Geology in the West Country blog

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This blog's main purpose is to publicise talks, field trips and events organised by West Country geological organisations. It also includes links to blogs and webpages of similar groups from around the UK. Discussion is encouraged and anything of general geological interest is included. Latest stories include Rock Circus in Box takes shape and Chemical ghost of 120million years-old bird.

There is a live link to the Geology in the West Country on the Outcrop homepage so that you can keep up to date with what is happening in our region.

Gypsum (CaSO4 - 2H2O)

Minerals of the Avon region 

Gypsum (CaSO₄ – 2H₂O)

Colour:    

               White but may also be clear, or stained pink or orange                       depending on the percentage of included iron minerals. One polymorph – Desert Rose – may be brown because of included sand.

Polymorphs:

                Selenite ( Serenity )

                Satin Spar

                Desert Rose

                Alabaster (Saccharoidal )

Crystal system: 

                May be tabular, prismatic, acicular, fibrous, granular or             massive.

Specific gravity: 

                2.31 – 2.33

Hardness: 

                1.5 – 2 on the Moh scale

Group:

                Gypsum is a sulphate group evaporite mineral

Location: 

                Aust Cliff*.

                Please follow the Geologist's Code here.
                       

Gypsum - Satin Spar

BRSUG B2317

Gypsum - Selenite

BRSUG B6236 

Gypsum - Desert Rose

BRSUG B3472

Gypsum - Alabaster

BRSUG B3932

The four pictures above are from the Geology Collection, University of Bristol. Larger pictures are here.

Paragenesis

There are very large deposits in the UK, the biggest is in East Sussex, with several seams in the Jurassic Purbeck beds. There are others in Staffordshire, Cumbria and Yorkshire. Gypsum is soluble in water but is unusual in that it becomes less soluble as the temperature rises. The deposits formed as seas or saline lakes dried out. It normally occurs as a massive rock or as crystals but can also form on the surface, as sand, where it is exposed to strong winds, such as the White Sands Monument in New Mexico.

General

        The Fauld gypsum mine in Staffordshire was the location of the biggest conventional explosion in either of the two world wars. in 1944, 3,500 tonnes of explosive blew up , killing 77 people and forming a crater 300ft deep.

         Because of its solubility in water and the fact that there are many shallow gypsum seams under Ripon, the city is known for an average of one subsidence event per year where solution cavities in the gypsum migrate to the surface and cause holes to open up. Details here.

         

Local exposures

        The best known local exposure of gypsum is at Aust Cliff*. The mineral occurs as both irregular masses and geodic nodules in the Mercia Mudstone. Here, in its alabaster massive form, it has the sugar lump or saccharoidal appearance, white or pink in colour.

It also occurs in secondary concentrations at the contact between the mudstones and underlying beds, as the ‘Satin Spar’ fibrous form which is invariably pure white.  

Uses

        Gypsum ( Alabaster ) was used in Somerset for ecclesiastical carvings but its main use now is for the production of dry lining boards for the building industry.

        It is also used as a soil conditioner for heavy, poorly draining, soils where the included sulfur (c. 15%) also aids plant growth by reducing the alkalinity. 

        It is a small constituent of Portland cement where the proportion controls the set time. The gypsum for this use in the UK invariably comes from the Sussex mine. It is said, by the Sussex miners, that every house built in the UK since about 1900 contains some gypsum from their mine.

        It also, of course, has a medical use as ‘plaster of Paris.’

        A side effect from the work to reduce the quantity of sulfur dioxide emitted from power stations is that a great deal of gypsum is produced by the desulfurisation process and so reduces the quantity required to be mined. This has, conversely, resulted in an increase in demand for calcium carbonate in the form of Limestone. The chemical reaction is

CaSO₃ (solid) + H₂O (liquid) + ½O₂ (gas) → CaSO₄ (solid) + H₂O.

Further details here. 

Richard Kefford

*Aust Cliff is a SSSI and removal of specimens from the cliff face is both hazardous and illegal.

References:

Geology Collection, University of Bristol.

RIGS of the Month [July] - Cliff Quarry, Compton Martin

RIGS of the Month – July 2012

Cliff Quarry – Compton Martin

Dedicated to the work of Cliff Salter.

