Bristol U3A Geological trip to Lydney and Huntley.

We set off early to make sure we could enjoy our trip to Lydney and retreat from the foreshore before the tide rushed in, “faster than a galloping horse.”
We arranged to meet at Lydney Harbour at 1030. As it happened, I was the last one to arrive and I was a quarter of an hour early.
After ensuring that we were all suited and booted for the treacherous Severn Foreshore, we set off.
Anna was leading the trip and had previously carried out a recce but the reeds had grown so much since then that it was difficult to find the way without being able to see the ground ( mud ).
We fought our way through the reeds, over a couple of mud banks and then found ourselves on the rocky foreshore which was mainly made up of very grippy red  mudstones of the Raglan Marl Group.
We surmounted these and then had to pick our way over some mud banks. From the erosion surface of the mud banks, deposition layers could be seen so our assumption was that the Severn foreshore was prograding in this area.
Then we came to a beach area underlying a cliff. This is what we had come to see. This is the famous Lydney Cliff SSSI – see citation below.
We walked further on to the point to investigate the strata and cliffs. We also worked out where the Severn Railway Bridge was before it was destroyed in 1960.

You will be glad to hear that we got back to the Harbour before the tide came in.

Height of image approx 3m.

65a2bd80-c76e-4af6-aaa4-eb81d5dbf9c9

Lydney Cliff showing nodular calcrete.

Calcrete notes

Soil formation – pedogenesis – in the Silurian and Devonian periods

Soils started to form in the Silurian from the products of chemical weathering from high ground resulting in minerals and clays being deposited in basins. Early organic soils containing plant material have been found in the Early Devonian Rhynie Chert, near Aberdeen, where fossils of early vascular plants have also been found. Before this, in the late Silurian, there were only inorganic, shallow, microbial protosoils. The development of deeper and stronger plant rooting systems on ArchaeopterisLepidendropsis/Protostigmaria),  and Rhacophyton in the Late Devonian resulted in deeper soils containing more organic material by the process of pedoturbation.

Calcrete genesis in the Devonian period.

Clays from the smectite group are found in Devonian palaeosols. These clays take up water and swell in wet conditions and so shrink when they dry out because of evaporation. This results in vertical fissures which allow fluids to penetrate the soil. With each hydration / evaporation cycle, the concentration of calcium and magnesium  carbonate minerals increases and these saturated mineral solutions react with the soil altering the aluminium based clays to either calcretes or dolocretes. This alteration is post deposition as can be seen by the limited penetration down from the surface. Older calcretes tend to have been altered to dolocrete.
Calcrete, also known as Caliche in Latin America,  cannot form in the present climate of the British Isles as it needs a mean wet season rainfall of 100-500mm and a mean annual temperature of 16 – 20oC. It is forming in modern times where this climate occurs in places such as Arizona USA, Gilgai Australia, Kankar India and areas of Mexico.
Calcrete is the formation of a Duricrust from calcium carbonate, as other minerals are involved, they are called ferrocrete etc.
In the Atacama Desert in northern Chile, vast deposits of a mixture, also referred to as caliche, are composed of gypsumsodium chlorideand other salts, and sand, associated to salitre (“Chile saltpeter”). Salitre, in turn, is a composite of sodium nitrate (NaNO3) and potassium nitrate (KNO3). Salitre was an important source of export revenue for Chile until World War I.
These deposits were mined because of their high nitrate content. They supplied the material for nearly all the explosives used in WW1. At it’s height, Chile was exporting 25 million tonnes a year.
These deposits are the largest known natural source of nitrates in the world, containing up to 25% sodium nitrate and 3% potassium nitrate, as well as iodate minerals,     sodium chloride, sodium sulfate, and sodium borate (borax). The caliche beds are from 0.2 to 5.0 m thick, and they are mined and refined to produce a variety of products, including sodium nitrate (for agriculture or industry uses), potassium nitrate, sodium sulfate, iodine, and iodine derivatives.
This mining was superseded by industrial production of explosives and fertilisers in Europe using ammonia produced by the artificial fixation of nitrogen using the Haber–Bosch process.
In the UK, Calcrete occurs in the Upper Silurian and Lower Devonian rocks and so are used as a marker for the start of the Devonian period.
Depending on the location, it is called Variously: Bishop’s Frome Limestone, Chapel Point Calcrete etc.

The lower part of the cliff consists of mudstones of the Raglan Marl Group but above this, for most of the cliff height it consists of the Bishops Frome Limestone, which is a nodular calcrete.  Towards the top of the cliff are cyclothems of nodular calcrete and siltstone / mudstones of the St Maughan’s Group.
Below the current beach level, fish fossils have been found. The cliff is eroding quite fast as shown by the number of nodules on the beach.

3731a2ab-8d51-42f6-ac0c-a5b9a0510c24

More details can be found in the GCR at

BGS.  England and Wales sheet 233. Solid and Drift.

Lydney 1

Extract from sheet 233 showing Lydney Harbour, a cross section of the anticline and the Silurian / Devonian transition.
We had a long discussion under the cliff about calcrete and it’s polymorphs. Then we wandered further along the shoreline investigating the various strata and noting the calcrete in the cliff above us. It was then time to turn back and retrace our steps to Lydney Harbour. We then decided it was time for lunch so out came our sandwiches.

Then it was time to set off for Huntley Quarry. This took us about half and hour, we regrouped by Huntley Church. Unfortunately the garden centre which used to provided parking, drinks and food is no longer. We set off up the hilly path by the side of the school into a beautiful wood. We followed the path along to the Geological Reserve which is owned by Gloucestershire Geology Trust. We read the interpretation bards along the way so we knew a little of the structural geology by the time we got to the quarry. This was just as well as nature had invaded the quarry and some of the rock faces couldn’t be seen. Never mind, we could see and access the main face so spent some time investigating the many features of sites 2 and 3. Site 1 was completely covered by vegetation.

daf3b13a-3293-4f4f-91e5-618d45422ce3

I won’t go into the detail of the structural geology here as that would just be repeating the text from the excellent guide that is available from Gloucestershire Geology Trust – £2.00 + £2.00 P +P.

Suffice to say, it will keep geologists and other interested people investigating for hours as the rocks vary from the Late Ordovician 485 – 443 mya to the Triassic 251 – 201 mya.  These include volcanic ash. Then there were the tectonic movements of the Blaisdon Fault and the accommodation movements, including thrust faults, that were partly caused by the Variscan Orogeny 390 – 310 mya.

