The Bristol 'tsunami': Flood or fallacy?

This post was originally featured on http://betweenarock.co.uk/

30^th January 1607*.
The day dawns sunny and bright. You are ploughing a field in your smallholding deep in the Somerset Levels. As the sweat drips down your back, you hear a distant rumbling sound but think nothing of it; the wind has been blowing a gale all night. Suddenly, a shout from a neighbour makes you look up in alarm. At the end of the far field you see a great cloud hugging the ground, light dazzling off the whiteness. At first you are confused: is it fog, or smoke from a fire? But then you realise, it's water. Within ten seconds, the tumbling, roaring mass has advanced the length of the paddock. You try to run but it's too late. Knocked off your feet by the force of the wave, your head dips below the surface and you inhale a lungful of salty water...
^{*The exact date depends on whether you have a preference for the Julian or Gregorian calendar}

From eyewitness reports, this is what it felt like to be caught up in the most catastrophic flood ever to hit western Britain. Striking in January 1607*, its effects were felt all over the south-west of England, extending over 570 km of coastline from Barnstaple to south Wales and as far inland as Glastonbury (approximately 22km). Contemporary sources put the death toll at over 2,000, though modern estimates have revised this to 500 - 1000¹. The water flow is said to have been so fast "... that no gray-hounde could have escaped by running before them." But what was the cause?

Contemporary woodcut depicting the scene in Monmouthshire on 30th January 1607.

Prior to a modern-day brush with fame, the Bristol Channel Floods were variously attributed an extreme spring tide (the maximum extent of a tidal range that occurs when the Earth, Moon and Sun are in alignment, roughly every fortnight), a storm surge (high water levels associated with a low pressure weather system) or a combination of both. This type of coastal flooding is relatively common in the UK; a particularly deadly occurrence in 1953 killed 307 people in East Anglia.

The tsunami hypothesis was first proposed in 2002 by two academics (Haslett & Bryant - see references 2,3 and 4), and followed up in a series of subsequent papers by the same authors. Their re-interpretation of the events unintentionally coincided with the devastating Boxing Day tsunami of 2004, and so was perfectly poised to percolate the national consciousness. Numerous media articles publicised the theory, and the floods were featured in two BBC2 TV programmes (Timewatch - "The Killer Wave of 1607" and "Britain's Forgotten Floods").

Flood plaque in Goldcliff parish church, Newport. Reads "1606. On the XX day of January even as it cames to pass it pleased God the flud did flow to the edge of this same bras [brass], and in this parish theare was lost 5000 and od pownds besides xxii [22] people was in this parrish drown.". Photo credit: Robin Drayton.

Of course, publicity is not the mark of whether a theory is right or wrong, but proving this particular watery dispute one way or the other has been hindered by a couple of confounding conundrums: the subjectivity of historical sources and the ambiguous nature of tsunami deposits.

At the turn of the 17^th century, literacy levels in the UK were still relatively low. There were no newspapers (or Twitter!), thus first-hand accounts are mostly limited to privately printed pamphlets which tend to offer contrasting reports. For example, the weather on the day in question is conflictingly described as being "most fayrely and brightly spred", "tempestuously moved by the windes" and in the grip of "a mightie storm". The most supportive evidence for a tsunami comes from "Gods [sic] warning to the people of England" , a publication funded by the Church. Its coverage of the event is predictably zealous, describing the flood as a "universal, punishment by Water."
As geologists, the obvious solution would be to look to the rock record; however, tsunami deposits are notoriously tricky to identify because their physical markers are incredibly hard to distinguish from other sources of coastal flooding. Pro-tsunami authors Haslett & Bryantt cite sand "storm" layers in sediments, erosion of salt marshes, vortex pools, and imbricated boulder dumps as supporting evidence for a 'killer wave'; all features imply rapid deposition from a forceful flow of water. Their proposed mechanism for the tsunami is either a submarine landslide or earthquake in the sea between Ireland and Cornwall.

Prof. Simon Haslett atop imbricated boulders in the Severn Estuary. Photo was taken during filming of the BBC2 programme “The Killer Wave”. Source: http://profsimonhaslett.blogspot.co.uk

Perhaps the most compelling evidence against the tsunami hypothesis is that severe flooding in Norfolk is documented on the same day. Most tsunami models agree that it is geometrically impossible for the effects of a tsunami to wrap around the entire coast of England. It seems like the most plausible cause of the floods is a storm surge imposed on an unusually high spring tide. Indeed, the Severn Estuary has the second highest tidal range in the world. The contemporary reports of windstorms driving up the seas is reminiscent of storm surges in New Orléans during Hurricane Katrina in 2005.

