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.


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.’

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

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


 Complete Crinoid fossil


Zaphrentites Corals




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.

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.


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


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.” interesting site about lead working in Bristol.

© 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

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.’


Tufa Dams                 


Spring line


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.

DSC00018 (1)

 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.

DSC00023 (1)

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