Glenroy, a narrow precipitous glen in Kilmonivaig parish, Inverness-shire, traversed by the Roy, a salmon and trout stream that flows 14¼ miles south-westward, till at Keppoch, 3½ furlongs below the Bridge of Roy and 12¾ miles NE of Fort William, it falls into the Spean, descending in this course from 1100 to 290 feet above sea-level. It lies in the Lochaber district; and its great interest arises from the three distinctly-marked terraces, known as the 'Parallel Roads of Glenroy,' which can be traced almost continuously on both sides of the valley. Each forms a gently sloping shelf from 3 to 30 feet wide, and the most striking characteristic of all is their absolutely constant level. The highest (1144 to 1155 feet) can be traced from the col (1151 feet) at the head of Glenroy - which forms the lowest part of the watershed between the Roy and the Spey - to Bohuntine Hill, near the mouth of the glen. The second shelf (1062 to 1077 feet) runs parallel to the first, but can be traced round Glen Glaster, which opens into Glenroy just below where the first road terminates. This second road corresponds in height to the col (1075 feet) at the SE end of Glen Glaster, which is part of the watershed between the Glaster and the Feitheil, a small tributary of the Spean. The third and lowest shelf (850 to 862 feet) can be traced right round Glenroy, Glen Glaster, and Bohuntine Hill, and away eastward along Glen Spean to a little below Loch Laggan. It corresponds in height to the col (848 feet) at Muckall above Loch Laggan, which forms part of the watershed between the Spean and Mashie Water, a tributary of the Spey. In Glen Gloy, to the W of Glenroy, is another similar road at a height of from 1156 to 1173 feet. The col at the head of this glen, which looks over to Glenroy, is 1172 feet above sea-level.
The constant level at which each of these roads remains suggests at once that they have been the shores of former lakes or seas. The marine theory advanced by Darwin, who regarded the glens as former arms of the sea, is not now generally accepted. The hypothesis which ascribes them to fresh-water lakes was first brought forward by Macculloch (Trans. Geol. Soc. Lond., vol. iv., 1st ser.), and taken up by Sir T. Dick-Lauder (Trans. Roy. Soc. Edinb., vol. ix.). They were forced to assume the former existence of gigantic barriers of débris, which were washed away by the water, after it had been kept for long periods at the heights of the various shelves. The difficulties of this theory are, first, where did the débris come from; and, secondly, where did it disappear to, without leaving even the slightest trace of its existence. Such difficulties do not exist in the bold speculation of Agassiz, who finds the necessary barrier in a huge glacier which slid down from Ben Nevis. This theory is supported by the numerous evidences of former glaciers in the district. Suppose a glacier to fill all the lower portion of Glenroy up to where the highest road terminates above Glen Glaster. The water collected in the glen could escape only at the col at the head of Glenroy. Let the glacier now recede till it reaches the points where the second road terminates on Bohuntine Hill. Glen Glaster will now be open, and the waters will be discharged over the lower col at the head of that glen. Let the glacier now recede quite out of Glenroy, and stretch across Glen Spean below the bridge of Roy. The water will then fall to the level of the lowest possible outlet, which is the col at Muckall above Loch Laggan. See R. Chambers' Ancient Sea Margins (1848), and Prof. Tyndall's Lecture in the Popular Science Review (1876), with authorities there cited.—Ord. Sur., sh 63, 1873.
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