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The Pamphlet Collection of Sir Robert Stout: Volume 22

III. Glaciation

III. Glaciation.

From Sumhurgh Head northwards to Hermaness in Unst, we find everywhere the clearest evidence that Shetland must have been at one time smothered in ice. The ice-worn islets along the shoreline, the polished and striated surfaces on the low grounds, the abraded and fluted appearance of the highest hills on the Mainland, alike point to the action of a thick mass of ice, which must liave enveloped the isles. It is quite true that over considerable areas much of the evidence is obscured by a thick covering of peat; but wherever the peaty covering has been worn away, there are convincing proofs of that intense abrasion which we are accustomed to meet with in highly glaciated regions.

Before describing the proofs of glaciation in the different islands, it may be well to state, as the result of our observations, that most of the roches moutonnées and striations indicate the movement of an ice-sheet across the islands from the North Sea to the Atlantic; but, in addition to this, there is satisfactory evidence for maintaining that, as the climatic conditions gradually ameliorated, the primary ice-movement gave place to that of local glaciers, which moved off the high grounds in the ordinary way, depositing their terminal and lateral moraines as they shrank back into the hills.

Along the eastern seaboard of Unst the direction of the ice-markings varies from W. to W. 20° S. From Norwick to Harolds-wick numerous striæ occur on the cliff-heads running W. to W. 20° S., some of which were found on the top of a cliff 500 feet high; while in the southern parts of the island the average trend is W. 30° S. In Fetlar the general direction of the striæ along the northern coast, from Gruting Bay to the promontory of Urie, is W. 30° S., though they vary from W. to W. 30° S. Two exceptions to the foregoing examples were found on the west side of the island—one on a glaciated surface of serpentine west of the promontory at Urie, running north and south, and the other on gneiss at the north-west comer of the island, pointing N. 10° W. These instances, however, have no connexion with the main set indicating the general glaeiation of the island.

Again, on the north-east coast of Yoll the striæ point W. 25° S., harmonizing with the direction of those found on the south side of the island of Unst; but on the western seaboard from Sandwick to the Noup of the Graveland the trend varies from W. 30°–39° N. In spite of this variation we are convinced, from evidence obtained in the Mainland, that these instances belong equally to the period of primary glaciation. It would seem that the ice-sheet abutted on the eastern seaboard of Shetland with a S.S.W. and S.W. trend, and after reaching the crest of the Mainland it swung round to the N.N.W. and N.W.

Along the north-west coast of Whalsey, between Skaw Taing and Symbister, the average direction of twenty-one instances is S. 28° W., page 791 varying, however, from S.W. to S. 15° W., the variation being due in many eases to inequalities of the ground; while on the southeastern shore the trend varies from S. 15° W. to S. 23° W. Now it is apparent, on a moment's consideration, that the direction of the striæ would have been widely different had the island radiated its own ice, and had the glaciation been purely local. Both on the north-west and south-east shores the striæ are either parallel with the long axis of the island or cut obliquely across it; and hence, in order to produce these striæ, there must have been, during the primary glaciation, a mass of ice moving in that particular direction.

In addition to this, there is evidence to prove that this island possessed local glaciers at a later period; for to the north of Challi-setter ice-markings occur, trending N. and N. 10° E. Close by these later striations, numerous small moraines are seen on the gentle slope which flanks the central ridge in the northern portion of the island.

There is, perhaps, no district in Shetland where the intense abrasion typical of glaciated regions is so patent as in the out-skerries of Whalsey. When sailing from the latter island to the Skerries, we were struck with the ice-worn aspect of the numerous little domes of rock projecting above the water. Housay, Brury, and Gruna may be described as large roches moutonnées which have been ground down, bared, and striated in a wonderful manner. Prom the top of the little hill south of the schoolhouse, one sees all round a succession of bare hummocks and domes of rock, destitute of any drift-covering, and with little vegetation, revealing unmistakably the great pressure to which the islands have been subjected. In Gruna the striae vary from S. 10° W. to W. 42° S.; in Brury, on the top of the highest hill, S. 35° W.; and in Housay, S.S.W. to S.W.

