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.

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

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.

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.

RIGS of the Month [May] - Portishead foreshore

RIGS of the Month - May
Portishead foreshore

SITE SPECIFIC INFORMATION

Accessibility: Park on esplanade near Lido. All public access.

Risks: Best accessed on a falling tide as tides can reach base of cliff.
Hard hats should be worn near cliffs
Slippery rocks in tidal areas, steep slopes on Battery Point.
Strong tidal flows.

 

 Location map (numbers refer to sites in text). BGS Sheet ST 47 Clevedon and Portishead.

All photos from this post can be viewed in a larger format - https://picasaweb.google.com/charly.stamper/PortisheadPhotos

 

The foreshore at Portishead offers an opportunity to walk through a sedimentary succession from the Lower Carboniferous through to the Mid-Lower Devonian. There is excellent exposure of the Portishead beds, the rocks that form the core of the Mendips, and the area has been known for many years for the famous Woodhill fish beds where examples of early Devonian fish fossils have been found. Interesting features along the coast include Variscan folding, fossils (including fish scales from the Devonian fish beds), cross bedding, faulting and calcrete formation. 




1 – Battery Point

Black Rock Dolomite (BRD). Part of the Black Rock Group of the Carboniferous Limestone Series from the Tournasian Stage (359 – 345 Ma) of the Carboniferous Period (equivalent to the Lower Dolomite in the Forest of Dean). Here, the strata dips 40/50º to the North East.

Near to the old lighthouse there is an outcrop of hard Black Rock Dolomite (BRD). This limestone was laid down in a shallow calm sea and was later dolomitised by magnesium saturated fluids, resulting in partial replacement of the calcium with magnesium and destruction of much of the shelly matter. In places the rock is stained red-brown from the overlying Triassic sediments and also contains some chert horizons. The BRD is surrounded by Dolomitic Conglomerate which consists of limestone clasts cemented in a matrix; this material was eroded from the BRD and deposited in a skirt around Battery Point during the Triassic. 

 

Contorted strata can be seen from the seaward side of the old lighthouse. The beds were deformed as part of the Variscan Orogeny and are thought to be part of a south verging anticlinorium (a vast elongated anticline with its strata further folded into anticlines and synclines) with a normal northern limb and an overturned southern limb.

The Merchant Navy memorial stone on Battery Point is a large chunk of Roach Stone from the Portland series. Many fossils can be seen in it, especially bivalves and examples of the ‘Portland Screw’ -  a turreted gastropod - Aptyxiella portlandica.

2 - Battery cliff

The Lower Limestone Shales are also from the early Carboniferous and consist of alternating lenticular limestones and calcareous siltstones. The limestones contain skeletal debris and well preserved fossils whereas the siltstones are intensely bioturbated. The beds are dolomitised and red stained throughout.

3 - Woodhill Bay

The transition from Carboniferous to Devonian cannot be seen as there is a shallow valley to the South of Battery Point which is infilled with Triassic sediments. The limestone units in Woodhill Bay are made up of a mixture of highly abraded hematic skeletal debris and well preserved fossils of articulated crinoid stems and fairly common examples of the tabulate coral Vaughania (Cleistopora) which is the index fossil for the Lower Limestone Shales. These have been affected by the Triassic alteration and so have been heavily dolomitised and stained red by the iron oxides. There are many small folds to be seen on the beach which appear to be part of the larger fold system. 

  Triassic sediments in Woodhill Bay containing well-preserved crinoid stems from the Lower Limestone Shales

4 - Kilkenny Bay 

At the Southern end of the sea wall, cliffs of the Upper Old Red Sandstone (ORS) (including and including the Woodhill Bay fish beds) are well exposed. This is the Portishead formation which dips at low angles to the North. This means that the rocks get progressively older to the South, eventually changing into the Black Nore Sandstones (BNS) of the Lower Old Red Sandstones which continue to be exposed as far to the South as Charlecombe Bay. The total thickness of the ORS in this area is thought to be 900 – 1200 m.

The Woodhill Bay fish bed formation consists of sandy siltstones and sandstones where beds vary in thickness between 0.5 m and 3 m. Many fish have been discovered here in the past and fish scales can still be found.

The ORS is associated with the erosion of the Caledonide mountains formed by the collision of Avalonia, Baltica and Laurentia to form the ORS Continent, an event known as the Caledonian Orogeny which occurred about 425 to 395Ma. Most of the ORS deposits seen in this area exhibit complex cross bedding. Plotting the current directions has shown that the main source of the pebbles in the deposits is to the North West. It has been proposed that the source of many of these pebbles is the Precambrian Mona complex of Anglesey

Two post-Triassic faults can be seen, with the Triassic conglomerate being down thrown with respect to the Devonian strata forming a graben between them. A discontinuous cover of Triassic Dolomitic Conglomerate blankets the ORS strata and, in places, excellent examples of angular unconformity are present.

Angular unconformity between Devonian Old Red Sandstone and coarse Triassic Dolomitic Conglomerate in Kilkenny Bay

Calcrete at Portishead
Calcrete can been seen in the Lower Devonian cliff exposures of BNS.  This calcrete formation is known in the South Pembrokeshire area as the Chapel Point Calcrete. It extends across South Wales and can be seen on the shore cliffs of the Severn at Lydney, where it is known as the Bishop’s Frome Limestone. For more info on this topic, click here.
 
Richard Kefford & Charly Stamper

References

- Savage R.J.G. 1977. Geological Excursions in the Bristol District.
- Barclay W.J. 2005.  Introduction to the Old Red Sandstone of Great Britain (GCR) Chapter 1. 

- BGS. 1968 Geological Sheet ST 47  Clevedon and Portishead. 

- BGS. 2004 England and Wales Sheet 264  Bristol.