RIGS of the month [May] - Troopers Hill, Bristol

RIGS of the Month - May
Troopers Hill, Bristol

Fig. 1: Location of Troopers Hill, St George, Bristol. Postcode BS5 8BL.

Click here for directions and public transport options (external website)

SITE SPECIFIC INFORMATION 

Location: BS5 8BL

Accessibility: The entrance at Malvern Road provides the easiest access for those with limited mobility. Some of the slopes on this path may mean that some wheelchair users need assistance. The remainder of the paths on the hill are unsurfaced.

Topography: Hilly, grassed footpaths
Restrictions: Troopers Hill is owned and managed by Bristol City Council, there is free unrestricted access.

This article has been adapted from Andrew's post on the Friends of Troopers Hill website - the original version can be accessed here and includes a detailed stratigraphic log. Eilene Stonebridge also contributed a great deal of material to the site (see section on geology) and wrote an accompanying leaflet which can be downloaded for free - link at the bottom of this post.

Introduction
Troopers Hill’s geology is unusual in Bristol. Much of the city lies on Carboniferous Limestone, but Troopers Hill is dominated by sandstone of the Pennant Measures which are sedimentary rocks formed in tropical swamps some 300 million years ago. In places, the sandstone can be seen on the surface, both as natural outcrops and old quarry faces.

The combination of sandstone and local industry has resulted in acid soils, which are rare in Bristol. This has encouraged a wealth of plants to flourish, that are found nowhere else in the City.

Much of the shape of the hill is the result of quarrying for the sandstone which was used as building stone throughout Bristol. The largest quarry was the gully in the centre of the hill which was worked until the end of the nineteenth centuary. The humps and bumps above Troopers Hill Rd are where the unsuitable stone was tipped.

As well as sandstones the Pennant Measures include mudstones, shales, clay and coal seams. In the past coal has been dug where it outcroped on the hill and both fireclay and coal have been taken from deep mines under the hill.
 
History

Troopers Hill is made up of a thick pile of layers of rock which must have been
well known to the generations of miners and quarrymen who extracted coal, fireclay and sandstone from them for centuries. However, much of that knowledge has been lost and no one now is quite sure what lies under the hill.

There are at least 9 coal seams there and some of these were exposed at the surface. Local geologist Tom Fry recalled one seam 5-6ft. thick revealed on the west side of the hill in 1968, but all the other seams are probably much thinner. He also remembered that a very thin seam was worked by unemployed
men in 1913 on the hill near the bottom of Troopers Hill Road, where sadly a friend of his died after the shaft collapsed. 

The rocks are sandstone, mudstone, coal and clay, and they all dip to the south at between 25 and 45 degrees. They belong to the Upper Carboniferous Coal Measures, and are near the base of the Pennant Sandstone. The rock types, structures and fossils suggest that they were formed when the area was covered by a forested swamp. At this time (about 300 million years ago) the area is thought to have been near the equator with a tropical climate. The rocks were next squeezed by huge forces, which crumpled them into a large arched fold that runs east-west through Kingswood. The Trooper's Hill rocks lie on the south side of this structure. 

Pennant Sandstone

The sandstone at Troopers Hill is called Pennant, since the early geologists took the name traditionally used by miners and quarrymen. The Pennant forms a thick mass of sandstone in the middle of the local Coal Measures around Bristol, as well as in the Forest of Dean and South Wales. The name is thought to have been derived from the Welsh words penn and nant, meaning head of the valley.The rock is well exposed in the Gully Quarry, just south of Trooper's Hill chimney.

The sandstones exposed on the north side of the Gully display cross bedding, indicating that the original sand grains were carried by running water in rivers. 

On the south of the Gully the rocks are coarser grained with coal pebbles and fossil imprints of tree trunks, suggesting that at times the water was flowing fast enough to erode sediment and move large pieces of wood.

