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The Pamphlet Collection of Sir Robert Stout: Volume 22

VIII. Conclusion

VIII. Conclusion.

1. Summary of the Evidence regarding the Primary Glaciation.—

We must now, very briefly, recapitulate the evidence regarding the primary glaciation of Shetland, in order to show the conclusions which may be justly drawn from the facts, and also to determine the relation which the glacial phenomena of these isles bear to the glaciation of Norway and Scotland.

It has been shown that in the islands of Unst, Fetlar, Whalsey, the Outskerries, Bressay, and along the eastern sea-board of the Mainland and Yell, there is one uniform system of ice-markings trending W.S.W., S.W., and in some cases S.S.W.; while in the western districts of the two latter islands, as well as in Meikle Rooe, Papa Stour, and Foula, the striæ swing round to the N.W. and N.N. W. From a careful examination of the striated surfaces and the Stossseite of the roches moutonnées, it is evident that the agent which produced them must have crossed the islands from the North Sea to the Atlantic. Fortunately this conclusion is page 807 placed beyond all doubt by the distribution of the Boulder-clay, as well as by the dispersal of the stones in this deposit. On the western sea-board of Unst the Boulder-clay contains fragments of serpentine, gabbro, and graphitic schists, all of which occur in situ on the east side of the Vallafield range. Moreover the relative distribution of the serpentine and gabbro stones in this deposit on the western shore is in direct proportion to the relative areas occupied by these rocks to the east of the watershed. It follows, therefore, that the agent which glaciated Unst must have crossed the watershed, carrying the bottom-moraine up the slope, and depositing it in the lee of the range. In Fetlar, blocks of gabbro and serpeutine are likewise found in the Bouldor-clay on the west coast; while along the east coast of Yell, blocks of gabbro occur in this deposit which have been brought from Unst and Fetlar, testifying alike to the same westerly movement.

The evidence derived from an examination of the Boulder-clay sections on the Mainland is equally conclusive; for it matters not whether we cross the northern, central, or southern portions of the island, we are compelled to admit that the ice-flow during the primary glaciation must have been towards the Atlantic. In the central part of Northmavine it has been clearly proved that the Boulder-clay partakes of the physical character of the rock-formation on which it rests, while a certain percentage of the stones is derived from localities which lay in the path of the glaciating agent. Abundant evidence has been adduced to show that the quartz-felsite area between Tanwick and Roeness Voe has been invaded by the diorite stones, while the area occupied by the bedded porphyrites has been invaded by the quartz-felsito and diorito stones. Moreover it is particularly observable that the blocks do-rived from the successive areas occupied by these rocks, which are present in the Boulder-clay, diminish in number in proportion to the distance from their parent source.

Again, in the long tongue of land which stretches from Scalloway southwards to Fitful Head, blocks of the Old-Bed-Sandstone rocks occurring on the eastern sea-board are found, not only on the tops of the highest hills, but also in the Boulder-clay on the western shore. Moreover the distribution of the fragments of the Lerwick Sandstone, Brenista Flags, and basement-breccia in the Boulder-clay north of West Quarff is in perfect harmony with the relative areas occupied by these subdivisions of the Old Bed Sandstone south of Lerwick. The same relationship holds true in the district between Maywick and Fitful Head. It is clear, therefore, that the glaciating agent must have overflowed the watershed, as we found to be the case in Unst.

2.Insufficiency of Icebergs or Coast-ice to account for the Phenomena.