General view of Cliff Quarry East face
Picture credit Richard Kefford

A larger picture here.

Geological map showing location of Cliff Quarry.
Picture credit - BGS.

                                                              A larger picture here

Gallery of quarry, fossils and crystals here.

Picture credits Richard Kefford.

Please follow the Geologist's code.
http://www.brerc.org.uk/rigs_site/geologists_code.htm

Location: Near Compton Martin ST 541 568

Access: Turn South into lane in Compton Martin called ‘The Coombe.’ Follow lane to end and then follow short section of footpath ahead up into Compton Wood where Cliff Quarry is on the left. Parking is limited in the village.

Risks: Keep away from rock faces. Hard hats should be worn.

Topography: Rough ground, steep slopes and unstable cliff faces.

   

Oxwich Head Limestone at East Cliff Quarry. 

Picture credit Richard Kefford.


A larger picture here

General description

        There are two parts of Cliff Quarry.  Access is better at the East as the West Quarry is overgrown.

       The East Quarry is adjacent to the 36 mile (58 km) ‘Limestone Link’ footpath. This path connects the Carboniferous Limestone of the Mendip Hills with the Jurassic Limestone of the Cotswolds and so bridges more than 100 million years of geological time.


       The quarry was used to supply hard road stone but was closed in 1956 as the big Eastern Mendip quarries had better transport links to the main markets in the South East of England.


        Slightly to the East at ST 543 566 are the Compton Martin ochre mines.
Note.
        The ochre mining industry of the Avon RIGS area will be the subject of a future feature on this blog.

Geological context.

        The quarry is cut into the Oxwich Head Limestone (previously known as Hotwells Limestone) which was deposited in a shallow shelf sea during the Asbian – Brigantian  stage of the Early Carboniferous, 327 – 334 million years ago. The Southern part of the British Isles then lay just South of the equator. It had a tropical seasonal climate, probably driven by monsoons.

        The upper part of the quarry exposes Triassic Dolomitic Conglomerate rocks of the Mercia Mudstone Group. These form a drape of varying thickness around most of the Mendips. They also form infilled wadis that were cut by flash floods during arid Triassic times. The adjacent ochre mines were developed in this formation.

        The quarry is located on the North Eastern limb of the Blackdown pericline which is one of four that are set en echelon to form the structure of the Mendip Hills. 


        The rocks in the quarry dip about 40⁰ to the North East. The strata of the Blackdown pericline dip more steeply to the North than to the South as it is an asymmetric anticline.


        The Oxwich Head Limestone Formation is up to 183 m thick in the area of the quarry. The type section is at Oxwich Head on the Gower. It is part of the Pembroke Limestone Group.

A      Cross section of Blackdown Pericline.      B

Picture credit - BGS

A larger picture here

                                 

Key to geological units on cross section. B à A

MMG -       Mercia Mudstone Group

CM    -       Coal Measures

DCG  -       Dolomitic Conglomerate - can be seen in quarry

QSG  -       Quartzitic Sandstone Formation

OHL  -       Oxwich Head Limestone - can be seen in quarry

CHI   -       Chinastone

CDL   -       Clifton Down Limestone

ChO  -       Cheddar Oolite Member

ChL   -       Cheddar Limestone Member

BO    -       Burrington Oolite Subgroup

VL     -       Vallis Limestone

BRL   -       Black Rock Limestone Subgroup

AvG   -       Avon Group ( Lower Limestone Shales.)

PO    -       Portishead Formation ( Old Red Sandstone )

Scientific importance

        This quarry is important because of the previous fossil content and the knowledge that this has added of the geology of this early part of the Carboniferous. This includes the discovery of a new species.


        The work was carried out on the fossils by Cliff Salter, who lived in Compton Martin. His collection of fossils is now held by the British Geological Survey (BGS) at Keyworth, Notts..

        This work has been recorded by Murray Mitchell and published by the Mendip Society.

   

        The many fossils that have been found in a section of this formation in the past are listed here pps 489 - 490

        A scientific paper based on the work carried out by Cliff Salter is here. It describes a new species, Cyclus Martinensis. It is named after the village of Compton Martin. See picture on page 489 here

          A common coral fossil called Lithostrotion ( Siphonodendron ) can be found in this quarry.