 

Fossilised wood in Pennant Sandstone. Photo credit: Steve England

Pennant was quarried at many sites in the local coalfields and provided stone for thousands of buildings, including the chimney at the top of Trooper's Hill. It sometimes splits into thin slabs which were much used for paving, kerbs, steps, gravestones, and, in earlier times, for roofing.

Mudstone
The mudstones were known as duns by the miners. They are less well exposed but can be seen in the paths to the south of Troopers Hill, and they indicate a period when the waters in the swamps must have been still. They had no commercial value. 

Coal 
The miners called the coal seams “veins” and gave them names which were used at each pit in the area where they were found, from Newton St Loe, near Bath, to St Phillips Marsh, in Bristol. Some seams thinned to nothing and others split, which made the naming uncertain in places. Each seam is thought to be the compressed remains of deposits of peat, built up from the remains of the trees of the forest. 

Crewe's Hole Pit was sited at the south end of Troopers Hill, and is marked by the remains of he chimney of the mine engine house. Air Balloon Pit was next to Air Balloon Road, about half a mile north of the Hill. The mining geologist John Anstie was employed to gather information on all the local coal mines for the Royal Commission on Coal. He collected some details of what had been found in these pits, although both were disused when he visited the area in about 1870 and he was concerned that the information might not be completely reliable. He also observed that “the outcrops of the Devil's seam, Buff and Parrot seams, follow parallel lines about 500 yards to the north of those of the Millgrit and Rag seams ... (and at) ..Trooper's Hill .. the shallow works on all of them are clearly traceable.” He noted that the Air Balloon Pit only worked for three or four years as the seams were too variable to be worked economically. 

The engine house in Troopers Hill Rd, c. 1914. Reproduced by kind permission of the Bristol Reference Library.

Tom Fry was brought up near Troopers Hill and knew the area very well. He recalled one seam 5-6ft. thick revealed on the west side of the hill in 1968 before the council used the area as a rubbish tip. Tom recalled that in around 1910, his father used to warn him of the dangers of the hill, telling him that it was all undermined. His father worked in the Fireclay Mine. Tom also recalled the collapse of an old sloping shaft in the floor of the main quarry, the sides of which had been walled with mortared stone, and that he could trace the sites of at least six shallow mines on the Hill.

Fireclay

Fireclay is usually found beneath a coal seam and is basically the clay soil where the trees grew which provided the peat deposits. Fireclay is rich in clay minerals which means that when fired into bricks or tiles, they can withstand high heat. Tom Fry noted that the Troopers Hill fireclay was used to produce high quality terra cotta tiles that were used in many local buildings. 

Three fireclay beds are known to have been worked beneath Trooper's Hill. 
 

Andrew Mathieson

Further reading

Downloadable leaflets on the geology, wildlife and history of Troopers Hill are available at http://www.troopers-hill.org.uk/leaflets/index.htm

References
Anstie, 1873, The Coalfields of Gloucestershire and Somersetshire, London
Bristol Fireclay Company Co Ltd, 1911, Section of the Mines at Troopers Hill
British Geological Survey 6 inch map Sheet ST 67 SW
Buckland and Conybeare, 1824, Transactions of the Geological Society of London (available online through Google Books)
Cornwell, 2003, The Bristol Coalfield, Landmark Publishing
Fry, Accounts of my Earliest Years (up to 1922) www.troopers-hill.org.uk/memories
Kellaway and Welch, 1993, Geology of the Bristol District, HMSO
Prestwich, 1871, Report of the Royal Commission on Coal (in) Gloucestershire and Somersetshire, HMSO (available online through Google Books)

The original walk was funded by our Stepping Forward Sustainability Grant, through the BIG Lottery Community Spaces Programme www.troopers-hill.org.uk/steppingforward

RIGS of the month [March] - Middle Hope

RIGS of the Month - March
Middle Hope, Kewstoke, Somerset

Fig 1: Locality map. Blue arrow shows location of RIGS on OS map (original scale = 1:25,000).

