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Home , Carboniferous Limestone, Great Elm, Mells River, Mendip District, Mendip Hills, Mendip Way

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East Mendips

Objectives The East Mendips shows all the key stages in the geological evolution of the and area, with Palaeozoic rocks (, , ) folded and uplifted, and then forming palaeo-islands in the Triassic, and overlain unconformably by Triassic and sediments. The trip focuses on sites around , and especially , 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 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 ( & Frome) 1:25 000 Explorer Sheet 142 ( & 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 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 . 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 village. Close to the old bridge across the river are the old 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 , located at Blackdown, North 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 and 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 overlain by sandstones and . 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 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 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 - 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 , 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 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 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. When Buckland and Conybeare (1824, p. 225) visited Vallis Vale, its steep-sided valleys were quiet and secluded, and these authors noted that ‘[b]eautiful sections may be seen in the precipitous sides of these valleys, exhibiting the oolitic strata in an absolutely horizontal position, reposing on the truncated edges of highly inclined strata of mountain .’ De la Beche (1846, p. 287) repeated similar observations when he wrote that Buckland and Conybeare had observed how Vallis Vale and Murdercombe ‘show the inferior oolite in nearly horizontal beds upon the upturned edges of the , with occasionally an interposed portion of a conglomerate referable to the lias, and containing organic remains’. De la Beche (1846, p. 288) illustrated the Carboniferous-Middle Jurassic unconformity (now commonly referred to as the De la Beche Unconformity) in the old quarry opposite the confluence of the Egford Brook and (ST 7557 4918). The first detailed geological accounts were provided by the noted local geologist Charles Moore (1867, 1876), as well as by McMurtrie (1885), in a rather obscure paper. However, McMurtrie (1885, pp. 103–106) was perhaps the first to highlight how the Carboniferous limestones along the banks of the Mells River were overlaid successively by Rhaetian, Lias and 5

Middle Jurassic rocks, corresponding to an inferred steeply rising palaeotopography of the eroded top of the Carboniferous limestone (Fig. 1B). The geology of Vallis Vale was mentioned by numerous geologists in the twentieth century (reviewed by Ronan et al. 2020), and it was detailed in field guides by Reynolds (1912), Savage (1977), Duff et al. (1985, pp. 135–139), and Farrant (2008).

Itinerary We suggest an itinerary that starts at Moon’s Hill Quarry, then stops at Holwell on the Shepton Mallet to Frome road (A361), and then site around Vallis Vale. You can start at Hapsford Bridge and then explore round the Vallis Vale before looping round Whatley Quarry, and then to Great Elm and Tedbury Camp. Visitors can concentrate on the geological story or the industrial archaeology, or a bit of both. If you wish to visit Moon’s Hill Quarry or Whatley Quarry, you must make prior arrangements with the owners (details on their web sites).

Moon’s Hill Quarry The quarry (ST 665460) is 3.42 hectares in area and is designated as a Site of Special Scientific Interest (SSSI) for its excellent exposure of the Silurian-aged andesites. From Shepton Mallet or Frome, drive along the A361 to Cranmore and turn north for ‘Waterlip, ’ at a small crossroads. Continue for 2 km along a road named ‘Tansey’ on the map, then Chelynch Road, and then Long Cross Bottom. Immediately after the ‘Stoke St Michael’ sign, turn right into the Wainwright Quarry Site entrance, and then immediately turn right into Somerset Earth Science Centre car park where cars and coaches can be parked. The Somerset Earth Science Centre was opened in 2009 and offers exhibits and information about the geology of the area, as well as hosting classes and conferences on geological topics. It is funded by the main quarrying companies in the Mendips (Aggregate Industries, Hanson, Morris & Perry Ltd., Tarmac, Wainwright) and has multiple purposes, including general geological education for people of all ages and seeking to recruit young people into the quarrying industry. Moon’s Hill Quarry (Fig. 3A) exposes a 300–400m thick sequence of greenish-grey andesites, rhyodacites, tuffs and agglomerates that are sandwiched between tuffaceous shales and mudstones. These clastic and volcaniclastic sediments have yielded a fairly rich, shelly fauna referrable to the Lower Wenlock Series (Sheinwoodian Stage). This igneous suite is designated as an SSSI because it represents the sole example of Wenlock-aged volcanic rocks (other than bentonites) in England. Further, in the British Isles only the Dingle Peninsula in Ireland shows such excellent exposures of volcanic rocks of Wenlock Age. They are considerably jointed, sheared, and veined by calcite and green epidote. 6

Active quarrying means the appearance of rocks in the quarry changes all the time, but bedding can be seen, marked by tuff horizons, oriented subvertically thanks to the subsequent folding. One tuff horizon immediately below some of the quarry buildings forms a vertical band about 3 m wide in which soft, dark purplish tuff has been sheared, and veined by calcite parallel to the bedding. The andesites are compact, dark purple or sometimes dark green in colour, with prominent dark-green augites. The rock is much shattered and faulted, showing in places strings and patches of epidote. Flow structures are sometimes visible on weathered surfaces. In thin section (Reynolds 1907, p. 231), the andesite groundmass is microlitic, comprising minute feldspar needles that are roughly parallel with each other, following flow directions around the phenocrysts of pyroxene, augite, and feldspar. The pyroxene phenocrysts may be up to 2 mm across, but are generally smaller, and they frequently enclose small patches of epidote and are sometimes intergrown with the augite. The feldspars are sometimes so altered that all sign of twinning and cleavage has disappeared; in other examples they are much fresher and exhibit twinning and low extinction angles, showing they are probably oligoclase-andesines. Epidote is prominent as an alteration product. The tuffs are finer grained and bedded, and they commonly display lamination and cross- bedding, indicating reworking and redeposition by water (Fig. 3B). In places, they are associated with marine fossils such as brachiopods, bivalves, gastropods, crinoids, and trilobites (Reynolds 1907, pp. 226–227). Coarser units include agglomerates and conglomerates, both comprising closely packed larger clasts in an ashy matrix. The agglomerates contain volcanic bombs, rounded, glassy pieces of andesite ranging in length up to 40 cm. Most of these bombs are well rounded thanks to their trajectory through the air as they solidified. The agglomerates show no sign of bedding and large and small blocks are jumbled together irregularly. Conglomerates (Fig. 3C) are full of well-rounded clasts of andesite, as well as quartzite and of micaceous grit or sandstone, but no airfall debris such as volcanic bombs. At the top of the quarry, especially on the north side, the overlying, bedded sandstones of the Upper Devonian Portishead Formation can be seen (Fig. 3D). Return to the Cranmore on the A361 and turn east towards Frome. Drive 7.6 km to Holwell and turn left past the quarry entrance on the left, and park in the village.

