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”

© Richard Kefford         2020         Eorðdraca.         

My books are for sale here:         Richard

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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A Dragon’s Walk

We dragons had previously arranged to meet for one of our periodic peripatetic preprandial perambulations. This one had been arranged by Brimdraca so we flew in from the South, East and North to the Weyr meeting place at the South land cliff of Worleburry Hill. There were no Green dragons from the West here today.

Once we had assembled we moved as a Weyr towards the access steps to the hill, stopping to chat to a guardian at the base of the steps. He was a futurologist or archaeologist as they are known in these modern times.

It was quite a steep climb up the hill. How it had changed since the last time we were here. We stopped often for a breather, being careful not set the vegetation alight, exertion can make dragon breath quite inflammatory.

The steep of the slope decreased as we gained the summist plateau. This prevented us from seeing too far out into the estuary of the Severn.We stopped and Baridraca explained the circular patches on the earth, a dearth of growing vegetation under the unnamed trees. The futurologist whom we saw later on our walk explained that they were Holm Oaks which were planted here some years ago as they are a species introduced from the Mediterranean area in the 16th century and are resistant to salt. 

As they are not native, the futurologist explained that he was overseeing a group of volunteers who were slowly clearing the trees and brush with the aim of returning the hill to calcareous grassland. Rare native specimen trees such as Hornbeam will be left in situ.

We also had a discussion about the relict buildings on the small Birnbeck Island which we now overlooked.

After a brief rest we continued our walk up the gently sloping woodland path.

As we slowly walked along, enjoying the warm sunshine now burning off the mist my mind went back to our last visit some 380 million years ago. 

The climate was different then as the area was near the equator. The deltas to the North were covered by tree ferns giving cover to gigantic insects. There was an unpleasant sulphurous smell of anaerobic decomposing vegetation in the air – nothing to do with our hot breath of course.

This was before the land had been uplifted so there was a level area of very wet lime mud. I wondered if the footprints that we had left behind us in the mud would still be here. Yes, they were! We could see the 93 remaining footprints which had later been lined with the indurated lime mud by the early humans many centuries later. The futurologist still thought that these dolines had been dug by the humans as storage pits.

It would have been interesting to have returned after our first visit some time in the late Carboniferous or early Permian to see the Variscan Orogeny form the ridge as the proto-continents of Gondwana jostled for position.

My dreams of the past slowly faded as we arrived at the fort which looked like several piles of stones until the futurologist explained the structures to us. The stones were rocks from the area – Goblin Combe Oolite.

We marvelled at the futility of the humans, labouring to return the fort to it’s “natural state”. What would this state be? Lime mud that we had seen in the early Carboniferous? A volcano rising from the sea, spreading basalt over the land? A new hill arising from the muddy plain? A treeless limestone promontory?  A calcareous grassland? A wood of trees imported from the South? Climate change would decide – probably a Savannah style dry dusty ridge like a South African Kopje.

We turned and retraced our steps back to the steps and slowly walked back to the meeting place – favouring our ageing knees on the steep downess.

It was now time for refreshments so we all got into the Weyrcar and hunted for food and drink. We found an oasis in Dr Fox’s where we recovered after our exertions before flying off to our respective eyries.

© Richard Kefford 2020 Eorðdraca

My books are for sale here:  Richard

Geology Book launch

I had a wonderful experience recently.

I had an e mail from BRERC ( Bristol Regional Environmental Records Centre ) which is the central repository for the environmental data for the region. The Bristol Region covers the West of England area, with Bristol and Bath at its heart. To the North it incorporates Thornbury and Wickwar; to the south, chew Valley Lake and the edge of the Mendip Hills; to the west, Weston – super – Mare and the estuary of the Severn; and to the east are the Cotswold Hills.

The e mail said that BRERC’s latest book “Geological Sites of the Bristol Region” had been published and copies were available to be collected at Bristol Museum.

I rushed over there and duly received my FREE “Author’s copy”, cover price £19.50. To say that I was chuffed is perhaps an understatement.

My contribution to the book was an article on the Redcliffe Caves and a geological poem – Search this blog for “Earth Song.”

I encourage you all to rush out and buy a copy because it is an excellent and well written book by a class act of Earth Scientists, and me, but I understand if you wish to think before buying because of the price. It is a reference book.

This book 5 in BRERC’s series of environmental books.

More information about BRERC can be gained from their web site:

http://www.brerc.org.uk

Here is a sample page from the book.

Geo book 2

© Richard Kefford         2020         Eorðdraca.         

My books are for sale here:         Richard

Tour of Clifton Bridge – including vaults.

 Some history of the vaults.

