VII. Conclusion.

The evidence now adduced regarding the glacial phenomena of Orkney is of the utmost importance in solving the question of the extension of the ice in the North Sea. We have already referred to the remarkable uniformity in the trend of the ice-markings throughout the islands, which, with certain exceptions, vary from W.N.W. to N.N.W. From the manner in which these striations maintain their persistent north-west trend, irrespective of the physical features of the country, it is evident that the agent which produced them must have acted independently of the islands.

Nay, more, the dispersal of the stones in the Boulder-clay leaves no room for doubt that the ice-sheet must have crossed the islands from the North Sea to the Atlantic. It is no doubt true that the lithological varieties of the Orcadian rocks are not so numerous as in Shetland, and hence the corroborative evidence of the northwesterly movement is not so abundant. Still in those cases where the geological structure of the ground permitted us to test with certainty the direction of the ice-carry, we were driven to the conclusion that the ice-flow must have been towards the Atlantic. In Westra the Boulder-clay sections contain striated blocks of red and white sandstone, which have been derived from Eda, and it is particularly observable that they diminish in number as we move towards the north-west. In Shapinshay blocks of the slaggy diabase from the south-east corner of the island occur in the Boulder-clay near Galtness; and so also in the Mainland, the red and white sandstones which cross the centre of the island are represented in the moraine profonde on the shore between Houton Head and the Loch of Stennis. Yet, again, to the west of the axis of crystalline rocks at Stromness, smoothed blocks of gneiss and granite are found in considerable abundance.

Fortunately, however, we have additional evidence which enables us to demonstrate, not only that the ice-movement must have been from the North Sea towards the Atlantic, but, what is of still greater moment, that the ice which glaciated Orkney must have come from Scotland. In the numerous sections of Boulder-clay described in this paper we have had occasion to refer to the occurrence of smoothed and striated stones of dark-grey limestones full of plant-remains, oolitic limestone, calcareous breccia, chalk, chalk- page 661 flints, fossil wood, pink granite, porphyritic felstone, &c., all of which are foreign to the islands.

According to the opinion of Mr. Carruthers, F.R.S., the blocks of dark-grey limestone with plant-remains in all probability belong to the Calciferous Sandstone series. He has identified a well-marked specimen of Lepidostrobus in one of the blocks, though it is not distinct enough to bo named specifically. Lithologically the boulders resemble some of the thin limestone bands in the Cementstone series of Central Scotland; and the nearest locality to Orkney where these rocks occur in situ is in the county of Fife. With reference to the Secondary rocks, Professor Judd, F.R.S., states that, besides the chalk and chalk-flints, he detected amongst our collection some specimens which resemble some of the Secondary rocks of Scotland. Two specimens of the calcareous breccia from the Boulder-clay in Odin Bay "very closely resemble parts of the Upper Oolites of Sutherland," and two other blocks are probably from the same locality. Moreover, he adds that the specimens of oolitic limestone very possibly come from some part of the Secondary series in Scotland.

In addition to these, wo observed, in the Odin-Bay section, large blocks of a remarkable rock which seems to be petrified wood. It has a curious fibrous structure and is very calcareous; indeed under the microscope it appears to be mainly made up of crystals of calcite, though occasionally there are portions where the structure is still retained. Blocks of the same rock, however, occur in the Caithness Boulder-clay, which show traces of organic structure under the microscope. On dissolving a small piece of the rock a large residue of coaly matter was obtained, which ignited with a strong flame. It would appear that this rock is largely burnt for lime in Sutherlandshire, where it is washed out of the Oolitic shales.

In all probability most of the blocks of granite, felstone, gneiss, quartzite, and schist which occur throughout Orkney, save those in the Stromness district, have been derived from the northeast of Scotland, though they possess no special characteristics which might enable us to identify them with any particular locality.

