On the origin of the parallel roads of Lochaber : and their bearing on other phenomena of the glacial period / by Joseph Prestwich.

  • Joseph Prestwich
Date:
1879
Catalogue details

Licence: Public Domain Mark

Credit: On the origin of the parallel roads of Lochaber : and their bearing on other phenomena of the glacial period / by Joseph Prestwich. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    the Spean, and resting on Moeldhu which is opposite, shut up the lower end of Strath Spean; while a third glacier issued from Glen Treig, and barred the valley of the Spean higher up. The last two glaciers served as barriers to the Glen Roy lakes, all of which he considered had extended into the opposite part of Glen Spean. On the retreat of the glacier of Glen Treig, the lake-waters were lowered, and, spreading into upper Glen Spean, escaped eastward through Glen Spey. In proof of the exist- ence of this glacier, Agassiz pointed out that the striae on the rocks in Glen Spean, opposite Glen Treig, were parallel with the axis of Glen Treig and transverse to the direction of Strath Spean. But his allusion to the phenomena of the parallel roads was incidental to the general question, and the subject was not discussed in detail. Adopting the glacial theory and general views of Agassiz, Mr. Jamieson, in 1863, with an intimate knowledge of the drift phenomena of the north of Scotland, sought, in a further investigation of the district, for the more special conditions applicable to this particular case.* Scotch geologists had in the meantime come to the opinion that the first great glaciation of the country had been succeeded by a warm interglacial period, which was followed by another period of considerable cold, during which only local glaciers descended from the greater mountam ranges. Mr. Jamieson concluded (although apparently with some hesitation) that it was during this latter period t that the lakes of the Lochaber district originated. He also, like Agassiz, attributes their formation mainly to two great glaciers, but differently disposed. One issuing from Glen Arkaig, and crossing the Great Glen,—at that time probably filled with ice from its summit level at Loch Oich to Fort William,—closed Glen Gluoy, and then flowed with the main stream over the shoulder of Strone-y-Vaa to near Tiendrish. Whence he concluded (p. 246), “ that the Glen Arkaig glacier not only blocked up Glen Gluoy, but also largely contributed to close the mouth of Glen Spean”—aided on the opposite side by a glacier from Coire ’n Eoin. The other glacier issuing from Glen Treig, “ and protruding across Glen Spean until it rested on the hills upon the north side of that valley,” blocked up the pass of Glen Glaster, while one main stream, turning westward, extended to the entrance of Glen Roy, where it served to dam the Glen Roy lake during its two higher levels, and another passed up the Spean Valley and through the pass of Makoul into the basin of the Spey. “ This,” he observes, “ would cut off all outlet to the eastward, both by Glen Glaster and Makoul, and, so long as the icy barriers maintained a sufficient * “ On the Parallel Roads of Glen Roy, and their Place in the History of the Glacial Period,” Quart. Journ. Geol. Soc., vol. xix., p. 235, 1863. See also Mr. Jamieson’s “ Ice-worn Rocks of Scotland,” Ibid., vol. xviii., p. 164, 1862; “ History of the last Geological Changes in Scotland,” Ibid., vol. xxi., p. 161, 1865 ; and “ Last Stage of the Glacial Period in Great Britain,” Ibid., vol. xxx., p. 328, 1874. t Sir Charles Lyell, who adopted the hypothesis of Mr. Jamieson, was of opinion not only that the lakes were formed in times long subsequent to the principal glaciation of Scotland, but also that they might have been as late, especially the lowest one, as that portion of the Pleisto9ene period in which Man coexisted with the Mammoth.—‘Antiquity of Man,’ 4th edit, p. 312.
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