Directions: After parking at the end of Beach Road, take the steps up to the top of the headland and keep right, away from the trig point. Go through the kissing gate in the wall and immediately turn left. Head down the bank and turn left again (heading east), keeping parallel with the coastline. Proceed for ~300m until you read a small pebbled beach.

SITE SPECIFIC INFORMATION 

Location: Roughly BS22 9UR (GR = ST 324661. Marked as "Swallow Cliff" on 1:50,000 OS Map)

Accessibility: Car park at the end of Beach Road. Footpath with steep steps.

Risks: Check tides before heading down onto the beach. Beware of falling rocks beneath cliff faces
Topography: Hilly, grassed footpaths
Restrictions: The site is a designated as a SSSI so please do not hammer or sample from the exposures

Fig. 2: View of exposures looking directly west from ST 32466. The oldest Black Rock Limestone unit lies to the north (out of the field of view - see fig. 3).

Introduction
The geology of Avon is dominated by sedimentary rocks, but the coastal exposure at Middle Hope offers the chance to see volcanic deposits intercalated with Carboniferous Limestone. The mix of sediments and volcanic deposits allows accurate reconstruction of the sequence of events that took place in this area some 350 million years ago (Ma). Ash layers within fossiliferous sediments record the onset of volcanic activity in a shallow sub-marine setting, before proceedings culminated in an extrusive pillow basalt (fig. 3). Such volcanism is typical of the early Carboniferous in the UK; most vents were small and short-lived, having only a localised impact on their surroundings.

Outcrop description
The younging direction is roughly north-south. Carboniferous Limestone is gradually disturbed and then overwhelmed by volcanic deposits before reappearing at the top of the sequence. The entire succession has been tilted due to the huge compressive forces associated with formation of Pangea during the Variscan Orogeny at the end of the Carboniferous period.

Fig.3: Schematic representation of Middle Hope geological setting. Outcrop is located below the storm wave base (SWB) on a carbonate platform. FWB = Fair-weather wave base. After Faulkner (1989).

Unit name colours correspond to labelling on fig.2

Limestone (pre-volcanic)
The oldest unit at this exposure dates from the early Carboniferous (~355 Ma), and forms part of the Black Rock Limestone group. Bedded at roughly 20cm intervals, it is fine-grained and highly fossiliferous, containing abundant crinoid ossicles, brachiopods and corals fragments (fig.4). In places the sediments have been bioturbated by ancient burrowing organisms and yield a variety of trace fossils. The preservation of these delicate structures indicates these sediments were deposited in a low energy sub-marine environment, below the storm wave base of an offshore carbonate ramp (fig. 3).

Fig. 4: The oldest limestone unit at Middle Hope dips 30º to the south and contains abundant fossils. Image on right shows close up of crinoid ossicles in long and cross section.

Interbedded tuffs and limestones 
About 10m to the south, more tilted beds appear out of the shingle. This part of the outcrop consists of interbedded limestones and tuffs, the latter heralding the onset of volcanism in the area. In general, the grain size of the tuffs increases up-section, corresponding to increasing intensity of volcanic activity. The first beds are red in colour and are rich in bioclastic material. The alternating thin-bedded limestones, some of which have ripples and cross-bedding, show that the volcanic outbursts were sporadic and of limited duration. Small lapilli-rich layers can be found in the tuff and conglomeritic deposits indicating there may have been pyroclastic currents or debris avalanches flowing down the carbonate ramp.

Fig. 5: Image on left shows interbedded limestone and tuff. Volcanic activity was impersistent, allowing reestablishment of the carbonate platform in between eruptive episodes. Image on right is a conglomeritic sub-layer, possibly resulting from pyroclastic or debris flows. 