Hunt

After much discussion, we walked along to Ackers Quarry to see the beds of Triassic Bromsgrove sandstone. We then returned to the cars and the trip home after a very interesting day

References

Guides

Huntley Quarry  – Geological Reserve Guide – Gloucestershire Geology Trust

Huntley Quarry – Teacher’s guide – Key stage 3 notes – Gloucestershire Geology Trust

Lydney Town and Harbour – Trail Guide – Gloucestershire Geology Trust

Maps

Outdoor Leisure 14 – Wye Valley & Forest of Dean – Ordnance Survey

Monmouth, England and Wales sheet 233 – Solid and Drift – BGS

Geological Conservation Review GCR

Lydney OS SO 652017 – http://www.jncc.gov.uk/page-2731 – Fossil fishes of Great Britain. Chapter 3: Late Silurian fishes sites of the Welsh Borders. – GCR

Information on Calcretes
Various sources inc: Dr Dave Green field trip of 9th March 2008
Dr Nick Chidlaw, “Soil Evolution, Arid and semi arid climates, diagrams”Advertisements

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Winford Ochre & Iron Oxides

We met at Red House Farm where Melanie and Lionel Patch had kindly allowed us to park our cars while we went off to inspect the quarry.

The idea of the trip was two fold. One was to look at the industrial archaeology and secondly to look at and understand why the ochre was there and how it related to the surrounding geology.

After a short briefing on the area’s geology we formed up in single file, because of the narrow roads with blind bends and set off to find the quarry. We got as far as the end of the farm track when someone spotted a mill stone so we had to have a quick look and discussion before moving on with promises to see more mill stones in the quarry.Unlike the lead and zinc ores, many of the iron ore deposits are secondary deposits. Intense weathering of the iron pyrite-rich Coal Measures, and other iron bearing rocks during Permian and Triassic times released the iron into the groundwater. The iron was subsequently redeposited as many thin discontinuous veins of haematite or pyrite, within the Carboniferous Limestone and the Dolomitic Conglomerate, ( MMG /MMMF ) and especially along the unconformity between the two. Many of these pyrite veins have now been altered to form limonite or ochre. Ochre also occurs infilling cavities in the Carboniferous Limestone and Dolomitic Conglomerate, or as a replacement ore-body, where metal-rich ground-waters have chemically replaced the host rock with iron ore.

The colour of ochre depends on the type of iron oxide and the impurities in the clay.

Yellow ochre is normally Limonite, ( rust ) which is a hydrated form of Goethite. As the percentage of Haemetite increases the colour changes from yellow to orange, red, purple and finally black.

Iron56 is the most common isotope of iron. About 91.754% of all iron is iron-56. … This means that as the Universe ages, more matter is converted into extremely tightly bound nuclei, such as 56Fe.

Iron is a “special” element because of its nuclear binding energy. The idea is that when you fuse two light elements together, you get a heavier element plus energy. You can do this up to iron. Similarly, if you have a heavy element that undergoes fission and splits into two lighter elements, you also release energy. Down to iron. The physical reason for this has to do with the balance between nuclear forcesand the electromagnetic force.

Due to the way these energies work, and because iron is thus thought of as the most stable, if you want to get energy from fusion or fission, your best bet is to use atoms that are farthest away from iron — very light (like hydrogen) or very heavy (like uranium).

As a side note, this is also why Type 2 supernovae happen — the star can no longer gain energy from fusion because it can’t fuse past iron, so the outward pressure from energy generation stops and the star collapses. 

This will happen to the sun soon – as the sun runs out of hydrogen and starts producing heavier elements until it gets to iron and then fusion will stop and the sun will collapse – in about 5 billion years time…

Glossary – Minerals

Botryoidal / Reniform

Texture or mineral habit is one in which the mineral has a globular external form resembling a bunch of grapes as derived from the Greek botruoeidēs. This is a common form for many minerals, particularly haematite, the classically recognised shape.

Boxwork

Honeycomb pattern of limonite (a mixture of hydrous iron and manganese oxide minerals) that remains in the cavity after a sulfide mineral grain has dissolved. The boxwork may be spongelike, triangular, pyramidal, diamond – like, or irregular in shape and may be coloured various shades of ochre and orange through dark brown. The colour and shape of the boxwork can sometimes be used to identify the dissolved sulfide minerals

Druse

Refers to a coating of fine crystals on a rock fracture surface, vein or within a vug or geode.

Limonite – FeO(OH) – nH2O

This a hydrated version of Goethite. It is a major component of rust and is yellow to brown in colour. Mined as yellow ochre. eg Winford quarries.

Pyrite – FeS2

Iron sulfide – fool’s gold. Often found in anoxic, shallow seas. Easily oxidised so specimens often decay.

Siderite – FeCO3

Iron carbonate.  48% iron so a valuable iron ore

Haematite – Fe2O3

This mineral is one of the most important ores of iron. It can vary in colour from metallic grey to bright red. It is a form of ferric oxide Fe2O3.. It  is the oldest oxide of iron ever to have formed on the earth. Its occurrence is widespread in rocks and soils. It is harder than pure iron. It has been used throughout history as a pigment.

It occurs in several forms. – Botryoidal or kidney ore, magnetite, iron rose and specularite

Goethite  – (FeO(OH) 

Is a hydroxide of iron that has also been used as a pigment – brown ochre. Its chemical formula is (FeO(OH). It contains iron of ferric form.Its main use is as iron ore and is also the source mineral for yellow ochre. Colour is yellowish to dark brown and black.

Most often in botryoidal, reniform, or stalactitic aggregates of radiating crystals or ball-like crystals. Also grainy, in veins, concretionary, oolitic, and in earthy masses. It often assumes the shape of other minerals forming a pseudomorph in place of the original mineral or as a coating above it. It is the main component of rust and bog iron ore. It forms prismatic needle-like crystals ( Needle iron ore ) acicular.

Geode

Are geological secondary formations within sedimentary and volcanic rocks. Geodes are hollow, vaguely circular rocks, in which masses of mineral matter (which may include crystals) are secluded. The crystals are formed by the filling of vesicles in volcanic and sub-volcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same, or other minerals precipitated from water, groundwateror hydrothermal fluids.  Sometimes known as “Bristol Diamonds” in the Bristol area.

