Geology Group Diary (31)

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    Anonymous

    The Geology Group met at Merlin's Bridge Village Hall at 10.30 am on Wednesday 9 May 2018. The topic for the meeting was
    BRITISH LOWER CARBONIFEROUS ROCKS.
    By the end of Devonian times the Caledonian mountains had been eroded down to lowlands and the Old Red Sandstone continent was inundated by the shallow shelf seas of the Rheic Ocean that separated Avalonia from Gondwana. This ocean was closing during the Carboniferous and continental collision would produce the Variscan Fold mountains extending from the Appalachians through the Pyrenees and the Atlas to the Urals.
    During the Lower Carboniferous Period (363-325 Ma) Britain lay in the equatorial region and so experienced hot wet tropical conditions suitable for the growth of coral reefs on the margins of the seas in which carbonates were deposited on offshore ramps. St George’s Land  formed a land barrier stretching from Wales across to Belgium; whilst marine limestones were laid down in the Mendips and South Wales, across much of Ireland  and in the Pennines to the north. The shallow shelf seas were rich in marine life particularly corals such as Caninia (single coral) and Lithostrotion (colonial coral) and numerous brachiopods including Productus and Spirifer. Crinoids also grew in profusion. In North Yorkshire, Northumberland and the Midland Valley of Scotland deltaic sediments were deposited from rivers flowing off the northern landmass.
    South West England.
    The Mendip Hills lie to the south of Bristol and extend WNW-ESE from Weston super Mare to Frome in Somerset. This axial trend reflects the influence of the Variscan earth movements that produced a major asymmetrical anticlinal structure plunging eastwards. Since the main thrust came from the south as the Rheic Ocean closed, the dips on the north side of the Mendips are much steeper than those on the south. Since the oldest rocks lie in the core of an anticline we find the Upper Old Red Sandstone in the centre of the Mendips where it forms the upland known as Black Down. The Carboniferous Limestone has been eroded over the centre of the anticline and now outcrops on the flanks of the Mendips. The northern outcrop is well seen in Burrington Combe which is one of several valleys cut into the limestone. However, during Triassic times flash floods would have dumped vast quantities of limestone debris and sand into these valleys. Today erosional remnants of the Dolomitic Conglomerate (which in fact is a breccia) can be seen resting unconformably on the sides of Burrington Combe. Thus much of the Mendips have been exhumed from beneath a Triassic cover.  The steeply dipping limestone beds are well seen at the Rock of Ages which is where the Rev.Montague Augustus Toplady is said to have sheltered in a cleft during a storm in 1763. According to the apocryphal story he received divine inspiration after which he wrote the classic geological hymn ‘Rock of Ages’! Aveline’s Hole is a fine example of a swallow hole that follows the inclined bedding plane and leads down to two large chambers where the remains of a Mesolithic cemetery have been found.
    Cheddar Gorge on the south side of the Mendips was cut by meltwater during cold periglacial phases of the Pleistocene Ice Age when permafrost would render the limestone impermeable. In warmer interglacial times the waters would descend into underground passages and caves leaving the gorge dry, as it is today. The caves contain the remains of animals that lived in Devensian times and the bones of ‘Cheddar man’ in Gough’s Cave have been dated to around 7150 BC.
    Ebbor Gorge lies about 8 kms to the south west of Cheddar where the Ebbor Thrust has forced the Carboniferous Limestone over Namurian quartzite that is exposed by a stream near the entrance to the gorge. The thrust zone extends along the SW margin of the Mendips and demonstrates the powerful effect of the Variscan earth movements. Wookey Hole near Wells is a magnificent swallow hole leading to some 25 underground caverns through which flows the River Axe. Human and animal remains indicate that the caves have been used since Palaeolithic times. Wookey Hole is a SSSI but like Cheddar, it is highly commercialised and a tourist’ honey pot’.
    The Avon Gorge which is spanned by the Clifton Suspension Bridge, cuts through a complete sequence of Lower Carboniferous rocks. In 1905 Vaughan pioneered the use of an assemblage of brachiopods and corals to create fossil zones K,Z,C,S & D to subdivide the strata [Cleistopora, Zaphrentis, Caninia, Seminula,& Dibunophyllum]. Today these zones have been superceeded by a classification based on 6 cycles of marine sedimentation during the Lower Carboniferous.
    The Pennines
    Whilst there is a conformable transition from the ORS into the Lower Carboniferous strata to the south of St George’s Land; to the north there is a marked unconformity between the Carboniferous and older rocks. This suggests that whilst the sea advanced steadily in the south providing an unbroken record of sedimentation; in the north uplift of the basement blocks (Askrigg and Alston blocks) meant that the sea did not cover them until midway through the Lower Carboniferous. Only in the basin areas between the blocks was there continuous cyclic sedimentation of Yoredale mudstones, sandstones and thin limestones.
    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, and Yoredale beds 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.
    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.          John Downes 

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