Open University Geological Society (London Branch)
At each event we ask for a volunteer to record their experiences and impressions. These articles are published in our newsletter and eventually appear here as an archive of our activities. Note that these are only provided for reference, and are not endorsed by Event Leaders. Interpretations can change often and rapidly in Geology, any information stated should be verified from appropriate sources before being quoted.
The Paleocene-Eocene Thermal Maximum • Crossrail Visitor Centre and Tottenham Court Road Station Upgrade • Soft-Ground Tunnelling through London • Duncton, Burton Park and Bignor • The 2010 eruption of Erta Ale, Afar, Ethiopia • Solnhofen • Kensworth Quarry, Dunstable Downs & Totternhoe Quarry • The Thames Barrier • Storm Surges Thames Estuary • The Green Chain Walk • Faringdon, Abingdon and the Oxford University Museum • Kurdistan • Boxgrove and Bracklesham • Carbon Capture and Storage • Ramsgate • Shetland • Northampton Building Stones Walk • The Basement of the London Basin • Reigate and Godstone caves • Wander up the Wandle • Meteorites • Isle of Wight • Herne Bay • Members' Evening • Geological problems London-Brighton Railway • Dinochlia
Lecture : The Paleocene-Eocene Thermal Maximum : Geochemical Investigations of a 46 million year old Global Warming Event - By Alex Dickson
4 February 2012
Alex is a Post Doctoral Research Associate (PETM) at the Open University, "currently working to apply novel isotope systems (Os, Mo and Re) to understand changes in ocean oxygenation and continental weathering during periods called hyperthermals".
He therefore gave us an up-to-the-minute account of current research and thinking on the causes, development and rapid decline (a mere 200,000 years!) of this episode of rapid global warming and consequent extinction of numerous species, concentrating on the Arctic and North Atlantic Oceans.
During the PETM temperatures were 5-8°C above those of today, accompanied by a very abrupt Carbon isotope excursion, a stronger hydrological cycle and benthic extinction, along with enhanced adaptation in the surface ocean and evolutionary changes in terrestrial flora and fauna.
Questions: the possible causes of the CIE: the release of methane hydrates, volcanic intrusion, methanogenesis from peatlands? How big: 2-7‰? Where did it come from? What was the effect on ocean chemistry? How extreme did the climate become? How was the carbon sequestered after 200 ky?
A result of increased oceanic productivity in greenhouse conditions is the development of anoxia and euxinia in the deep ocean. The Arctic Coring Expedition (ACEX) is studying this phenomenon, with a control in Tajikistan to ensure the results are global and not due to local conditions in the Arctic.
Molybdenum isotopes are a proxy for sea-floor anoxia and Osmium isotopes a proxy for weathering and volcanism, so the work is to expand the isotope database. The North Atlantic Igneous Province, by creating a land bridge, might have triggered the PETM, through uplift of continental margins. Molybdenum isotope redox reconstruction is being applied to the PETM for the first time, showing a much more reducing ocean than today.
The lecture was accompanied by numerous charts and graphs, and provoked a number of questions.
Crossrail Visitor Centre and Tottenham Court Road Station Upgrade
16 February 2012
Over thirty of us assembled at 2pm in the foyer of the Crossrail Visitor Information Centre at 16-18 St Giles High Street – surrounded by brochures, fact sheets, display boards, station models and a screen for showing videos. Ursula Lawrence, Geotechnical Engineer for Crossrail, was our leader for the day.
Introduction to the Crossrail Project
Ursula introduced the Crossrail Project by showing us a number of videos – including one all about Tunnel Boring Machines (TBMs), an animated 3D Typical Station Escalator/ Access; a film showing the existing but disused Connaught Cut-and-Cover Tunnel below the London Docks, which will be refurbished and deepened to accommodate Crossrail trains, and a film about the Canary Wharf Station Box.
If you want to watch them again (and read much more besides) then go to the following Crossrail websites:-
Visit to Tottenham Court Road Station Upgrade Site
From there, we were organised into groups of 5 or 6 to go over the road to the Centre Point Steps for a bird’s eye view looking west over the excavation for the new London Underground Eastern Ticket Hall for the upgraded Tottenham Court Road Station. It will serve the Northern Line, the Central Line and Crossrail. The excavation is at present approximately 31.5 metres wide by 51 metres long.