Gossan

Rust-coloured oxide and hydroxide minerals of iron and manganesethat cap an ore deposit. Gossans form by the oxidation of the sulfide minerals in an ore deposit and they thus may be used as clues to the existence of subsurface ore deposits. especially if distinctive boxworks are present.

In addition to hydrous oxides of iron and manganese, gold and silver in the native (natural, nearly pure) state and various sulfate, carbonate, and silicate minerals can occur in gossans. The hydrous oxide minerals occur as the residuum when sulfide minerals are dissolved from the outcrops; they are either indigenous (i.e., fixed at the site of the original sulfide mineral) or transported. Indigenous hydrous oxides indicate the presence of copper, whereas transported hydrous oxides indicate its absence or its presence in very low proportion to iron and manganese. 

Harptree Beds.

Found on Felton Common and behind Leighdown Farm ( quarry ). Also in large beds on the Mendip Plateau where across much of the central Mendips, outcrops of the Jurassic Lower Liassic and Inferior Oolite limestones have been replaced by chert. These cherts are known collectively as the ‘Harptree Beds’. These cherts are very hard and have been quarried from a small outcrop behind Leighdown Farm – near the “Crown” pub. – use unknown. These cherts have been formed by the process of Metasomatism. ie they have been metamorphosed but by hot gas or fluids rather than the more usual heat and pressure. This happened during the Missisippi Valley Mineralization when the mineralisation of the Mendips took place.

Vug,  

A small to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartzcalcite, and other secondary minerals. Open spaces within ancient collapse breccias are another important source of vugs. Vugs may also form when mineral crystals or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids. The inner surfaces of such vugs are often coated with a crystal druse. Fine crystals are often found in vugs where the open space allows the free development of external crystal form. The term vug is not applied to veins and fissures that have become completely filled, but may be applied to any small cavities within such veins. Geodes are a common vug-formed rock, although that term is usually reserved for more rounded crystal-lined cavities in sedimentary rocks and ancient lavas.

Processes

Levigation using an edge runner mill

  is the process of grinding an insoluble substance to a fine powder, while wet. The material is introduced into the mill together with water, in which the powdered substance remains suspended, and flows from the mill as a turbid liquid or thin paste, according to the amount of water employed. The amount of grinding depends on the particle size required. The slurry is then fed to settling tanks where the water is drawn off.

Calcining

is also used to mean a thermal treatment process in the absence or limited supply of air or oxygen applied to ores and other solid materials to bring about a thermal decomposition.

There are examples of the mill stones used for levigation, near the entrance to the quarry. They are granite, which is unusual. Most mill stones are made from Carboniferous Sandstone from the Eponymous Millstone Grit strata from Derbyshire and Cumbria.

This was an edge runner type of mill where the stones are mounted vertically and run around a deep dish. The stones were often fitted with iron tyres to extend their life. It is difficult to carve this size of stones from granite so they are “frenched” – this means that parts are connected by iron staples – it does not mean that the stones came from France. Their origin is unknown but as there are large feldspar crystals in the granite and Darmoor is not too far away, it seems likely that they were made from the Giant Granite on Dartmoor, which is the biggest emplacement of granite in Britain.

There were also four water powered mills along the Winford Brook. These were used for processing the ochre from the many pits and mines in the area. It is worth noting that some mills were only used for yellow ochre as yellow was so easily contaminated by the darker shades.

One of the mills at Upper Littleton was used to produce gunpowder, using the Saltpetre that was shipped into Bristol from India. This mill was built to replace the city centre mill near the present site of Temple Meads Railway station. ( Knight’s Templars meadows )  This was in the medieval building Tower Harratz. This is now under the foundations of the old Bristol and West building. It was thought too dangerous to keep a gunpowder factory in the centre of Bristol so all ships carrying explosives, including saltpetre, had to unload their cargoes at the Powder House at Pill before sailing up the Avon to the Bristol Docks. It may be that the encircling mines and quarries near Winford gave a ready market for some of the gunpowder. Stocks were kept in the explosives store near the spoil tip which can still be seen.R

References

BIAS JOURNAL No 26 1993 Winford Ochre and Oxide Peter Addison

Earth Colours Marie Clarke, Neville Gregory & Alan Grey

Mendip and Bristol Ochre Mining.  – Available for ordering from the Mendip Cave Registry and Archive ( MCRA )

https://www.mcra.org.uk/wiki/doku.php?id=for_sale– £9.00

BGS Bristol Geology  Map S & D Sheet 264

BGS Bristol and Gloucester region geology – memoir,

OS Map. Bristol West and Portishead. Explorer No. 154
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© Richard Kefford 2020 Eorðdraca

My books are for sale here:  Richard

Charterhouse ore field on Mendip

Meeting up

We met where the road turns a sharp bend around Velvet Bottom. We dressed in waterproofs, woolly hats and gloves as, while the clouds looked fairly thin, there were sharp showers predicted during the day. The first topics of conversation were, as always, “Which route did you take?” and “ do you think the weather will hold?” Where have you been since we last met – Thailand, Djakarta and Bristol were the replies. We had a couple of showers but mainly walked under leaden skies.

Having sorted this out, the eight of us trooped up to a high point in the area – in the middle of the SSSI called Ubley’s Rakes Warren. This is an SSSI because of the rare lead resistant plants contiguous with the lime loving plants such as mosses and liverworts; and the underlying cave systems.

From this point it was possible to see the “Gruffy Ground”, a local names for the landscape left after many years of mining. The layout and formation of the Rakes was explained to us, as was the Mineralization – why was the lead ore, Galena, Lead Suphide, here in the first place? To understand this we had to go back 300 million years to the  Carboniferous Period.

Lead 1

Key to numbers
10 – Car park
11 – Smelting plant and flues
12 – Upper Flood Swallet
13 – Waterwheel Swallet
14 – Black Rock Limestone
15 – Culvert

Tectonic context

Partway through the Carboniferous Period there was the start of the Variscan Orogeny. This was felt in the British Isles as pressure from the South West, In this area it resulted in the uplift of the Mendips, the formation of Broadfield Down and lesser folding, examples of which can be seen on Portishead Beach.