Holwell village and quarries There are four main quarries around the tiny village of Holwell, now located on a loop of the old Shepton Mallet to Frome road and bypassed by the re-aligned A361 (Fig. 1). The first (ST 727451), housing the current Aggregate Industries Asphalt quarry buildings, is just north of the 7

A361 at the west end of Holwell village, and termed Quarry 1 by Savage (1977). A further quarry (ST 727453) is located to the north of this one, north of Horn Street, and now largely filled with water, Savage’s (1977) Quarry 2. A third quarry not numbered by Savage (1977) is further north yet (ST 725457). Savage’s (1977) Quarry 3 is south of the A361 (ST 725449), now often called the ‘Microlestes Quarry’ (Whiteside and Duffin 2017, 2020) because it was the location of the original field work by Charles Moore (Fig. 4A) and his discovery of fossil mammals. This quarry is currently occupied by Connor Construction South West. Unless you have sought permission in advance, you cannot enter these quarries, but you can see a neptunian dyke beside the road, as described below. All the Holwell quarries are in the Carboniferous Clifton Down Limestone Formation. The limestones dip 12–25° to the south, as they lie on the gently dipping southerly limb of the east- west-trending Beacon Hill pericline. The limestones have yielded abundant corals, brachiopods, crinoids, bryozoans, and even trilobites at one horizon. Savage (1977) gave a full account of the quarries, but these are not readily accessible now. He noted the appearance of numerous fissures in the Carboniferous limestone of Quarry 1, that penetrate vertically into the limestone, descending at least to the floor of the quarry or even deeper. These fissures vary in width from a few centimetres to several metres, and they generally trend WNW–ESE, in line with the strike of the major fold. “The fissures are often thickly lined with calcite which grew inwards from the wall in radiating clusters, as beef (sparry calcite) or dog-tooth spar; sphalerite and hematite are also to be found. Because of their E–W trend, the fissures are best seen on the east and west faces of the quarry; several still survive in Quarry I though dust from the quarry plant makes them difficult to distinguish. The top of the Carboniferous Limestone is peneplained and where exposed usually displays remains of epifaunal (e.g. oysters) and infaunal (e.g. boring molluscs and worms) biota… On the north face of the quarry the top is seen to be capped by a few metres of rubbly Inferior Oolite”. In Quarry 2, Savage noted that the Inferior Oolite cover was “rich in fossils”, including common valves of the bivalve Entolium as well as casts of the bivalves Trigonia, Lopha, Pseudolimea and Pholadomya. Terebratulid and rhynchonellid brachiopods also occur, as well as abundant fragments of the zonal ammonite Parkinsonia parkinsoni. In Quarry 3, Savage (1977) notes “a fan shaped mass of Dolomitic Conglomerate which stands proud as a large promontory on the south face of the quarry. The geometry of the conglomerate mass as seen during quarrying suggested a source to the north; the boulders are mostly of Carboniferous origin and well rounded indicating probable transport down a wadi in the rainy . The Dolomitic Conglomerate fan is seen on its west face to be cut by several E-W fissures, usually only 10-50 cms across and infilled with greyish clays containing Rhaetian fish teeth. The Dolomitic Conglomerate must have been 8 lithified between its deposition in Triassic times and the formation of the fissures; the infillings are not earlier than Rhaetic and may be as late as early Inferior Oolite”. If you cannot enter the quarries, you can trace the margin of the Mendip Island from the public highway, beginning to the east of the Holwell village bypass, and working back to the entrance of Quarry 1. The Holwell quarries lie close to the southern shore of the Mendip Island, and the unconformable cover by Mesozoic rocks can be seen in places. Indeed, there are sites that show a traverse from just offshore to onshore, and the coastline was steep at this point, shown by the fact that the Mendip Island did not change in shape here despite rising sea levels during the Rhaetian (Fig. 2). In an old quarry on the Marston Road, the A361 (ST 730449), in the direction of Frome, Nordén et al. (2015) reported a remarkable association of marine and terrestrial fish and reptile remains. Sedimentologically, the Marston Road Quarry showed (Fig. 4B) a typical Rhaetian succession resting on uplifted and eroded Carboniferous limestone, and the basal bonebed contains all the usual shark and bony fish teeth, scales and denticles, as seen in the basal bone bed at Aust Cliff (see pages XXX). There are also remains of the small marine reptile Pachystropheus, as might be expected, but with the unexpected addition of bones of a small lepidosaur, probably a sphenodontian, a terrestrial wash-in. In addition, this is one of the few sites where placodonts are recorded by specimens of their teeth. Placodonts lived on the shores, scouring oysters from the rocks and crushing them with broad, globular teeth. The placodonts indicate a near-coast location, possibly with a rocky shore, and the delicate bones of the lepidosaur would only have survived if deposited very close to shore. In a nearby site (ST 731448), Whiteside and Duffin (2021) reported further terrestrially derived fossils from bedded, marine Rhaetian: some teeth of haramiyid mammals and jaws of the small rhynchocephalian Penegephyrosaurus, further evidence for the wash-off of terrestrial animals into the sea. Unfortunately, the Marston Road Quarry was nearly obliterated by widening and realignment of the A361 in 1985 (documented by Nordén et al. 2015), and it is little more than an overgrown depression at the side of the road. However, it lies less than 100 m from the first of many neptunian dykes on the island, and what was presumably land at the time. The “Holwell 1” neptunian dyke of Kühne (1947) and “COL-4” of Wall and Jenkyn (2004, fig. 8) is an obvious feature (Fig. 4C) at the north of the road that stands out as a low cliff running from the main entrance to the quarry (ST 724451) behind the houses and the old pub on the north side of the old highway to beyond the Holwell Brook (ST 728449). The dyke varies from 1.5–2 m wide and is filled with red siltstones and marls, as well as orange-brown micrite containing fossil debris and other clasts (Fig. 4D). Wall and Jenkyns (2004) attribute the red mudstones (their fill type 1) to the same sources as filled the Triassic-aged fissures around Bristol, this probably Rhaetian in age (Whiteside et al. 2016). They attribute the orange-brown micrites (their fill type 2.2) to a 9 lowermost Jurassic (late Hettangian-early Sinemurian) age, based on enclosed marine fossils at other sites. In recent unpublished studies, we have found a mix of terrestrial microvertebrate fossils and some typical marine Rhaetian shark and bony fish teeth and scales from this fissure. So, whether the neptunian dyke COL-4 represents two phases of tectonics, in the Rhaetian and Hettangian-Sinemurian, separated by some 5 Myr, is to be considered. This is only one of some 25 neptunian dykes identified by Wall and Jenkyns (2004, fig. 8) in the various Holwell, Quarries, all oriented roughly east-west, parallel to the high shoreline (Fig. 2), and perhaps activated at different times through the Late Triassic and Early Jurassic. In summary, the Holwell quarries are designated as an SSSI because they represent an internationally important site for Rhaetian, Lower Jurassic and Middle Jurassic fissure fillings. The Rhaetian fissure fillings have yielded the richest assemblage of vertebrate faunas known from the British Triassic. The site is famous as the site where Moore (1859, 1867, 1876, 1881) and Kühne (1947), collected specimens of Rhaetian mammals such as the morganucodontid Eozostrodon and the haramiyidans Thomasia and Theroteinus, some of the oldest known in the world. Fissure deposits have also yielded fishes and reptiles, including the chondrichthyan fish Duffinselache and Pseudocetorhinus, the rhynchocephalians Diphydontosaurus, Gephyrosarus and Penegephyrosaurus, the trilophosaurid Variodens, a procolophonid, and a placodonts, the first record in Britain (Nordén et al. 2015; Whiteside and Duffin 2017, 2021). The Lower Jurassic fissure fillings yield ammonites and brachiopods which are important in dating. Further, there is a three-dimensional 'wadi-fill' of Triassic age and the regionally important Carboniferous- Middle Jurassic (Inferior Oolite) unconformity.