Massive tuff (Middle Hope Volcanic Member)
Above the bedded units lies the Middle Hope Volcanic Member. Greeny-black in appearance, this massive unit varies between 5-10m in thickness. Lapilli-rich zones appear in places and calcite veining associated with late Carboniferous tectonics is prominent. Near to the basal contact with the interbedded tuff there is evidence for metasomatism (chemical alteration by hydrothermal fluid); here, brown patches contain highly altered fossil fragments which have a distinctive green colouration.
 

Fig. 4: Middle Hope Volcanic Member. Massive green-black tuff deposit with extensive calcite veining. Close up shows metasomatised basal contact with underlying limestone.

Pillow basalt
Immediately above the tuff is a fine-grained pillow basalt, the result of an extrusive sub-marine lava flow. It varies in thickness up to 4.5m and good pillow shapes can be made out towards the base. The upper surface of the basalt is full of voids (amygdales) up to 10cm in diameter; when the lava was molten, these would have been filled with gas. Some have been stretched parallel to the flow direction of the lava. Later alteration has infilled the amygdales with calcite.

Fig. 6: Pillow basalt. Outcrop in left image is about 3m in height. Close up on left shows amygdales (voids left by gas bubbles) elongated parallel to flow direction which have been infilled later by calcite.

  

Limestone (post-volcanic)

After the extrusion of the basalt, the intensity of volcanic activity receded and is again recorded in gradually fining tuff layers interbedded with fossilerous limestone. The cliff deposits then grade into Black Rock Limestone, and finally Gully Oolite (which can also be found in the Avon Gorge).


Charly Stamper

Quaternary geology

The capping deposits on the cliff are a Pleistocene aged fossil cliff and shore platform. The raised-beach deposits have been correlated with the last interglacial, and this is the only Pleistocene raised-beach site where such a sequence can be demonstrated and as such is of fundamental importance to quaternary geology.

 
Biological
Middle Hope is designated as an SSSI for both its geological and biological features. The headland supports a calcareous grassland community with a restricted British distribution (Festuca species and Dactylis glomerata).

References:

- Faulkner TJ (1989) The early Carboniferous (Courceyan) Middle Hope volcanics of Weston-super-Mare: development and demise of an offshore volcanic high. Proc. Geol. Ass., 100(1), 93-106.

- Volcanic rocks of the Bristol region, Speedyman DL. in Geological excursions in the Bristol District. Savage RJG (1977). University of Bristol.  

The Hot Well, Bristol

The Hot Well spring, Bristol
From rags to riches and back again - 
the story of the Hotwells spa
 

The old Hotwell House on the banks of the Avon. Built in 1696, it held a pump rooms and lodgings for visitors. After the terminal decline of the Hot Wells spa, it was demolished in 1822 and no remnants of its former glories remain, the site being adjacent to The Portway.

INTRODUCTION

The King’s Spring in Bath has been exploited by humans since 836BC, most famously by the Romans who built the first baths, and then later during the spa age of the 18^th century. Less renowned is the eponymous spring of Hotwells in Bristol. At its peak in the Georgian era, the “Hot Well” served a fully functioning pump room and hot baths, and provided the catalyst for much of the development in Clifton. Today, the spring has diminished in flow and is only visible at low tide as a trickle emanating from the banks of the River Avon.

The modern-day resurgence of the Hot Well from the banks of the Avon as visible at low tide. All photo credits: Charly Stamper

HISTORICAL BACKGROUND

The spring was first mentioned in 15^th century historical records, and by the 1630s it was being regularly visited by society. The thermal water emerged on both sides of the Avon, roughly opposite the intersection the The Portway and Bridge Valley Road. It was contemporarily described as being "milky white" and was thought to have restorative properties, particularly for "hot livers, feeble brains and red pimply faces". During the 17^th century the spring was relatively inaccessible, for there was no formal path and a descent from Clifton involved “200 slippery steps”.

