Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

The Pamphlet Collection of Sir Robert Stout: Volume 22

III. Glaciation

III. Glaciation.

The glacial phenomena of Orkney completely establish the double system of glaciation which we found to obtain in Shetland. There is satisfactory evidence for maintaining that during the primary glaciation the Orkney Islands must have been overridden by a mass of ice which moved from the North Sea to the Atlantic; but towards the close of the Glacial period, when the great mer de glace had retreated from the Orcadian coast-line, local glaciers must have lingered for a time in the valleys of Hoy and in some of the more elevated parts of the Mainland.

Though these islands do not comprise any districts that might be compared with North Mavine or the promontories of Lunnasting in Shetland, which are dotted all over with finely preserved roches moutonnées and rock-basins, nevertheless a careful search along the cliff-tops reveals numerous instances of glaciated surfaces and ice-markings. The latter, however, are not so abundant as we found to be the case in Shetland, which may be satisfactorily explained by the rapid disintegration of the flagstones when long exposed to atmospheric waste.

In the island of Westra the average direction of the striae in the eastern part of the island is W. 20°–30° N. Close by Noltland Castle, at the roadside, the trend is W. 20° N., on the north-west face of Cleat hill N.W., and immediately to the east of the same hill W. 30° N. At Rackwik, on the eastern shore, the ice-markings vary from W. to W. 20° N., while in Tuquoy Bay they point W. 10° S.

A careful examination of the striated surfaces on the hills west of Pierowall proves that the ice must have been slightly deflected as it impinged on the eastern slopes, the lower portion moving in the direction of the northern coast-line, while the higher strata streamed westwards over the hill-tops towards Nonp Head and Russitaing. On the north-eastern face of the hill south of Ourness several examples were noted pointing N. 30°–35° W., but in the gap between the hills the direction is W. 5° S.

Perhaps one of the most interesting features connected with the glaciation of Westra is the freshness of the ice-markings on Nonp Head (240 feet) and along the cliff-tops to the south. A few yards to the north and south of the highest point of this bold headland, finely preserved stria; were observed on grey flags, where the thin Boulder-clay had been recently removed by the action of the sea, trending W. to W. 3° N. Above Ramna Gio the direction page 652 varies from W. 10° N. to W. 10° S.; at Russitaing, W. 20° S.; near the Red Hare, W. 10° S.; near Inganess, W. 15° 8. to W. 18° N.; and again, in the bay south of Inganess, a well-marked instance points W. 12° N.

In some parts of the island of Eda the proofs of glaciation are marvellously fresh, more especially on the surfaces of the harder sandstones. From the finely glaciated surfaces and numerous roches moutnnnées in the centre of the island north of Lonton it is evident that the ice must have overtopped the hills in its northwestward march. On the east slopes of the Stennie hill the direction of the striæ is W. 20°–25° N., and not far to the south W. 40° N.

Along the eastern coast, between Calf Sound and Lonton Bay, the ice-markings point N. 20°–30° W., while between the Kirk of Skail and the Veness promontory the average direction is W. 35°–40° N. In one remarkable instance, on the shore about a mile to the south of the Kirk of Skail, striæ were observed on a highly inclined rock-face trending north and south, while on the cliff-top the direction is W. 35° X., the former being evidently due to local deflection. Along the western coast the general direction of the ice-movement is in perfect harmony with that just described. In the neighbourhood of Warness, which forms the south-west promontory of the island, the trend is W. 13° N., while to the west of the Wart of Eda, on the cliff-tops, it varies from W. 28° N. to W. 43° N.; and again, to the north of Seal Skerry, W. 40° N. One of the best examples to be met with in the island occurs in the bay east of Fara's Ness, where a small stream enters the sea. This burn has cut down through a deposit of shelly Boulder-clay to the polished pavement on which it rests; and along the stream-course the firm lines produced by the ice-chisel may bo seen to advantage on the glaciated surfaces of the sandstones. The direction of these instances is N. 27° W., but on the shore, close by the mouth of the stream, the trend is W. 38° N.

Notwithstanding the widespread covering of blown sand which envelops the greater portion of the island of Sanda, we succeeded in finding abundant traces of the ice-movement. In the Burness peninsula striated surfaces are numerous along the coast-lino, about a dozen instances occurring between Hermaness Bay and the Holms of Eyre, which, with one or two exceptions, point W. 10°–15°N. To the west of Loch Roo the direction is W. 25° N.; and not far from the Saville boulder, on the eastern shore of the peninsula, the trend is N.W.

On the shores of Kettletoft Bay the average direction is W. 10°X.; inland from this bay towards the Free Church it varies from W. 20°–40° N., while in Bacaskeal Bay it is N. 32° W. This north-westerly movement is equally borne out by the evidence obtained in the southern part of the island; for in the bay west of Hack Ness the ice-markings point N. 30° W., and on the western shore between Spur Ness and Stranquoy N. 8°–17° W.

