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 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