—Perhaps some may attribute the numerous striated surfaces, as well as the Boulder-clay, to the action of icebergs or coast-ice on a sinking area; but a little consideration will show that either of these causes is quite inadequate to explain the phenomena. We have shown that over the whole of Shetland the glaciating agent must have conformed to the inequalities of the surface, descending into page 808 the smallest hollows and overflowing the projecting knobs of rocks, indicating in an unmistakable manner that the agent must have pressed steadily and firmly over the whole area. Nay, more, the islands have been grooved and striated in one determinate direction, while rocky slopes have boon likewise abraded; and from the manner in which the stria; run obliquely up the hill-face, it is evident that the agent must have ascended the slopes, and ultimately overflowed the high grounds. Now it is hardly necessary to point out that neither coast-ice nor icebergs are capable of producing such results as these. It is impossible to conceive that icebergs or coast-ice could press steadily on a wide archipelago like Shetland, so as to plane down the inequalities on the surface; far less could they produce this uniform system of striation. "We may well ask, by what means could floating ice or coast-ice ascend a rock-slope several hundred feet high, leaving at the same time indelible impressions of the upward movement? Such an occurrence would be a physical impossibility.

Again, the phenomena of the Boulder-clay are quite at variance with the floating-ice theory; for if this deposit be due to the droppings of icebergs or coast-ice, then assuredly it would have been more or less stratified; whereas, from one end of Shetland to the other, the Boulder-clay, with but few exceptions, is quite amorphous. If it be really a marine deposit, how could it possibly partake of the characters of the rock-formation on which it rests, and how could the relative ingredients diminish in number in proportion to the distance from their parent source?

Further, the occurrence of blocks in the Boulder-clay on the western sea-board of Unst and the Mainland, which must have crossed the watershed to reach their present position, is still less explicable by this hypothesis. For if the high grounds of Unst or the Mainland were submerged so as to allow a free passage for icebergs in their westward career, where are the areas of gabbro, serpentine, or Old Red Sandstone which could have supplied the materials found in the Boulder-clay? Even if we suppose that ice rafts drifted off the eastern sea-board laden with such materials, we must suddenly invoke a special subsidence of several hundred feet at least, both in Unst and in the Dunrossness area, to enable them to cross the watershed. But this improbable supposition still leaves unexplained the relationship which exists between the relative distribution of the stones in the Boulder-clay on the west coast, and the relative areas occupied by the rock masses. For these reasons, therefore, and others Which it is not necessary to specify, it is impossible to reconcile the glacial phenomena of Shetland with the theory of icebergs or coast-ice.

3. Shetland glaciated by Scandinavian Ice.

—Similar phenomena to those now referred to have been observed and described again and again in Scotland and other highly glaciated regions, where they have been almost universally ascribed to the action of land-ice. It is not necessary for us to show how the uniform system of striation, or the rounded outlines, or the close relation between the Boulder-clay and the rocks on which it rests, are satisfactorily ex- page 809 plained by the passage of land-ice over Shetland. It is sufficient for our present purpose if we show that, during the general glaciation of Scotland, Boulder-clay was transported across important hill-ranges by the ice which radiated from the Grampians. On the south of the Sidlaw range, as well as on the south side of the Ochils, the Boulder-clay contains fragments of schist, gneiss, and granite, which must have been transported from the Highlands. Further, on the top of Allermuir hill small patches of Boulder-clay were observed by Dr. Croll containing striated stones derived from the Highlands to the north-west. It is evident, therefore, that the Scotch ice-sheet was powerful enough to override such important ranges as the Sidlaws, the Ochils, and portions of the Pentlands, and must likewise have rolled forward the bottom moraine, depositing it in the Iee of the hills. And if such was the ease in Scotland, then why may not the same thing have happened in Shetland? Indeed, had Shetland formed a part of the western sea-board of Scotland, there would have been no hesitation in ascribing the striated surfaces and the Bouldor-clay to the action of land-ice.

