Downloaded from http://trngl.lyellcollection.org/ at University of California-San Diego on June 2, 2015 HVINGSTON—ICE-ACTION IN LOCHABER. 39 No. III.—BOULDERS AND OTHER EVIDENCES OF ICE-ACTION iff LOCHABER. By COLIN LIVINGSTON, Corresponding Member, Fort William. With a Map [Plate II.]. [Read 12th April, 1900.] THE chief evidence of ice-action in Lochaber, as elsewhere through­ out Scotland, is, perhaps, the configuration of its glens and valleys, caused in large measure by the denuding action of the glaciers of which they were at one time the beds. The district is intersected by two main valleys, the best known, Glen More, which extends from the Argyllshire coast in the west, with the Island of Mull on one side and the district of Lorne on the other, to the Moray Firth on the east, lies along an anticlinal axis, with its two extremities occupied by the sea. The middle is partly occupied by a chain of lakes, Loch Ness on the eastern slope and Loch Lochy on the western. Loch Oich is really on the watershed, though it is naturally connected with the eastern slope, and artificially by the Caledonian Canal, with the basin of the Lochy in the west. The summit-level of the canal is only 100 feet above that of the sea, and the watershed at the bottom of the valley is some 20 or 30 feet higher. Loch Ness and Loch Lochy are both very deep—about 90 fathoms, and they are evidently due, in part at least, to the action of the ice, which first largely scooped out their beds, and then excluded from the depressions the detrital matter which now blocks the lakes at their outlet, and gradually, as the ice melted, gave place to water. The general direction of this valley is from S.W. to N.E. Between Fort William and Banavie, at a point occupied by what is locally known as the " Moin Mhor " or the " big moss," this valley is intersected by another one not quite so regular in its outline, though well enough defined, which lies almost due E. and W., but at the eastern end towards Badenoch trends considerably to the north. This eastern part contains Loch Laggan, formed evidently in the same way as Loch Ness and Loch Lochy, and the western holds Loch Eil, an arm of the sea, and the freshwater Loch Eilt. On the eastern side the bottom elevation rises to about 1000 feet, after which it drops towards the valley of the Spey. On the western side no part of the bottom of the valley Downloaded from http://trngl.lyellcollection.org/ at University of California-San Diego on June 2, 2015 40 TRANSACTIONS—GEOLOGICAL SOCIETY OP GLASGOW. rises above 400 feet, and the greater part does not exceed 100 feet. The sides of these valleys are formed of lofty mountains, many rising from 2000 to 3000 feet above sea-level. In Ben Nevis, which occupies the south-eastern angle formed by the inter­ section of the two main valleys, they reach their highest altitude —4406 feet—and several summits exceed 4000 feet. This configuration of the land determined the direction of glacial movement, so far as it is now traceable, but above an elevation of 2000 feet there is little remaining evidence of it. That at one time the ice-cap surmounted our highest mountains is more than probable, but evidence in support of this theory must be sought for elsewhere than on the mountain-sides, and requires a wide field. Here and there throughout the Lochaber district are to be found blocks of porphyritic agglomerate which may have been conveyed from the summit of Ben Nevis. If this was the actual site from which they were brought, the ice-sheet must, of course, have been of very considerable thickness above the present mountain-summits before it could bear such blocks along with it; and other indications are not wanting that the upper part of the ice-cap moved in great measure independently of the configuration of the valleys. The most striking evidences of the former existence of glaciers in the district are the well-known "Parallel Roads" of Lochaber, seen in Glen Gloy, Glen Roy, and Glen Spean. It is not intended to go here at any length into the consideration of questions con­ nected with the " Roads"; but it may be mentioned briefly that for a distance of over twenty miles they are distinctly marked in the various glens. They are parallel in the sense that their lines as they pass along the hillsides continue practically at the same relative elevation to each other, and this fact clearly indicates that they owe their existence to water, and are lake- margins. In his latest paper on the " Parallel Roads," Prof. Jamieson accounted for their formation in a manner that appears entirely to satisfy the requirements of the case. Briefly it amounts to this :—Owing to the excess of precipitation on the west coast as compared with that on the east, the sheet of ice which formed on the former was much thicker than that on the latter, beyond the Downloaded from http://trngl.lyellcollection.org/ at University of California-San Diego on June 2, 2015 LIVINGSTON—ICB-ACTION IN LOCHABER. 41 present watershed of Scotland. As a consequence, when the seasons meliorated, the surface of the ground was first laid bare in the east, and the denudation proceeded gradually westward. In the glens in question, which are west of the existing water­ shed, lakes formed behind the retreating ice-barrier, and the Parallel Roads are the margins of these lakes. The levels of the successive lines are found to correspond with the elevation of cols by which the water would be discharged into one part or other of the valley of the Spey as the retaining barrier gradually receded westward. The evidence to be met with, below an elevation of 2000 feet, that at one time the Lochaber valleys were occupied each by its own glacier, is abundant. It consists, in addition to the usual detrital covering of the hillsides, of—(1) Detrital Lines in the form of Lateral and Terminal Moraines, (2) Rock Striation, and (3) Boulders. (1) DETRITAL LINES. The lines of detritus are specially noticeable in the valley of the Spean at its junction with the Lochy. This is as might be expected from the fact that the Spean valley is the widest of our glens and has its sides formed of the highest mountains. From Spean Bridge to Fort William there is the small lateral valley of Auchindaull, connected with the united Spean and Lochy, where the lines are particularly evident. Within three miles of Spean Bridge four of these lines cross the public road from Fort William. In places they run into each other, but the general direction is for the most part preserved, curving in towards the Spean valley, which was the main bed of the glacier. One may be traced from near the base of Aonach More, a part of the Ben Nevis range, across a somewhat elevated moor into the Auchindaull valley, where, just above the gate to the farmhouse, it forms a ridge with the top horizontal, and at one time about 100 yards long, and known as "Tom na Brataich"—"the banner mound." It rose from 50 to 60 feet above the general level, but much of it now has been carried away for railway ballast, for which, from its gravelly character, it is specially adapted. The ridge is cut across by two trenches, evidently made for defensive purposes when the banner waved from its summit, Downloaded from http://trngl.lyellcollection.org/ at University of California-San Diego on June 2, 2015 42 TRANSACTIONS—GEOLOGICAL SOCIETY OP GLASGOW. as the name implies—possibly before the great battle of Inverlbchy in 1341, in which Donald Balloch defeated the Royal forces under the command of the Earls of Caithness and Marr. A short distance behind it on the moor is another detrital mound known as "Tor Sonnachan"—"the pallisaded mound." The top is enclosed by a turf wall, probably used for defence in connection with the same battle, as defensive positions of this kind do not appear to have been used in the ordinary clan fights. The line of which " the banner mound" forms part proceeds down towards the bottom of the Auchindaull valley, ascends the opposite side, and descends again towards the River Spean. In its descent it occupies the bottom of a small lateral valley for some distance, and here greatly resembles a great turf dyke about 30 feet high with a base of as many yards. On the opposite, or north side, of the Spean it is not to be seen near the river, but at an elevation of something like 100 feet above the river-bed it again assumes its dyke-like form, and may be followed almost to the extremity of the 850 feet " Parallel Road " on the shoulder of the hill above Spean Bridge—Meall na Luath. The public road, in a distance of not more than two miles from "the banner mound," is crossed by at least three other lines, well-marked and traceable for considerable distances across the moor on either side. As might be expected, their continuity is a good deal broken, but they still retain their identity. A mound of considerable size belonging to one of them has been cut into for the supply of road-material, for which it is well suited, as it consists of small, sub-angular chips of stone.