This pressure faded towards the end of the Permian Period and eventually reversed, putting the strata under tension. This resulted in crustal extension and actively subsiding rift basins. This tension and subsidising continued through until the late Jurassic. We saw isolated rocks with calcite-filled tension gashes.

A feature was the Somerset basin, which formed between the Avon platform to the North and the Cornish Platform to the South West. This Somerset Basin infilled rapidly with Jurassic sediments. The basin waters were squeezed out onto platforms via tension structures. These basinal fluids at C.1000C, saline, migrated into the platforms, reached impermeable ceilings, ponded and cooled. Ores precipitated out as they mixed with the cool ground waters. The minerals are therefore found in caves, tension structures, faults, joints and fissures.

Stratigraphy

The lead ore here was found in the Rakes that trend NW – SE. These are fissures in the limestone formed during the tension event mentioned above.  They rapidly filled with local minerals and erosion products. The galena lead ores found in these rakes are therefore placer or secondary deposits – ‘an accumulation of valuable minerals formed by gravity separation during sedimentary processes.’

https://en.wikipedia.org/wiki/Placer_deposit

The limestone here is Black Rock Limestone, BRL. It is dark, as its name suggests, is richly fossiliferous with crinoids, stems and ossicles, and Zaphrentites corals and some brachiopods.. Slicified limestones can be seen, pointing to localised metasomatism. ‘Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place.’ There are several examples of this across the Mendips, examples are Felton Common on Broadfield Down, near Bristol airport and the Harptree Beds to the South of Smitham Hill. The famous Devil’s punchbowl sink hole is developed in these beds.

A rich, varied flora has developed here because of the juxtaposition of alkaline limestone rocks with the acid loessic soils – wind blown sand, mainly from the Sahara, which are common on the Southern flanks of the Mendips

http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=BRL

The ground here may be damp from recent rain but there is little or no surface water because the BRL is permeable.

crinoid-1331665_960_720

 Complete Crinoid fossil

Zaphrentites

Zaphrentites Corals

Brachiopods

Brachiopods

Discussions

After a talk to help us understand the geological processes that had formed the lead ore deposits we walked into one of the rakes to find and observe the many fossils. We found many specimens of crinoids, corals and brachiopods.We then walked over to the deepest and most extensive rake to the East of the SSSI. We concluded that the scientific evidence showed that this trench in the BRL had been dug by the devil when he was trying to stop the lead mining by flooding the rakes.

It was interesting to note that the rocks on the North side of the rake were bedded and jointed while those to the Southern side were mainly massive – we did not arrive at an explanation for this. Our lichen expert pointed out how the lichens differed on the different vertical sides, presumable because of the different conditions such as sunlight and rain. We also had a short talk about how each type of lichen – one of thousands – was composed of a synergy between a fungus and an alga.

As we walked back to the reserve entrance, we saw several old mine shafts, protected by padlocked steel grids, thus emphasising what a dangerous area this is.

Lead processing

We then walked across to the car park which has an excellent information board and an imagined picture of the area when it was a working industrial landscape. Just by the car park we examined an outcrop of rocks from the Avon Group.

http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=AVO

These are Lower Limestone Shales and underlie the BRL. The LLSs are   impermeable because they are mudstones so there are an increasing number of puddles in this area and dams which form the lakes that were used as reservoirs for the water needed for washing the ores in the buddles. Near the car park are remains of the mine managers house which was called Bleak House – one of many we assumed.

After absorbing some of the information we walked along the ore tramway to the remains of an old smelting plant and flues which were in use until 1878.. Here a steam driven fan forced hot air over the lead-rich slag and slime from earlier mining operations. The vaporised lead condensed in the flues and was removed by hand, a particularly  unpleasant and dangerous job. We could see up to the end of the flues where they were still roofed.That made us feel how uncomfortable it must have been, bent over, probably in the dark with only candles, scraping the lead off the walls.

The dammed reservoirs had leats leading off from them and theses sank underground at the contact with the permeable BRL near the car park. There are two gated caves nearby, Upper Flood Swallet and Water Wheel Swallet. The water from both of these swallets and cave systems eventually emerges from underground at Cheddar. Walking towards the reservoirs, we came to the banks of slag left from the processing. The banks consist of lumps of black stones, some of which are shiny, like obsidian. It has a high lead, zinc and cadmium content and a low level of plant nutrients and so is poisonous to most plants. This means that the plants that do grow there are highly specialised and nationally rare. They are metal tolerant and form a low growing mat of lichens, mosses and tolerant vascular plants such as alpine penny-cress, herb Robert, and common whitlow grass. There are also many lichens of the Cladonia genus and several species that are normally found on siliceous rocks in upland areas.

References

During the preparation of this trip and the trip report, much use was made of the “Walkers’ guide to Western Mendip” and the associated geological map.
This was written by Dr Andy Farrant of the BGS, Keyworth, Nottingham, British Geological Survey. ISBN 978 085272576 4

Additions

Here are a couple of additions about smelting and refining lead from the Charterhouse mines.The slime referred to is the ‘Anode slime’ where valuable by products such as silver accumulate when lead electrolysis – using lead total loss anodes –  is used. This means that the slime may have a higher value than the basic lead. I think this is the process used to recycle car batteries.“The electrolytic refining of lead bullion from soluble anodes has been practiced for years in a number of large plants. Because of poor solubility, solutions have been restricted to the lead salts of fluosilicic acid, fluoroboric acid and amido-sulfuric acid. Metals with a higher electrochemical potential than lead (silver, gold, copper, bismuth, antimony, arsenic, and germanium) do not dissolve and accumulate in the anode slime that is processed to recover these valuable by- products.”http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.518.2254&rep=rep1&type=pdfAn interesting site about lead working in Bristol.http://brisray.com/bristol/lead.htmAdvertisements

© Richard Kefford         2020         Eorðdraca.         

My books are for sale here:         Richard

The Sea Dragons of Street

Yesterday I joined up with a friend of mine to travel to Street in Somerset to see the wonderful display of “Sea Dragons”

During the age of the dinosaurs, the ocean was home to many types of ichthyosaur or ”sea dragon”.

They appeared in the Triassic, reached their peak in the Jurassic, then disappeared in the Cretaceous – several million years before the last dinosaurs died out. 

Ichthyosaurs were among the first skeletons to be discovered by early fossil-hunters, at a time when theories of evolution and concepts of geology were in their infancy.