Vallis Vale Vallis Vale is a 23.9 hectare biological and geological SSSI, which was scheduled for conservation especially because of the De la Beche unconformity, where yellow-coloured Middle Jurassic limestones sit horizontally on top of an eroded palaeotopography of uplifted and steeply dipping Carboniferous limestones (De la Beche, 1846, pp. 287–288). The underlying Carboniferous limestones dip at around 30° and are assigned to the Black Rock Limestone Formation and the Vallis Vale Limestone Formation, a variant of the Clifton Down Formation. The top surface of the limestones is eroded, marking the original shape of the topography of the Mendip Island in the Triassic, and they are overstepped by Mesozoic sediments, all near- horizontal but with a slight NE dip, dating from the Rhaetian at the east end near Hapsford Bridge, to Inferior Oolite at the De la Beche Unconformity. Visitors can proceed through Vallis Vale, along the south bank of the Mells Stream, beginning at either end; we describe the stops from the Hapsford Bridge end, and they are numbered 1–4 (Fig. 5A). There are two key themes for the Vallis Vale sites: inspecting remnants 10 of the former industrial workings here (the map in Figure 5A shows the quarries and industrial buildings and rail lines in 1906; further details in Discussion), and tracing the progressive overstep of the eastern nose of the Mendip Island, shown by localities a, 2 and 3 in the cartoon cross section (Fig. 5B). At stop 1 (ST 761495), a section is seen through the Carboniferous-Mesozoic unconformity (Ronan et al. 2020, figs. 4, 5). This site is a low cliff, excavated around 1900 to build a small marshalling yard for the mineral railway (Fig. 5A). Here, there is 1 m of Carboniferous limestone capped by 3–4 m of Rhaetian, including the Westbury and Lilstock formations (Fig. 6A). The Westbury Formation is represented by 2 m of basal bone bed (Fig. 6B), thin limestones, conglomerates and mudstones, followed by 2 m of the Lilstock Formation (Cotham Member is a marly limestone and siltstone, and Member is a limestone with borings on top). The section is capped by 1 m of Inferior Oolite (Middle Jurassic, Bajocian). Here, the Rhaetian bone bed yielded microvertebrate remains of four species of sharks and two species of bony fishes, all of them typical of Rhaetian-aged bone beds (compare with Aust Cliff, pages XXX). The invertebrate fauna is especially rich, including bivalves, echinoids and barnacles, as well as trace fossils (Ronan et al. 2020). Older records included the classic Rhaetian bivalve Rhaetavicula contorta and the ostracod Euestheria minuta (Savage 1977). Heading west, and so up the shoreline of the eastern end of the Mendip Island, locality 2 (ST 760494, ST 760493) comprises two deep quarries, exploited for the Carboniferous limestone. Notice that the Carboniferous-Mesozoic contact unconformity has risen higher above the road level, and here the Rhaetian is only about 1 m thick. Continuing further west along the track, crossing the Mells Stream, and working round some factory buildings, the famous De la Beche unconformity (locality 3, ST 756492) is located up a small track north of the gravel track at the confluence of the Egford Brook and Mells River. On entering the quarry, the unconformity is sharp and beautifully displayed, the steeply dipping Carboniferous limestones capped by horizontally bedded, yellow-coloured Inferior Oolite (Fig. 6C). Note here how much higher the unconformity is than at localities 1 or 2 – we have now progressed higher up the palaeotopography of the eastern shore of the Mendip Island, and the Rhaetian is absent because of the sequential overstep of the palaeoisland by fitfully rising sea levels through the Rhaetian and Jurassic (Fig. 5B). It is possible to climb up the side of the section to inspect the Carboniferous-Jurassic unconformity and the overlying Inferior Oolite. The surface of the Carboniferous shows borings, and it was colonized by oysters. The Inferior Oolite is an oolitic limestone, rubbly, thin bedded or massive, with marly partings, very similar to the Stone, a classic building stone type quarried nearby. Savage (1977, p. 98) reported the fossils from the Inferior Ololite here as “including the bivalves Ctenostreon, Pholadomya and Lima; brachiopod Acanthothiris; echinoids Cidaris and Acrosalenia.” 11

Walk south along the track beside the Egford Brook to locality 4 (ST 758487). This is fenced off for safety reasons, but you can view the deep pit from the side, showing some 15 m of Carboniferous limestone, with the unconformity at the top. In terms of the progressive flooding of the Mendip palaeoisland, the Vallis Vale localities show not only the changing elevation of the unconformity, and hence evidence for minimally two phases of flooding – one in the Rhaetian (latest Triassic) and a second in the Bathonian (Middle Jurassic). At both levels, the underlying Carboniferous limestone was bored by sponges and worms producing the borings called Trypanites and Gastrochaenolites (Fig. 6D–F). The Rhaetian bone bed at Hapsford Bridge famously, and unusually, yields flat pebbles of Carboniferous limestone that have been successively bored, encrusted by oysters (Liostrea, Atreta) and then ripped up by storms and redeposited, as suggested by the cartoon sequence (Fig. 6H–J). Finally, two bored hardgrounds are exhibited, that dating to basal Rhaetian, some 205 Ma (at Hapsford Bridge), and that dating to the Bajocian, some 169 Ma (at the De La Beche unconformity).

