The Edinburgh Geologist
Issue no 23
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The Edinburgh Geologist | | ||||||||
by David Gould
The advance guard of the party left Edinburgh for Aberdeen on Friday 20 May, under the command of lan Hogarth in a hired minibus, fully laden as usual with provisions, and travelled overnight to Lerwick on the St. Clair, while the majority of the party flew by Loganair from Edinburgh to Tingwall on the 21st, and a second minibus was hired in Shetland. Base was set up at Wester Houll Chalets, magnificently sited overlooking Scalloway beside a large wind generator, which has since suffered severe damage in the winter storms. Shortly after gaining entry to the chalets and off-loading the provisions and luggage, the airborne party joined the seafarers who had been looking at the exhibits, including some very good mineral specimens, in the Lerwick museum. The remainder of the Saturday was spent examining the Old Red Sandstone exposures near Lerwick. The base of the succession consists of the Rova Head Conglomerate containing rounded clasts of granite, not presently exposed in the immediate vicinity, and Dalradian metasediments. Exposures just south of Lerwick town show cross-bedded sandstones cut by irregular calcite veins and folded into sharp monoclines; similar to those on Bressay, where they are associated with volcanic vents. The day was rounded off by a visit to the Broch of Clickhimin, a well-preserved pre-Norse fortified dwelling built on a peninsula jutting into a small loch. 
The afternoon itinerary took us first to the Loch of Spiggie, where highly weathered epidote-rich granite of the Spiggie Complex was seen in faulted contact with strongly folded phyllites of the Clift Hills Division (Upper Dalradian). Then followed another archaeological break at Jarlshof, where successive buildings of Celtic, Viking and Mediaeval farming communities have been excavated and opened to visitors. A brief very cold and windy stop was made at the Slithers, beside Sumburgh lighthouse, in order to examine calcareous layers in the greyish flaggy Old Red Sandstone. This was followed by an excavation programme at the Exnaboe Fish Bed. The location of the Fish Bed was easily discovered from the pile of detritus left behind by previous fossil fishermen, and carbonised fish scales proved quite abundant. On the Monday, the party made its way to Esha Ness, in the far northwest of the Mainland. This proved a good move, as the wind was from the east, and we received sunshine and shelter together. A stop was made at Mavis Grind, where a road cutting exposes wide scapolite veins in diorites of the Northmaven Complex and the Atlantic Ocean comes within 50 metres of the North Sea. We also viewed the Heads of the Grocken, where spectacular cliff scenery is developed in the Ronas Hill Granite. At Esha Ness to the west of the Melby Fault, the local Old Red Sandstone succession is younger than that in south-east Shetland. It consists largely of volcanic rocks, which were examined in a walk from Esha Ness to the Grind of the Navir. Particularly spectacular were a volcanic breccia at Esha Ness itself and a rhyolitic ignimbrite, with flattened and welded lapilli, at the Grind of the Navir. The coastal scenery was testament to the force and erosive power of the sea on exposed coasts. At the Holes of Scraada, for example, the sea has eroded a 500 metre long subterranean passage whose roof has collapsed, while at the Grind of the Navir there is a storm beach Iying 15 metres above sea level and 50 metres inland. The storm beach consists of rectangular blocks of rhyolitic ignimbrite, up to 5 metres in size, plucked from the cliffs; not a place to visit during winter storms. On the way back to Scalloway we visited the disused magnetite mine at Clothister, where specimens of massive magnetite were readily obtainable. The magnetite rock occurs within a fairly narrow band in a group of varied schists of uncertain provenance (the Queyfirth Group). Tuesday's trip to the Walls peninsula enabled the party to see yet another facies of the Old Red Sandstone, this time that lying between the Walls Boundary Fault and the Melby Fault. These rocks are older and more strongly folded than those to the east and west and have been intruded by Caledonian granitic rocks. The Walls Sandstone Formation consists of mainly fine-grained grey sandstones which exhibit graded bedding, ripple marking and large-scale desiccation cracks. Considerable argument was generated among the party as to whether the sedimentary structures indicate deep or shallow water sedimentation, but eventually a consensus was reached that at least some of the sedimentation took place in shallow water. In the afternoon, the party crossed the Melby Fault and attempted to find fossil fish remains in the Melby Fish Bed which is roughly contemporaneous with the Esha Ness volcanics. We had much less success fossil-hunting than at Exnaboe. Finally we travelled across the Melby Fault again to see the metamorphic rocks underlying the Walls Sandstone. The abundance of marble and calcsilicate rocks in the West Burra Firth Group (?Grenvillian) was noted, before rain and time forced a retreat. 