The land-ice which glaciated Scotland could only have come from Scandinavia, as the striated surfaces clearly point in that direction. And we must now briefly consider what grounds there are for believing that the Scandinavian mer de glace was powerful enough to invade the North Sea. The researches of Erdmann, Hörbye, Esmark, Holland, Törnebohm and Linnarsson have revealed to us the extent of the ancient glaciation of Norway and Sweden. They clearly show that Scandinavia was not glaciated by Polar ice moving southwards from the Arctic regions; for the ice-markings generally radiate from the great tablelands as they do in Scotland. It must have been buried underneath an ice-sheet which moved off the land in all directions. It has been generally supposed that this mer de glace must have broken up in the form of bergs when it reached the shallow North Sea; but fortunately we are now supplied with data which enable us to prove that this could not have been the case. If we take the estimate given by Holland for the minimum thickness of the ice in Sogne Fjord during the period of extreme cold, it follows that, instead of the ice breaking up in the form of bergs, it must have invaded the North Sea and moved in a westerly direction towards the Shetland Isles. He gives 6000 feet as the estimate at this point; and when we remember that the average depth of the German Ocean is about 240 feet, we can readily understand how such a mass of ice could never have floated between Norway and Shetland, much less between Norway and Scotland.

When this mer de glace impinged on the Shetland frontier, it would necessarily be deflected to some extent by the opposing high ground. Hence, as we move southwards from Unst, where the average trend of the ice-markings is W. 10°–20° S. towards Bressay and Lerwick, the deflection increases to S.W. and in some cases to S.S.W. But as soon as the ice reached the crest of the Mainland, it would naturally follow the path of least resistance, veering round to the N.W. and N.N.W. It is highly probable that this northing may be due in part to the resistance offered by the Scotch page 810 ice-sheet, which must have coalesced with the Scandinavian mer de de glace in the North Sea. That this union must have taken place is evident from the proofs of the deflection of the glaciers along the eastern sea-board of Scotland and England; and it would even now appear that the great Chalky Boulder-clay of East Anglia is a product of land-ice which moved inland in a north-east and southwest direction. These phenomena point to the existence of some constantly opposing force which was capable of overcoming the seaward motion of the Scotch and English glaciers. In other words, the two ice-sheets must have united on the floor of the North Sea, one great outlet for this ice-field being towards the north-west by the Pentland Firth and the Orkney Islands. When the Orkney Islands are examined in detail they will doubtless yield conclusive evidence in support of this north-west movement.

After the mer de glace had ceased to be confluent with the local glaciers of Shetland, the latter lingered on for a time, filling all the main valleys and flowing off the land in all directions. The deposits met with on the eastern coast of the Mainland between Lerwick and Boddom, and again between Colifirth Voe and Fethaland Point, must be attributed to this local movement; while the numerous moraine heaps sprinkled over the valleys indicate the immense quantity of debris which must have been borne downwards on the surface of the small glaciers.

4. Absence of Gravel Kames and Raised Beaches in Shetland.

Throughout the isles we searched in vain for those ridges of gravel which form such a notable feature in Scotland. Here and there the moraine mounds and the moraine debris, which is spread irregularly over the slopes of the hills, show signs of rudo stratification, while the stones are more or less waterworn; but no one would readily mistake them for true kames. Moreover there is a remarkable absence of raised beaches indicating changes in the relative level of sea and land. Though we examined the islands with considerable minuteness, we never found a trace of those familiar terraces which are so characteristic of parts of the Scotch coast-line. This is all the more remarkable, as the voes or sea-lochs are admirably adapted both for the formation and preservation of sea-beaches. We cannot help believing that, if such deposits had been formed, we must assuredly have met with some indications of them; and for this reason it seems just to infer that they never existed in Shetland. The remarks made by Professor Geikie in an article in 'Nature'* clearly show that their absence has an important bearing on the question of their origin. For if they be due, as Dr. Croll suggests, to the rise of the sea-level, owing to an accumulation of ice round the North Pole during the glacial period, then we should naturally expect to find them in localities which are so well adapted for their formation; but since this is not the case, we may infer that they indicate pauses in the gradual elevation of the land which must have been general over the whole of Scotland in postglacial times. It would seem, however, that Shetland did not participate in these general movements of upheaval.

* 'Nature,' vol. xvi. p. 414.