"I now turn to the alternative explanation (see _ante_, p. 145), viz., that the whole of the phenomena were produced by a mass of land-ice which was forced in upon Moel Tryfaen from the north or northwest, overpowering any Welsh ice which obstructed its course. This view is in harmony with the observations regarding the 'terminal curvature' of the slates, the occurrence of sharp angular chips of slate in the boulder-clay, and the coincidence of direction of these indications of movement with the carry of foreign stones. The few shells and shell-crumbs in the sands and gravels would, upon this hypothesis, be the infinitesimal relics of huge shell-banks in the Irish Sea which were destroyed by the glacier and in part incorporated in its ground-moraine or involved in the ice itself. The sands and gravels would represent the wash which would take place wherever, by the occurrence of a 'nunatak' or by approach to the edge of the ice, water could have a free escape. "Two principal objections have been urged to the land-ice explanation of the Moel Tryfaen deposits. An able critic asks, 'Can, then, ice walk up-hill?' To this we answer, Given a sufficient 'head' behind it, and ice can certainly achieve that feat, as every _roche moutonnée_ proves. If it be granted that ice on the small scale can move up-hill, there is no logical halting-place between the uplift of ten or twenty feet to surmount a _roche moutonnée_, and an equally gradual elevation to the height of Moel Tryfaen. Furthermore, the inland ice of Greenland is known to extrude its ground-moraine on the 'weather-side' of the nunataks, and the same action would account for the material uplifted on Moel Tryfaen. "The second objection brought forward was couched in somewhat these terms: 'If the Lake District had its ice-sheet, surely Wales had one also. Could not Snowdonia protect the heart of its own domain?' Of course, Wales had its ice-sheet, and the question so pointedly raised by the objector needs an answer; and though it is merely a question of how much force is requisite to overcome a certain resistance (both factors being unknown), still there are features in the case which render it specially interesting and at the same time comparatively easy of explanation. It seems rather like stating a paradox, yet the fact is, that it was the proximity of Snowdon which, in my opinion, enabled the foreign ice to invade Wales at that point. "A glance at the map will show that the 'radiant point' of the Welsh ice was situated on or near Arenig Mawr, and that the great mass of Snowdon stands quite on the periphery of the mountainous regions of North Wales, so that it would oppose its bulk to fend off the native ice-sheet and prevent it from extending seaward in that direction. [Illustration: Fig. 46.--Section across Wales to show the relationship of native to foreign ice.]