East Mendips-Vallis Vale

1 East Mendips Objectives The East Mendips shows all the key stages in the geological evolution of the Bristol and Somerset area, with Palaeozoic rocks (Silurian, Devonian, CarBoniferous) folded and uplifted, and then forming palaeo-islands in the Triassic, and overlain unconformably by Triassic and Jurassic sediments. The trip focuses on sites around Frome, and especially Vallis Vale, which shows a broad range of geology, including the classic ‘De la Beche Unconformity’ between the CarBoniferous and Jurassic, evidence for sea level rise and flooding of the Mendip Island in the Late Triassic and Jurassic, as well as extensive evidence of former quarrying and mining and the associated industrial infrastructure. Risk analysis Vallis Vale is a public parkland area with regularly used footpaths, and so generally offers no further risk beyond the usual issues of muddy slopes and brambles; most of the Vallis Vale sites are accessible to most people, including those in wheelchairs, by gravel tracks on the level. Moon’s Hill Quarry houses the Somerset Earth Science Centre, and both are open to the public and accessible. Tedbury Camp is also in public parkland, but it can be reached only by a rocky and steep climB, which requires some agility. The Holwell neptunian dyke can Be viewed from the puBlic highway, but Whatley Quarry is not open to the puBlic and is viewed from a puBlic footpath that requires some walking ability. Maps Ordnance Survey 1:50 000 Landranger Sheet 183 (Yeovil & Frome) 1:25 000 Explorer Sheet 142 (Shepton Mallet & Mendip Hills East) Geological Survey 1:50 000 Sheet 281 (Frome) Main references Farrant (2008); Ronan et al. (2020); Locations This trip involves four stops (Fig. 1), at Moon’s Hill Quarry (including the Somerset Earth Centre), Holwell, Vallis Vale and Tedford Camp (including Whatley Quarry). These can Be reached by public transport, with buses going past the first two, but driving is easier if all sites are to be visited. It is equally satisfactory to visit just Vallis Vale, Whatley Quarry and Tedbury 2 Camp as the core portion of the trip, using public transport. Take the train or bus to Frome centre. From Frome station, either walk to the Hapsford Bridge (ST 761496) or Egford (ST 757484) end of the Vallis Vale site, following either the A382 west and north through the centre of Frome, or footpaths via Packsaddle and Spring Gardens to Hapsford, and Somerset Road and Egford Hill to Egford. It is possible to park in a layby on the east side of the road at Hapsford Bridge or in the small car park just north of Elm Lane at Egford. Vallis Vale is a Y-shaped site, comprising steep-sided valleys of the east-west-running Mells Stream and the Egford Brook which joins it from the south. There are level walks on the gravel trackBeds of old mineral railways around the Vallis Vale site. It is easy to walk from Vallis Vale to Tedbury Camp along the side of the Mells Stream, a part of the East Mendip Way. The footpath starts on the south side at Hapsford Bridge, crosses the Mells Steam by an old railway bridge, runs along the north side past the De la Beche unconformity, and then crosses the stream to the south side, across the currently operational mineral railway to Whatley Quarry, past Great Elm village. Close to the old bridge across the river are the old iron works in Great Elm, as well as the entrance to the grounds leading to Tedbury Camp. Outline Geology The Mendip Hills form a roughly east-west series of round-topped hills, sometimes described as showing a ‘whaleback’ appearance. These comprise four periclines, somewhat dome-like anticlines, located at Blackdown, North Hill, Pen Hill, and Beacon Hill, from west to east. These were folded up during the Variscan (Hercynian) Orogeny in the Late CarBoniferous and Early Permian, when the southern supercontinent Gondwana was moving north against the northern supercontinent Laurasia. The collision of these supercontinents forced up a substantial mountain range stretching for thousands of kilometres from the eastern United States, across southern Ireland and England and through central Europe. The tectonic consequences were numerous upfolded structures, of which the Mendip Hills are the most substantial feature in the area, as well as thrust faults, where slabs of pre-existing rock were pushed northwards over older rock slaBs Below. In all cases, compression was in a roughly south-north direction, and so the fold axes are oriented east-west, and thrusts go from south to north. In the East Mendips, the Palaeozoic succession consists of Silurian-aged sediments and andesite lavas, the Devonian-aged Portishead Formation, and successions of Carboniferous limestones overlain by sandstones and coals. The periclines may have uplifted these older rocks to as much as 1500 m, but they were then eroded fast in the Permian and Triassic, exposing the older Silurian and Devonian rocks in the cores of the folds, surrounded to north and south by 3 steeply