38 Ice Marks in North Wales. [Jan., 



capable of doing this but ice. Lower down this same valley, on the 

 spur which separates it from Llyn dwythwch, are portions of mo- 

 raines deposited by the glacier when, during its greatest extension, 

 it descended to join that of the vale of Llanberis. The moraines 

 around Cader Idris are not so numerous nor so well marked as 

 those of Snowdon. There is, however, a very fine one circling round 

 Llyn y gader, under the highest peak ; and lower down, below 

 Llyn y Gafer, is- a rock-strewn slope, even more thickly covered 

 than that of Own Brwynog. Llyn Cai, situated in the tremendous 

 chasm on the south of Cader Idris, has also a small but very per- 

 fect moraine at its extremity, through an opening in which its 

 waters escape. I believe that the existence of moraines and moraine 

 matter, when as well marked as those of North Wales, is of itself 

 sufficient to prove that there has been a glacial period. There is, 

 however, much other confirmatory evidence. 



3rd. Eoches Moutonnees. — Glaciers are often many hundred 

 or even several thousand feet thick, and as they move slowly over 

 the surface their enormous weight, assisted by the gravel, pebbles, 

 and boulders frozen or imbedded in them, grinds down all sharp 

 angles, peaks, and jagged edges of the rocks, giving to them a more 

 or less blunted or rounded outline. The degree to which this 

 grinding away takes place must depend on many causes, such as 

 the weight of the glacier, its rate of motion, the material it carries 

 with it, the time it continues in action, and the hardness, toughness, 

 and original form of the rock itself. This peculiar effect of the 

 passage of a glacier is very easily recognized when once seen, espe- 

 cially if one studies the forms that the rock assumes by natural 

 weathering or by the action of water, both of which will be seen to 

 be very different from that produced by ice. In the valleys around 

 Snowdon and Cader Idris this form of rock-surface is continually to 

 be seen, and when, as is frequently the case, the comparatively soft, 

 slaty rocks can be compared side by side with hard greenstone or 

 grit, the first is found to be ground down to a smooth surface, 

 gently curved or rounded, while the second is left in irregular bosses 

 and lumps, all their asperities smoothed and rounded off, but not 

 ground down to an even surface. This can be well seen on the 

 banks of Llyn Padarn, near Llanberis. It is upon these roches 

 moutonnees that are often found the peculiar markings we have 

 next to consider. 



4th. Grooved and Striated Bocks. — During the process just 

 described, it frequently happens that grooves or scratches are made 

 upon the rocks by the hard materials imbedded in the bottom or 

 sides of #ie glacier. Owing to the enormous weight and slow motion 

 of glaciers, they move with great steadiness, and thus the markings 