      There is some further information on the implications of Cliff Salter's research here and here reproduced from  'Thornbury Geology Group Newsletter February 2012'. 

      

There is also a paper here that follows on from this.


The above texts, following on from Cliff Salter's work, show that the existing numbers of fossils from the Carboniferous seas may be badly skewed and may result in parts of the palaeontology text books being rewritten.


Acknowledgements.


Thanks to Richard Ashley and Tarquin Bolton for their advice on identification of the fossils in the gallery.

References

The Salter Collection, from Cliff Quarry. Murray Mitchell. The Mendip Society.

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

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

A Walker’s Guide to the geology and lanscape of western Mendip. 2008. Andy Farrant. BGS.

http://www.bgs.ac.uk/mendips/geology/geological_structure.htm

http://palaeontology.palass-pubs.org/pdf/Vol%2010/Pages%20317-321.pdf

Thornbury Geology Group Newsletter February 2012.

Richard Kefford. July 2012.

Avon Wildlife Trust

In the first of a new series, we catch up with local groups who are involved in geoconservation in the Avon region. First up, Avon Wildlife Trust.

From belemnites & brachiopods to bats & bullfinches...

Avon Wildlife Trust is committed to working for people and wildlife in the local area, and we manage 35 nature reserves, some of which are also sites with outstanding geological features. One reserve which has been at the centre of our recent 'Bats for Bath' project is Brown's Folly.

Brown's Folly, Bathford. Photo credit: Avon WT

Browns Folly Nature Reserve is a Special Area of Conservation (SAC) and Site of Special Scientific Interest (SSSI) that has rare calcareous grassland habitats, unique geology, ancient woodland and a cave system with fantastic bat populations, with 12 of Britain’s 18 Bat species having been recorded here.  The caves are the remnants from mining the local oolitic limestone which was extracted for building material, and which is known locally as 'Bath Stone'. Oolitic limestone gets its name from the spherical particles or 'ooids' it contains. This word comes  from the Greek word for 'egg' because ooids are thought to resemble fish eggs. You can see fossils  in the exposed stone - thick shelled bivalves such as Trigonia, found today in the Pacific, along with brachiopods, ammonites and belemnites which once swam in the sea which covered this area 170 million years ago.

Oolitic limestone hand specimen (left) and thin section (right). Photo credit: http://www.earthscienceeducation.com

The stone was used for building in Roman times, but intensive mining of the limestone beds in the valley was carried  out from the 17th to the early 20th century. The characteristic stone was used for the construction of most of Bath's famous buildings, as well as Buckingham Palace and many stately homes across the country. The different mines, which were driven in to the rock in the  area, stretched for many miles and contained stables, cart-roads and tramways. Stone was extracted by the 'room and pillar' method, by which chambers were mined, leaving pillars of stone to support the roof.  By the time the mines closed down in 1930 there were over 300 acres of interconnecting underground workings and tunnels, which were used for storing ammunition and government  records during the Second World War.

Bats are very fussy about their roosting sites - they need somewhere with stable temperature and ventilation levels, and the old mine tunnels under the woodland seem to suit them very well. The huge size of the mines means that areas deep down are less affected by outside variations in climate - a perfect spot to hibernate for the winter months. The Trust has installed bat grilles on the entrances of the mines in the woodland to make sure that the bats remain at peace within their hilly homes. Bats are protected by law, and the grilles also prevent access to the mines - protection for people as well as preserving peace for the bats. In spring and summer when the bats awake and come out to feed at dusk, the grassland glades of the reserve are rich with insects right on their doorstep.

Greater horseshoe bat - just one of the twelve species recorded in the caves at Brown's Folly. Photo credit: Avon WT

Brown's Folly in Bathford is one of our most popular reserves that receives around 5000 visitors each year with most people enjoying the fantastic views from the grassland plateau across to Bath and the surrounding area, and talking long walks through the ancient woodland full of rocky fissures, giant veteran trees, mosses and ferns, and open glades that all help support the bats roosting in the caves.

You can read more about Brown's Folly and other Trust nature reserves here.

Pat Ellingham
(Avon WT Director of Communications and Development)


Look out for more information on the geology of Brown's Folly - it is soon set  to feature on the blog as a RIGS of the Month.

