Geology Group Diary (03)

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    Anonymous

    Geology Meeting 11 November 2015  Notes on the geology of the Peak District.

    CARBONIFEROUS ROCKS OF THE PEAK DISTRICT

    The Peak District is the name given to the upland region of the Southern Pennines that extends through Derbyshire into North Staffordshire. The region was designated as Britain’s first National Park in 1951.  Geologically the Peak District is formed of an eroded anticline the core of which is made of Carboniferous Limestone. On the flanks of the anticline is the Millstone Grit overlain by Coal Measures.

    THE WHITE PEAK; CARBONIFEROUS LIMESTONE  (Dinantian age 363 – 325 Ma)

    Miller’s Dale and Monsal Dale through which flows the River Wye are classic steep sided wooded valleys cut into the Carboniferous Limestone. At Hobs House near the Monsal Viaduct, the limestone stacks yield a variety of corals including the colonial Lithostrotion and the solitary coral Dibunophyllum.  Brachiopods such as Productids and Spiriferids are also common in the reef limestones around Castleton, providing evidence of the existence of warm shallow seas in Lower Carboniferous times. These limestones have been subject to karstic weathering where deep potholes and caves have developed due to the solution action of rain water percolating through fissures and joints. Winnat’s Pass is an example of a collapsed limestone cave system. The Blue John caverns are also well known for their purple coloured fluorspar crystals. Fluorite is a gangue mineral in lead zinc veins in the Pennines, precipitated from hydrothermal fluids.
    In the southern part of the region the limestone is dissected by south flowing rivers in Dovedale and the Manifold valley where there are numerous features such as caves, potholes and reef knolls. The wooded valleys are cut into the bare upland plateau where surface drainage is minimal and dry valleys are commonplace.
    Volcanic activity during Lower Carboniferous times is recorded by several lava flows in the Wye Valley and also at Calton Hill near Buxton. Here, in what is now a disused quarry, the volcanic vent marks the site of an olivine basalt extrusion which shows excellent columnar jointing. Above the columnar basalt there are several contorted scoriaceous flows which appear to issue from several small vents.  The cindery surface is similar to present day Hawaiian aa lavas and the flows contain pyroclastic material such as blocks of columnar basalt and vesicular lava. The resulting agglomerate may represent fall back debris in the vents.  This SSSI is unique in that the lava contains olivine nodules (xenoliths) derived from peridotite in the upper mantle.

    THE DARK PEAK; MILLSTONE GRIT (Namurian age 325 – 315 Ma)

    The Castleton area shows the contrast between the light grey reef limestones and the dark overlying shales and gritstones of Namurian age. Mam Tor about 2 kms west of Castleton,
    is also known as the ‘Shivering Mountain’ because its south east face has been subject to numerous rotational landslips caused by waterlogged shales underlying thin sandstones. The A625 Sheffield to Chapel en le Frith road used to run along the foot of Mam Tor a section of it was permanently closed in 1979 due to continued slippage.
    Millstone Grit forms much of the high moorland in the Peak District. Kinderscout is a good example of gritstone scenery with roughly horizontal beds of coarse sandstone alternating with shales. The gritstone forms steep scarped ‘edges’ (escarpment) where the strata are exposed around the margins of the Kinderscout plateau. The Pennine Way starts at Edale and can be followed for 268 miles to Kirk Yetholm in the Scottish Borders. From Edale the well worn track leads up Grindsbrook Clough to the southern edge of Kinderscout where the gritstone is covered by thick peat covered moorland. Numerous ‘mushroom’ rocks stand sentinel above the escarpment. They have been carved by wind and rain into varied shapes as the joints have been widened leaving the harder gritstone standing proud.
    The Roaches in North Staffordshire are located in the SE corner of the National Park on the Leek – Buxton road. The structure is synclinal with outward facing gritstone scarps. There is a shallow coalfield within the syncline (Goldsitch Moss). Here the Millstone Grit shows excellent cross stratification, evidence of its deltaic origin when large south flowing rivers brought down vast amounts of sandy sediments during Namurian times.

    UPON THE PENNINE WAY
    John Souster

    Some load themselves like sherpas
    Upon the Pennine Way,
    And bowing under bulging packs
    With downcast eyes and doubled backs
    They trudge along unheedingly,
    Indifferent to the scenery
    Upon the Pennine Way.

    But let me travel lightly
    Upon the Pennine Way.
    I’ll walk alone, with time to see
    The berries on the rowan tree;
    To see the fern beside the fall
    And listen to the curlew call;
    To notice where the glaciers flowed
    And where the clanking cohorts strode
    Upon the Pennine Way.

    Yet each must manage somehow
    To walk his Pennine Way,
    And all will find it rough at times
    And none avoid some weary climbs.
    But when it draws to close of day,
    And when the maps are put away,
    And when we sit and journey back
    In memory along the track,
    Then not the one with speed his boast,
    And not the one who carried most
    Will finish wisest of the host
    Who tread their Pennine Way.

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