Popular Science Monthly/Volume 10/January 1877/The Parallel Roads of Glen Roy
|THE PARALLEL ROADS OF GLEN ROY.|
FROM a lecture recently delivered by Prof. Tyndall before the Royal Institution, we gather the following facts in regard to that natural wonder in Scotland, which for so long remained a puzzle to all investigators. There is an unusual interest centred around its history, from the time when the country-people explained it by their crude and half-mythical theories, to the time when it became a labor of love for the untiring efforts and acute observations of scientists.
The earliest published allusion to these roads was made in a work brought before the public a century ago, but no systematic description of them appeared before 1817. They are found in the district of Lochaber, Inverness-shire. On both sides of the steep, narrow glen through which the Roy runs, there are three perfectly horizontal and parallel roads, directly opposite on each side, those on one side corresponding exactly in elevation to those on the other. They are respectively 1,150, 1,070, and 860 feet above the sea, and are formed as shelves in the yielding drift which covers the sides of the mountains. They usually slope somewhat from the hill, and vary in width from one to twenty yards. The two highest stop abruptly at different points near the mouth of Glen Roy, although no barrier now remains to show any reason for it. At some points the grass on the shelves differs from that which is above and below, and, as the roads lie in the midst of heather-covered hills, the absence of the dark shrub from them adds greatly to their conspicuousness.
The terraces were originally supposed to have been made for the heroes whose deeds have been sung by Ossian. A less romantic view was that "they were designed for the chase, and were made after the spots were cleared in lines from wood, in order to tempt the animals into the open paths after they were roused, in order that they might come within the reach of the bowmen who might conceal themselves in the woods above and below." In 1816 Playfair believed them to be aqueducts for artificial irrigation. In 1817 Dr. MacCulloch discussed the probability of there having been lakes embosomed in Glen Roy at one time, and supposed that these roads were the margins of the lakes. It remained, however, for Sir Thomas Dick-Lander to bring forward the facts of the subject, and place them in a scientific light. Adjacent to Glen Roy is Glen Gluoy, along the sides of which there is a single terrace or road, having the same elevation on each side of the valley, and similar in all respects to the roads of Glen Roy. Wishing to see whether these two sides would be united at the head of the glen, and in what manner, he followed them into the mountains. As the valley gradually rose, he observed the shelves approaching each other more nearly; and finally, at the head of Glen Gluoy, he discovered a water-shed of exactly the same elevation as the road which swept around the glen. This height was found to be 1,170 feet, or 20 feet higher than the upper road of Glen Roy. From this watershed he passed through a lateral branch-valley to Glen Roy, descended to the highest road, and followed it up the glen as he had pursued the previous road. In the same manner he came upon a water-shed looking into Glen Spey, and of precisely the same elevation as the road. After this he dropped down to the lowest shelf, and followed it to the mouth of the glen. It did not end here, however, but doubled around the hills, and ran along the sides of the mountains which flank Glen Spean. Continuing eastward, he observed the Spean Valley gradually approaching the road until the two were on a level, when, as in the other cases, he discovered a water-shed.
From these facts, convinced that water alone could have produced the terraces, he saw that if the mouth of Glen Gluoy were stopped by a barrier the waters from the surrounding mountains would be collected in the valley until they had reached the water-shed, when any further rise would be prevented by the branch-valley, which would carry the additional water off to Glen Roy. As long, then, as the barrier remained, there would be a lake in Glen Gluoy, at the exact level of the road, which, by constant action upon the loose drift,
would be sufficient to produce the road. Now, if the mouth of Glen Roy should also be barred at the same time by a sufficiently high barrier, the waters would be collected behind it, the surface of the lake would rise till it reached the water-shed dividing Glen Roy from Glen Spean, when the superabundant water would flow into the latter valley. In this way the highest shelf of Glen Roy would be formed. If its barrier were now to be partly removed, so as to establish a connection between it and the upper part of Glen Spean, while the lower part remained blocked up, upper Glen Spean and Glen Roy would then be occupied by a continuous lake, the level of which would be determined by the water-shed discovered in Glen Spean. The water in Glen Roy would take a level corresponding to its new place of escape, and the lowest parallel road would be formed. The conclusions thus drawn would be strictly logical, if proof could be offered as to the existence of the barriers.