South Gloucestershire geology booklet - free download

The story of geology & landscape in South Gloucestershire was published in April 2007 hard copy format as a collaboration between South Gloucestershire Council, the Avon RIGS Group, BRERC and Bristol Museum. Although paper copies are no longer available, this excellent thirteen page booklet is still available as a free-to-download pdf (see link at bottom of post). It was also recently featured in the popular Down to Earth magazine.

Front cover

Fourteen local sites, including Aust Cliff, Huckford Quarry and Wick Golden Valley are featured. Each outcrop is described and interpreted with cross sections, location maps and annotated photos.

An example of a site description from the leaflet - Wick Golden Valley

 Download the pdf from the South Gloucestershire Council website

 Download booklet - 6MB

Furthermore, pdfs of all the site interpretation panels in South Gloucestershire can be accessed using this link  - Geological Conservation in South Gloucestershire (menu on right hand side of page)

Charly Stamper

'Lithostrotion'

Fossils of Avon RIGS region 

'Lithostrotion'

Please read the Geologist's code here:-

http://www.brerc.org.uk/rigs_site/geologists_code.htm

Name: Siphonodendron (formerly Lithostrotion) martini

Phylum: Cnidaria

Order: Rugosa

Horizon: Carboniferous

Photo credit - T. Bolton

Source rock: Clifton Down Limestone Formation

Age: Tournasian stage of the Lower Carboniferous Period (345 - 359Ma)

Locations: Avon George, Broadfield Down, Mendip Hills. Tickenham Ridge

Note: The underlined area is the type location for this Formation

Description of fossil

Siphonodendron martini corals were colonial, cylindrical and showed growth increment bands. Increment bands arise as a result of daily changes in light (which would affect the algae living in the outer cells of the organism) or temperature as well as monthly changes, associated with the lunar cycle. In the cool season or at night the corals secrete less calcite and so monthly and daily bands are visible on the surface of the corallites. Devonian corals studied at a different location show 400 daily bands in a year while these in the Lower Carboniferous have 391. This would indicate that the earth is slowing down on its rotational axis with tidal friction thought to be a major causal factor. The decrease in speed of the Earth's rotation is shown to accelerate in the Lower Carboniferous due to widespread occurrence of shallow shelf-seas.

Siphonodendron martini has a solid rod columella, major and minor septa (28 septa in each order) and dissepiments. In some areas many corallites did not reach maturity, only growing to 3mm. in diameter. Three ecotypes of the fossil are identified probably representing growth stages in the coral. The third ecotype is characterised by small radius, comprising only twenty septa and a single row of dissephnents. Some samples showed rejuvenescence i.e. the corallites reached adult dimensions then stopped growing only to recommence with smaller radius and more juvenile features. Measured growth was reduced from 4 mm a month in the limestone down to 2 mm a month in the calcareous shales where stunted growth is also observed to affect the diameter of the corallites. In the shale, rejuvenescence took place and there was little evidence of asexual breeding or monthly bands as the increase in nutrients from high levels of sedimentation is likely to have restricted breeding. Alternatively, a decrease in light levels  light may have affected the algae living in the outer cells of the organism, have reduced breeding and thus eliminated the monthly banding.

Photo credit - T. Bolton

Description of source rock ( Clifton Down Limestone )

Paleoenvironment

During the Lower Carboniferous the British Isles were just south of the equator and under water. As the Laurasian continent drifted north a large delta carrying eroded sediments from the north deposited its load in the sea, eventually killing the coral reefs and starting the Millstone Grit beds – forming the Quartzitic Sandstone Formation in the Avon RIGS area.

Lithology        
The lithology is dominated by calcite mudstones with a locally abundant but low diversity fossil assemblage. At Burrington Combe the formation is about 170 m thick, and three subdivisions can generally be recognised across the Mendip area. The lowest unit comprises a mixture of calcite mudstones, white oolitic limestones and dark splintery limestones. This interval is relatively expanded in the Cheddar area, where a 38 m thick dark limestone ('Cheddar Limestone Member') is overlain by a 58 m thick white oolitic limestone ('Cheddar Oolite Member'). The middle part of the succession is dominated by fine-grained, grey-black limestone with nodules and bands of chert and abundant remains of the coral Siphonodendron['Lithostrotion'] martini ('Lithostrotion Limestone'). Porcellaneous calcitic mudstones dominate the highest part of the formation, including locally developed algal mudstones and stromatolites, indicating deposition in a very shallow-water, near-shore or lagoonal environment.