As the popularity of spas increased in the Georgian era, so did the number of visitors to Lower Clifton. Initially, development focused on the area adjacent to the natural resurgence, with the building of a spa (old Hotwell House), entertainment complexes (Jacob Wells theatre) and genteel housing (eg. Dowry Square). In the 18^th century the spring’s reputed curing powers extended to venereal disease, tuberculosis and cancer.

Strangers' Burial Ground, Lower Clifton Hill. By the 1750s, the Hot Well acquired a reputation for curing tuberculosis; however, these claims were unfounded and an overflow burial ground was instated to cater for unfortunate commoners who came from outside of the parish.

The main dent in the Hot Well's popularity remained the distance and difficulty of access down the steep sides of the Avon Gorge, and in the mid 1780s Thomas Morgan embarked on an ambitious engineering project to bring the waters to the heart of Clifton. From Sion Row he drilled a shaft some 250ft through the Carboniferous limestone to tap the hot waters, supplying water to a new pump room with hot baths and a reading room, later to become the St Vincent Rocks Hotel (now Avon Gorge Hotel). By 1793, this diversion had become known as the “New Hot Well”.

In the following years, both springs began to cool, almost certainly as result of increased groundwater mixing. This coincided with a nationwide decline in spa popularity in favour of sea bathing, increase in subscription charges and end of the Napoleonic Wars, meaning British people were free to travel abroad. Several revival attempts in the 19^th century failed to capture former glories, and the spring is no longer commercially exploited. 

The Colonnade, Hotwells Road. Originally a shopping arcade, it was built in 1786 as an attempt at reviving the failing fortunes of the Hot Wells spa.

GEOCHEMISTRY AND HYDROGEOLOGY

Modern day studies of the hot springs of the Avon area have tried to shed some light on the source and science behind their existence. In 1993, the yield of the Hot Well was measured at 0.41 x 10^6 litres a day, about a third of the present-day flow recorded at the King’s Spring in Bath and enough to fill an Olympic-sized swimming pool in a week. Compositionally, the two springs are very similar, being rich in calcium and sulphates, though 25% of the Hot Well volume is cold groundwater, reflected in the relatively low average temperature of 24ºC.

The source of the two springs is thought to be rainfall in the Mendip Hills, some 15km to the south-west.  Measured carbon isotopes (∂¹³C) are consistent with storage in Carboniferous Limestone, and hydrogen and oxygen isotopes provide evidence that most of the water is meteoric in origin. The head at this elevated topography is high enough to force the water down beneath the Coal Measures to a depth of around 2.7km in the Bristol-Bath basin and heat the groundwater; silica geothermometers indicate the thermal component of the springs reaches a maximum temperature of 72ºC. 

Cross section showing flow of groundwater through Carboniferous Limestone from source in the Mendip Hills to resurgence in Bath and Hotwells (Andrews et al., 1982).

The overlying stratum of the Coal Measures is a proven aquiclude, so no upward migration can take place. The area north of the Mendips is heavily faulted and folded from both Mesozoic and Tertiary tectonics, so the water migration is unlikely to be direct. Tritium (³H) was produced by thermonuclear weapons testing in the 1950s and is used to identify any modern-day recharge in groundwater. Low tritium levels in the Hot Wells indicate only minor amount of mixing with of ‘recent’ waters, and the majority is likely to be up to 10,000 years old. At the end of its journey, the Hot Well resurges directly from Carboniferous Limestone into the channel of the Avon river.

THE FUTURE

Although the Hot Well spring has had its heyday, the King’s Spring at Bath remains at the heart of the city’s tourist trade. In 2011, two companies (Eden Energy and UK Methane Ltd) were given licenses by Mendip district council to begin a feasibility study for the controversial practice known as “fracking”. Concerns were immediately raised by councilors in Bath and led to a subsequent uproar in the local (and further afield) media. A specially commissioned British Geological Survey report concluded that the risk to the Bath springs was no higher than any other part of the UK, although critics point out that relatively little is still known about the subterranean flow of the groundwater. The energy companies are a long way off obtaining the planning permission needed to begin exploratory drilling, but the authorities would to well to bear in mind the role that human intervention had in the decline of the Hot Well spring.

