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

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

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

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

The Clapton Klippen

Photo credit  Mark Howson

Upper left  –  Sigillaria
Right           –  Calamites
Lower left  –  Lepidodendron

It was a mizzly sort of day but, as daft geologists, we decided to go anyway. We met at the agreed time under the motorway bridge. It was dry there, except for the rivulets running down the gutters on each side of the road. It was also dark so ferreting out our gear from the car boot was mostly feeling for the oh so familiar objects.

We dressed up for cold, wet weather and muddy ground, secured the cars and then set off up the familiar road which quickly morphed into a track. At the branch, we headed left through a kissing gate onto a rocky path that hugged the higher side of the field, up against the barbed wire that kept us out of the wood that was groaning in the erratic wind. The path was muddy with many rocks intruding through the red mud, their curiosity driving them up into the damp air above . We had a look at these rocks and speculated about their provenance. We identified them as Pennant Sandstone from the Carboniferous but why were they so separate instead of just being a massive exposure? Was there a quarry nearby ? Perhaps this was the discarded waste from the quarry that was graded as poor quality?

We came to another gate that led us uphill into the wood. It was a public path through private woodland that permission is required to visit, so we had asked for ,and were given, permission to divert from the path as we thought fit, to investigate any quarries or interesting whatevers that we spotted. The path curved to the left and we could see an outcrop over to the right. This was a valley curving away to the North about 5 metres deep and 10 metres wide at its crest. On the Southern side it was steep with several near vertical cliffs that looked like old quarry faces. Close inspection showed that the rock was Pennant Sandstone. We were now higher than the footpath along the edge of the wood so this tended to confirm our hypothesis that the questing stones we had seen on the footpath were indeed from a quarry – this one? Perhaps as surface treatment for a muddy path?

We started looking along the opposite side from the face as stone that was discarded was usually dumped away from the active face. We knew that the Pennant was usually  quarried for building and was prized for its prized ‘flats’. Any fossils were regarded with suspicion as “the devil’s work” or because they broke up the smooth style of the rock and reduced its quality as building stone. This was something we had learned – when looking for fossils in a disused quarry, always look for the rejected stone pile – unless, of course, the quarry is for road-stone, which is destined to be crushed and screened anyway so any fossils will have been destroyed in the process.

We found a ring of charred wood and rocks which had been clearly gathered for a fire – perhaps a barbecue? We started sorting through the stones and found an interesting rock with two fossils in it. We were delighted as my companion – a geologist – had found a Sigillaria specimen here a few weeks before. Just after we found this, a couple turned up from the local Court. We had invited them to join us on the fossil hunt. They told us that their house, near the top of the hill was built from Pennant Sandstone – most likely from this quarry. They had brought the previously found Sigillaria fossil as they wanted to bring it back and leave it at its birthplace.

We spent another half an hour or so looking for more fossils but it was not to be so, after many photos we carefully hid the fossils in a cleft in the old quarry face for others to find. There wasn’t much else to see as it is a small quarry. I would like to stress again that it is on private land and permission should be sought from the owner before entering it.

We then looked around the area because we were looking for the variously named Clevedon or Naish House fault as we knew it ran from Clevedon beach West along and through this area. We found clues – a change from woodland to a field used for grazing sheep above the quarry. As we walked up to the wire fence between the two, we found an increasing density of limestone rocks. These rocks had crinoid fossils in so were probably either from the Avon Group, Lower Limestone Shales or from the Black Rock Limestone. This demonstrated that the fault was in this area. The vegetation change from woodland to grazing fields followed the underlying geology change from acid to alkaline soil

It was now time to retrace our steps to the motorway bridge and  then walk along the side of the motorway towards Nicholas Wood which is a wood perched on a large, almost circular, mound. This one of the Clapton Klippen. Klippen is a plural German word that translated to the English, Cliffs. A Klippe is a peculiar feature where older rocks have been moved by faulting and erosion. The Oxford Dictionary of Earth Sciences defines it thus: A tectonic outlier produced by the erosion or gravity-gliding of one or more nappes. The front portions of the nappes become detached to produce the klippe structure. A nappe is defined as: from the French nappe, meaning ‘cover’ a thrusted mass or folded body in which the fold limbs and axes are approximately horizontal.

So this means that, as you approach Nicholas Wood and transition from a grazing meadow to wood land, you walk up a hill from a muddy red field into a dry ground wood. Rock clasts are randomly scattered on the surface and can clearly be seen to be Black Rock Limestone complete with Crinoid fossil ossicles.

Nicholas Wood covers just one of the Clapton Klippen. There are four others in the area – one of which has been quarried for its limestone. We debouched on to St Michael’s church path and walked back through the village then up Wood Lane to the cars.

The other Klippen will be investigated another day.

© Richard Kefford    2020                               Eorðdraca

My books are for sale here:      Richard

Trial post 1

Welcome to my rocky blog. Anybody know where this pic was taken?

 

This is Coire Lagan on the Isle of Skye. My favourite place.
Here you can see a classic glaciated corrie formation with these features and a wonderful view – on a clear day.
Gabbro
Roche Moutonee
Glacial striations
Dykes
Glacial Tarn
The Great Stone Chute.
Screes
Headwall

© Richard Kefford         2020         Eorðdraca.         

My books are for sale here:         Richard