1911190a0Nature191479419610916119011900028-0836196110.1038/1911190a0ukNatureNatureNATUREnatureNature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public./nature/journal/v191/n4794issueJournal homeArchiveCurrent issueAdvance online publicationPrivacy policySubscribeNature Publishing GroupCurrent issue1911190a0Continuation of the Great Glen Fault Beyond the Moray Firth
AU  - APPLEBY, ROBERT M.Department of Geology, University College, Cardiff.DURING 1944-45 I was stationed in Fair Isle, Shetland, during which time I was able to make anumber of general observations on the geology of the island which tend to support Dr. Flinn's hypothesis1 that the Great Glen Fault continues northwards, past the west coast of Fair Isle, to Shetland, where it is called the Walls Boundary Fault. Because of the inaccessibility of the Isle and its important position relative to the suggested course of the fault, it is worth while recording the observations; but it must be emphasized that they are recollections, supported by only a few photographs, specimensand notebook data.Fair Isle is composed mainly of a barren series of shales, sandstones and conglomerate of Old Red Sandstone type, which the Geological Survey doubtfully identify as Lower Old Red Sandstone2. A detailed search for fish-beds was made but without success. The beds dip approximately to the south-east[ast] with dips ranging from about 60 on the east side of the island (Fig. 1) to very nearly vertical on the west. On the east and north sides the rocks are unaltered, but towards the south-west corner of the island the shales and sandstones appear to pass into phyllites and low-grade schists containing quartz lenticles. Several of the pebble beaches in this part of the island were made up, almost entirely, of a dark, fine-grained, but porphyritic rock containing phenocrysts of plagio-clase, indicating that igneous rocks were probably in situ just off the coast, if not in the inaccessible cliffs of Malcolm's Head at the south-west of Fair Isle. Farther north on the west coast, one or more dolerite dykes cut the cliff in an east-west direction.
When compared with Orkney, to the south, and Shetland, to the north, the most surprising feature is the attitude of the rocks, for in Orkney and Shetland the Old Red Sandstone has low dips of the order of 40 or less in the limbs of the gently rolling anticlines and synclines4. The fact that the dips become increasingly steep and approach the vertical towards the west coast of Fair Isle would accord with the hypothesis that a large disturbance lies to the west.
A further point is that erosion on the east side of the island reflects the lithology and structure of the rocks, soft bands being removed and more resistant beds standing up as headlands, the whole relief being aligned along the north-east-south-west strike. On the west side, however, erosion has been along joints and minor faults, and erosion features are parallel to the direction of dip. This tends to mask the fact that the general trend of the west coast does not coincide with the strike of the rocks but is about parallel to the trend of the Great Glen Fault extension as suggested by Dr. Flinn.
Fig. 1. Old Bed Sandstone dipping to the south-east at a high angle on the east coast of Fair Isle, Shetland
Lastly, there is no large-scale shattering of the rocks in Fair Isle. This is also to be expected if the Great Glen Fault occupies the position suggested by Dr. Flinn, for in the Sell Voe area of Shetland there is shattering for distances of more than a mile from the fault, while Fair Isle lies about four miles east of the supposed position of the fault. The fault zone and the zone of shattering have been removed, as they are being rapidly removed at the present day at Seli Voe, Aith Voe and Brae.
Thus, the increase of the dip towards the west coast of Fair Isle, the trend of the west coast-line and the absence of large-scale shattering are three pieces of geological evidence which accord with Dr. Flinn's hypothesis.Flinn, , D., Nature, 191, 589 (1961).ISIGeological Survey, Geological Map of Scotland and North England. (Scale, 10 miles to the inch.) (London : H.M.S.O., 1948.)Wilson, , G. V., Summary of Prog., Geol. Surv. Great Britain forx 1933, Pt. 1, p. 73 (1934).Wilson, , G. V., et al., The Geology of the Orkneys (H.M.S.O., Edinburgh, 1935).