Straight ahead, over on the west side of the excavation, is a 4.5 m diameter shaft which will, in the future, house a lift shaft down to the Northern Line platforms below. It has until recently been used as an access shaft for the work on modifying the platform tunnels of the existing Northern Line Station.
To the right of the shaft is the excavation for Ticket Hall Plant Rooms and new Central Line access, with propped cantilever bored pile retaining walls either side. The tubular steel props are painted red and white. To the left of the shaft is the west wall of the excavation, formed of contiguous bored piles. These piles have been driven down between the Northern Line platform tunnels. This wall is not at present propped but excavation immediately adjacent to it is only down to the Thames Gravel. Nearer to us, excavation has been taken down to the surface of the London Clay. It would seem that the east half of the N/S Ticket Hall ground slab will be cast first – so that this wall can then be strutted off it whilst the west half is cast!
For those of us who are interested in fossils, it was disappointing that we could not go down and have a look at the London Clay!
A cascade of loose excavated material at the south end of the excavation hid from view the retaining wall beyond which the new
Northern Line and Crossrail escalator shafts have been built.
By the way, we were unable to see any of the asociated Crossrail works from the viewpoint. The route of the nearest Crossrail tunnel passes just south of the Ticket Hall excavation. The northern Crossrail Tunnels will be driven over the Northern Line Tunnels and below the new southern escalator shafts.
The Afternoon Lecture
Ursula introduced us to the many aspects of the design and construction of Crossrail – far too complicated to record in any detail and so only rough notes follow.
Auroch bones found.
Top-Down Box Construction Method for Canary Wharf Station.
Wall tied back along the top to anchor piles.
Sprayed Concrete Linings
Used for irregularly-shaped and sloping connecting tunnels, as well as the large station tunnels, excavated by electrically-driven conventional machines. Primary lining, water-proofing membrane and then a secondary and inner lining. TBMs will bore into the already existing station tunnels.
Formed by pushing precast concrete rings down by undermining, a conventional method of construction illustrated spectacularly by a time lapse video of the Bond Street site. See it again on : http://www.crossrail.co.uk/news/construction/construction-a-grout-shaft-for-bond-street-eastern-ticket-hall.
Some 1043 cable-percussion and rotary boreholes – total length 34 km – numerous contracts spread over 20 years. Supplemented
by another 653 from other sources, totalling 25 km.
Time lapse video of demolition at one of the Ticket Hall sites (TCR Western Ticket Hall?) very spectacular and caused some amusement.
Structural Geology at Farringdon Northern Boundary Fault and four N/S trending faults (including the Smithfield Fault). A very impressive block diagram showing strata down to the chalk, with each strata removed in sequence to reveal the underlying surface. Pronounced river channel in the surface of the LC (Proto-Fleet River?). Also pointed out to us that Thames Gravel terraces either side of the Fleet at different levels because of the downthrow at the fault.
Lecture : Soft-Ground Tunnelling through London - by Dr Ursula Lawrence (Geotechnical Engineer, Crossrail)
16 Feb 2012
Following our interesting introduction to Crossrail in the afternoon, we were privileged to hear Ursula again as she gave us a wealth of information with maps, illustrations and diagrams of the work begun in 2009, the commissioning of the first TBM (Tunnel Boring Machine) this year, the rewards and difficulties of completing the project and its phased opening in 2018.
The programme is daunting with the upgrading of 28 existing stations and 90 km of existing surface network in addition to 21 km of new sub-surface twin-bore railway through London and 8 new sub-surface box stations with different construction methods according to the local ground conditions, the whole stretching from Paddington to Abbey Wood.
She then talked of Tunnelling Strategy with a total of 8TBMs, mostly working in London Clay, but also Thanet Sands, River Terrace Deposits and Chalk at Woolwich, completing 100m a week if all goes well. These enormous machines were illustrated with photographs and cross-sections showing the Archimedes screw evacuation mechanism and the way in which curved concrete panels are lifted into place to make the tunnel lining as the machine advances. In the short tunnels in stations, the concrete lining will be sprayed on by machine.
There are many problems associated with the Geology under London: nodules in the clay, discontinuous formations associated with a shallow marine environment, Blackheath cobbles, alluvial channels of which some still have water, and Chalk with sheet flints under the Thanet Sands. There are engineering challenges due to major faults, some only recently discovered; sensitive structures above the route, and archaeological remains which have to be preserved; finally there may be unexploded ordnance from WWI and WWII.