The famous fossil hunter Mary Anning discovered the first complete fossil of an ichthyosaur in the cliffs near Lyme Regis, Dorset, in 1810.

Her discovery shook up the scientific world and provided evidence for new ideas about the history of the Earth.

The display we went to see was a private collection of Icthyosaurs at Street which is fitting as they were all found in the quarries in Street where the Jurassic Blue Lias building stone came from. This stone can be seen in many of the buildings around the town. The collection was only open for a week so we jumped at the opportunity to see these wonderfully preserved fossils. Details of the collection can be found here:Sea Dragons of Street Exhibition

If you get a chance, I would certainly suggest you go. There are some 15 – 20 specimens there and they are very well preserved – even down to the colour of their teeth. They are mounted in near vertical display cases – without glass! – so all the details can clearly be seen.

Picture credit – Chris Martin

© Richard Kefford         2020         Eorðdraca.         

My books are for sale here:         Richard

The quest for the Golden Spike

Geology is sometimes seen as a boring, stuffy science dominated by scruffy old men with beards – not by me, I hasten to add –  so how about this for a change of image to change and expand your mind?

Geology is the study of the Earth and, as the earth has a long history of some 4.56 billion years, it makes sense to split that long history into chunks – just like English history is split into the Stone age, Bronze age, Jacobean, Tudor, Elizabethan etc. Because geology is international and the same rocks are exposed at different places, in different countries around the world, ages and stages have to be decided internationally. This done by the International Commission on Stratigraphy ( ICS ) – a part of the International Union of Geological Sciences ( IUGS )

Screen Shot 2017-07-03 at 08.16.16

Nearly everyone has heard of  Jurassic, Triassic and Devonian times but the problem is defining the start and length of each of these. These are also split into Epochs which are often called – not always – Upper, Middle and Lower. These are then split into Stages, then Zones and then sub Zones. The transition from each of these to the next is called a point.

For the purposes of this piece we will talk about the point that is at the transition from the Hettangian to the Sinemurian Stage in the Lower Jurassic Epoch, in the Mesozoic Era, in the Phanerozoic Eon. This point has been researched and defined to be 199.3 +/- 0.3 million years ago ( mya ). This defined point is also called a Golden Spike. It is shown as such on the International Stratigraphic chart. It is more fully known as a Global Boundary Stratotype Section and Point ( GSSP )

Screen Shot 2017-07-01 at 12.57.50

I’ll come to how this is worked out later but the problem now is finding a place somewhere in the world that exposes this point. It has to have several attributes: The succession must be exposed, it must be complete – and have several others. Semur-en-Auxois in Eastern France was proposed in 1842 but later research found that the strata was incomplete. Then in 18956-1858 then in 19061 and 1971 the coastal cliffs west of Lyme Regis were proposed. A later study in 1972 and then in 1984 found an exposure on the West Somerset coast that had five times the strata thickness of Lyme Regis. This exceptionally complete sequence, as best exposed north of East Quantoxhead, was proposed as the GSSP for the base of the Sinemurian Stage in 1995. Among the known sections across the Hettangian / Sinemurian boundary elsewhere in the world where sedimentation is believed to be continuous, the GSSP at East Quantoxhead offers the most complete succession of relatively well preserved ammonites. This section meets the requirements of the ICS for a GSSP. It has therefore been accepted as a GSSP by the Sinemurian Boundary Working Group, the ISJS and the ICS – almost unanimously – and was finally ratified by the IUGS in August 2000.

The location of the Golden Spike is described as 0.9 m above the base of bed 145 coinciding with the first appearance of the genera Vermiceras and Metophioceras. These are types of ammonites.

Fig-1-The-main-chronostratigraphic-subdivisions-of-the-Jurassic-System-the

This is the first ever Jurassic Golden Spike in the UK. There have since been others, one in Robin Hoods Bay on the Yorkshire coast and one provisional one on Skye. The complete worldwide list is here.

http://www.stratigraphy.org/gssp/

Going back to how the age of this boundary was worked out:

The Cliff section at East Quantoxhead – and other exposures of the same age around the world contain fossils of ammonites. Ammonites are very useful for establishing comparative dates as they were spread around the ancient world and they evolved at a very fast rate. This means that different ammonite fossils occur in quite precisely known strata. These have all been charted so that a stage change eg Hettangian to Sinemurian in the lower Jurassic can be defined by the evolution of fossils.

This is explained in more detail here

There are, of course, other checks that can be carried out to confirm this. These include: Ostracods, foraminifers, palynomorphs, magnetostratigraphy and gamma ray log.

If you do follow this quest to East Quantoxhead, please keep well clear of the very fragile cliffs as there are often rock falls.

This coast is also a Site of Scientific Interest ( SSSI) so no hammering or fossil collecting from the cliffs is allowed. Please leave this beautiful nature for future generations to enjoy.

If you wish to find out more, I suggest you download the paper by Gert Bloos and Kevin Page ( March 2002 ) Global Stratotype Section and Point for base of the Sinemurian Stage ( Lower Jurassic ) which is freely available on the web.

