The Group's aim is to identify, survey, protect and promote geological and geomorphological sites in the former County of Avon - the modern unitary authorities of Bath and North East Somerset, Bristol, North Somerset and South Gloucestershire. RIGS are selected for their educational, research, historical and aesthetic value.

Friday, 29 June 2012

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

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

Friday, 22 June 2012


Fossils of Avon RIGS region 

Please read the Geologist's code here:-

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 )


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.

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


- 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

Saturday, 9 June 2012

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:-

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’.

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.

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.

- 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

RIGS of the Month [June] - Aust Cliff

RIGS of the Month - June
Aust Cliff
Tracing an ancient drowned desert


Location: GR = ST565895 South (downstream) of Severn Bridge.
Accessibility: Parking on B4461 Aust Wharf road at Old Passage. Via steel gate with stile to concrete causeway. Limited access to wheelchair users.
Risks: The cliffs are dangerous. Beware of falling rocks. Beware of the tides. Beware of mud flats.
Topography: Level estuarine foreshore beneath cliffs.

All photos from this post can be viewed in a larger format - 

Geological and geographical maps for Aust. Click on map for bedrock descriptions.

The river cliff at Aust is a spectacular outcrop of Mid and Late Triassic to Early Jurassic sedimentary rocks, an impressive geological archive for tracing the drowning of an ancient hot, arid desert between ca 221 and 195million years ago.

Aust Cliff south of the River Severn Bridge. Fallen rocks on the foreshore have produced teeth of primitive sharks, remains of ichthyosaurs, pleisiosaurs and terrestrial dinosaurs.
The succession is clearly visible in this gently arching anticline by the striking changes of colour in the strata. The red beds of the Mercia Mudstone Group (formerly called Keuper Marls) form the Branscombe Mudstone Formation (206 - 221 Ma) from the cliff base. These pass up to the green-grey beds of the Blue Anchor Formation (206 – 221 Ma), (formerly called Tea Green Marls). Above the Blue Anchor Formation rest the dark then lighter grey beds of the Penarth Group: the Westbury Formation and Cotham Member, (formerly called Rhaetic Beds) from the Late Triassic, or Rhaetian (206 – 210 Ma). At the cliff top are the light brown beds of the Blue Lias Formation from the Early Jurassic, (195 – 210 Ma).

There was extensive sedimentation in the region during the Early and Mid Triassic, burying the older Carboniferous Black Rock Limestone landscape. This underlying limestone platform is partly visible at low water in the curving, tilted outcrops on the river bed upstream of the Severn Bridge, best viewed from the pedestrian walkway. Notice also how the descending arc of the Aust Cliff anticline and the succession upstream continues across the estuary in Sedbury Cliff (ST555930). It is also worth noting the correlation between the formations at Aust Cliff and those at Garden Cliff at Westbury-on-Severn, the North Somerset and South Glamorgan coasts.

The interpretation panel at the end of the concrete causeway to the foreshore informs us that Aust Cliff is a Site of Special Scientific Interest and, of course, a RIGS.

The beds of blocky, red-brown dolomitic mudstones and siltstones of the Branscombe Mudstone Formation are coloured by ferric oxides adsorbed onto fine-grained sedimentary particles in well-oxygenated environments. Green-grey interbedded deposits can also be seen in places. Well-rounded sand grains in the sediment suggest abrasion in a wind-blown, local environment. The sediments have been deposited in water, and the presence of evaporites (gypsum is abundant here) and celestine suggests extensive hypersaline, enclosed, ephemeral lakes or playas.

Gypsum deposits in a long horizontal seam and vertical veins in the base of the red beds at the far end of the foreshore. Gypsum varieties: satin spar, a white fibrous form, selenite, and alabaster, a very fine-grained, white or tinted form, are present. Vertical cracks were possibly caused by folding.
The pale green-grey dolomitic silty mudstones and siltstones of the Blue Anchor Formation were also formed in lakes or inter-tidal flats, depositing clays and silts. The green colour is due to the adsorption of ferrous oxides by sedimentary particles in a waterlogged environment. Halite (salt) pseudomorphs have been found, indicating hypersaline conditions.

These two formations of the Mercia Mudstone Group are devoid of macrofossils.

