VII. Freshwater Lochs and Voes.

The freshwater lochs abound chiefly in the Mainland; and in certain districts they occur in great numbers. They are due either to the irregular deposition of the Boulder-clay or moraine-matter, to hollows in the peat, or to rock-basins which have been eroded by the ice. Indeed they are so abundant in some of the rocky districts as to recall portions of the north-west of Sutherlandshire. At present we are only concerned with those which occupy rock-bound hollows, and which are the result of glacial erosion.

These occur most abundantly on the rocky plateau of Roeness, in the diorite-area of Northmavine, on the rocky headlands north and south of Vidlon Voe, and in the district of Walls. In each of these localities the sheets of water, with certain exceptions, fill eroded hollows in the rocks; and, from the manner in which their rocky margins are grooved and polished, from the freshness of the roches moutonnées which encircle them, there can be little doubt they have been eroded by the ice during the general glaciation. From one of page 805 the hills north of Magnusetter Voe, in Northmavine, we eounted about twenty small lochs in the heart of the diorite-area.

On the promontory of Lunnasting they likewise occur in great numbers, varying in size from basin-shaped hollows to locls more than a mile in length. Their long axes coincide with the strike of the underlying gneiss; but, owing to the scooping-agent having crossed the lines of stratification nearly at right angles, their outlines are very irregular. Similar strike-basins are to he found on the promontory between Vidlon and Dourye Voes; and from the manner in which they are hemmed in by roches moutonnées on every side, it is impossible to resist the conclusion that they are duo to ice-action. The lochs now referred to must have originated during the primary glaciation, because there is no evidence that the later glaciers ever overflowed the headlands of Lunnasting.

The voes or sea-lochs are among the most interesting features of the Shetland Isles; and the question of their origin is not free from difficulty. Flowing, as they do, for miles into the heart of the country, it sometimes happens that only a narrow isthmus is left to prevent the waters of opposite shores from uniting. Yell is nearly bisected by the Whalofirth and Reafirth Voes; and a submergence of a few feet would separate Northmavine from the Mainland, and allow the waters of Sulem Voe to flow westward into St. Magnus Bay. Sometimes the voes are flanked by gentle slopes of Boulder-clay, as we have frequently indicated; at other times they are bounded by steep walls of rocks, as in the well-known Roeness Voe. Many of the most characteristic sea-lochs lie along the lino of strike of the metamorphic rocks, of which the Weesdale, Stromness, Whiteness, Dales and Laxfirth Voes may be cited as the best examples; but there are others which have no connexion with the lines of stratification. As a rule, they are found to merge into narrow valleys draining the high grounds, the width of the voes being in direct proportion to the size of the valleys. This relationship would seem to indicate that these narrow fiords are submerged land-valleys which existed long before glacial times. In the course of our traverses in Shetland, we heard frequent testimony pointing to the conclusion that the ridge-shaped contour which is so prevalent in the Mainland, Yell and Unst, likewise extends along the sea-bottom; and it is highly probable that it is due to the same cause in both cases. If this be true, then these fjord-valleys may have been carved out by the ordinary agents of denudation when the floor of the sea which now surrounds Shetland formed dry land. Both in Scotland and along the cast coast of England the evidence derived from buried river-channels would lead us to believe that these countries stood at a higher level in preglacial times than they do now; and we may well believe that Shetland shared in the same continental conditions. The absence of shells in the Boulder-clay seems to strengthen this conclusion.

At any rate the agents of denudation would be guided in their operation in a large measure by the strike of the metamorphic rocks; and if there was a wide area of land round what now constitutes the Shetland archipelago, they would accomplish greater page 806 results, as the size of the rivers would be in proportion to the area of drainago. We have seen also that some of the voes and inland valleys coincide with the outcrops of bands of limestone, the erosion of which would be aided by chemical agencies.

There can be no doubt, however, that the sea-lochs in Shetland were deepened by ice-action during the primary glaciation; indeed numorous instances have been cited in this paper where the great mer de glace took advantage of the existing hollows in crossing the island, This produced, in certain instances, fjord-basins, of which we shall adduce two examples. The soundings given in the Admiralty chart show that Sulem Voe, which is one of the largest of the sea-lochs in the Mainland, measuriug upwards of seven miles in length, varies from 10 to 15 fathoms in depth between Foula Ness and the mouth of Voxter Voe. Beyond the latter point, however, to the head of the voe, the depth suddenly increases to 21 and 25 fathoms. This increase of 60 feet in depth at the head of the sea-loch is doubtless due to the intense abrasion caused by the ice as it impinged on the rocky isthmus of Mavis Grind. We have already pointed out how distinctly the east face of this narrow isthmus has been polished and striated; and this fjord-basin helps us to realize still better the erosive power of this agent. Still another instance occurs in Roeness Voe; for at the bend north of Urie Firth the depth varies from 102 to 138 feet, while about two miles further down the loch shallows to 42 feet.

There is one peculiar feature connected with these voes which may be dismissed in a few words. It frequently happens that spits of gravel are thrown up by tidal action near the head of the sea-loch. These banks are seen in all stages of formation in Shetland, sometimes extending a third, a half, or nearly the whole of the way across the loch. Ultimately the voe is crossed by a continuous bank of gravel which isolates the upper part; and this isolated portion is converted into a sheet of brackish water.