Geology Group Diary (16)

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    The Geology Group met at Merlin's Bridge Village Hall on Wednesday 11 January 2017 at 10.30 am. The topic was 'The GEOLOGY OF THE MENDIPS AND SURROUNDING AREA'.
    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 River Mells flows through Vallis Vale about 1.5 kms west of Frome. Here the steeply bedded Carboniferous Limestone is overlain by horizontal beds of Inferior Oolite (Middle Jurassic). This is a classic unconformity that was first described by Henry De la Beche (1796-1855) the first director of the Geological Survey. Here the limestone erosion surface shows worm borings and bivalves and immediately above is the basal conglomerate of the Inferior Oolite marking a time gap of some 150 Ma.
    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.
    Portishead Point is formed of Carboniferous Limestone overlain unconformably by the Dolomitic conglomerate, whilst the coastal section along Kilkenny Bay shows the Dolomitic conglomerate resting directly on the Old Red Sandstone. The structural trend of the Mendips  is clearly demonstrated in the Weston super mare area where the three headlands of Brean Down, Worlebury Hill and Middle Hope are all formed of Carboniferous Limestone. At Swallow Cliff on the north side of the Middle Hope headland the limestone contains a volcanic horizon formed of a basal tuff overlain by some 10 metres of basaltic pillow lavas. Thus it appears that during Lower Carboniferous times whilst the limestones were being laid down in shallow shelf seas fringed with coral reefs, some volcanoes were erupting submarine basalt lavas.
    Aust Cliff is where the first Severn Bridge was built in the 1960s. The cliff itself is formed of Red Triassic marls at the base overlain by Rhaetic shales and thin bedded Liassic limestones, but these relatively soft rocks rest on a foundation of Carboniferous limestone. This extends out beneath the River Severn and the first pier of the bridge is built on the solid Ulverstone Rock. Unfortunately Carboniferous mudstones covered by Triassic sandstones outcrop on the Beachley side of the river and so the second pier had to be sunk 15 metres down into the mudstones to secure a firm foundation.

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