The Glaciation of the Shetland Islands.
The Glaciation of the Shetland Isles.
In his valedictory address as President of the Edinburgh Geological Society, at the close of the session 1879-80,1 Mr. Milne Home reviewed our recent paper on "The Glaciation of the Shetland Isles."2 In his address he not only called in question our conclusions regarding the direction of the glaciation of these islands, but likewise referred to the discordance between the observations of Mr. C. W. Peach and ourselves. As much of this adverse criticism is based on a misconception of the real nature of the evidence bearing on the question, we are anxious to reply to some of the points in the address which might mislead those who are unacquainted with the subject.
In our paper we endeavoured to show that there were at least two periods of glaciation in these islands; the one being coincident with the climax of the Ice age, during which the islands were buried underneath the Scandinavian mer de glace, while the other was characterized by local glaciers which radiated from the high grounds in the ordinary way. We stated as the result of a careful examination of the striated surfaces, and specially of the dispersal of the stones in the Boulder-clay, that, during the primary glaciation, the Scandinavian ice-sheet abutted on the eastern seaboard of Shetland with a W.S.W. and S.W. trend, and after reaching the crest of the Mainland, it swung round to the N.W. and N.N.W.
With reference to this statement, Mr. Milne Home makes the following remark: "Even on the east coast of the Shetlands, -where the striations should show a N.E. direction, there is no uniformity in that direction. Near the south end of the group, viz. at Bressay and Lerwick, as the arrows on the map show, the direction of the striæ is not from N.E. to S.W., but from N.W. to S.E."
1 1 Trans. Edin. Geol. Soc. vol. iii. part 3, p 357.
2 2 Quart. Jouni. Geol. Soc. vol. ixxv. p. 778.
No reference is made in the foregoing sentence to the fact that the south-easterly striæ at Lerwick and at certain localities in Bressay belong to the period of local glaciation. Neither is any allusion made to the existence of cross-hatches along the shore at Lerwick; the older markings running S.W., and the newer ones S. 40° E. Moreover, we distinctly pointed out in our paper, that the southwesterly movement of the ice-sheet during the primary glaciation in the Lerwick and Quarff districts is placed beyond doubt, by the occurrence of striated blocks of Old Red Sandstone grits and flags in the Boulder-clay on the west side of the island near Quarff. On the other hand, we indicated that the presence of striated fragments of schists and slates from the Cliff Hills, in morainic deposits in the neighbourhood of Lerwick, points to a local radiation of the ice which was only powerful enough to invade the north-western part of the island of Bressay.
Regarding the direction of the ice-flow in Unst, Mr. Milne Home says: "Also at the north end of the group of islands, viz. in Unst, though the authors of the paper represent by contour lines, and also by the text, the direction of the movement to have been from N.E. to S.W., considerable doubt must bo felt on that point because of the contrary testimony of Sir. C. W. Peach, as given in the British Association Reports for 1864. Mr. Peach states 'that ruts and striae fell under his notice in North Unst, on the cliff at Hagdale in Haroldsnick Bay; the direction being nearly W.N.W. and E.S.E.' Mr. Peach says 'that in ascending the Muckle Heog Hill, which reaches a height of at least 500 feet, he found the W.N.W. end vertical and polished, to the depth of 150 feet.' "
The discrepancy between the observations of Mr. C. W. Peach and ourselves may be best explained by quoting from a letter dated November, 1880, which he has kindly forwarded to us, and which he has permitted us to use in our reply to this address. "I send you a copy of my paper on Shetland, read before the Royal Physical Society, Edinburgh, in which I stated that the stria; on the Muckle Heog, Unst, ran nearly W.N.W. and E.S.E. In the closing sentence of that paper I also stated that all the bearings are by compass, no allowance having been made for variation. This should be taken into consideration and the deviation allowed for as far as Shetland is concerned. Since I wrote that paper, having seen much more of the glaciation of Scotland and thought more about it, I have seen cause to alter my opinion as to the direction of the drift over Shetland, viz. the opposite of what I inferred in my paper to the Royal Physical Society. At the time I wrote (1864), I was much puzzled, when examining the Boulder-clay near Hammer in Balta Sound, to find mingled with the striated stones of serpentine, numerous striated stones of gabbro from Balta Island; and then at the haunted burn of Watlea, where the black shales are exposed and in which lies the Boulder-clay containing smoothed and striated stones of serpentine in abundance, when beyond the Skaw to Saxaford Hill I met with no trace of serpentine or gabbro stones, although I searched rather carefully. I now feel quite satisfied, page 3 that although I noticed the bearing of the striæ right, I was wrong as to the direction the drift came from. At that time I was full of dredging matters, and my mind ran so much after Hydrozoa, Polyzoa, Crustacea, Mollusca, etc., that I had little time for examining the glaciation of the islands, and hence the oversight and neglect of the warnings of Hammer and Watlea, for which I am sorry."