Tedbury Camp We explore the Bajocian-aged unconformity in more detail at this location. It is possible to reach the Tedbury Camp site by walking from Vallis Vale west along the Mells River or driving to Great Elm, crossing the river and parking in the rough layby at Fordbury Bottom (ST 749492). From this layby, walk through the kissing gate and follow the track (part of the East Mendip Way) over a narrow metal bridge just before reaching the railway line and turn sharp left, still on the East Mendip Way, along a muddy path that parallels the stream. After 200 m leave the East Mendip Way and take the right fork that climbs steeply up a loose and stony path to the eastern edge of the quarry floor Tedbury Camp is an promontory hill fort, occupying 24 hectares and close by an extensive Carboniferous-Jurassic unconformity site (ST 747489), which is a large area at the top of the hill, sitting surrounded by dense trees all round, and marked on the Ordnance Survey map as a disused quarry. The site was actively quarried in the early 20th century, when the thin overburden of Jurassic was stripped off. Carboniferous limestone was removed for building and road metal, but the site can never have been profitable, and it closed in the 1960s. The quarry owners used it intermittently to store gravel, but it was only in the 1980s that geologists became aware of the site. Thanks to the endeavours of Charles Copp, then a PhD student at Keele University, the Nature Conservancy Council cleared the site, and ensured it was preserved by making it an SSSI and including it in Duff et al. (1985) because of its great educational value. As you walk around the site, you can see the Clifton Down Limestone Formation, dipping NNW at 45–55o, beneath your feet (Fig. 7A, B). The individual beds form long parallel ridges 12 across the site, and these give some understanding of the extraordinary processes of folding and subsequent planning by erosion. Here and there, minor folds can be seen in the underlying Carboniferous, and it yields a variety of typical fossils, including solitary and colonial rugose corals (e.g. Lithostrotion), productid brachiopods, and crinoid ossicles. These fossils occur scattered and in fossil beds, and other features include oolitic limestones, algal stromatolites, dolomitised areas and a chert unit in the northeast corner of the quarry. Importantly, the peneplaned top Carboniferous surface marks the unconformity with the overlying Jurassic, here represented by 4–6 m of Inferior Oolite (upper Bajocian, Middle Jurassic, 169 Ma; Fig. 7A). Locally, around the Beacon Hill pericline, the Inferior Oolite can reach greater thicknesses, up to 20 m, and it is best seen at Doulting, where the Doulting Stone has been quarried as a building stone for over 500 years. The Inferior Oolite here begins with 1–2 m of poorly bedded, nodular, oolitic limestone containing abundant fossils. Then follows 25 cm of brown clay, and a 50–80 cm lenticular bed of oolitic limestone. The sequence is capped by 2–4 m of poorly bedded oolitic grainstone. The fossils, as documented by Charles Copp, and listed by Whiteley (2008), include abundant bivalves (Ctenostreon, Liostrea, Lithophaga, Pholadomya, Pleuromya, Trichites and Trigonia) and brachiopods (Acanthothyris, Kallirhynchia and Sphaeroidothyris). Occasional echinoids (Crotoclypeus and Nucleolites) have been found in life position on the unconformity surface and above Hardground B, and the ammonite Parkinsonia is reported. Other fossils include poorly preserved pleurotomariid and procerithiid gastropods, corals and serpulid worms. Many of these fossils are found clustered on the unconformity surface, at the base of the Inferior Oolite succession. The unconformity surface, which dominates the site, is worth investigating (Fig. 7A). Following the classic work by De la Beche (1846), who identified borings in the coeval unconformity at Vallis Vale, Cole and Palmer (1999) provided a detailed description of the Tedbury Camp site. They noted three kinds of borings (Fig. 7C, D) as well as numerous encrusting molluscs. The three borings are the large Gastrochaenolites and two species of Trypanites, the smaller being T. weisei and the larger T. fosteryeomani. Gastrochaenolites was bored out as a living crypt by a rock-boring bivalve perhaps like the modern Lithophaga, and its borings leave shallow pits often filled with younger sediment after the animal vacated its home. The Trypanites borings are usually assigned to worms. The whole process of planning of the top-Carboniferous surface, flooding by the sea, and occupation as a hardground by borers and encrusters is dated as Bajocian, based on the evidence of the overlying sediment (Cole and Palmer 1999). The encrusting oysters are mainly Liostrea and Ctenostreon.

