llyn Bochlwyd ; this 

 forms a characteristic hanging valley with a mountain torrent ripping 

 its course down to Llyn Ogwen. Next comes Llyn Idwal in Cwm 

 Idwal, flanked by the grand precipices of Glyder Fawr and Y Garn. 

 Then from south to north follow Cwm-clyd, Cwm Cywion, Cwm- 

 goch, Cwm-Bual, Cwm Perfedd, Cwm-graianog, and Cwm Ceunant. 

 In most of these cwms there are relics of their own small glaciers, 

 especially well seen in Cwm-graianog, and it is significant that the 

 change of slope marking the truncation of the spurs is practically 

 always at a height above sea-level of 1,250 feet; and further, this 

 altitude marks the limit of glacial striae incised by the great glacier. 

 Of Cwm-graianog Ramsay remarks: " But in none of the tributary 

 valleys north of Llyn Idwal are the signs of a small glacier so 

 distinct as in Cwm-graianog below the steep slopes of Moel Perfedd, 

 It is a small craggy valley over half a mile in length looking across 

 jSTant Ffrancon. On the east the felspathic porphyry of Moel 

 Perfedd rises in a rough peak, and on the west the great bare ripple- 

 marked strata of the Lingula grits dip towards the hollow at an 

 angle of 48° or 50°. 



" At the mouth of the valley above the steeper descent to ITant 

 Ffrancon, a small but beautifully symmetrical terminal moraine 

 crosses the valley in a crescent-shaped curve, that once passed from 

 200 to 300 yards up the eastern side of the glacier. On this side 

 almost every stone of the moraine is a fragment of the felspathic 

 rock of Moei Perfedd, having been shed from the edge of the glacier 

 by a part of the ice that had that mountain as its source. Further 

 "west along the moraine, the material becomes mixed with fragments 

 of grit and slaty sandstone, and, by degrees, passing to the western 

 side of the valley, the moraine matter consists entirely of pieces of 

 the Lingula beds that form the crags of Carnedd-y-filiast. ... In 

 Cwm-graianog the whole is formed of large angular loose stones 

 mixed with smaller debris. The largest of these lies on the top of 

 the moraine, from 450 to 500 feet above Nant Ffrancon. It was 

 originally 11 yards long, 9 broad, and about \^ high, and when 

 entire must have weighed nearly 300 tons. . . . Inside the moraine 

 the bottom of the valley is covered with glacial rubbish and heaps of 

 loose blocks." ' 



In marked contrast with these cwms is the even unbroken slope 

 that bounds the eastern side of Nant Ffrancon and forms the ridge 

 known as Pen-yr-Oleu-wen. But striae can be seen on the rocks 

 below Braich-du at a similar height to those on the opposite side of 

 the valley. These facts afford evidence of the maximum thickness 

 of the glacier that filled Nant Ffrancon. The present level of the 

 alluvial tract is 700 feet above sea-level, but the valley is partly 

 filled up with boulder-clay and peat, possibly together 40 feet thick. 

 The ice was therefore certainly not less than 700 feet thick. It 

 enveloped all the land lying at altitudes lower than 1,250 feet, for no 



1 Op. cit., pp. 83, 84. 