After lunch, beside a sheltered gravel storm beach linking the point of Fethaland to the mainland, we visited the new lighthouse on the point, where we were rewarded with a view of Gruney Island (another Old Red Sandstone outlier) and the far off cliffs of Yell. To the west of the Oueyfirth Group we traversed the Sand Voe Group, which consists mainly of psammites with thin bands of hornblendic rocks, all of which dip at moderate angles to the east. These rocks are regarded as Moine with Lewisian inliers. As we proceeded westwards, the rocks became progressively more sheared, until we reached the Wester Keolka Shear Zone. This is a much more deep-seated and steeper dislocation than the Moine Thrust (which occupies an equivalent position in north-west Scotland), which separates the Sand Voe Group from the sheared orthogneisses of the Wilgi Geos Group (Lewisian). The orthogneisses were at first not very convincing due to the intense shearing, but the last outcrop of the day, on the west coast of the Fethaland peninsula, showed a less sheared and more believable Lewisian gneiss. Thursday saw the party reunited for the long haul to Unst, which necessitated a 7 am start to catch the two ferries. The Dalradian metasediments of the Valla Field Block, which forms the west coast of Unst, were seen at Burra Firth before the party went on a good plowter through the very muddy Queyhouse talc quarry, in sheared and altered ultrabasic rocks of the lowest ophiolite nappe. The boundary of this nappe was seen at the Taing of Norwick, where hornblende schists of the Saxa Vord Block are separated from serpentinites of the ophiolite nappe by a 1-2 metre wide band of talc-schist. We then walked along the shore to try to find outcrops of the Skaw Granite, a foliated rock with 5cm phenocrysts of red microcline, and although we failed, abundant waterworn boulders were seen and a few were collected. After lunch, the return southward was punctuated by a visit to some of the many disused chromite excavations north of Baltasound, where good specimens of chromite and kammererite (a violet chromium-bearing mica-like mineral) were collected, but the chrome garnet uvarovite proved elusive. Lack of time and deteriorating weather meant that attempts to visit the Muness Phyllites and the gabbros and sheeted dykes of the ophiolite were abandoned in favour of a brief sightseeing stop at Muness Castle. Friday's excursion started with a drive along the road from Bixter through Aith to Voe, mostly just east of the Walls Boundary Fault. After noting psammites of the Moine Yell Sound Division cut by monzonites of the Spiggie Complex, the party examined in detail the metamorphic aureole caused by the intrusion of a circular hornblendite/diorite plug into pelites of the Scatsa Division (Lower Dalradian). After a stop at the craft shop in Voe, lunch was taken on the beach at Lunna House (headquarters of the "Shetland Bus" during the Second World War). The spectacular Valayre Gneiss was seen, containing prophyroblasts of red microcline 5-8 cm in size. The gneiss is a marker horizon which lies close to the Moine-Dalradian boundary east of the Walls Boundary Fault. The party then visited Vidlin, where a gossan, with spectacular colours caused by iron oxide minerals, marks the outcrop of a Cu-Zn-Pb-bearing massive sulphide horizon. This provided a good sunbathing location, and the troops proved difficult to rally for the final locality of the day, just off the Vidlin-Lunning road, where large crystals of staurolite, kyanite and sillimanite in Dalradian pelites were enthused over and collected by many. On the Friday evening the party assembled in the Hogarth chalet for a small celebration. Your correspondent was initiated into the mysteries of the Strontian Hammer by being most surprisingly presented with this trophy "at the first attempt and without even trying"! The citation included encouraging the party to reach the last and best outcrop of Lewisian on Fethaland, and research into the effect of penny whistle music on nesting fulmars (from a safe distance!). Saturday's schedule only permitted a brief excursion because the flight from Tingwall was scheduled for lunchtime. One minibus load made the trip to Hamnavoe on West Burra, where the wide spread Spiggie Complex was seen yet again, this time in the guise of a granite with large (5cm) microcline phenocrysts. This made an interesting comparison with the Skaw granite and the Valayre Gneiss. Monzonite and hornblendite of the complex were also seen as well as another storm beach and some seals swimming in the sea. The airborne party managed to leave despite the low cloud and mist only a few miles to the east. The seafarers were able to stay on Shetland till Sunday evening, but their activities were hampered by fog and low-lying cloud for most of the time. In all, the excursion was
very successful and Andrew McMillan performed very well despite his rather sudden
elevation to leadership. Author: David Gould, British Geological Survey, Edinburgh. >> top of page[Return to Edinburgh Geologist index] |
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