Discussion Mesozoic flooding 13

On this trip, we have seen different evidence of the progressive rise of sea level from latest Triassic through the Mesozoic. Indeed, Farrant et al. (2014) identify ancient shorelines by planing of the Carboniferous top surface and subsequent colonisation as hardgrounds by marine organisms such as bivalves, sponges and worms that bored into the limestone, as well as colonisation by oysters and other encrusting marine invertebrates. They noted three steps, corresponding to the Middle Jurassic, Late Jurassic and mid Cretaceous, and Ronan et al. (2020) added a fourth, corresponding to the base of the Rhaetian, 205.7 Ma. Further evidence of coastal sites and progressive flooding of the Mendip island, with reduction in its size, comes from the neptunian dykes at Holwell and the close proximity to inshore bonebed sites on the Marston Road, associated with finds of the oyster-eating placodonts. We have seen two of the peneplaned surfaces, at Hapsford Bridge in Vallis Vale, and Tedbury Camp. Both provide excellent evidence for hardground formation and subsequent reworking. The hardgrounds presumably developed after sea levels had risen, and marine organisms colonised the limestone seabed, some of them boring into the underlying Carboniferous rock, leaving the characteristic Trypanites and Gastrochaenolites borings (Figs. 6D–J; 7C, D). Other organisms may have occupied softer sediment in neighbouring areas, or attached directly to the seafloor, but leaving no trace. Then, as sea levels remained high, covering the hardground, in certain cases, most notably at Hapsford Bridge, in the earliest Rhaetian, storms occurred and the storm surge ebb current ripped off pieces of the bored hardground limestones, and these were tumbled, dumped, encrusted by oysters, and eventually incorporated into the marine sedimentary layers (Fig. 6H–J). Similar phenomena have been noted in the Late Jurassic and Early Cretaceous (Farrant et al. 2014), so we can envisage the Mendip Island, reducing in size through the Rhaetian as sea levels rose (Fig. 8A), and then the coastline retreating inland in stages through the Jurassic and Cretaceous (Fig. 8B), as sea levels rose at certain times, sediment was deposited, and overall base level continued to rise. Each documented burst of sea level rise is associated with a hardground with borings and encrusters (De la Beche 1846; Coles and Palmer 1999; Farrant et al. 2014; Ronan et al. 2020), occasionally with evidence of storm activity and redeposition of eroded hardground (Ronan et al. 2020), and neptunian dykes in certain cases, especially in the Rhaetian and Early Jurassic (Wall and Jenkyns 2004). Neptunian dykes are known from several sites in the Mendips, including Holwell, as we have seen, and they tend to fill east-west tension clefts formed in the Carboniferous Limestone while it was covered by the sea (Robinson, 1957; Savage, 1973). Marine sediments were swept into the fissures together with remains of terrestrial animals that were living on the shoreline, and the mixing of marine and terrestrial fossils is a characteristic feature. The fact that sediments filled the fissures while they were being formed indicates that the tectonic activity 14 was not Variscan, but Rhaetian to Jurassic in age, corresponding to the ages of the infilling faunas (Robinson, 1957; Savage, 1993). The origin of the fissures along east-west-trending faults or joints is confirmed by the fact they are all parallel with each other and their margins are sharp, even cutting through boulders in the older rock (Wall and Jenkyns, 2004). It is likely they filled by rapid injection of sediment, shown by the absence of any bedding suggesting gradual accumulation and the fact that any sedimentary structures are parallel to the fissure walls (Wall and Jenkyns, 2004). These authors suggest the fissures opened beneath a stack of sediment under the sea, thereby creating a void and a reduction in pressure, which in turn caused sediment to be sucked in; sediment injection associated with submarine faulting is the dominant model for formation of neptunian dykes (Wall and Jenkyns, 2004). Times of intense fissuring seem to correlate with times of high sea level. These phenomena can be seen at many localities across the Bristol and Somerset areas, but the East Mendips perhaps show them best, as we have seen. The overall geological model for the Wessex Basin is one of continuing rifting from Rhaetian to Bajocian, a span of 45 Myr, with pulses of extensional tectonic activity causing faults, joints, and injection-filled fissures, and coinciding with marine transgressions (Wall and Jenkyns, 2004). The coincidence of rifting and transgressive episodes suggests a causative model, that as water depths increased, underwater basins sank beneath the weight of sediment and water, and the sinking led to more filling and more sinking (Wall and Jenkyns 2004; Lovegrove et al. 2021).

Quarrying activities at the eastern end of the Mendips It is perhaps unusual to see such extensive geological exposures at inland sites, but this is possible because of extensive quarrying across the eastern end of the Mendips. The Carboniferous limestones have been quarried for a long time across the Mendip Hills, and this often cut down through overlying Mesozoic sediments. Early stone quarrying was a small-scale operation, often conducted by hand and in numerous locations close to where the stone was needed, but from Victorian times and the arrival of the railways, quarrying became more and more concentrated in ever larger quarries that had good transport links. Industrial activities at Vallis Vale filled the small with lime kilns, iron workings, and woollen mills, all powered by the fast-flowing waters of the tiny Mells River. The oldest examples include the iron workings at Great Elm village (ST 738488), possibly dating back to 1500, and then taken over by the Fussell family in 1744. The Fussells’ Lower Works, located on the north side of the Mells Steam became a substantial enterprise, with 250 employees, and a further five sites nearby. However, the industry declined and closed down by 1895. 15