Steve mentioned to me that he had heard that there were huge vaults under the Leigh Woods abutment of Clifton suspension Bridge. I wasn’t sure of the veracity of his claim and so did what we always do, which is look it up on Google. I would also see if there was any chance of having a look.

DSC00086

He assured me that the story was correct. Ray Brown had been contracted to renew the paving of York Stone around the two towers on the bridge. He knew that the foundations for the Bristol side tower had been stepped to reduce the stone required as the bedrock was close to the surface. He started digging to prepare the ground and came across a couple of big timbers. He hoiked them out and found that they had been covering a chamber – or vault – about 1.5 metres high. Being a contractor and not directly employed by the bridge company, he decided it was safe to have a bit of a lark, so got hold of a demonstration skeleton and propped it up at the far end of the vaulted chamber. He then told the bridgemaster of his find. The bridgemaster was not amused. Ray admitted that it was just a joke and got on with his paving work.

DSC00145

When he had completed the Bristol side, he moved over to the Leigh Woods side and started the same job there. Again he found some timbers so he hoiked them out but this time he found a 2ft diameter vertical shaft. He reported this back to his good friend the bridgemaster and he arranged for a volunteer, Guy Barrett, to be lowered down the shaft to see what was there. They found near horizontal shafts peeling off the vertical shaft in two directions and at two levels. It was very difficult to squeeze around and enter these but they eventually managed it and found several huge vaults under the bridge towers. There were twelve separate chambers in total on three levels. It was then decided to drill a doorway into the side of the abutment to allow a full inspection of the vaults and to later allow guided public tours.

Until the discovery of the vaults in 2002, the abutment was thought to be solid, filled with rubble, rock debris and lime cement. How wrong can you be?

The tour

DSC00088

Steve and myself booked a tour on Tuesday 24th July 2018. This tour was led by Dave who is a retired structural civil engineer so he was well up to answering the questions from the group of ten of us. The first half of the tour was on the surface covering the long history of the on / and off building of the bridge. This was very interesting but we had really come to see the vaults. After an hour, we had finished the surface tour and so trooped up to the visitor centre to be kitted out with a hi vis waistcoat and a hard hat. We also had to sign a safety waiver saying that we accepted the risks involved and were fit enough to cope with climbing a vertical ladder and negotiating a narrow passage.

DSC00133

We then followed Dave down a gravel path that followed the slope of the gorge side to the new entrance door which had been cut out using boring tools and strengthened by rock bolts. We also had to negotiate our way down  a near vertical ladder some 3m high. Dave switched on the lights inside the vault and led us into vault No 4. It was very impressive. A high vaulted chamber constructed of local stone from the adjacent limestone quarries on the banks of the avon. The exposed external facing stone was dressed Devonian Sandstone that had been carried to the site from quarries on the Leigh Court estate. This was loaded into the redundant celestine dramway from the estate down to Miles dock. This had previously been used to load barges with celestine, strontium ore, for transport down river to Avonmouth for export to Germany for sugar processing. The stone was then loaded into barges for the short trip up river to the bridge site. The stone was then hauled up a dragway to its point of use.

DSC00139

The vault was cool compared to the outside and there was still some dripping from the roof in spite of the recent drought. The most impressive feature of the vaults was the plethora of pencil calcite stalactites hanging from the vault roof. They are up to 7 m long. There are, of course, the complementary dumpy stalagmites on the floor of the vaults.

After gazing our fill of vault 4, we crouched and shuffled through a short, low tunnel to vault 5. This was similar to the previous vault but a lot bigger and the 2ft shaft entries could be seen. After the necessary inspecting, understanding and picture taking it was time to retrace our steps back to the visitor centre to return our hi vis jackets and hard hats. 

A very interesting and worthwhile tour.

Further reading and links.

https://www.cliftonbridge.org.uk/vaults – excellent videos.

https://www.cliftonbridge.org.uk/event/hard-hat-tour-leigh-woods-abutment

© Richard Kefford         2020         Eorðdraca.         

My books are for sale here:         Richard

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

Time

Time

We have plenty of time. We can shrink it or stretch it out just by our actions. Think back to when you are doing something that really interest you, everything else shrinks to the background. When you have completed what you wanted to do so desperately, you look and realise that  a lot of time has passed without you realising. In the flow they call it. But wait, do we really have plenty of time. There is plenty of time, about 13.8 billion years has passed since the universe was born but we do not have access to all of it. Once we were born we are allowed perhaps 100 years. That is not a large fraction of the time that this universe has been around. In fact 100 divided by 13.8 billion is about 7.246376812 x 10-8so we are but a pinprick in the overwhelming space time of the universe. Einstein then worked out that time can be varied by the speed at which you move – time dilation effect. Don’t ask me to explain it, just look it up.