In Glen Spean there is a large quantity of detritus, and Sir Thomas Dick-Lander supposed that this had at one time been heaped up by some unknown convulsion. As he could not account for the middle road of Glen Roy in the same manner, he assumed that at a certain point—the level of this road—the barrier which had been wasting away held its ground for a sufficiently long time to form the road. But, on the same principle, there would naturally have been a greater number of roads in this glen, and additional roads in the other glens. A weakness was thus admitted into the theory which was immediately attacked by Mr. Darwin. He believed that the whole region had once been covered by the sea, and that, in the upheaval of the earth, there were pauses during which these roads were formed. But this would not account for the sea being higher in one of the glens than in another, nor for the unequal number of terraces by which the mountains are belted. As soon as Mr. Darwin detected these fallible points, he abandoned his theory.
In 1847 the Dick-Lander hypothesis received new strength from a discovery made by Mr. Milne-Home. There is a lateral glen, called Glen Glaster, running eastward from Glen Roy, which had escaped the notice of Sir Thomas Dick-Lander. Mr. Milne-Home entered this glen, pursued a branch of it extending to the southeast, and came upon a water-shed exactly level with the second Glen Roy road. On the same theory as before, when the barrier should be properly removed, the water in Glen Roy would sink to the second road, and the surplus water would escape over the Glen Glaster water-shed into Glen Spean. But this mode of explanation could not yet be accepted, for there is scarcely a trace left of the immense quantity of detritus that would have been necessary to form the barriers. Nor could the detritus have been swept away by glaciers, for there have been no glaciers in these valleys since the retreat of the lakes.
At the time when Sir Thomas Dick-Lander was making his investigations, the action of ancient glaciers was not understood. The subject had been pursued in Switzerland, but it was not till 1840 that unmistakable marks of glacier-action were pointed out in Great Britain by Agassiz. He visited Glen Roy, and, having detected the traces of glaciers, pronounced these to have been the barriers blocking up the glens. This theory was afterward examined and confirmed by Mr. Jamieson. "It was their ascription to glacier-action," says Prof. Tyndall, "that first gave the parallel roads of Glen Roy an interest in my eyes; and in 1867, with a view to self-instruction, I made a solitary pilgrimage to the place, and explored pretty thoroughly the roads of the principal glen." At different places he found that the effects of the lapping of the water on the more friable portions of the rock are still perfectly distinct. Several months ago he again visited the place, prior to delivering a lecture upon the subject. The entire ground was thoroughly explored, and the principal hills were found to be intensely glaciated. The collecting-ground of these glaciers, which blocked up the valleys, were the mountains south and west of Glen Spean—among others, Ben Nevis. These lofty mountains encounter the southwestern Atlantic winds, and deprive them of their vapor. During the glacial epoch this vapor was precipitated as snow, which slid down the slopes, while every valley and recess kept up a constant supply of glaciers into Glen Spean, filling it to an ever-increasing height. There would of course be ice in Glen Spean, and water to the north of it, as the winds in passing north would be partly dried and warmed by the liberation of their latent heat. As long as the supply was in excess of the consumption, the dams closing the glens would increase in height. As the weather grew warmer, the opposite would be true. For a long time the conflict would continue, retarding indefinitely the disappearance of the barriers, but the ice in the end would have to give way. "The dam at the mouth of Glen Roy, which probably entered the glen sufficiently far to block up Glen Glaster, would gradually retreat. Glen Glaster and its water-shed being opened, the subsidence of the lake 80 feet, from the level of the highest to that of the second parallel road, would follow as a consequence." "In presence, then, of the fact that the barriers which stopped these glens to a height, it may be, of 1,500 feet above the bottom of Glen Spean, have dissolved, and left not a wreck behind; in presence of the fact insisted on by Prof. Geikie, that barriers of detritus would undoubtedly have been able to maintain themselves had they ever been there; in presence of the fact that great glaciers once most certainly filled these valleys—that the whole region, as proved by Mr. Jamieson, is filled with the traces of their action—the theory which ascribes the parallel roads to lakes dammed by barriers of ice has, in my opinion, an amount of probability on its side which amounts to a practical demonstration of its truth."