Geology Group diary (17)

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    The Geology Group met at Merlin's Bridge Village Hall at 10.30am on Wednesday 8 February 2017. There was an excellent turn out of 18 members. This month's topic was:
    Edinburgh has long been seen as the cradle of Geology which developed as a scientific discipline during the Scottish Enlightenment in the late 18th century. James Hutton (1726-1797) was a physician and natural philosopher who spent much of his life studying the rocks around Edinburgh and formulating theories as to their origin.  Bishop Ussher (1581-1656) using biblical data had calculated that the earth formed in 4004 BC, but Hutton was convinced that the earth was millions of years old with “no vestige of a beginning and no prospect of an end”. We now know that the earth is some 4.5 billion years old. Hutton argued that the uplift, erosion and deposition took enormous lengths of time and that present day physical processes were the key to understanding the past. In his book Theory of the Earth Hutton put forward this idea of uniformitarianism; that the earth’s surface had been changed immeasurably slowly as opposed to catastrophism or violent upheaval. He also considered that the earth’s interior was hot and that molten rock was erupted at the surface which he explained with reference to Salisbury Crags in Edinburgh. This idea brought him into conflict with the current Neptunist view that all rocks had been precipitated out of the Great Flood.
    Charles Lyell (1797-1875) was a Scottish geologist who popularised Hutton’s ideas in his 3 volume Principles of Geology which in turn inspired Darwin on his voyage in the Beagle. The frontispiece of Lyell’s book showed a drawing of the Temple of Serapis in Pozzuoli in Southern Italy. Three columns of this Roman ruin still stand and a line of mollusc borings can be seen about a metre above the base of the columns. The evidence suggests that the building had slowly sunk beneath the sea and later been uplifted which Lyell used as support for the concept of uniformitarianism.  Lyell also produced numerous scientifical papers on volcanoes, earthquakes and stratigraphy and enthusiastically advocated Hutton’s dictum that ‘the present is the key to the past’.
    Igneous rocks in Edinburgh. During the early Carboniferous Period (354-323 Ma) considerable volcanic activity occurred in the Central Lowlands of Scotland and in Edinburgh where Arthur’s Seat Volcano erupted explosively. Today we can see the agglomerate (broken pieces of lava and ash) in Queen’s Drive (Holyrood Park) which blocked the volcanic vent. Some of the basalt lavas that formed the volcanic cone can be seen on Whinny Hill to the east of the city. Castle Rock on which Edinburgh Castle stands, is the site of a parasitic vent. Salisbury Crags are formed of a massive dolerite sill intruded into the west side of the volcano. At Hutton’s Section (SSSI) you can see the contact between the dolerite and the underlying Carboniferous sandstone which has been ripped up and baked by the intrusion. Bass Rock in the Firth of Forth is an eroded volcanic plug marking the site of another Carboniferous vent.
    Hutton’s Unconformity, Siccar Point In 1788 Hutton visited Siccar Point on the Berwickshire coast, 40 miles from Edinburgh near to the village of Cockburnspath (on the A1 south of Dunbar). Here he discovered two totally different types of sedimentary rock in contact with each other. The underlying Silurian greywacke was steeply dipping and its surface was eroded. Resting on this surface was a horizontal basal conglomerate of Old Red Sandstone. Hutton inferred from the sharp junction between the strata that an enormous time interval was required for the underlying rocks to have been uplifted, folded and eroded before the overlying conglomerates and sandstones were laid down. The fundamental geological principle of ‘deep time’ was thus established and it would later be shown that a time gap of some 65 million years was represented by the Siccar Point unconformity.
    Oil Shale in West Lothian Oil production began in Scotland in 1851 when James Young opened a refinery near Bathgate where he processed local cannel coal by heating it in a large retort to extract oil which could then be further refined into paraffin, lubricating oil and motor spirit (petrol). At the time whale oil was used for oil lamps but this was soon replaced by cleaner paraffin lamps. Factories also needed oil to lubricate machines and by the turn of the century motor spirit was being used to fuel the internal combustion engine. Supplies of local coal were soon used up and so Young then turned to the oil rich shales that were plentiful in West Lothian coalfield. Today the legacy of the oil shale industry can be seen in the huge tips or’bings’ of burnt shale that litter the landscape. About 8 tonnes of shale were required to produce 10 barrels of oil. The slow decline of the industry began in 1924 when a refinery was opened at Grangemouth on the Firth of Forth that was designed to process crude oil imported from the Persian Gulf. Refined oil could then be produced at a fraction of the cost of Scottish shale oil.
    What is Oil Shale? Cannel coal or candle coal is a hard bituminous coal that burns with a bright flame and was used for domestic fires in the 19th C. It was formed in coal swamps on Carboniferous deltas from plant remains (tree ferns, horsetails, club mosses). Oil shale is also highly bituminous but is formed in a lagoonal environment at an earlier stage in the cycle of sedimentation or cyclothem during Upper Carboniferous times (311-290 Ma). However, repeated marine submergence and terrestrial emergence ensured that repeated cyclothems containing coal seams and oil shale bands were formed.
    Forth Bridge. The famous cantilever railway bridge was opened in 1890. The site at Queensferry was where the Firth of Forth narrows and where there were firm foundations for the bridge piers. A massive dolerite intrusion extends from North Queensferry to Inchgarvie and West Lothian oil shales outcrop on the south side. In 1964 a suspension bridge was opened for road traffic and a new Queensferry bridge is due to open in 2017.

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