Charly Stamper

REFERENCES AND FURTHER READING

Andrews JN, Burgess WG, Edmunds WM, Kay RLF & Lee DJ (1982) The thermal springs of Bath. Nature 298: 339-343.

Atkinson TC & Davison RM (2002) Is the water still hot? Sustainability and the thermal springs at Bath, England. Geological Society, London, Special Publications, 193: 15-40.

Clifton and Hotwells Conservation Area Character Appraisal (2010) Bristol City Council http://www.bristol.gov.uk/sites/default/files/assets/documents/clifton-and-hotwells-character-appraisal.pdf

Gallois RW (2007) The formation of the hot springs at Bath Spa, U.K. Geol. Mag. Vol. 144, 741-747

Jones, D (1992) History of Clifton. Phillimore.

Kellaway, GA (1993) The hot springs of Bristol and Bath. Proceedings of the Ussher Society, 8, 83-88.

Mowl, T (1991) To Build the Second City: Architects and Craftsmen of Georgian Bristol. Redcliffe Press Ltd.

Smith NJP & Darling WG (2012) Potential problems within the Bath and North East Somerset Council and surrounding area with respect to hydrocarbon and other exploration and production. British Geological Survey Commissioned Report CR/12/055, 26 pp.

Baryte (BaSO4)

Minerals of the Avon region

Baryte   (Barium Sulphate – BaSO₄)

Colour: White but is often coloured by other minerals to a pink or pale brown

Polymorphism: Forms a series with Selestine

Name: From the Greek for weight due to it’s high specific gravity      

Crystal system: Orthorhombic with up to 70 forms

Specific gravity: 4.5 measured (4.47 calculated)

                           

Hardness: 2.5 – 3.5

Group: Barite Group

Association: Fluorite, calcite, dolomite, rhodochrosite, gypsum,                        sphalerite, galena, stibnite.

Occurrence: A gangue mineral in low-temperature                                         hydrothermal veins; in residual deposits from weathered barite-bearing limestones.

Local location: Stancombe Quarry, Flax Bourton (Working limestone quarry)

Cross section of Baryte vein -Stancombe Quarry

Picture credits Richard Kefford

Larger pictures can be seen here

Paragenesis #1 - Bilbao supergene type

During the Late Triassic, iron rich saline oxidising water leached through the rift basins formed during the early Permian (~290 Ma) to the late Jurassic (~150 Ma).

Paragenesis #2 - Mississippi Valley Type ( MVT )

In the Mendip – Bristol vein field, baryte has been deposited by hydrothermal fluids in tension cracks and fissures in the Carboniferous Limestone. The primary minerals in these fissures are Galena and Sphalerite. Gangue minerals such as Baryte and Calcite occur in banded formations where the veins pinch out. This is known as the Mississippi Valley Type (MVT) and took place during the Middle Jurassic (~170Ma).

Baryte with associated galena and calcite - Stancombe Quarry

Uses:

By far the greatest use of this mineral ~80%, is for the production of drilling mud for use in oil exploration. The main reason for this is that it is very heavy and so helps to prevent blow outs in the drilling stage of the exploitation of an oil reservoir. It is also chemically inert. The specification for drilling mud includes a requirement that the specific gravity should be 4.2 or greater.

Non drilling applications of barytes are comparatively small, although still important because of their higher value. High purity grades of barytes with fine and controlled particles sizes are used as fillers in marine and industrial paints, in brake lining/ friction materials and in plastics. A specialised use of barytes based on its high density and ability to absorb radiation, is as an aggregate in dense concrete for shielding applications in the nuclear industry and hospital radiation departments.

Bladed rosettes of Baryte on associated red ochre

Sample found by Leon Sparrow at Winford

Sources:

Barytes is produced in England and Scotland. In England it is now only produced as a by product of fluorspar mining and processing. In Scotland, barytes is extracted as the sole mineral from the Foss Mine near Aberfeldy. 



