III. Denudation.—One of the most striking geological features of the district is the amount of denudation to which the rocks have been subjected. Slioch itself is a splendid monument of denudation, standing, as it does, a gigantic cone, in isolated grandeur, the rocks that once reached the same altitude around him having been swept off by gigantic denuding forces, of which we have now little conception. The same denuding processes have been at work, as already remarked, on the Torridon peaks round Loch Torridon and Loch Inver. But Scotland has been subjected to extraordinary denuding forces all over its surface, from John o' Groats to Galloway; such peaks remaining as wonderful monuments both of what once existed and of what has been swept away. Other remarkable examples of denudation are given in this work. Such forces have been active since the birth of time.
IV. Rock Junctions.—In the district, there are several noteworthy junctions of the rocks of the great geological epochs deserving examination.
One has already been mentioned, that on the shore near the Free church at Gairloch, between the Torridon and the Cambrian, strikingly clear and impressive from the perfect unconformability between the two series, and their extraordinary dissimilarity in character. The composition of the breccia may here be easily examined, from its wave-worn bareness, and the fact perceived that it has been formed of pieces of the Hebridean floor immediately beneath, with foreign matters included.
Another equally remarkable junction of the same two systems, hitherto unnoticed, occurs three miles from this one, across the Gairloch, at a beautiful spot called Shieldaig of Gairloch. Just before descending on the mansion, the road enters a narrow pass, having a steep cliff on the right. This precipice consists, in the lower portion, of the Hebridean, and in the upper, of Torridon conglomerate. The line of union, halfway up the cliff, is clear from the road, and on reaching it, you can insert your hand between the two systems and crawl along their junction. The components of the conglomerate are here much more rounded than at Gairloch. This Torridon forms an isolated patch, on both sides of the road, about a quarter of a mile in length and two or three hundred yards in breadth. It is eminently worth a visit, and is easily reached by the pedestrian.
Another striking junction, also undescribed and little known, occurs between the road and the sea, about a mile from Poolewe, not far from Tournaig. There, in a peat bog, an isolated patch of Hebridean rises to the surface, through the Torridon, which surrounds it. It is not more than three or four hundred square yards in area, and is the only gneiss in the broad expanse of Torridon sandstone, which lies on this side of Loch Ewe between Inveran and Greenstone Point. A fine conglomerate of the Torridon firmly adheres to the surface of the rough gneiss, on the outer edges of the bare Hebridean, and fills up its irregularities in a telling way.
Another junction of the same rocks occurs on a small cape formed of gneiss, called Craig an t'Shabhail, which juts into Loch Maree about a hundred yards from Inveran. There a still finer conglomerate is seen, in a thin hard layer, sticking to the surface of the gneiss, evidently the tenacious remnants of a thick bed that has been scraped off by the powerful denuding forces once so active in this region.
Another capital very unconformable junction between the gneiss and the conglomerate is found on Loch Torridon, where the isolated patch of Hebridean that towers into Alligin crosses the loch and forms its Narrows. In the bed of a burn, not far from the school, and in a ridge above it, the two rocks may be easily traced in contact for a considerable distance, and the composition of the brecciated conglomerate easily examined. Similar junctions exist on both sides of this loch at the Narrows, some of them near Shieldaig of Applecross being very good,—all examined by me many years ago.
Between Gairloch and Poolewe, in a hollow to the west, just before the road rises to its summit level, a detached mass of Torridon sandstone, referred to elsewhere, may be easily observed by the traveller. It forms a thick deposit, with a bold precipitous front facing the south and east, the horizontally bedded red sandstone contrasting well with the grey gneiss that surrounds and underlies it. It also bears well-marked traces of the lateral pressure of ice on its sides next the road.
V. The Valley of the Hundred Hills.—No geologist or traveller should miss traversing the picturesque road between Kenlochewe and Loch Torridon, for its wonderful scenery of unsurpassed grandeur and loneliness, and its splendid exhibitions of the Torridon sandstone, crested by the contrasting pale Quartzite, as seen in Beinn Eay, the Grey Heads, and Liathgach. No sea loch in the Highlands is encircled by such mountain masses, mighty, mural, precipitous, and profoundly impressive.
About halfway to Torridon, on the left hand, the eye is arrested by an extraordinary, if not unique, assemblage of hillocks, closely set along the bottom of a glen which opens on the road. These are generally round and peaked, and consist of loose stony débris. They caught the eyes of the observing Celts of old, who named the place the Coire Cheud Cnoc, the "Corrie of a Hundred Hillocks." The explanation of their number and character seems not far to seek. It will be observed that, opposite this valley, on the right, lies the steep narrow glen that separates Liathgach from Beinn Eay. Out of this has issued an immense glacier, as proved by the abundant scratches that point into it, which pushed its ice right across the strath we are in, against the hills on the other side and up into the valley with the hillocks. As is well known, the surface of a glacier is traversed by numerous runnels, which gush over its icy front, bearing with them the débris that constantly falls on the glacier from its enclosing walls. These streamlets thus deposit a series of conical hummocks of this débris, which gradually cover the ground as the ice retreats, similar to those in the corrie in question. Examples of such glacial hillocks may be found, by the uninitiated, in the sketches of Norwegian glaciers in Campbell's "Frost and Fire." On the Liathgach glacier, the amount of detritus would be unusually large, from the steepness of the hillsides and the constant waste of the sandstone, and still more, from the superabundant débris of the rapidly disintegrating Quartzite in the precipitous Beinn Eay.