Vallis Vale is lined with quarry faces which have become overgrown in the past hundred years. After the final abandonment of quarrying in the 1950s, remaining mineral railway lines were removed, and the gravel track beds turned into footpaths; this is a popular site for walkers (Prudden, 2006). The first commercial activity in Vallis Vale was the production of lime for mortar; the Somerset Carboniferous limestones were widely used for this purpose. Limestone was extracted from locations on hillsides, either by using crowbars and picks to remove blocks or by blasting, and it was then brought downhill using gravity to the limekiln. Large quantities of broken limestone would be piled in, with wood and charcoal, and then lit. The bottle-like shape of the limekiln, with air intakes around the base, allowed very high temperatures, about 1000oC, to be reached, at which point the limestone (calcium carbonate) released carbon dioxide, becoming calcined from CaCO3 to CaO, which is quicklime. If the quicklime was mixed with water, it was ‘slaked’ to become calcium hydroxide, Ca(OH)2, which was used to make lime mortar for building, or whitewash by addition of water and glue to the slaked lime. In addition, the Somerset limestones were used to produce poultry grit and concrete (Loupekine, 1956). Remains of eight lime kilns can be found in the Vallis area. Stone quarrying was a long-established industry in the Mendip Hills, providing a diversity of building stones (Stokes, 1999), but it began rather late in Vallis Vale, in 1893. That year marked the formation of the Somerset Quarry Company (Foundations of the Mendips, 2017) to work quarries in the valley of the Mells River beside Hapsford Mill, on the Vallis Road (A362). The company was formed by James Dovell Armstrong, a local farmer, and Jonathan Drew Knight, a Frome maltster. They took a 30-year lease of the Vallis quarries from the Earl of Cork and Orrery and acquired the former woollen mill at Hapsford Bridge to use as stone crushing plant. The company worked four quarry faces along the south bank of Vallis Vale (Fig. 2A, localities 1–3), and the 41-man team produced over 100 tons of rock a day of which 10 % was burnt for lime in two quarry kilns and the rest was crushed for road building (Thornes, 2015, p. 75). The limestone was transported in wagons called ‘tubs’ and drawn by horses along narrow- gauge rail lines to Hapsford Mill. There, the full tubs of stone were lifted to a higher level by a wire system and dumped into the hoppers of the stone-crushing machinery, which was powered by the water flow of the Mells River or, when the flow was insufficient, by a steam engine (Anonymous, 1898). Soon, these horse-drawn wagons were replaced by steam engines on the narrow-gauge rail lines. As the business grew, and became a registered company in 1907, the operation expanded to quarrying six faces along the banks of the Mells River and Egford Brook, and eventually 12, which more or less connected continuously (Fig. 2A, between sites 2–4). The stone was shipped 16 out along the narrow-gauge railway lines, which connected up Vallis Vale past Hapsford Mill eastwards to join the standard-gauge -to-Frome line of the Great Western Railway which runs past the Vale (Atthill, 1984). The 1906 and 1936 Ordnance Survey maps show the narrow-gauge railway running on the south bank of the Mells River at Hapsford, right beside the Rhaetian section, and crossing to the north bank of the river to branch eastwards beside Hapsford Mill (Fig. 2A). The lines extended for some 4 km (2.5 miles) and were worked by two small four-wheeled Ruston Hornsby diesel locomotives. The Somerset Quarry Company became Roads Reconstruction (1934) Ltd., and then Hanson PLC in 1989. The narrow-gauge line was replaced in 1943 with a standard gauge system (Thornes, 2015). Quarrying in Vallis Vale reduced after the war and ceased in the 1960s. Meanwhile, New Frome Quarry, later renamed Whatley Quarry (UK National Grid Reference, ST 731479), opened in the 1940s to the west of Vallis Vale, generated over 2.5 million tonnes yearly from the 1980s onwards, and remains one of the two largest quarries still in operation in Somerset (Quarry Faces, 2019). Because of the small scale of the Vallis Vale quarries, and the fact that the Carboniferous limestones were covered by considerable thicknesses of Rhaetian and Jurassic sediments that had to be removed, Roads Reconstruction (1934) Ltd. planned a new, larger quarry. They bought Mells Green Farm in 1938, which was the beginning of what was then called New Frome Quarry. As this quarry, located 2 km WSW of Vallis Vale came into production, a new rail line was extended up the valley of the Fordbury Water. For several years, stone was transported from this new quarry along the tiny mineral rail lines to Hapsford Bridge for crushing, but this limited production substantially. In 1960, the operation was acquired by Thomas Roberts (Westminster) Ltd., and in 1965 they built a substantial new crushing plant in New Frome Quarry, by then known as Whatley Quarry, capable of handling a million tonnes of limestone per year. By massively increasing the output of this single quarry, the company closed its other limestone quarries at Vobster and , and the crushing plant at Hapsford became redundant and was dismantled. In 1974, a substantial new rail link was opened into the quarry by the new owners, Amey Roadstone Corporation (ARC), and the old mineral lines around Hapsford were removed. Capacity was increased to 10 million tonnes of limestone per year, largely used as ballast for the western and southern regions of National Rail, and a huge new plant was opened in 1987. ARC was acquired by Hanson PLC in 1989, and Whatley Quarry continues to expand, being one of the two largest active quarries in Somerset. Over a century then, the Vallis Vale-Whatley site has seen limestone production increase from some 30,000 tonnes per year in 1893 to 10 million tonnes per year in 1987. There were other quarries nearby, at Vobster and Bilbao in Mells parish to the north-west, associated with collieries that extracted coking coal from the Upper Carboniferous Coal 17

Measures there. Vobster and Bilbao quarries were part of the Roads Reconstruction (1934) Ltd. company that owned the Hapsford Bridge operation. In 1934, Quarry (Merehead) began operations to the southwest, and it currently produces 6 million tonnes per year, largely sent out along its own rail lines, as is the case for Whatley Quarry. The coal and limestone were used by the Westbury Iron Company, opened in 1857. The Carboniferous limestones quarried in Whatley Quarry include much of the Lower Carboniferous succession, comprising the Black Rock Limestone, Vallis Limestone, Burrington Oolite, and Clifton Down Limestone formations. The stone varies from fairly chemically pure oolites to mudstone-rich and cherty limestones. Whatley Quarry exploits limestones on the northern flank of the Beacon Hill Pericline, a substantial east-west trending that forms the backbone of the Mendips and the Mendip Island in the Triassic and Jurassic, with Silurian- aged andesite and Devonian sediments at the core, and overlain by Carboniferous limestone, dipping here at 60 to 70° and in some cases, beds are overturned. Minor faulting is significant, and the quarried-out eastern part of Whatley Quarry carried a cover of Inferior Oolite (Middle Jurassic). Moon’s Hill Quarry was purchased by John Wainwright in 1897 and continues under that ownership. The quarry exploits the andesite which is largely used as hardcore in road building. Holwell Quarries (centred around ST 726450) comprise several pits to north and south of the A361 Frome to Shepton Mallet road. There are five main quarries, Cree’s Quarry south of the A361, Coleman’s Quarry to the west of Holwell village and currently open, North Quarry and Bartlett’s Quarry to the north, and Lime Kiln Quarry beside Brook to the north-west; maps are provided by Savage and Waldman (1966, fig. 1), Wall and Jenkyn (2004, fig. 8) and Whiteside and Duffin (2017, fig. 1). The quarries were worked and expanded through the 1800s and 1900s for roadstone material, but this is no longer actively quarried and only the roadstone asphalt coating plant remains.

References Anonymous, 1898. Vallis Vale Limestone Quarries. The Quarry 1898, 153–156. Atthill, R. 1984. Old Mendip. Bran’s Head, Frome, 164 pp. Buckland, W. and Conybeare, W.D. 1824. Observations on the south-western coal district of England. Transactions of the Geological Society of London 2, 210–316. Cole, A.R. and Palmer, T.J. 1999. Middle Jurassic worm borings, and a new giant ichnospecies of Trypanites from the Bajocian/Dinantian unconformity, southern England. Proceedings of the Geologists' Association 110, 203–209. De la Beche, H.T. 1846. On the Formation of the Rocks of South Wales and South Western England. Memoirs of the Geological Survey of 1, 1–296. 18