Take the white cliffs of Dover as an example. They are made of the coccolith calcite plates that are formed by coccolithophores, which are aquatic, single-celled algae. They are marine and live as phytoplankton in the photic zone of the open ocean, where they are a major source of food and a significant producer of oxygen. They are very small, about 2-5 micron in diameter. These plates form the majority of the chalk.

Now some simple maths. 

The chalk cliffs of Dover are some 80M high. The chalk was laid down during the Cretaceous period which lasted 80 million years. A simple calculation shows that, on average it took one thousand years to deposit 1 mm of chalk.

So, if you lived in the Cretaceous, you wouldn’t even notice that chalk was being deposited. In fact the household dust that builds up and needs removing every week or so – depends how house proud you are  – is deposited at a much faster rate than the chalk cliffs of South England. I’ll leave you to do the maths on this one. It goes to show one of the principles of geology – most processes happen very slowly and so require a great deal of time – luckily, there is plenty.

So we now have some idea of the time we are dealing with, even if we cannot directly relate to them. One of the problems with geology and dealing with these scales is that it makes you realise how insignificant humans are.

Here are some facts that might bring this home to you.

Modern humans  – Homo Sapiens – have existed on Earth for a short time – about 200,000 years. Dinosaurs existed on Earth for a little longer time – about 135 million years

Maths again!

This means that dinosaurs lived on the Earth some 675 times as long as modern humans have. Do you think humans will be around 135 million years in the future? No, me neither!

So, in spite of “dinosaur” being used as an insult for an out of date person, they did quite well and have been some of the most successful species on Earth.

A small diversion. I once went to a working quarry a few years ago in Gloucestershire, for a look around. Quarries are fascinating to geologists as they open a 3D window in the rock under our feet. I went to the mine manager’s office, as he was going to show me around. It was a warm day so the office door was propped open by a flat piece of rock. I looked at the rock – as you do – and saw that there were markings on the surface. As a conversation starter I asked him about them. ‘Oh those are dinosaur footprints, we come across a lot of those. They are a nuisance because they are really damage to the surface of the flat laminations so it is just a piece of waste rock to us’. I had a closer look and saw that it was as he said – it had the three toed foot. I realised after I had left the quarry that I should have asked him if I could have one of his scrap rocks with the foot print.

Another problem when you are dealing with geology is that you can become a little blasé about these scales. In the area where I live there are several limestone ridges that were deposited in the Carboniferous ( 359 – 297  million years ). It is simple to find fossils in these rocks so when you break a lump open and see a fossil coral there you realise that this fossil has been waiting there for over 300 million years – just waiting for you. Then you might come across some Triassic ( 250 – 201 million years ) and you start thinking that these rocks are fairly young. But when you see an archeologist on TV going on about “very old Roman finds that are two thousand years old”, you start to realise what deep time is. 

Then you might go to North West Scotland and place your hand on some Lewisean Gneiss and realise that it is over 3 Billion years old – two thirds of the age of the Earth. What a privilege to be able to see and touch these ancient rocks!

Language.

Geology is an international science so there are some lovely words to play with. Here are a few of my favourites.

Paleoproterozoic
Rhaetic
Solifluction
Slickenside
Batholith
Olivine
Wolfram.
Galena
Tourmaline
Subduction
Obduction
Lithification
Coccolithophores
Phytoplankton

As a writer, I often use some of these words in stories, sometimes as the names of characters. Slickensides sounds like a hairstyle. I remember writing about Solly Fluction who was an author who wrote detective novels.  I think my favourite is still Paleoproterozoic – it just rolls off the tongue.

© 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

Earth Song

Precambrian 

I was in hell being bombed in the Hadean. 
I was just alive in the Archean. 
I was long present in the Proterozoic. 

 Paleozoic 

I was changed by life in the Cambrian. 
I brought order to the Ordovician and 
I just survived the Silurian. 
I nearly drowned in the Devonian, all those fish! 
I made coal in the Carboniferous, delta, changes. 
I was probably present in the Permian desert dust storms. 

 Mesozoic 

I was a playa in the triple, arid Triassic, 
I evolved with many ‘ites during the Jurassic. 
I chalked the Cretaceous, fashioning forans and flints. 

Cenozoic 

I nearly perished in the Paleogene, 
I numbered all the Neogene. 
I quaked in the sometimes chilly Quaternary. 

Holocene 

I have lived so long, it may seem perverse but 
I want to live to the end of the universe. 

I am the worse for wear and war weary. 
I am your home, your Earth, cherish me dearly. 

© Richard Kefford 2020 Eorðdraca

My books are for sale here:  Richard