watt's thesis

 

 

 

 

 

 

 

 

Back Sand, Ollaberry

 

Walls Boundary Fault

Significance: The Walls Boundary Fault has a long and complex history and is likely to have been linked to the Great Glen Fault, the major transcurrent fault that cuts Scotland. This fault is likely to been active at various times from the Devonian (170km sinistral movement) through until the Jurassic (65km dextral movement).

            The Walls Boundary Fault (and its splay, the Nesting Fault) lie within broad crush zones of shattered rocks that include both mylonite and fault gouge. Mylonitization is likely to have occurred during Devonian/Carboniferous movements and the fault gouge during Triassic/Jurassic movements. Since mylonite is formed at a much deeper level within the Earth’s crust than fault gouge this implies a considerable amount of erosion during this period to lift the fault to a relatively higher level in the crust.

Locality 1. Back of Ollaberry, Otter Hadd to Back Sand. HU 371 805 to HU 372 810. Exposure of the Walls Boundary Fault.

A shallow depression with a boggy floor is seen to run north cutting across the headland; this marks the line of the fault.  At Back Sand a high vertical cliff of dark red material, which extends north for about 100m, dominates the eastern side of the bay. The dark red 'granitic' material, which appears to form the cliff face, is in fact a covering of fault gouge about 0.5m thick. Analysis of this material has shown that it is a hematite-stained isotropic paste containing fragments of analcite and quartz.

            The eastern side of the bay forms a broad zone of cataclasis and folding within metamorphic rocks (semipelitic granulites and mica-schists). Folding of these rocks here is chaotic varying in scale from centimetre to several meters. Intense shearing has ground down rock so in places it now forms a clay layer between bands of schist.

Locality 2.  Clothister Quarry. HU 342 728.  Clothister Magnetite Mine.

            The metamorphic rocks between the Northmaven Complex and the WBF are a highly dislocated series of quartzites, garnetiferous semipelitic schists and green schists associated with siliceous limestones.

At Clothister Hill there is a bed of graphite-schist associated with an epigenetic deposit of magnetite. The orebody consists of very pure massive magnetite (iron content 60-70%) with an exceptionally low phosphorus content (0.5%) enclosed in a skarn consisting of garnet, hornblende, pyroxene and epidote.

The Geological Survey discovered the orebody in 1933 and the first adit mine was opened by the Scottish Home Department during investigations between 1941 and 1943 with a view to exploit the deposit. This lenticular deposit was found to be about 53m long and 3m wide with a north-south trend and a steep westerly dip. The estimated volume of the orebody was 4000 cubic metres giving a possible 20,000 tons of ore. Between 1953 and 1957 Deering Shetland Mining Ltd., a subsidiary of Deering Products Ltd of Canada, extracted ore by adit and opencast methods. The ore was for use in the manufacture of heavy mud for a coal flotation process. Between 6,000 and 10,000 tons of ore was obtained at an extraction rate of 300-400 tons of crushed rock per month.

Locality 3. Scord Viewpoint. HU 412 317. Car park.

            The rocks of the Clift Hills Division make up the long ridge of hills, part of the ‘spine’ of Shetland running from Laxfirth, north of Lerwick, to Scousburgh and Fitful Head near Sumburgh.

            The large quarry above Scalloway is Shetland’s major roadstone quarry and shows colour variations in the Clift Hills Phyllite. Note the near vertical dip of the rocks in the quarry. These rocks (like the rest of the East Mainland Succession), were once horizontal beds of sediment that have been metamorphosed and folded. With the exception of the granitic intrusions the rocks within the Succession get older (i.e. stratigraphically lower) towards the west. The whole succession is probably part of the limb of a fold on a scale greater than Shetland.

            The thin band of the Asta Spilite follows a NNE-SSW trend through the valley floor and is the base of the Clift Hills Division. This is succeeded to the west by the Laxfirth Limestone at the top of the Whiteness Division that forms the west shore of the East Voe of Scalloway and most of the valley floor and is cut by the Nesting Fault.

Scalloway lies at the end of the Tingwall Valley that follows the plane of the Nesting Fault (a splay of the Walls Boundary Fault and part of the Great Glen Fault system). There is about 16km dextral displacement along the fault, i.e. the rocks on which Scalloway stands were once relatively 16km to the SSW. To the west of the fault a ridge of psammite runs through the middle of the village followed in turn by another band of limestone, the Girlsta Limestone.

            The hill beyond Scalloway, the Hill of Berry, is a sheet of granite formed during the period of main regional metamorphism (~500 Ma) and intruded and gneissified Lower Dalradian psammite which forms the hillside above the village.