Geology Group Diary (10)

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    The Geology Group met on Wednesday 8 June at 10.30am at Merlin's Bridge Village Hall.
    Here are the notes for the session.

    The West Riding of Yorkshire includes most of the Central Pennines that are dissected by several rivers which are tributary to the River Ouse. These rivers flow through and form the Yorkshire Dales from Swaledale in the north; Wensleydale; Nidderdale; Wharfedale; Airedale to Calderdale in the south. For the most part the rocks are Carboniferous Limestone, Yoredale beds and Millstone Grit and they form a faulted tilted structure known as the Askrigg Block formed of Lower Palaeozoic basement rocks. The block dips gently north eastwards but the higher western side is marked by steep scarps along the line of the Dent Fault and the South Craven Fault. The maximum upthrow along the Craven faults is about 1600 metres. Giggleswick Scar (NW of Settle) is a good example of a fault scarp along the line of the South Craven Fault. The younger Namurian gritstones of the Craven lowlands lie on the downthrow side of the fault whilst the older Carboniferous limestone forms the upthrow side of the fault.
    The Lower Palaeozoic inliers are small areas of basement rocks (Ordovician and Silurian) that have been exposed by erosion along the north side of the North Craven Fault. The best example is the Ingleton inlier in the valley of the River Twiss where the river cascades over Thornton Force. Here the Carboniferous Limestone (363-325 Ma) rests unconformably on the highly inclined Ingletonian slates (Ordovician, Arenig 493-476 Ma), representing an age gap of at least 110 Ma. The slates originated as Ordovician mudstones that were uplifted, folded and metamorphosed by the Caledonian orogeny, then eroded to an undulating surface over which the early Carboniferous seas deposited a basal conglomerate followed by thick bedded limestones. The Crummack Dale inlier is best known as the source of the Norber erratics. These huge boulders of Silurian gritstone have been plucked by ice from the head of Crummock Dale and deposited on the surrounding limestone surface. The height of the limestone pedestal below the boulder shows the amount of solution weathering that has taken place since the Ice Age.
    Malham Tarn also rests on impervious Silurian slates, but notice that its outfall disappears underground when it reaches the limestone at the North Craven Fault. Malham Beck issues as a Vauclusian Spring at the base of Malham Cove, a natural amphitheatre with a well developed limestone pavement above it. The clints (blocks) are cut by rectilinear grykes (fissures) that form as solution weathering widens the joints in the horizontally bedded limestone.
    Cawden Hill near Malham village is one of several reef knolls that lie along the southern margin of the Mid Craven Fault. They are formed of calcite mudstones with some shelly limestones rich in corals, crinoids and brachiopods. These reef knolls clearly formed  on the margins of the Lower Carboniferous seas where the water was shallow above the Askrigg Block.
    Another spectacular karstic feature is Gaping Ghyll, a large swallow hole that takes the water of Fell Beck off the slopes of Ingleborough. This peak is capped by impervious Millstone Grit and Yoredale beds but as the water drains off on to the underlying limestone it disappears down numerous sink holes (swallow holes) including Gaping Ghyll which has the highest underground waterfall in Britain (98 metres).
    The strata in the Yorkshire Dales are gently dipping to the northeast so the Great Scar Limestone is well exposed in the Craven District but farther north the overlying Yoredale beds are more common. Yoredale is an older name for Wensleydale where alternating thin limestones, shales and flagstones occur in rhythmic succession. Such cycles of sedimentation reflect the fluctuating sea levels above the Askrigg Block towards the end of Lower Carboniferous times. Differential erosion of the strata on the valley sides has produced a stepped topography with the limestone forming prominent terraces. Also where the rivers cross the harder rocks there are often spectacular waterfalls. Hardraw Force can be visited through the Green Dragon Inn in Wensleydale and lower downstream on the River Ure are a series of rapids known as Aysgarth Falls.
    Swaledale was a major lead-zinc mining area reaching peak production in the late 18th C. The most important mineralised zone occurred on the north side of Swaledale, particularly around Gunnerside Gill. The veins run approximately east-west along pre existing faults in which hydrothermal minerals were precipitated. Gangue minerals such barytes and fluorite also occur in the veins and some old mines have been reworked in recent years in order to obtain these minerals. The source of the hydrothermal fluids is considered to be a large granite intrusion deep below the Askrigg Block and the rising granite also provided the necessary buoyancy to elevate the block above the surrounding lowlands that occupy ‘basin’ structures where great thicknesses of sediment accumulated in Carboniferous times in contrast to the relatively thin cover over the Askrigg Block.

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