Richard Kefford

References:

www.bgs.ac.uk/downloads/start.cfm?id=1349 

http://www.aditnow.co.uk/documents/Bridford-Baryte-Mine/BARYTES-MINE.pdf

Chidlaw, N. (2012) Metamorphism and Mineralisation in the Bristol - Mendip area

Geo-gardening at Trendlewood Quarry, Nailsea

Geo-gardening - Sunday 3rd February 2013

On Sunday 3^rd February 2013 a Joint Gardening Force (JGF) from Friends of Trendlewood Park (FoTP) and Avon RIGS Group descended on Trendlewood Quarry in Nailsea and proceeded to spend a few hours clearing ivy and other encroaching vegetation from the lower parts of the quarry face in three areas.

Before worked started. Photo credits: Richard Kefford. More photos at

http://tinyurl.com/bwd6shw

These three areas were selected to demonstrate the differing bedding types – from varves to massive -  in this Pennant Sandstone of the Downend Formation and the differing types of deposition – point bars and channel deposits.

Geo-gardeners at work

This is the first part of the implementation of the quarry management plan that was agreed at a site meeting between N Somerset Council, Friends of Trendlewood Park and Avon RIGS Group. The second part which will be carried out by the N Somerset Green Team will consist of clearing the upper levels of the face and removing a few selected trees that are in danger of falling and have extended their roots into the joints and fissures rock close to the face and so are damaging the exposure.

Meeting one of the site natives

See here for details of the geology of this site and its location.

A cleared area on one face

Richard Kefford

Earth Heritage magazine

Earth Heritage magazine - free download

Earth Heritage is a magazine produced twice yearly to stimulate interest in geodiversity and a broad range of geological and landscape conservation issues within the UK and further afield. It is produced by Natural England, Scottish Natural Heritage, Countryside Council for Wales and the Geologists' Association. 
 

 
You can download the latest issue for free here. 

Trendlewood Quarry Nailsea

RIGS of the Month – January 2013

Trendlewood Quarry, Nailsea

Pennant Sandstone quarry face in Nowhere Wood
( Double click on picture for larger view )

More photos at  http://tinyurl.com/b7k53sy  Photo credits Richard Kefford

Location:             Trendlewood Park, Nailsea            ST 479 702

RIGS citation:   

‘Best surviving exposure of Carboniferous Pennant Sandstone in the Nailsea Coalfield.’

 It is designated as a RIGS because of its aesthetic and education value

Access: 

Head North from Nailsea and Backwell railway station, passing St Francis school on the right. Turn right along a public footpath just after the crescent to enter Tendlewood Park. Follow the path through the wood then take a path to the right that slopes down on to the quarry floor. Follow the path until the quarry faces can be seen ahead and to the right. Some parking is available in the crescent road.

Risks: 

Keep away from steep rock faces with loose material that can result in rock falls. Hard hats should be worn when approaching the face.

Topography:     

Path through woodland, unstable rock faces.

General description:

This is a disused quarry that was used to supply building stone to the local area. It was in use until 1930.

It is located in Nowhere Wood which is part of Trendlewood Park. This park is owned by North Somerset Council and managed jointly by the owners and a local group of volunteers known as Friends of Trendlewood Park.

http://www.friendsoftrendlewoodpark.org.uk/blog/

Entrance to Trendlewood Community Park

Geological history

The closure of the Rheic Ocean by the end of the Carboniferous Period (~300 million years ago) caused the Variscan Orogeny, resulting in folding of the strata in our area when ‘Nailsea’ was just north of the equator. This produced high mountains which were then quickly eroded, with the detrital material transported north to be laid down as Pennant Sandstone in deltaic environments.