Regardless of the cause, it is important to consider the impact that a repeat of the 1607 floods would have today, in order to mitigate against future disasters. The Severn estuary is home to the (active) Hinkley Point and (closed) Oldbury nuclear power stations, and is the proposed site of the controversial Severn Tidal Barrage. Other notable infrastructure includes two motorway bridges, a working port (Avonmouth) and half a million people living in Bristol alone! One risk assessment puts the cost of such an event at £7 - 13 billion¹.

In the wake of the 2004 Boxing Day tsunami, the UK government recognised they did not have a quantitative assessment of threat to the UK. This was despite another infamous tsunami study⁵ (the results of which are now viewed with scepticism) which predicted that a landslide off La Palma would generate waves "higher than Nelson's column" and smash into the west coast of Britain - mass media loved it. Happily for us, the government reports conclude "tsunami-type events [affecting the UK] are unlikely to exceed those anticipated for major storm surges", and "all major centres of development on coasts and estuaries have defences that have been designed to withstand such surge waves."

Should we have these in Bristol City Centre?

Despite their assurances, a small part of me feels pretty smug about a living and working a good 50 metres above sea level!

Charly Stamper

References
[1] "1607 Bristol Channel Floods: A 400-Year Retrospective" - Online publication by Risk Management Solutions.
[2] Bryant EA & Haslett SK (2007) Catastrophic Wave Erosion, Bristol Channel, United Kingson: Impact of Tsunami? The Journal of Geology: 115, p. 253-269.
[3] Bryant EA & Haslett SK (2002) Was the AD 1607 coastal flooding event in the Severn Estuary and Bristol Channel (UK) due to a tsunami? Archaeology in the Severn Estuary. 13: 163 - 167.
[4] Haslett & Bryant (2004) The AD 1607 coastal flood in the Bristol Channel and Severn Estuary: historical records from Devon and Cornwall (UK). Archaeology in the Severn Estuary. 13: 81 - 89.
[5] Ward, SN & Day, SJ (2001) Cumbre Vieja Volcano; potential collapse and tsunami at La Palma, Canary Islands. Geophys. Res. Lett. 28-17, 3397-3400.

Geo-gardening - Sunday 21st October 2012

John Toller was joined by a very small group from WEGA to clean up the RIG site at Itchington which the group had agreed to maintain. The site demonstrates a small but excellent exposure of the "Bristol Time Gap". This shows the angular unconformity which separates the almost horizontal Triassic Mercian Mudstone Group, locally Dolomitic Conglomerate, from the older, underlying and tilted Carboniferous Limestone, which is known locally as Clifton Down Limestone. This was formed approximately 334 to 341 million years ago in the Carboniferous Period. 

The Carboniferous Limestone was deposited in warm shallow seas covering much of the country. This was followed by the Coal Measures and then thick layers of sandstone lain down by river deltas until the start of the Permian Period. No Permian rocks are present here, but during that period the continents were all brought together to form one huge landmass, called Pangaea.

The Carboniferous rocks here were folded and compressed to form mountains, then weathered and eroded down to produce more sediments. The layers overlying the limestone here are products of that erosion during the Triassic Period, resting on the worn down limestone surface. The environment by then had changed to mainly desert, resulting in characteristic reddish sands, clays and debris as seen here, contrasting with the hard grey limestone. The time gap between the two layers is at least 50 million years, enough time for a whole mountain range to be created and worn down.  

The rock face had become covered in a fine variety of plants and saplings which were threatening to completely engulf it. The many ash sapling were especially unwelcome as the strong roots can quickly penetrate the joints and fissures in the rock to break down the exposure. After a couple of hours determined clearance the unconformity was looking very much more obvious, as John is pointing out in the photo. Next year it will all need doing again or the ash will take hold, so keep the clippers handy! 

To admire this site, take the turning  from the A38 at the Grovesend road junction near Thornbury signposted to The Slad and Itchington. Keep travelling until you are about to go under the M5 and see the exposure on the right. Close by you can also see a beautifully flat natural bedding plane of limestone forming a bank. The exposures are fenced off and there is always a danger of falling rocks, so keep at a safe distance. 

Photo credit: Sandi Shallcross

Sandi Shallcross

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.

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

RIGS of the Month [June] - Aust Cliff

RIGS of the Month - June
Aust Cliff

Tracing an ancient drowned desert

 

SITE SPECIFIC INFORMATION

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 - 

http://tinyurl.com/aust-cliff

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 30^oN, 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

Maps

OS Bristol & Bath Sheet 172 1:50 000 Series

References

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.