The excavated material, loaded on to lorries and ships, will be taken to Wallasea Island and transformed into mud flats to provide a haven for wild birds.
Many thanks to Ursula for an impressive tour de force. The website: www.crossrail.co.uk is very informative.
Duncton, Burton Park and Bignor - with David Bone
4 March 2012
Such a lovely part of the world, but such unpromising weather! First stop: Duncton Hill viewpoint car park, but it was raining hard and who could blame the members of the group who didn’t leave the comfort of their cars at this point, to hear the introductory overview of the geological setting.
We stood on the scarp slope of the South Downs in West Sussex, with a rather murky view over the Wealden successions to the North. We were on the Middle Chalk and below us was the Lower Chalk. The Upper Greensand bench, with its minor escarpment, could be seen at the foot of the main South Downs escarpment, then the clay vale of the Gault Clay, followed by the Lower Greensand formations: the Folkestone Formation, the Sandgate Formation and the distant sandstone ridge of the Hythe Formation.
The next stop was in a field at the bottom of the hill. This was on the Greensand bench near Duncton Mill. Behind us, hidden in woods, were the remains of 19th century lime kilns and chalk quarries. The lime may have been used on fields or it may have been used for lime wash. In those days, internal walls would be painted with a lime wash every two to three years, as lime has anti-bacterial properties. The Lower Chalk is also good for finding ammonites.
Below us, a spring emerged from beneath a cliff of the permeable Upper Greensand onto the Gault Clay. Gault Clay is soft and impermeable marine mud, so water runs over its surface and erodes it. The flow rate of the spring is consistent, even in dry years and the water quality is continuously high. We were invited to decide whether the hollow above the spring was of a natural size, or whether it had been unnaturally enlarged by human excavation. The latter seemed likely, but for what purpose?
Further downstream, by Duncton fishpond (photo 1), there were small buildings (photo 2) made from blocks of ‘malmstone’, a calcareous siltstone from the Upper Greensand. This was an important local building stone. It often weathers with a brown coating. It was not quarried, but the sunken lanes nearby have sides cut back from the road and this may have been the source of it. Malmstone does contain some fossils, typically marine shells.
We were now informed of the suspicion that the hill behind the spring had indeed been cut back, but not for malmstone, rather for tufa, although there are now only tiny fragments of tufa in the streambed. Were you aware that calcareous tufa is a precipitation of minerals from cold fluids, whilst travertine is a hard, compact variety of calcareous tufa, a precipitation from hot fluids in volcanic regions? Tufa forms when carbon dioxide comes out of solution, allowing calcium carbonate to be deposited and although it looks weak and friable, it forms a perfectly useable building stone.
The examples of modern tufa from West Sussex that we were shown had fronds, stems, twigs, leaves and other small pieces of detritus, beautifully outlined by a coating of calcium carbonate. The stone was highly porous and pale grey in colour (photo 3).
We walked up through Burton Park, past a stately home once owned by the Courtauld family and now converted into luxury flats. The destination was the church at Burton Park, rededicated in 2003 to St Richard of Chichester (photo 4).
This church was originally built in Saxon times and various ages of construction could be observed in its walls. It was very interesting to see blocks of the tufa being used as building stones here (photo 5).
The pitched or herringbone stonework represents Saxon work (photo 6).
One block of tufa had a mammilated or reniform shape (photo 7).
Other building materials in the walls were flint, two types of iron stone, tiles (possibly of Roman origin) and there were oyster shells packed into the joints. The tufa, too, may originally have been used in Roman times and been reused here.
The two types of ironstone (also known as iron grit, carstone or iron pan) were dark red ironstone and bright red iron stone. The dark ironstone is from the Folkestone Formation sandstone. It formed along lines of water percolation. The brighter red ironstone separates the Folkestone Sands from the Gault Clay, along the junction. The sandstone formed in a higher energy environment than the clay.
The use of ironstone and malmstone in churches has been mapped. It follows the local geological outcrops. Steyning, not far away, may have been another source of tufa. It was a port in medieval times and stone from France was brought up river. On such a wet, cold morning the prospect of a warm and dry hostelry for lunch was very inviting. The Cricketers pub in Duncton fitted the bill nicely. We peeled off the dripping outer layers of clothing and settled down to a cosy meal.