Geological chronostratigraphic chart

Geological context

The quarry is cut into Pennant Sandstone, a lithology that was deposited in a river system with point bar and channel deposits. The cross bedding directions show that the provenance of the material was from the SSE. The rivers carried eroded material from high ground that was upthrust during the Variscan Orogeny that occurred during the late Carboniferous period.

Formation of cross bedding dune structures

The Sandstone is part of the Downend Formation which is up to 660m thick in the Somerset Coalfield. It consists mainly of sandstone with some mudstone. Some coals appear in the lower part; Graces seam in the Nailsea Coalfield for example.

The Pennant deposition took place during the Bolsovian (Westphalian C) time which is 308 – 311 million years ago.

The Downend Formation is part of the Pennant Sandstone Group and is also known as part of the Upper Coal Measures Group. It is exposed as the uppermost strata in the Nailsea syncline. There is also a small exposure at the road cutting at Bucklands batch, which is passed on the road down the hill to the station. This exposure clearly shows the dip of the strata forming the southern leg of the Nailsea syncline.

Other exposures of Pennant Sandstone in the area occur at Conygar Quarry, Clevedon (private land), at a disused quarry, now Cloud Hill Industrial Estate and at Highbury Hill, near the waterfall. Both are near Temple Cloud. There is also a disused quarry where building stone for Bristol was extracted, at Troopers Hill in St George.

http://www.troopers-hill.org.uk/

Lithological description

Green-grey and blue-grey, feldspathic, micaceous. Lithic arenites ( “Pennant “Sandstones ) of southerly provenance, with thin mudstone/siltstone and seatearth interbeds and mainly thin coals; the lithologies are commonly arranged in fining upwards channel-fill sequences.

Thickness

c. 275m in the east of the coalfield  [c. SO 25 03 ] to c. 1350m in the Swansea area [SS 73 94]

330m maximum in the Nailsea area.

Geographical limits

Outcrops widely in the South Wales Coalfield, from near Llanelly  [SN 40 00] in the west to Pontypool  [SO 25 03] in the east. It is also present in the Forest of Dean and Bristol coalfields, and in the subsurface in the Oxfordshire and Berkshire coalfields.

Type area

Formation named after the predominant “Pennant” sandstone facies of the South Wales coalfield, which provides a “type area”.

Lithology of Pennant Sandstone

It is classified as a sandstone or arenite which means that the grains are 0.0625mm – 2mm in size. It is feldspathic which means it contains clasts ( grains) of feldspar, an aluminosilicate mineral which makes up some 60% of the Earth’s crust. It is also micaceous, meaning it contains a small proportion of clasts of biotite or muscovite mica which is a hydrated aluminosilicate mineral.

Quartz makes up the majority of the grains, which are cemented by silica.

This composition suggests that Pennant sandstone is composed of the detrital remains of granitic rocks.  Granites are igneous rocks which form volcanic plutons below the surface which may later be upthrust or exposed by erosion of their roof. The grains have polished surfaces which shows that they were transported by water (airborne grains have a ‘frosted’ surfaced). The quartz grains survived the transport because they are very hard, being composed of silicon dioxide. The rock itself is fairly soft.

In some areas the grains are covered with different iron oxides which accounts for the different colours seen. Red colouration is from iron whereas purple is indicative of managanese.

Uses of Pennant sandstone

It has a long history of use as a building stone. As the grains are very hard and resistant to wear it is used as a high skid resistant road material, especially at bends, traffic lights etc.

Richard Kefford

References

http://en.wikipedia.org/wiki/Pennant_sandstone

http://en.wikipedia.org/wiki/Sandstone

http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=PES

Green, GW. (1992) British Regional Geology. Bristol and Gloucester region. BGS.

Published by NERC. ISBN 0 11 884482 2

BGS. England and Wales Sheet 264. Solid and Drift Geology Map. 1:50 000 series.

BGS. Classical areas of British Geology. Geological sheet ST 47 Solid and Drift.

Clevedon and Portishead. 1:25 000 series.