The candid admissions contained in this letter enable us to account for the discordance between the recorded observations of Mr. C. W. Peach, in 1864, and ourselves. We visited the locality at Hagdale, referred to by Mr. Peach, sen., and confirmed the accuracy of his observations so far as the magnetic readings are concerned. When due allowance is made for the magnetic deviation, the true direction of the ice-flow at Hagdale is nearly E. and W., as noted by us. Along the eastern seaboard of Unst, however, the striæ vary from W. to W. 30? S.; the westerly trend being more prevalent in the northern part of the island. From the foregoing letter it is also evident that Mr. C. W. Peach had observed certain facts connected with the dispersal of the stones in the Boulder-clay which unquestionably point to the westerly movement of the ice. He noted the occurrence of gabbro stones from Balta Island in the Boulder-clay at Hammer, and striated serpentine fragments in the Boulder-clay at Loch Watlea to the west of the serpentine area. These facts are not referred to in any of the papers which ho wrote on the subject at that time, doubtless for the simple reason that they are inexplicable on the hypothesis which he then adopted of an ice-movement from the W.N.W. and N.W. Had he found time, in the midst of his dredging operations, to traverse the western shore of Unst, between Woodwick and Belmont, he would have met with still more convincing proofs of this westerly movement in the presence of serpentine and gabbro stones in the Boulder-clay, which must have been carried across the water-shed. Indeed, so abundant are these striated fragments in this deposit on the west coast, that it is impossible to escape the conclusion, that the ice must have crossed Unst from the North Sea to the Atlantic.
In 1868, Mr. C. W. Peach informed Dr. Croll1 that a minute examination of the shelly Boulder-clay of Caithness, continued for several years, had led him to the conclusion that the ice must have crossed the low grounds of that country from the S.E. towards the N.W. His faith in the north-westerly movement in Unst seems then to have wavered, but no subsequent opportunity was afforded him of re-visiting Shetland to examine the evidence anew.
1 Geol. Mag. 1870, p. 212.
Mr. Milne Home further says: "With regard to the west coast of the islands, where the markings are N.W. and S.E., the authors state that these indicate a movement from the S.E. But the nature of the evidence to show that the movement was from the S.E. and not from the N.W. is not given."
We are at a loss to understand how any one who has attentively read our description of the Boulder-clay sections, could possibly conclude that the nature of the evidence for the north-west movement on the west side of the Mainland is not given. We described with considerable minuteness a series of Boulder-clay sections1 extending across Northmavine from Ollaberry by Hillswick, Braewick. Tanwick, to the Grind of the Navir and similar sections along the banks of Roeness Voe. On referring to the map accompanying our paper, it will be seen that the lithological varieties of the rock-masses along these lines of section are so distinct as to render it an easy matter to determine the direction of the ice-movement, from the dispersal of the stones in the Boulder-clay. We distinctly indicated that the ice-carry between the diorite area east of Hillswick and the cliffs north of the Grind of the Navir was towards the north-west. We pointed out that the quartz-felsite and granitic area was invaded by the diorite stones, and the area occupied by the contemporaneous porphyrites and tuffs was invaded by the diorite and quartz-felsite stones; the relative ingredients diminishing in number in proportion to the distance from their parent source. Furthermore, in the peninsular tract which lies to the west of Weesdale, we stated that corroborative evidence is obtained of this north-westerly movement on the west side of the Mainland. To the east of the north and south bounding fault which crosses the peninsula from Aiths Voe to Bixetter and Selie Voes, no trace of the altered Old Bed Sandstone rocks are to be found, either in the Boulder-clay or on the surface, while numerous blocks of the epidotic syenite and the associated gneisses and schists are met with to the west of the fault. And so also in the island of Papa Stour numerous striated blocks of the altered Old Bed rocks from Sandness Hill are commingled in the moraine profonde with fragments of the local porphyry and contemporaneous diabase porphyrites, while in the neighbourhood of Melby the Boulder-clay sections may be searched in vain for blocks derived from Papa Stour. It requires only a moment's reflection to see that the phenomena would have been precisely the reverse of what we have just described, had the ice-movement been from the north-west, as Mr. Milne Home imagines. Indeed, as we stated in our paper, "the evidence obtained from the Boulder-clay along these lines of section completely refutes the theory that these north-westerly striæ could have been produced by ice coming from the North Atlantic."
1 Quart. Journ. Geol. Soc. vol. xxxv. p. 796, et seq.
Mr. Milne Home concludes his review by stating that "the authors of this paper, besides maintaining that the Shetlands were glaciated by a mer de glace from Scandinavia, have gone so far as to suggest that the whole of Scotland underwent a glacial invasion from the same quarter; and they give reasons for this opinion which are not very intelligible."
The only ground for this statement is the following sentence in the conclusion of our paper: "The land-ice which glaciated Scotland could only have come from Scandinavia, as the striated surfaces clearly point in that direction." Owing to an unfortunate printer's error, for which we are sorry, the word Scotland in the foregoing sentence has been substituted for Shetland; an error which is self-evident to any ordinary reader after a careful perusal of the context. We do not believe that any part of Scotland was ever over-ridden by the Scandinavian mer de glace; indeed, there is not the slightest evidence in support of such an hypothesis. So far from this being the case, we have advanced sufficient evidence to prove that the Scotch ice-sheet must have spread far enough over the floor of the North Sea as to over-ride the Orkney Islands.1
We have now disposed of the various points in this address which are likely to mislead the general reader. We have spent our annual holidays for four years in working out the glacial phenomena of Shetland, Orkney and Caithness, with a view to determine the question of the extension of the ice in the North Sea during the Glacial period. In the course of these traverses we have amassed a great amount of detailed evidence, which cannot readily be incorporated in the pages of a scientific publication like the Quarterly Journal of the Geological Society. We have had to content ourselves with merely summarizing the evidence. We can only state, however, that our repeated traverses have left no escape from the conclusion, that during the climax of the Glacial period, the direction of the ice-movement in Shetland, Orkney, and Caithness was from the North Sea and the Moray Firth towards the Atlantic.
Stephen Austin and Sons, Printers, Hertford,
1 Quart. Journ. Geol. Soc. vol. xxxvi. p. 648.
1 Quart. Journ. Geol. Soc. vol. xxxvi. p. 648.