dipping Carboniferous rocks, which are generally mirrored to north and south of the Mendip Hills. The Silurian volcanic series comprises 350–640 m of andesites, tuffs and agglomerates overlying Wenlock-aged sediments, and in turn overlain by the Devonian-aged Portishead Formation (Green 2008). These are particularly well seen in Moon’s Hill Quarry which exploits the thickest part of the succession in the core of the Beacon Hill Pericline. The volcanic series thickens to the east, and the andesites were derived from volcanoes located to the east or northeast of Frome that were active during late stages of closure of the Iapetus Ocean, part of the Caledonian Orogeny, when the microcontinents Laurentia (mainly North America plus Scotland), Baltica (mainly the Baltic area plus Russia), and Avalonia (with the southern parts of the UK) fused. The Iapetus suture zone, marking the zone of fusion between Laurentia to the north and Avalonia to the south, runs through the Southern Uplands of Scotland and Northern Ireland, and further south there was an island arc running through the Lake District and the Irish Sea, and another around the Bristol Channel and Welsh Borders. As volcanism proceeded in phases, the andesites are the erupted lavas, the tuffs are ashes that are here reworked by rivers, and indicate pauses in eruption and exposure of the surface as land, with water flow eroding and reworking some deposits. The agglomerates comprise volcanic bombs, lumps of lava hurled through the air, as well as ash and broken fragments of lava, representing further evidence for pauses in eruption and exposure of the land surface. Towards the end of the Caledonian Orogeny, the area remained an upland surface, and was further folded in the Middle Devonian, and the Upper Silurian to Middle Devonian is aBsent. In places, the Silurian volcanic series is overlain by the Upper Devonian Portishead Formation, deposited in rivers. Sea levels then rose in the Carboniferous, inundating the area and depositing the Lower Carboniferous limestones, and in places then Upper CarBoniferous coal- bearing sediments. These Palaeozoic sediments were then folded and uplifted in the Late CarBoniferous and Early Permian Variscan Orogeny, and then eroded and weathered subaerially in the Triassic. For much of the Triassic, fine-grained mudstones of the Mercia Mudstone Group covered lower-lying areas around the Severn Estuary, as seen at Aust Cliff (see pages XXX), but in higher areas south of Bristol and especially in the Mendips, there was little deposition of Triassic red beds and the areas were presumably eroding uplands at the time, with minimal deposition. The Holwell, Vallis Vale and Tedbury Camp sites provide detailed information on the Rhaetian to Middle Jurassic transgression of south-west England (Fig. 1). Much of England was flooded by an advancing ocean, initially the Rhaetian Transgression, beginning 205.7 Ma, and followed by continuing inundation through the Early and Middle Jurassic. The Rhaetian transgression was proBaBly triggered by the break-up of Pangaea, marked by emplacement of 4 the Central Atlantic Magmatic Province and major rifting on the Afro-European and North American sides (Wall and Jenkyns, 2004), and it flooded across Europe from the Tethys Ocean to the south. In the Mendips-Bristol-South Wales area, the folded Carboniferousn uplamds formed numerous palaeoislands, of which the Mendips Island was the largest (Fig. 2). The inundation and accumulation of latest Triassic and Jurassic sediments were accompanied by extensive faulting along basin margins and fissuring of karstic uplands which became infilled by palaeosols and other terrestrial sediments, sometimes associated with small vertebrate fossils (Wall and Jenkyns, 2004; Whiteside et al., 2016; Lovegrove et al., 2021). The rising seas separated upland areas as distinct palaeoislands, of which the long, east-west- extending Mendip Island was one. Near the coastlines of these palaeoislands, bedded marine Rhaetian and Jurassic sediments accumulated in horizontal layers. In some places, contemporary faulting meant that sediment was sucked into the faults, forming neptunian dykes, sometimes containing marine fossils within terrestrial areas, such as at Holwell. In other areas, such as at Hapsford Bridge and Tedbury Camp, the underlying Carboniferous surfaces were planed flat By erosion and Became hardgrounds, Bearing aBundant Borings made into the limestones, as well as sometimes becoming covered with encrusting oysters, and then also, as sediment accumulated on these hardgrounds, mixtures of marine and terrestrial fossils, as well as bored hardground pebbles torn up by storm activity. Here, we see a Rhaetian-aged and a Middle Jurassic hardground, dating to about 205 and 167 Ma respectively, showing how the marine inundation of the Mendip Island proceeded to flood the island from east to west as sea levels rose in steplike increments.

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