Duff, K.L., McKirdy, A.P., and Harley, M.J. 1985. New Sites for Old; A Students’ Guide to the Geology of the East Mendips. Nature Conservancy Council, Peterborough, 189 pp. Farrant, A. 2008. A Walker’s’ Guide to the Geology and Landscapes of the Eastern Mendip. British Geological Survey, Keyworth, 76 pp. Also available at https://www2.bgs.ac.uk/mendips/home.htm. Farrant, A.R., Vranch, R.D., Ensom, P.C., Wilkinson, I.P., and Woods, M.A. 2014. New evidence of the Cretaceous overstep of the Mendip Hills, Somerset, UK. Proceedings of the Geologists’ Association 125, 63–73. Foundations of the Mendips, 2017. British Geological Survey website (accessed January 2021) https://www.bgs.ac.uk/mendips/home.htm. Green, G.W. 2008. Volcanic stratigraphic architecture of the East Mendip Silurian Inlier, Somerset, UK. Proceedings of the Geologists’ Association 119, 339–350. Kühne, W.G. 1947. The geology of the fissure filling “Holwell 2”; the age determination of the mammalian teeth therein; and a report on the technique employed when collecting the teeth of Eozostrodon and Microclepidae. Proceedings of the Zoological Society of London 116, 729– 733. Loupekine, I.S. 1956. Mining and quarrying in the Bristol district, 1955. Proceedings of the Bristol Naturalists’ Society 29, 155–161. Lovegrove, J., Newell, A.J., Whiteside, D.I., and Benton, M.J. 2021. Testing the relationship between marine transgression and evolving island palaeogeography using 3D GIS: an example from the Late Triassic of SW England. Journal of the Geological Society in press. McMurtrie, J. 1885. Notes of Autumn excursions on the Mendips. Proceedings of the Bath Natural History and Antiquarian Field Club 5, 98–111. Moore, C. 1867. On abnormal conditions of secondary deposits when connected with the Somersetshire and South Wales Coal Basin; and on the age of the Sutton and Series. Quarterly Journal of the Geological Society of London 23, 449–568. Moore, C. 1859. On Triassic beds near Frome, and their organic remains. Report on the Twenty- Eighth Meeting of the British Association for the Advancement of Science 1859, 93–94. Moore, C. 1867. On abnormal conditions of secondary deposits when connected with the Somersetshire and South Wales Coal Basin; and on the age of the Sutton and Southerndown Series. Quarterly Journal of the Geological Society of London 23, 449–568. Moore, C. 1876. [The geological characteristics of Vallis Vale and the Holwell Quarries]. Proceedings of the Somerset Antiquarian and Natural History Society 21 (1), 25–29, 31–37, 61–63. Moore, C. 1881. On abnormal geological deposits in the Bristol district. Quarterly Journal of the Geological Society 37, 67–82. 19

Nordén, K., Duffin, C.J., and Benton, M.J. 2015. A marine vertebrate fauna from the Late Triassic of Somerset, and a review of British placodonts. Proceedings of the Geologists’ Association 126, 564–581 Prudden, H.C. 2006. New uses for former mineral workings in Somerset. Geoscience in South- west England. Proceedings of the Ussher Society 11, 249–251. Reynolds, S.H. 1907. A Silurian inlier in the eastern Mendips. Quarterly Journal of the Geological Society of London 63, 217–240. Reynolds, S.H. 1912. A Geological Excursion Handbook for the Bristol District. J.W. Arrowsmith, Bristol 224 pp. Robinson, P.L. 1957. The Mesozoic fissures of the Bristol Channel area and their vertebrate faunas. Journal of the Linnean Society of London, Zoology 43, 260–282. Ronan, J., Duffin, C.J., Hildebrandt, C., Parker, A., Hutchinson, D., Copp, C. and Benton, M.J. 2020. Beginning of Mesozoic marine overstep of the Mendips: the Rhaetian and its fauna at Hapsford Bridge, Vallis Vale, Somerset, UK. Proceedings of the Geologists' Association 131, 535–561. Savage, R.J.G. 1977. The Mesozoic strata of the Mendip Hills. In: Savage, R.J.G. (Ed.), Geological Excursions in the Bristol District. University of Bristol, Bristol, pp. 85–100. Savage, R.J.G. 1993. Vertebrate fissure faunas with special reference to Bristol Channel Mesozoic faunas. Journal of the Geological Society, London 150, 1025–1034. Stokes, P. 1999. Yesterday’s Mendip. Mendip’s Past: A Shared Inheritance. Council. Doveton Press, Bristol 61 pp. Thornes, R. 2015. Quarry Faces: The Story of Mendip Stone. Ash Tree Publications, Newton Abbot, 215 pp. Wall, G. and Jenkyns, H. 2004. The age, origin and tectonic significance of Mesozoic sediment- filled fissures in the Mendip Hills (SW England): implications for extension models and Jurassic sea-level curves. Geological Magazine 141, 471–504. Whiteley, M.J. 2008. Tedbury Camp Quarry (https://geohubliverpool.org.uk/tedbury/index.htm). Whiteside, D.I. and Duffin, C.J. 2017. Late Triassic terrestrial microvertebrates from Charles Moore’s ‘Microlestes’ quarry, Holwell, Somerset, UK. Zoological Journal of the Linnean Society 179, 677–705. Whiteside, D.I. and Duffin, C.J. 2021. New haramiyidan and reptile fossils from a Rhaetian bedded sequence close to the famous ‘Microlestes’ Quarry of Holwell, UK. Proceedings of the Geologists’ Association in press. 20

Whiteside, D.I., Duffin, C.J., Gill, P.G., Marshall, J.E.A., and Benton, M.J. 2016. The Late Triassic and Early Jurassic fissure faunas from Bristol and South Wales: stratigraphy and setting. Palaeontologia Polonica 67, 257–287.

21

A 3 62

Holcombe

e l a Coleford Vobster llis V a Mells V Tedbury Camp

Whatley Frome Quarry Whatley

Stoke St Michael Moon’s Hill Quarry

Nunney 1 A36

Holwell N

9 5 3 A 0 500m

Oxford Clay & Kellaways fms Charmouth Mudstone Fm sandstones in Coal Measures Burrington Oolite Formation Coalbrookdale Formation Cornbrash Formation Blue Lias Formation Middle Coal Measures Fm Vallis Limestone Formation tuf Forest Marble Formation Westbury Formation Lower Coal Measures Fm Black Rock Limestone Fm andesite Mercia Mudstone Group - Quartzitic Sandstone Fm Frome Clay Formation marginal facies conglomerate Avon Group mudstone Mercia Mudstone Group - Oxwich Head Limestone Fm Portishead Formation Fuller’s Earth Formation mudstone and halite Inferior Oolite Group Pennant Sandstone Formation Clifton Down Limestone Fm fault

Fig. 1. Geological map of the East Mendips area, showing major roads and the four stops. Drawn by Susan Marriott from BGS data from Digimap © Ordnance Survey and British Geological Survey.