The island of Stronsa likewise supplies conclusive evidence page 653 regarding the direction of the ice-movement; for in Odin Bay, where an important section of Boulder-clay occurs, which we shall describe presently, the striæ point W. 15°–35° N.; between Kirk-buster and Finga the trend is W. 10°–40° N., at Burgh Head W. 40° N., and north of Holland W. 40° N. On the western coastline, on the shores of Rousholm Bay, the direction varies from W. 41° N. to N. 40° W.; and on the shores of Linga Sound it is W. 40° N. It is of the utmost importance to note the perfect agreement in the trend of the ice-markings in different parts of this island, because it indicates a persistent movement in one determinate direction.

A careful examination of the striated surfaces on Shapinshay confirms in a remarkable manner the evidence regarding the ice-flow during the primary glaciation in the northern islands. Along the west coast, between Galtness and Stromberryness, the direction varies from W. to N.W., while on the shores of Veantro Bay, which indents the northern part of the island, the markings point N.W. and N. 35°–40° W. Further along the eastern coast-line, between Gioness and the school-house, the direction is W. 30° N., and the same trend is observable southwards towards the church; near Foot stria: were noted pointing N. 30° W., and close to Haco's Ness N. 20° W.

It is impossible, within the limits of this paper, to describe the various instances we met with in the Mainland, and we will therefore merely indicate the general trend in different parts of the island. On the glaciated surfaces of granite and gneiss north of Stromness numerous examples occur trending W. lo°–20° N. and W. 12° S. Immediately behind the town the direction varies from W. 8°–40° N., while on the moorland between Yesnabae and the Loch of Stennis, as well as at the Ring of Brogar, the same variation is observable from W. 12° N. to N.W.

On the hill-slopes overlooking Gorsness and the island of Gairsa the average direction of several examples is N. 25°–30° W., and along the coast-lino from Irland Bay to Houton Head the trend varies from W. 12°–42° N. One instance occurs in Irland Bay pointing W. 32° S., which probably belongs to the later glaciation.

In Kirkwall Bay, a short distance to the east of the pier, beautifully striated flagstones may be seen where Boulder-clay has been recently removed by the action of the sea, the striæ running N. 6° W. and N.N.W.; and so also on the surfaces of the flagstones in the Scapa Quarry the direction is N. 8° W. Along the shore from Scapa to Howquoy Head the average direction of several examples is N. 30°–35° W., and near St. Mary's the trend varies from N.W. to N. 25° W.

In the southern islands striæ are not so abundantly found, owing to the readiness with which the soft yellow sandstones and marls crumble away when long exposed to the denuding agencies. In South Ronaldshay several examples occur, the general direction of which is W. 20° N. These may be seen on the cliff-tops near Stow Head and Halcro Head by removing the coating of Boulder- page 654 clay. Even on the cliffs of the island of Hoy, overlooking the Atlantic, striated surfaces have been observed by Professor Geikie at a height of 600 or 700 feet above the sea-level.

The evidence now adduced regarding the ice-movement proves beyond all doubt that the islands have been glaciated in one determinate direction, independently of their physical features. A glance at the striæ map accompanying this paper (PI. XXVII.) shows the remarkable uniformity of the ice-flow in the different islands. Here and there, where local causes interfered with the general movement, slight deflections are met with; but, on the whole, the prevalent direction varies from W.N.W. to N.N.W. A careful examination of the numerous striated surfaces convinced us that the ice-sheet must have crossed the islands from the North Sea to the Atlantic. Indeed no one who reflects for a moment on the physical features of the islands could reasonably attribute the striations to a local radiation of the ice. If we except Hoy, these scattered islands contain no mass of elevated ground which is capable of giving rise to a local ice-sheet. So far from this being the case, we shall have occasion to refer to the absence of any indications of the existence of local glaciers in most of the islands towards the close of the Glacial period, a phenomenon which is doubtless due to this very cause. On the contrary, when we view the persistent northwesterly trend of the striations in connexion with the physical features, when we consider that the glaciated surfaces along the cliff-tops, as well as the roches moutonnées on the hill-slopes, prove that the islands must have been overflowed by the ice, we cannot resist the conclusion that the ice-movement during the primary glaciation originated beyond the limits of Orkney.

Fortunately the dispersal of the stones in the Boulder-clay amply confirms the foregoing conclusions regarding the north-westerly movement of the ice, while the presence of Scotch rocks in the same deposit enables us to demonstrate that the ice-sheet which crossed this group of islands must have radiated from the mainland of Scotland.