Now it ought to be borne in mind that chalk, chalk-flints, and various rocks of Jurassic age are found in the Boulder-clay of Caithness, and also in the same deposit in the low grounds of Banffshire and Aberdeenshire, where it possesses the same physical characters as in Orkney, and likewise contains fragments of shells. It seems perfectly reasonable to conclude, therefore, that the Boulder-clay in these widely separated localities must be ascribed to a common cause, or, in other words, to the action of land-ice. Indeed no one who attentively examines the sections in Orkney would ascribe them to the action of icebergs or coast-ice. We have already discussed the objections to the marine origin of the Shetland till, and the very same arguments apply with equal force to the present case.

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Moreover, on referring to the chart showing the probable path of the ice in the North Sea, which accompanies this paper (PI. XXVII.), it will be seen that it is impossible to escape this conclusion. The ice, which radiated from the north-east of Scotland, not only filled the basin of the Moray Firth, but likewise spread over the low grounds of Banffshire and Aberdeenshire. The researches of previous investigators point to this conclusion; and quite recently, during the prosecution of the Geological Survey of the south side of the Moray Firth, additional facts have transpired which tend to confirm it. Further, in the neighbourhood of Dunbeath, on the Caithness coast, the stria; gradually swing round till they run parallel with the shore, eventually bending inland till they point towards the northwest, in harmony with the trend of the ice-markings in Orkney. Clearly, then, the ice must have been deflected so as to override the low grounds of Caithness, as pointed out long ago by Dr. Croll. Similarly in Forfarshire and Kincardineshire, the ice which moved off the south-east slopes of the Grampians, on reaching the coastline, was bent round in a N.N.E. direction, as indicated on the chart. A glance at the chart will also show how the land-ice was deflected along the south-east coast of Scotland, as described by our colleague Dr. James Geikie, F.R.S. Now these marked deflections undoubtedly point to some opposing force which was capable of overcoming the seaward motion of the Scotch ice-sheet. Had it been allowed to follow its natural pathway then the phenomena would have been widely different.

The results of our investigations in Shetland prove that the Scandinavian mer de glace not only invaded the North Sea, but likewise overlapped that group of islands in its march to the Atlantic. The presence of this mass in the bed of the German Ocean furnishes a satisfactory explanation of the phenomena above referred to; for the two ice-sheets must have coalesced on the sea-floor, and the combined ice-field would naturally take the path of least resistance. In other words, one portion would flow north-westwards by the Orkney Islands, while the southern portion would flow in the direction of the English coast, as laid down on the chart. In all probability the dividing line would be somewhere opposite the basin of the Forth.

We can quite well understand therefore how the Scotch ice-sheet, as it crept outwards along the bed of the Moray Firth towards the North Sea, must have pushed along the marine shells and silt which it encountered on the sea-floor. These would be commingled with the moraine profonde which had gathered underneath the ice-sheet; and the shells would ultimately be smoothed and striated precisely like the stones in the bottom moraine. Hence the occurrence of Scotch rocks together with shell-fragments in the Orkney Boulder-clay is what we would naturally oxpect; and in the light of the foregoing reasoning all difficulty as to the explanation of the phenomena disappears. It is not necessary for us to assign the precise localities from which the various foreign rocks have been derived; it is sufficient for our present purpose if we show, as has been done, that page 663 they may have come from the basin of the Moray Firth or the eastern counties of Scotland lying to the north of the basin of the Forth. The presence of blocks of limestone of Calciferous-Sandstone age in the Odin-Bay section in Stronsa seems to indicate that a portion of the ice which crossed Fife was deflected to the north; and even if the Saville boulder should prove to be of Scandinavian origin, its position in the north of the group is quite in keeping with the path which would be followed by the Scandinavian ice.

It is a significant fact that nowhere in the Shetland Boulder-clay did we find a vestige of the Secondary rocks of Scotland; and though the evidence is merely negative, it nevertheless confirms the foregoing conclusions. We are inclined to believe also that the absence of marine shells in the same deposit, which we noted in our previous paper, may probably indicate that a portion of the present sea-floor round Shetland formed dry land during the climax of glacial cold. We see, therefore, how the glacial phenomena of Orkney furnish a striking confirmation of the views advocated by Dr. Croll more than ten years ago.

Though we visited nearly all the islands of the group and traversed the greater part of the coast-line, we found no trace of gravel kames or raised beaches indicating recent changes in the relative level of sea and land.