A glance at the map will show that the instances now adduced coincide in direction with those occurring in Whalsey, and, with the exception of a littlo more southing, they agree with those in Unst, Fetlar, and on the east coast of Yell. In the case of the Skerries this south-westerly trend has a marked significance, inasmuch as no one can possibly dispute that the glaciating agent must have been quite independent of the islets. It is equally clear that the markings are not due to the action of any local sheet radiating from the Mainland of Shetland. Apart altogether from the fact that the position of the roches moutonnées, as well as a minute examination of the striated surfaces, convinced us that the ice crossed the Skerries from the north-cast towards the south-west, there are other reasons why these markings cannot be attributed to any such local cause. When we come to discuss the evidence supplied by the Mainland in regard to the extent of the later glaciation, we shall see that there is satisfactory ground for maintaining that the later glaciers did not spread far beyond what is now the coast-lino of that island. Moreover, the direction of the later glacier movement on the east side of the Mainland is at variance with the trend of the striæ occurring in page 792 the Skerries. For these various reasons, therefore, we are justified in inferring that the glaciation of these outlying islets is due to the action of an ice-sheet originating far beyond the sphere of Shetland.

On the eastern seaboard of Northmavine, in the Mainland, between Ollaberry and North Rooe, the general trend of the ice-markings is in a south-westerly direction. On the north shore of North Rooe Bay two sets of stria: were observed—one pointing S. 40° W., belonging to the primary glaciation; the other S. 30° E., produced by later glaciers moving down the bay. Near Fethaland Point two sets of striæ were observed, which clearly prove the general movement of the ice during the primary glaciation, and at the same time a separate movement of the lower portions of the mass caused by an undertow. On the headland north of the fishing-station the striaæ run N.W. and N. 20° W.; while on the south side of the bay, about a mile from the fishing-station, the markings on the cliff-heads point N. 6° W., N. 10° E., N. 20° W., indicating a varying movement in a northerly direction. On ascending the polished slope which overlooks the foregoing examples, the direction is S. 10°–35° W. This divergence is readily accounted for by supposing that the lower current moved in a north and north-west direction, while on the slopes of the ridge the upper current moved towards the south-west in harmony with the general movement along the eastern seaboard of the Mainland.

Again, in the upper part of Roeness Voe, the striæ point W. and W. 10° N.; but on descending the sea-loch they swing round to the north-west, the instances near the mouth of the voe trending N. 20°–28° W. The same northing of the striæ is splendidly seen on the area occupied by the interbedded volcanic rocks between Braewick Bay and Hamna Voe, the direction varying from N.20°W. to N.W.

Along the highroad from Ollaberry to Mavis Grind, numerous instances were observed which likewise indicate a passage of ice from the North Sea towards the Atlantic. On reaching Sulem Voe from the north, the eye at once fixes on a large roche moutonnée of diorite, which rises to a height of 200 feet above the sea-loch, and the surface of which is finely polished and striated, the markings pointing W. 5° S. And so, also, the narrow neck of land at Mavis Grind is similarly grooved; indeed, over the whole of the district round Hagrister and Islesburgh and north of Magnusetter Voe, the ice-worn aspect of the hills is very apparent, the smooth slopes looking to the east, while the rough slopes face the west, indicating the direction from which the ice came.

On the eastern shores of the districts of Nesting, Lunnasting, and Delting there is no lack of evidence regarding the glaciation, as striæ are plentiful, and in certain areas there is but a scanty covering of peat and herbage. It is difficult to convey an adequate impression of the singularly bare and mamillated appearance of the tract of ground which forms the peninsular headlands of Lunnasting. Bare dome-shaped hills, dotted all over with lochs, occur in the page 793 tract between Dourye and Vidlon Voes; and the same features are apparont on the rocky promontory north of the latter sea-loch. Indeed, so perfect and so abundant are the roches moutonnées that it may be correctly described as by far the finest district on the Mainland for studying the effects of the primary glaciation.

