Geology Group Diary (09)

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    Anonymous

    The Geology Group met on Wednesday 11 May 2016at Merlin's Bridge Village Hall.

    The topic for this month was

    GEOLOGY AND SCENERY IN THE NORTH WEST HIGHLANDS OF SCOTLAND

    This region of Scotland has some of the most spectacular geology and scenery in the whole of the British Isles. It also contains some of Britain’s oldest rocks; the Lewisian gneisses that have been radiometrically dated to around 3300 Ma.  This compares with 4000 Ma for the oldest Precambrian rocks in the Canadian Shield. Two geological surveyors, Ben Peach and John Horne, conducted detailed mapping in the NW Highlands in the late 19th century that first revealed the large scale thrust structure of the rocks that were emplaced on the Precambrian foreland.

    Lewisian Gneiss.
    The highly deformed and metamorphosed rocks are exposed along the coast from the SE corner of Skye to Cape Wrath and in the Outer Hebridean islands of Lewis and Harris. It is considered that these rocks were originally a sequence of magmatic intrusions and that sometime after 3000 Ma they were subjected to intense heat and pressure and converted into coarsely banded gneiss rich in quartz, feldspar and mica. Later around 2200 Ma the rocks were intruded by a series of mafic dykes that are particularly well exposed near the village of Sourie. About 2 kms north of Laxford Bridge on the A838 there are road cuttings where the Lewisian gneiss is cut by sheets of pink pegmatite that pinch and swell indicating that the gneiss was hot and semi molten at the time of the intrusion.

    Torridonian Sandstone
    The gneisses were uplifted and folded over millions of years and by 1000 Ma a deeply eroded Lewisian landscape had emerged. Then in the late Precambrian the Torridonian sandstone was deposited, resting with a marked unconformity on the eroded Lewisian rocks. The coarse grained red aeolian sandstones are horizontally bedded and have gradually been stripped off  much of the Lewisian basement leaving an exhumed topography with isolated Torridonian mountains such as  Canisp, Suilven, Quinag and Stac Pollaidh The ice scoured surface of the Lewisian gneiss in coastal Sutherland produces a ‘knock and lochan’ topography (Cnoc is gaelic for small hill) where numerous small lakes occupy glacial hollows that are interspersed with ice moulded hillocks and erratics of Torridonian sandstone.

    Cambrian Sediments
    Around 544 Ma the region was inundated by the Cambrian sea which laid down the white Eriboll Quartzite unconformably on the eroded Torridonian sandstone surface. The quartzite can be seen in the road cutting at Skiag Bridge where there is also the distinctive ‘pipe rock’ Here the beds are full of vertical tubes made by burrowing worms (Skolithus), one of the earliest forms of life in the Cambrian. The Durness Limestone overlies the quartzites. It is interesting to note that the trilobite Olenellus lapworthi occurs in the sands and siltstones above the Pipe Rock and this fossil has affinities with North American trilobites. The plate tectonic evidence suggests that NW Scotland was part of Laurentia (NE North America) in Cambrian times and was separated from the rest of Britain by the Iapetus Ocean.

    The Moine Thrust Zone
    The coastal region of NW Scotland is separated geologically from the rest of the Highlands by a zone of low angle faults known as the Moine Thrust Zone which was formed during the Caledonian orogeny in late Ordovician times. However, in the 19th C geologists were not aware of this structure and so the rock sequence caused major problems because it appeared that the unaltered Cambrian sediments passed conformably upwards into high grade schists.  The Law of Superposition indicates that the schists must be the youngest rocks in the sequence, but it was hard to explain how unaltered Cambrian sandstones and limestones could be overlain by highly metamorphosed schists. There was much controversy as to how this sequence could have formed. In 1883 Archibald Geikie, Director of the Geological Survey sent two experienced surveyors (on horseback) to this wild remote area of Scotland to carefully map the rock outcrops. As a result Peach was able to demonstrate that the eastern Moine schists (late Precambrian in age) had been thrust westwards over the rigid foreland block of Lewisian gneiss, Torridonian sandstone and Cambrian sediments by a series of large scale low angle thrust faults.(Moine, Glencoul and Sole thrusts). The thrust planes are separated by smaller imbricate faults and duplex structures. Thus we see how the older schists were moved many miles by major thrusts and placed on top of younger Cambrian rocks! A visit to the nature reserve at Knockan Cliff shows the position of the Moine Thrust which is marked there by a fine grained flinty rock called mylonite, produced by rock grinding along the thrust plane.

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