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.