22

Cromhall Deposition of Both Tytherington Woodleaze No Westbury Deposition No Cotham Deposition

Fissure locality Yate

¯N 10 km Portishead Bristol Durdham Down

Clevedon

Bristol Airport Bath

Weston-super-Mare

Radstock Batscombe

Cheddar Emborough

Highcroft

Windsor Hill Holwell Frome Wells

Shepton Mallet

Fig. 2. The Bristol-Mendips island archipelago in the Rhaetian. The map shows the palaeoislands during the time of deposition of the lowest Westbury and uppermost Cotham beds, and the shades of green indicate the size of the palaeoislands at the beginning of the Rhaetian transgression (base Westbury Formation; pale green) and some 2–3 million years later, at the time of deposition of the Cotham Member (dark green). The sea is blue. Fissure fill localities and described bonebeds are marked with red and orange dots respectively, and built-up areas are outlined in black. This revised map of the palaeoislands is based on a detailed GIS study of field and borehole data by Lovegrove et al. (2021).

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A B

D

C

Fig. 3. Moon’s Hill Quarry volcanics. (A) View into Moon’s Hill Quarry in 2010, looking down from the viewing platform; (B) cross-laminated, reworked tuff; (C) block of volcaniclastic conglomerate; pebbles are well rounded but do not include volcanic bombs here, so suggesting this has been reworked by water transport; (D) view northward showing red-coloured sandstones of the Devonian Portishead Formation overlying volcanic tuffs. Photographs courtesy of Ian West. 24

A

B

C D

Fig. 4. Holwell quarries neptunian dykes and bedded Rhaetian. (A) Moore’s diagram of the folded Carboniferous limestone penetrated by 17 neptunian dykes, in Holwell Quarry 3, the so- called ‘Microlestes Quarry’, south of the A361; (B) the Marston Road marine site, just 100 m from the neptunian dyke, showing Carboniferous overlain by Westbury Formation (black) with the basal bone bed, and Inferior Oolite (grey) above; (C) fresh exposure of the neptunian dyke behind the houses on the north of the old Holwell main road; (D) close-up if the Carboniferous limestone host (yellow, top) and the dyke (red, with calcite crystals, below); Photographs (B) by English Nature, 1985; (C, D) by MJB, summer 2019. 25

Fig. 5. The context of Vallis Vale, showing the Vale in 1906, when the quarries and mineral railways were active. (A) The four locations to be visited are numbered (1, Hapsford Bridge Rhaetian site; 2, neighbouring quarry also showing Carboniferous-Rhaetian unconformity; 3, the site of the De la Beche unconformity between Carboniferous and Inferior Oolite (Middle Jurassic); 4, Egford quarry, comprising a great thickness of Carboniferous limestone). (B) The Rhaetian overlies the Carboniferous limestone along much of the banks of the Mells River, from localities 1 to 3, but disappears between 2 and 3. For A, © Crown Copyright and Database Right 2018. Ordnance Survey (Digimap Licence); from Ronan et al. (2020). 26

A B

C

E

D Gastrochaenolites Trypanites

G

Gastrochaenolites

Atreta 10 mm

F Liostrea H I J

Fig. 6. Geology of Vallis Vale. (A) The roadside section at Hapsford Bridge, showing 1 m of Carboniferous limestone at the base, overlain by the Rhaetian. (B) Beds 2–6 of the Westbury Formation, showing the basal bone bed, level with the hammer head, overlain by muddy limestone and limestone beds. (C) The De la Beche unconformity, which is slightly overgrown by vegetation, showing the yellow horizontally bedded Jurassic Inferior Oolite lying unconformably on the steeply dipping Carboniferous limestone. (D–J) Reworked pebbles of Carboniferous limestone from the basal beds of the Westbury Formation. (D, E) One pebble (BRSMGCd384), showing Trypanites and Gastrochaenolites borings. (F, G) A larger pebble (BRSMG Cd382), showing Trypanites and Gastrochaenolites borings, overlaid by encrusting bivalves Liostrea and Atreta. (H–J) Inferred history, as worms and bivalves make Trypanites and Gastrochaenolites borings, presumably in earliest Rhaetian times (H), and then some surface 27 layers of the Carboniferous pavement is stripped off, and the fragments tumbled and abraded, before settling on the seabed and becoming encrusted by bivalves (I), and finally burial in basal Westbury Formation sediments (J). Images all from Ronan et al. (2020).

A B

C D

Fig. 7. Tedbury Camp Carboniferous-Jurassic unconformity. (A) Overview of the site, showing upended beds of Carboniferous limestone planed flat, and the overlying Inferior Oolite cover seen in the far distance. (B) Vertical view of the ends of the Carboniferous limestone beds. (C, D) Vertical sections of the unconformity surface, showing borings of Jurassic-aged organisms into the underlying Carboniferous limestone, with examples of cross-cutting Trypanites weisei (C)and Trypanites weisei with a larger Gastrocxhaenolites crypt (arrowed). Photographs from Wilson44691, Wikimedia (A, B) and courtesy of Martin J. Whiteley (C, D). 28

A 10 km

Batscombe Mendip Island

Deposition of Emborough Both No Westbury Highcroft Deposition No Cotham Windsor Hill Holwell Deposition Fissure locality

B C

Hardground encrusters and borings

Neptunean dykes

Rhaetian (205 Ma) Bajocian (169 Ma) Oxfordian; Albian (Tadhill) Oxfordian (160 Ma) Albian (110 Ma) Bajocian (Tedbury)

Rhaetian (Hapsford)

Fig. 8. The flooding of the Mendips in cartoon form. (A) Mendip Island, showing the maximum size at the beginning of the Rhaetian (pale green) and the reduction in island size (dark green) during Cotham times, perhaps 2 million years later. (B) Evidence for moving coastlines at the eastern end of the palaeoisland, as sea levels rose through Rhaetian, Jurassic and Early Cretaceous times. (C) Vertical view of rising sea levels at the east end of the Mendip Island, summarising the key evidence for coastal conditions in hardgrounds, encrusters and borers, and neptunian dykes.