© Richard Kefford 2020 Eorðdraca

My books are for sale here: Richard

The Iron Mine

This is the report of a geology trip by the Bristol U3A Geology Group to the Providence iron mine at Long Ashton. Long Ashton New Providence Iron Mine Ashton Hill Iron Mine No.2 ST 535 709 New Providence Mine “Mine below Providence Mine. Part choked entrance in pit. 19C red ochre mine. Fine passage with deads and pit props leads to Red Rift with bedding chamber and pool. 2m active micro gour slope, cave pearls and calcited twigs in main passage.” Iron Ore is mainly Hematite,  FeO3 and Red Ochre When the mine was worked out, many miners went to work in the nearby Durnford Limestone Quarry. Iron in the form of hematite and earthy red ochre was mined at Providence Iron Mine, in a field known as the Iron Plantation. Yields varied from 600-3000 tons of ore per annum between 1858 and 1878. Mining continued here until the First World War. Reports from cave explorers in the 1950s refer to an enormous main rift (ST 5350 7093) leading to a partially choked adit entrance (ST 5370 7070) and further workings below. The Miners Rest on Providence originated as a cottage, owned by the BEAMES family, where miners could obtain refreshment. Providence mine also produced Baryte – Barium Sulphate  – Ba SO4 Notes The iron minerals here are in veins in the Hotwells Carboniferous Limestone. This known as part of the Bilbao Supergene Mineralisation . There is a lot of info on this on the Internet. ( Google Bilbao Supergene Mineralisation for more info )The crustal extension in the Early Permian ( C. 290 Ma) to the Late Jurassic ( C. 150Ma ) created rift basins.  This was caused by the crustal relaxation after the Variscan Orogeny, These created more stable platforms between the rift basins. The basins subsided and infilled rapidly with sediment. Basin waters squeezed out onto platforms via tension structures thus allowing mineralised hot waters to flow up into the Triassic sediments, leaving mineral deposits. New Providence Iron mine is in one of these giving rise to localised iron ore sediments.  There is another vein of iron ore shown on the BGS map. Sheet 264. but we saw no sign on the surface that this deposit had been worked. Unlike the lead and zinc ores, many of the iron ore deposits are secondary deposits. Intense weathering of the iron pyrite-rich Coal Measures, and other iron bearing rocks during Permian and Triassic times released the iron into the groundwater. The iron was subsequently redeposited as many thin discontinuous veins of haematite or pyrite, within the Carboniferous Limestone and the Dolomitic Conglomerate, and especially along the unconformity between the two. Many of these pyrite veins have now been altered to form limonite or ochre. Ochre also occurs infilling cavities in the Carboniferous Limestone and Dolomitic Conglomerate, or as a replacement ore-body, where metal-rich ground-waters have chemically replaced the host rock with iron ore. Iron Oxides There are several oxides of iron and each has several polymorphs so iron is a complicated subject. Iron56 is the most common isotope of iron. About 91.754% of all iron is iron-56. … This means that as the Universe ages, more matter is converted into extremely tightly bound nuclei, such as 56Fe. Iron is a “special” element because of its nuclear binding energy. The idea is that when you fuse two light elements together, you get a heavier element plus energy. You can do this up to iron. Similarly, if you have a heavy element that undergoes fission and splits into two lighter elements, you also release energy. Down to iron. The physical reason for this has to do with the balance between nuclear forces and the electromagnetic force. Due to the way these energies work, and because iron is thus thought of as the most stable, if you want to get energy from fusion or fission, your best bet is to use atoms that are farthest away from iron — very light (like hydrogen) or very heavy (like uranium). As a side note, this is also why Type 2 supernovae happen — the star can no longer gain energy from fusion because it can’t fuse past iron, so the outward pressure from energy generation stops and the star collapses.  This will happen to the sun soon – as the sun runs out of hydrogen and starts producing heavier elements until it gets to iron and then fusion will stop and the sun will collapse – in about 5 billion years time. Haematite – Also spelled Hematite. This mineral is one of the most important ores of iron. It can vary in colour from metallic grey to bright red. It is a form of ferric oxide Fe2O3.. It  is the oldest oxide of iron ever to have formed on the earth. Its occurrence is widespread in rocks and soils. It is harder than pure iron. It has been used throughout history as a pigment. It occurs in several forms. Botryoidal or kidney ore, magnetite, iron rose and specularite Goethite   Is a hydroxide of iron that has also been used as a pigment – brown ochre. Its chemical formula is (FeO(OH). It contains iron of ferric form.Its main use is as iron ore and is also the source mineral for yellow ochre. Colour is yellowish to dark brown and black. Most often in botryoidal, reniform, or stalactitic aggregates of radiating crystals or ball-like crystals. Also grainy, in veins, concretionary, oolitic, and in earthy masses. It often assumes the shape of other minerals forming a pseudomorph in place of the original mineral or as a coating above it. It is the main component of rust and bog iron ore. It forms prismatic needle-like crystals ( Needle iron ore ) acicular. Limonite. This a hydrated version of Goethite. It is a component of rust and is yellow to brown in colour. Mined as yellow ochre. eg Winford quarries. Pyrite Iron sulfide FeS2 – fool’s gold. Often found in anoxic, shallow seas. Easily oxidised so specimens often decay. Siderite Iron carbonate FeCO3.  48% iron so a valuable iron ore Glossary for 17th January 2019 Botryoidal / Reniform Texture or mineral habit is one in which the mineral has a globular external form resembling a bunch of grapes as derived from the Greek botruoeidēs. This is a common form for many minerals, particularly haematite, the classically recognized shape. Boxwork Honeycomb pattern of limonite (a mixture of hydrous iron and manganese oxide minerals) that remains in the cavity after a sulfide mineralgrain has dissolved. The boxwork may be spongelike, triangular, pyramidal, diamondlike, or irregular in shape and may be coloured various shades of ochre and orange through dark brown. The colour and shape of the boxwork can sometimes be used to identify the dissolved sulfide minerals Druse Refers to a coating of fine crystals on a rock fracture surface, vein or within a vug or geode. Ferrocrete A form of Calcrete where iron is emplaced instead of calcium Fluting is a process of differential weathering and erosion by which an exposed well-jointed coarse-grained rock such as granite or gneiss, develops a corrugated surface of flutes; especially the formation of small-scale ridges and depressions by wave action. Geode Are geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely circular rocks, in which masses of mineral matter (which may include crystals) are secluded. The crystals are formed by the filling of vesicles in volcanic and sub-volcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same, or other minerals precipitated from water, groundwater or hydrothermal fluids.   Gossan Rust-coloured oxide and hydroxide minerals of iron and manganese that cap an ore deposit. Gossans form by the oxidation of the sulfide minerals in an ore deposit and they thus may be used as clues to the existence of subsurface ore deposits. especially if distinctive boxworks are present. In addition to hydrous oxides of iron and manganese, gold and silver in the native (natural, nearly pure) state and various sulfate, carbonate, and silicate minerals can occur in gossans. The hydrous oxide minerals occur as the residuum when sulfide minerals are dissolved from the outcrops; they are either indigenous (i.e., fixed at the site of the original sulfide mineral) or transported. Indigenous hydrous oxides indicate the presence of copper, whereas transported hydrous oxides indicate its absence or its presence in very low proportion to iron and manganese.  Pseudomorph In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another. The name literally means “false form”. Scalloping A sedimentary structure superficially resembling an oscillation ripple mark, and having a concave side that is always oriented toward the top of the bed. Also known as a scallop. Variscan Orogeny A geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. It is seen in our area as pressure from the South West towards the North East. Vug,  is a small to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartzcalcite, and other secondary minerals. Open spaces within ancient collapse breccias are another important source of vugs. Vugs may also form when mineral crystals or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids. The inner surfaces of such vugs are often coated with a crystal druse. Fine crystals are often found in vugs where the open space allows the free development of external crystal form. The term vug is not applied to veins and fissures that have become completely filled, but may be applied to any small cavities within such veins. Geodes are a common vug-formed rock, although that term is usually reserved for more rounded crystal-lined cavities in sedimentary rocks and ancient lavas.[2]  © Richard Kefford 2019  Eorðdraca My books are available for sale here:      Richard

West Tanpit Wood

West Tanpit Wood – Lower Failand

A circular walk from St Bartholomew’s Church.

