VI. Erratics.

From an examination of the numerous boulders scattered over the Mainland and the other islands, it is evident that their dispersion belongs to two distinct periods of glaciation. We saw none which cannot be satisfactorily accounted for by the double system of glaciation already established, without invoking the aid of coast-ice or icebergs.

Along the slopes of the Vallafield ridge in Unst we observed numerous blocks of serpentine, some of them measuring 5 feet across, which were dispersed during the primary glaciation. In the Mainland they occur in dozens on the rocky plateau of Roeness, on the diorite area north of Mavis Grind, and on the rocky headlands of Lunnasting.

In the valleys draining the eastern slopes of the Roeness plateau, and in the low hills between Colifirth Voe and Fethaland, blocks of pink quartz-felsite are strewn, sometimes on the moraine heaps, on page 804 roches moutonnées, or on the drift-slopes, all of which were distributed by the later glaciers.

Again, boulders of the Northmavine diorite are scattered over the low ground between Hillswick and the Grind of the Navir, while blocks of metamorphic rocks were carried by the great mer de glace and the later glaciers from the Leas of Deal and the surrounding heights onto the diorite area of Busta Voe. The peculiar band of nodular gneiss on the promontory of Lunna has supplied boulders which can be followed westwards towards the head of Swining Voe, and the ridge overlooking Dourye Voe, in harmony with the primary ice-movement.

West of Weesdale, blocks of the porphyritic granite in Bixetter Voe, as well as boulders of gneissose rocks from the Weesdale hills, are strewn over the area occupied by the altered Old-Red-Sandstone rocks : while boulders of the Sandncss-hill quartzites have been borne seawards to Melby.

Perhaps the most interesting scries of erratics occurs on the ridge of high ground which extends from Scalloway to the Wart of Skewsburgh, where small blocks of the Brenista Flags and the Lerwick Sandstones, varying from a few inches to a foot across, are exposed in places where the peat has been worn away. These have been carried from lower to higher levels; indeed they have been carried to the tops of the highest hills along this tract. We have already incidentally referred to this remarkable fact, and to the occurrence of similar blocks in the Boulder-clay on the west coast, and as erratics on the slopes of the hills. Those which are found on the western sea-board are much larger than those on the bill-tops, as they sometimes measure 3 feet across : many of them still show ice-markings. We believe that their occurrence in the drifts on the west coast and as erratics on the hill-tops is duo to the same eause, viz. to the westerly movement of the great mer de glace, which was powerful enough to override the watershed.