Richard Kefford

References

- British geological Survey – Lexicon of rock units

- Natural History Museum – British Natural History
- The Black Country Geological Society – Newsletter No. 129.  June 1998.
- BGS. 1:50.000 series. Bristol 264 (S & D)
- BGS. 1:25,000 series Clevedon and Portishead. Geological Sheet ST 47.
- Post by T. Bolton on www.ukfossils.co.uk/ forum - photo credits

Celestine (SrSO4)

Minerals of the Avon region 

Celestine (SrSO4)

Colour: Light blue (but may also be colourless, yellow or brown)

Crystal system: Orthorhombic

Specific gravity: ~3.96

Hardness: 3-3.5

Location: Yate, Aust, Clifton

Please read the Geologist's Code here:-
http://www.brerc.org.uk/rigs_site/geologists_code.htm

Celestine (SrSO4). Specimen on left shows distinctive light blue colouration and prismatic form (source: Mexico). Example on right is from Yate, nr. Bristol. http://www.bristol.ac.uk/centenary/look/cabinet/celestine.html

Celestine (also referred to as 'celestite' or 'spar') is a principal strontium (Sr) ore and was heavily quarried in South Gloucestershire in the 20th century. It is characteristically light blue in colour and its name is derived from the Latin caelestis meaning ‘heavenly’ or ‘from the sky’.

Paragenesis

Although celestine can form in hydrothermal veins, it is most commonly found in sedimentary deposits. In the South Gloucestershire, it is found in the Mercia Mudstone (a unit of mudstone and siltstone of Triassic age, sometimes known as the Keuper Marl). The Mercia Mudstone was deposited in an arid environment where saline lakes formed in desert basins. The evaporation of water from these playa lakes produced evaporite deposits of gypsum (CaSO4.2H2O), halite (rock salt) and celestine (both primary and as a replacement mineral). The extreme enrichment of Sr in the waters, and resulting prolific deposition of celestine in the Bristol area, is postulated have come from the dolomitisation of aragonite in the Carboniferous sediments; aragonite may contain up to 8,000 ppm Sr in comparison to 400 ppm Sr in calcite. Celestine is differentiated from other evapourite minerals by its high specific gravity and relative hardness.

Local exposures

The most abundant deposits of celestine in the Avon region are in the Yate area (Charfield, Leechpool and Goosegreen), Leigh Court in Bristol, and at Aust Cliff*. The mineral occurs as both irregular masses and geodic nodules in the Mercia Mudstone, and in secondary concentrations at the contact between the mudstones and underlying beds, such as Coal Measures or Silurian sediments.

Uses

Celestine is the world’s primary strontium ore. Quarrying of celestine took place around Yate from the 1820s to 1990s and it was exported to Germany for use in the refining of sugar beet. At one time, 90% of the world’s celestine yield was extracted in Yate; unfortunately, the main quarry site is now buried under Yate shopping centre.

Celestine quarrying in the Yate area - Bristol Mineral & Land Co.  c.1950. Photo credit: http://www.sgmrg.co.uk

In spite of former glories, extraction of the dwindling Yate celestine deposits is no longer economically viable and the majority of the world’s supply of Sr metal is now sourced from celestine and strontianite (SrCO3) from Mexico and Germany. Sr is used in pyrotechnics (it produces the crimson-coloured flame in fireworks) and in the manufacture of display screens (dense Sr-doped glass shields the viewer from x-rays).

Charly Stamper

*Aust Cliff is a SSSI and removal of specimens from the cliff face is both hazardous and illegal.

References:

- Deer WA, Howie RA, Zussman J. An introduction to the rock forming minerals, 2nd. ed. (1996)
- Green GW. British Regional Geology: Bristol and Gloucester District, 3rd ed (1992).

- Wood MW & Shaw HF (1976) Chem. Geol. 17: 179-193.

- South Gloucestershire Mines Research Group http://www.sgmrg.co.uk/celestine.php

- Yate Heritage Centre http://www.yateheritage.co.uk/history-of-yate/beneath-our-feet.htm