Devonian Portishead Beds – Upper Old Red Sandstone

Devonian Black Nore Sandstone – Lower Old Red Sandstone

Basal Carboniferous Shirehampton Beds – Lower Limestone Shales of the Avon Group

Triassic Mercia Mudstone Marginal Facies – ‘Dolomitic Conglomerate.’

Quarry 

Tufa Dams                 

Springs

Spring line

Unconformity

Carboniferous fossils

Park on the verge outside St Bartholomews, Lower Failand. 

Best map is OS Sheet 154 Bristol West and Portishead.

Enter the field via a stile directly opposite the Tee junction. Walk down the hill, keeping close to the hedge field boundary on your right. At the right hand corner of the field surmount another stile and then walk downhill to a gate under a big tree. After the gate there is another with a wall stretching off to your right for some 50M. Examine closely the stone blocks of which the wall is made. This is quite a coarse sandstone, as a hand lens will show, and also has many clasts of vein quartz included. There are several clues as to what material it is and where it came from. The cross bedding indicates it was laid down in a river – Fluvial sandstone. The grains of sand are polished rather than frosted – so again it is water, not air, borne. The clasts, or pebbles, included in the matrix are of hard quartz but have been eroded so they are rounded or sub rounded, indicating that they have travelled a long distance. There are also some brown pebbles of Jasper. Putting all of this together, it is thought that these pebbles have come a long distance in a powerful river from the North West. Some pebbles have been identified as coming from the Mona complex in Anglesea which is the site of a Pre Cambrian ophiolite, approx. 611 mya – a subduction zone where the ocean sediments of the descending slab are scraped off by the continental plate. This process is known as obduction. There will have been some Andesite extruded above the subduction zone as the entrained seawater heats up as the oceanic slab descends into the deeper, hotter earth.  

This is the Black Nore Sandstone of the Lower Old Red Sandstone. It was probably laid down in the Emsian  Stage of the Devonian Period, 407.6 million to 393.3 million years ago ( mya ). It has minimal fossils in it. The reason for looking at the wall is that there are no exposures or quarries in this strata where it can be seen in situ. The sites of the quarries are surmised but are not definitively known. ( NOTE: See below )

To summarise, volcanic rocks were eroded from the andesitic volcanoes of the Caledonian Orogeny, the mica and feldspar were softer and so were eroded away, leaving the harder quartz grains as the rocks matured. They were transported across a vast desert plain by braided rivers to their present location. The clasts were rounded into pebbles as they were tumbled in the rivers.

Walk to the Eastern end of the wall and then turn left and follow the path. Follow this path to the corner of Summer House Wood. Then walk along the edge of the wood until emerging onto the verge of the A369. Turn sharp left and walk a few yards until there is a footpath entrance on to the tractorway. Follow this until a footpath appears on the left into the wood. Follow this path until the old quarry appears on the left. Climb up to approach the quarry face. BEWARE STEEP DROPS IN THIS AREA. The Angular Unconformity between the dipping Black Nore Sandstone ( Devonian Lower Old Red Sandstone ) and the Dolomitic Conglomerate ( Triassic Mercia Mudstone Marginal Facies ) which rests on it , can clearly be seen. This is a time gap of up to about 200 million years. This unconformity can also be seen on Portishead foreshore and across this region. Have a look at the old building across the stream which used to be occupied by the Rosewell family. It is an old mill, rumoured to be a snuff mill.

Follow the path until a footpath and sign appears on the left. Follow this path up the hill and walk back to the concrete trough. If you look up to the right, you will see a field exposure of the BNS. Walk back up the hill to the BNS wall.

Now follow a hedge back up the hill to the road but this time follow a hedge line heading further to the East, keeping it on your left. Check the capping stones on the top of the wall by the cattle trough – are they all Black Nore Sandstones – without fossils? Look at the drop on the other side of the wall. Was this an old quarry? During the winter, when the leaves are off the hedge plants, a wall can be seen in the middle of the hedge. This wall becomes more distinct as the road at the top of the hill is approached. A close inspection will show that it was built using similar stones that have already been seen. Use the stile to get back on the road where a National Trust interpretation board for the Failand Estate can be seen, close to the hedge.

Directly across the road there is a track with a public footpath sign. Follow this track down a steep hill, passing some cottages on the left. At the bottom of the hill, rejoin Sandy Lane , turning right to follow it down to the ford by Mulberry Farm. The farm house garden wall is partly built on exposed bedrock. These are the Portishead Beds and more exposures will be seen later in the walk. Look at and identify the rocks in the wall. The 1837 tithe map shows a tan yard opposite the farm. It is believed that the tannery was built and run by the St Augustine monks who also created and ran the fishery at Abbots Pool.

Turn right at the Farm and follow the footpath through a gate, keeping the wood and stream on your left. The stream is called Markham Brook. It flows into the River Avon at Pill. Go through a gate into the wood. Just in front of you is a bridge across the stream. Cross the bridge, to the left a small pump house can be seen. A look inside will show the pump housing while in the side of the stream an iron pipe can be seen. This worked on the hydraulic ram principle. The pressure in the pipe from higher up in the stream increased until it was high enough to trigger the ram with an audible thump – thus pumping water up the hill to a storage tank near to where it was needed. One of these tanks can be seen by the side of the road on the way back to the church. The use of this water supply to Lower Failand continued until the 1950’s. The tile on the top of the pump house is embossed with ‘Danger. Baldwin. Electricity’ so assumedly the pump was converted from a hydraulic ram to an electric pump at some stage.