The average trend of the ice-markings in the districts now referred to is W. 35° S., though they vary from W. to S.W. The position of the roches moutonnées leaves no room for doubt as to the direction of the ice-movement. In Swining Voe, which lies to the west of Vidlon Voe, there is a gentle Boulder-clay slope on the east bank, and a steep rock-face on the west bank, rising to a height of from 400 to 500 feet. Notwithstanding this steep slope, the whole rock-face is splendidly glaciated; and, strange to say, the stria; do not run parallel with the coast-lino but obliquely across it, the direction being nearly south-west. In one remarkable instance, about halfway down the voe, on a glaciated surface, which slopes downwards into the sea-loch at an angle of 65°, stria; were observed which could be traced from the water-level up the rock-face at an angle of 25° with the surface-plane of the sea-loch. We shall point out presently how the dispersal of the stones in the Boulder-clay completely substantiates this south-westerly movement of the ice.

The tract of country which stretches from Weesdale westwards to Melby and Walls presents the same glaciated aspect, though in many places the roches moutonnées have been much broken up by atmospheric waste. Nevertheless the rounded outline of the hills testifies to the moulding of the whole tract by ice, while the stria; have a marked north-westerly trend, quite in keeping with the northing already referred to on the western shores of Northmavine. Not only so, but the highest ground in the centre of the Mainland is likewise ground down and striated. The ridge which extends from Weesdale hill (842 feet) to Scallafield (916 feet) reveals the finer lines as well as the flutings of the ice-chisel wherever the peat is worn away, the direction varying from W. 28°–40° N. Near the gap in the ridge overlooking the head of Weesdale Voe, the polished surfaces and striations are as fresh as if the ice had but recently passed away. Further, the same north-westerly trend is met with on the banks of Olna Voe, east of Meikle Rooe, and in the numerous sea-lochs opposite the isles of Papa Little and Vementry.

In the districts of Lerwick and Quarff, on the eastern seaboard, there is conclusive evidence of the existence of two systems of ice-markings, the one set belonging to the general glaciation trending in a south-westerly direction, and the other set belonging to a later period, indicating a movement in a south-easterly direction, produced by local glaciers. Indeed, so severe must have been the later glaciation in the neighbourhood of Lerwick, that most of the instances belonging to the primary system were well-nigh effaced by it. Both the abundance and the freshness of the stria; belonging to the later system plainly indicate the power of the local glacicrs in this neighbourhood; but we shall see presently that at no time were they large enough to override the island of Bressay. Several interesting page 794 examples of cross-hatches were observed near the fort at Lerwick, also north of the docks, and again near the village of Sound, the older markings running S.W. and the newer ones S. 40° E. to E. 40° S.

In the long tongue of land stretching southwards from Quarff to Sumburgh Head, the striæ belong mainly to the later glaciation, the direction varying from E. 29° S. to S. 34° E.; about half a mile from Boddom, however, by the roadside, some examples occur in which the trend is W. 3°–9° N., produced by ice moving in a westerly direction.

From the evidence we obtained in Bressay, it is clear that the south-westerly system is the one which is most prominently marked in that island; indeed, so abundant are the ice-markings belonging to the early glaciation, that some parts probably escaped the movements of the later glaciers altogether. This much is certain, that the local glaciers of the Mainland were only able to override the north-western portions of Bressay. Along the eastern coast, from Heogan to the lighthouse, as well as by the roadside from Cullonsbro to Gardie, the trend varies from W. 20° S. to S. 30° W. But on the slopes east of the Wart the later system points S. 20° E. to E. 16° S.

In the island of Meiklo Rooe, which lies to the west of the Mainland, the average trend is N. 30° W.; in Papa Stour it varies from N. to N. 28° W.; while in Foula, the most isolated of the Shetland group, situated about 18 miles to the S.S.W. of the village of Walls, well-marked striations were observed, running N.W. and W. 30° N.

Altogether we recorded upwards of three hundred and twenty instances of striations in the Shetland Isles, the great majority of which belong to the primary glaciation.