The Pamphlet Collection of Sir Robert Stout: Volume 22
One of the most interesting problems connected with glacial geology is the explanation of the glaciation of those groups of islands which lie at some distance from the north-east corner of the mainland of Scotland. It is now almost universally admitted, by those who have carefully weighed the evidence, that during the maximum cold of the glacial period, Scotland, Ireland, and the greater part of England wore buried underneath an ice-sheet, which moved off the high grounds towards the sea-level. This has been clearly proved by the careful mapping of the ice-markings indicating the trend of the old glaciers, as well as by a minute examination of the stones in the Boulder-clay which accumulated underneath the ice, and was rolled along with the onward motion of the mass. So far most geologists are agreed; but when the glaciation of the Orkney and Shetland Isles has been discussed, it has given rise to considerable difference of opinion. Doubtless this want of uniformity has been largely due to the imperfect evidence hitherto obtained from the isles regarding the direction of glaciation and the nature of the various superficial accumulations. There has been no systematic examination of Shetland, or even of Orkney, with a view to determine these questions; and hence the absence of reliable observations has given scope for some latitude of opinion, and has likewise retarded the final settlement of the question.
The group of islands to which this paper especially refers may be said to form a broken rampart running nearly north and south for a distance of about 70 miles. The isles are about 200 miles distant page 779 from the Norwegian coast-line at Bergen, and about 86 miles from the north-east corner of Scotland. Though they are thus completely isolated from both countries, it will be shown that their physical history is to some extent associated with that of Scotland and Norway.
The earliest references to the dispersion of boulders in these isles were made by Dr. Hibbert, who inferred that "the great diluvial wave which swept over the low elevations of the whole of Scotland and England had in the latitude of Shetland a north-easterly origin, or, in other words, that it had a south-westerly direction"*.
Moro recently certain observations on the glacial phenomena of Shetland were made by Mr. O. W. Peach, who visited Lerwick, the outskerries of "Whalsey, and the island of Unst; and at each of these localities he noted the ice-worn aspect of the rocks, the striae, and the existence of Boulder-clay†.
To our colleague, Dr. Croll, belongs the merit of having first suggested the probability of the North Sea being filled with ice, enveloping alike the Orkney and Shetland groups of islands. This suggestion was first thrown out in a paper on "Glacial Submergence," which appeared in the 'Header' of the 14th Oct. 1865. In a subsequent paper "On the Origin of the Caithness Boulder-clay"‡, he pointed out that the Scandinavian and Scotch ice-sheets probably united on the floor of the North Sea, and thence moved northwards and northwestwards towards the Atlantic. He showed that in all probability the enormous mer de glace which pressed out on all sides from Scandinavia, produced, in virtue of its greater size, a slight deflection of the Scotch ice, and caused it to override portions of the mainland. He indicated that in all likelihood both the Orkney and Shetland Isles were overtopped by the combined ice-sheets in their onward march towards the Atlantic.
In the autumn of 1876, one of us visited Shetland with the view of determining the question whether the glaciation of that group of islands had any connexion with that of Scotland and Norway. From the traverses then made, it was evident that these isles had been glaciated by Scandinavian ice, though in certain areas it seemed as if a more recent local glaciation had well nigh effaced all traces of the original movement§.
The rich variety of rocks in Shetland renders it a comparatively easy matter to determine the direction of the ice-movement; but in order to insure accuracy it seemed desirable to map out approximately the areas of the respective rock-formations. During our leave of absence from official work in the summer of 1878, we therefore returned to the isles for the purpose of accomplishing this end with as much minuteness as time would permit. We were induced page 780 to work out the succession of the representatives of the Old Red Sandstone as developed on the Mainland, as well as the relations of the associated contemporaneous and intrusive igneous rocks, on account of the important evidence which they furnish regarding the ice-movement. While pursuing this object, we were fortunate enough to discover in the Walls district a rich series of plant-remains in rocks which have been hitherto considered as forming part of the metamorphic series. The general character and physical relations of these altered rocks will be briefly described in a subsequent page (p. 785).
* Edinb. Journ. of Science, vol. iv. pp. 85-91.
† Brit. Assoc. Report, 1864, p. 59. It should be remembered that Mr. C. W. Peach gives the magnetic readings in his paper; and hence, in order to obtain the true direction of the ice-markings, due allowance must be made for the magnetic deviation.
‡ Geol. Mag. vol. xvii. pp. 209 and 271. The fullest exposition of Dr. Croll's views is given in 'Climate and Time,' chap, xxvii.
§ Nature, vol. xv. p. 139.