The transition from the Blue Anchor Formation to the darker fossiliferous shales and pyritised, shelly limestones of the Westbury Formation marks the change to a stagnant, anoxic, brackish shallow sea or coastal lagoons. At the base of the Westbury Formation is the famous Westbury Bone Bed, 15 cm thick blocks of a conglomerate mainly of small lumps of green-grey siltstone, quartz pebbles, and a concentration and diversity of well preserved vertebrate fragments, all cemented into a sandy matrix.

Pieces of the Bone Bed and the moulds and casts of bivalves can be found in rocks that have fallen from the cliff onto the foreshore. For a list of fossil finds click here.

The Cotham Member, named after Cotham House in Bristol, has provided specimens of flora and fauna distinct from the Westbury Formation. These species, including the well-known algal derived Cotham Marble from the uppermost horizon, indicate fluctuating lake levels. For native rock click here.

There is an ongoing lively debate based on geological and geochemical evidence from the Cotham Member in the area, and from St. Audrie’s Bay and Lavernock Point for seismic, volcanic, meteor impact events, and their association with an end-Triassic mass extinction.

For a closer look at the Westbury Formation, a display of Bone Bed blocks and the Cotham Member, visit the nearby RIGS at Manor Farm (ST576894). A detailed description of the Late Triassic strata at Manor Farm may be found in the reference below, Radley & Carpenter, 1998. There is also a good display of Aust Cliff rock and fossil specimens in the Bristol City Museum.

Manor Farm ‘borrow pit’ was restored in 2002 and retains the Westbury Formation (bottom), Cotham Member and Blue Lias Formation (top). Despite the weathered facies there is much to see, including a good display of the Westbury Bone Bed. Private land. Enter with the owners’ permission.

The Aust Cliff top is mostly overgrown, but the Blue Lias interbedded limestones stacked above the Cotham Member are visible in places. The fossil record, which includes ammonites of Hettangian age, indicates a developed marine environment.

There is a remarkable fault in the cliff face before the bridge footings. It helps with identifying the strata from the foreshore, and is a final point on the trail to realise that Britain’s palaeogeography in Early to Mid Triassic Pangaea was a hot, arid desert at latitude 30oN, North Africa today. There followed the marine transgression in the Late Triassic and Pangaea rifted apart. In Early Jurassic Laurasia, Britain was submerged.

A normal fault with the downthrow to the right of the fault highlighting the succession. Above the dropped green-grey Blue Anchor Formation, the darker grey Westbury Formation and light grey Cotham Member trace the marine transgression to the Blue Lias Formation brown interbedded limestones (top right).
There are several normal faults visible in the cliff from the foreshore, and some of these give rise to springs. Retracing the trail, and along the concrete causeway, a faulted section has a promontory at the base where spring water rich in calcium carbonate is precipitating a type of limestone, forming mounds and domes of tufa. Geological and geochemical processes are continuing today.

Mounds and domes of tufa form by precipitating calcium carbonate from spring water emerging near the cliff base.

John Byles


OS Bristol & Bath Sheet 172 1:50 000 Series


Hamilton, D., Aust Cliff, Geological Excursions in the Bristol District, Ed. Savage, R.J.G., 1977.

Radley, J.D. & Carpenter, S.C., 1998, The Late Triassic Strata of Manor Farm, Aust, South Gloucestershire, Proceedings of the Bristol Naturalists' Society, 58:57-68.

Chidlaw. 2000. A Commentary on Geology and Scenery in the West of England by A.E. Trueman. Allegro.

Benton, M., Cook, E. & Turner, P., 2002, Permian and Triassic Red Beds and the Penarth Group of Great Britain, Geological Conservation Review Series, No. 24, Joint Nature Conservation Committee, Peterborough.

Simms, M.J. 2007, Uniquely extensive soft-sediment deformation in the Rhaetian of the UK: Evidence for earthquake or impact? Palaeogeography, Palaeoclimatology, Palaeoecology 244 407–423

Gallois, R.W. 2009, The lithostratigraphy of the Penarth Group (late Triassic) of the Severn Estuary area. Geoscience in South-West England, 12, 71-84.

Deenen, M.H.L. et al., 2010, A new chronology for the end-Triassic mass extinction. Earth and Planetary Science Letters 291 113–125.