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.’
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.
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.
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
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