Follow the stream until you see a second bridge. Cross this bridge, turn to the right and follow a path along a gully until a fallen tree can be seen. Look to the right at the stream and look for a tufa dam in the stream. The water has passed through the limestone, dissolving carbonate minerals. Where it passes over a cascade, carbon dioxide is released. The minerals come out of solution and are deposited as carbonate rock. This slowly builds up to form a tufa dam. This is a similar way that stalactites are formed in Limestone caves and stromatolites in shallow warm seas. These are relatively rare features in the UK so please do not disturb this example in any way. A separate, more detailed explanation, is in the appendix.

West Tan Pit Wood is so called because leather was tanned using the clean water. Pits were dug and lined with oak and used for leather tanning. The tannins leached from the oak bark to soften the hides.

Return to the bridge and walk on to a ’Tee’ junction with another path, noting the sandstone crag exposure to your right. These are Upper Old Red Sandstones from the Devonian period and are known as the Portishead Beds. These are younger than the Black Nore Sandstones previously seen. The also have a different habit in that there are minimal pebble inclusions and the cross bedding is more defined. These deposits were laid down during the Famennian stage of the Upper Devonian period 372.2 to 358.9 million years ago Subtracting the end of the Emsian stage, 393.3,  from the start of the Famennian, 372.2, you get a gap of about 21 million years. During this time either nothing was deposited or something was deposited and then was subsequently eroded away. Either way, there is a time gap between the two strata, this is called an unconformity.

Turn right on to the other path, noting the carved wooden sign. Follow the path to a gate which allows entrance to a grassy area with an artificial circular pond A rest may be taken on a thoughtfully provided seat to enjoy the pool with its backdrop of a small cliff of the sandstone. Walk further on, taking the right hand fork across the grass to see a natural-looking pool with no apparent water supply, even though water is flowing out. It may be fed through a hidden pipe from the spring-fed stream in the garden. This is one of the springs and is flowing out of the Limestone overlying the Sandstone. The Limestone is permeable because of the many joints so the water can flow through it but cannot enter the impermeable Lower Limestone Shales of the Avon Group so emerges at the surface as a spring and runs downhill as a stream.

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 Tufa dams

Follow the path up a steep incline to a path junction at the end of the road, in front of a large garage. The two houses there are called Ferney Row. 

Progress up the hill then turn right and cross the field, keeping the hedge on your left. A gate into a wood will appear. Pass through the gate, which may be surrounded by deep mud and follow the track noting the springs on the hillside to your left and and the rock in the track bed. This is the limestone which rests conformably on the Devonian Sandstone. This is Carboniferous Limestone – Lower Limestone Shales from the Avon Group. This is younger than the Devonian Sandstones. As its name implies, this was laid down in the Carboniferous period, in fact it is the basal strata of the Carboniferous succession. At the beginning of the Carboniferous – approx. 360 mya – the arid terrestrial environment of the Devonian gave way to shallow marine conditions – a marine transgression. The Mendip area became part of a broad, southward shelving, shallow tropical sea that stretched from Belgium westwards into Pembrokeshire. The initial flooding of the region produced the mud-rich Avon Group (Lower Limestone Shale), This is up to 150 m thick in the western Mendips. The dominant lithology is fissile mudstone with limestone inter-beds. The mud-rich nature of the succession reflects the environmental transition from arid desert to shallow sea. Conditions were too turbid to allow the growth of corals, which are a feature of much of the lower Carboniferous succession, but other marine fossils such as crinoids, brachiopods and bryozoans became well-established and are a significant component of limestones in the lower part of the succession, including a marker-horizon known as the ‘Bryozoa Bed’. Ripples, scours and cross-bedding in the limestones show that deposition occurred in a shallow, high energy environment, and some of the limestones are distinctly reddened due to high concentrations of the iron mineral haematite. The higher part of the formation contains greenish-grey shales and black crinoidal limestones, which were probably deposited in a slightly more open-water marine setting.

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Crinoid ossicles in Avon Goup Lower Limestone Shales

Keep an eye on the verges and the track bed, you will be very unlucky if you do not see some brachiopod and Crinoid fossils there. Possibly, if you are lucky, some Bryzoa.. The dip of the strata may be seen in the track bed.

Continue along the lane, pass a wooden bungalow on your right and eventually arrive at Oxhouse Lane complete with the Forestry Commission Wood notice board. Turn to the right and follow the lane back up to the church. Just after leaving the track, you will see on the right an exposure of the Portishead beds showing that this is very close to the contact between the Limestone and Sandstone – hence the springs in this area. The road will lead you up the hill. Halfway up the hill, look back to the hill on the other side of the valley and the road to the wood. You will see a ridge running across the field. This is called a break of slope and marks the transition to the harder Black Rock Limestone from the softer Lower Limestone Shales. The harder and softer rocks are eroded at different rates so forming the ridge. This is called differential erosion. Just before you reach a footpath off the the right, you may be able to see another small pump house hidden in a patch of woodland. When passing Failand House to your right, an inspection of the gate posts will reveal that they are made from Black Nore Sandstone. The house is owned by the National Trust but there is no public access. 

Arriving back outside St Batholomew’s Church, it is worth having a look at the building stones. The church was built by Richard Vaughan in 1887. The areas that require freestones – window frames, statue niches etc. are made from Bath Stone. This is a cream Oolitic limestone from the Great or Inferior Oolite, probably from one of the quarries on Dundry Hill. The walls are built from the local Black Nore Sandstone, the pebbles can clearly be seen. The colour of the walls also hints at the Old Red Sandstone. Inside, the font is also made from Bath stone. The steeple can be seen to be cream rather than red so is probably made from Bath stone as the individual blocks would need to have been shaped during the building process.

It is always worth looking at churches, from the geological point of view, as they were usually built mainly from the closest available suitable stone to reduce costs. Transport was more expensive than the quarrying costs. They are therefore a marker for the quarries and rock to be found locally.

There is a booklet, available for 10p at the church or a web site – see appendix1 at the end of the booklet.

It is also interesting to see that the churchyard is bounded by a wall made from the same Devonian stone. However this wall is topped by a different sandstone. This is the Pennant Sandstone from the Carboniferous period. This gives a delightful colour contrast to the main mass of the boundary wall.

© Richard Kefford    2020                               Eorðdraca

My books are for sale here:      Richard