We spent the afternoon at the 3rd century AD Roman villa at Bignor, just a few miles away. The villa, the largest discovered in Britain, lies in wonderful agricultural land, with Stane Street Roman road just a few hundred yards away. Its wealth in Roman times was based on agriculture. The villa was built on the Greensand bench and the predominant building stone is Upper Greensand. Roman farms and villas have been discovered every few miles around the area.
George Tupper, a local farmer, discovered Bignor Roman Villa while ploughing his land in 1811 and it is still owned by the Tupper family today. The preservation of the villa over the years has been rather quirky by modern standards. After first being uncovered, it was left to weather and deteriorate. Nevertheless, large expanses of amazing mosaics survived. Then, in Georgian times, the stone lying around the excavations was used to build walls directly onto the Roman walls of the villa, to support a protective roof.
There are columns on site made of oolitic limestone. They might be Bath Stone from the Cotswolds, but might equally be continental, as the nearby River Arun was navigable in Roman times. The mosaic tiles are probably made of Purbeck chalk (white), clay tile (red) and Kimmeridge shale (black). A sarcophagus in the grounds is made of Hythe Formation sandstone. The stone for the piscina is probably from Purbeck. A quern stone on display is made of Lodsworth Stone, a local variety of sandstone from the Hythe Beds near the village of Lodsworth. It is green, very hard and heavily bioturbated. More excavations took place from 1985 to 1990.
The site makes a wonderful trip out in sunny weather, but I have never seen it crowded. It is less well known than sites owned by English Heritage or the National Trust and definitely worth a visit.
Before we left, David treated us to glasses of locally produced sparkling wine, including an award winning 2003 vintage from the Nyetimber vineyards of Sussex. I imagine that our Roman forebears might have enjoyed just such a glass of wine, all those centuries before us.
Lecture : The 2010 eruption of Erta Ale, Afar, Ethiopia - by Lorraine Field
15 March 2012
Lorraine from the University of Bristol, engaged on a thesis on the Magmatic Evolution of Dabbahu Volcano, Afar, gave a talk illustrated by videos, maps and photographs, of the occasion in November 2010, when a group of researchers from the Afar Rift Consortium arrived in this remote region in the north west of the Afar, intending to emplace a cyclometer on Erta Ale, only to find the volcano erupting. She had amazing footage of the eruption, causing you to fear for the safety of the photographer.
Erta Ale is a shield volcano with two magmatic segments, whose base is below sea level. There are two lava lakes, of which the one in the south pit had reactivated with overspills about every two hours continuing until November 22nd and building a rim on the edge of the crater. The basalt is porphyritic and vesicular, emitting toxic gases. The eruption was preceded by a large earthquake in the Gulf of Aden and the eruptive period lasted from November 11th to December 12th when the lava lake was back to pre-eruptive levels.
Geochemically the magma is similar to that of Hawaii, indicating shallow crystallisation. Pre-eruption temperatures have been calculated as c.1150° C; viscosity was low. Its composition can’t be related to that of the previous eruption in 1973, so there must be at least two chemically distinct sources of magma.
The last part of the talk compared it with the Nyamagura eruption in the Congo, on the western flank of the East African Rift. Nyamagura is also a shield volcano with two parasitic cones of which the western was active from 6th to 28th November and the eastern from 28th November to 5th February. It is one of the most active volcanoes on the planet. The lavas are very alkaline-rich and there are scoria cones 100m high. Once again we were treated to a dramatic video of the eruption. All recent eruptions have been basaltic, but earlier ones were rhyolitic.
There will be a series of Volcano Live on BBC2 in four parts in July, and for the Afar Rift, I refer you to the website:
Solnhofen - by Chris Duffin
19 April 2012
A historian of Geology and well-known author and lecturer, whose specialist research concerned fossil fish, Chris first of all introduced us to the stone quarried at Solnhofen in Bavaria between Nuremberg and Munich. A late Jurassic limestone, it was used from the late 18th century until 1880 to produce lithographic prints, a process invented by Alois Senefelder in 1796. Today it is used as a paving stone, which means that it is continually being quarried and new discoveries made.
The paleogeographical environment is that of a sponge reef and lagoon, a region of gradual uplift resulting in thinly-bedded limestones. It is a world-famous conservation lagerstatte with an amazing collection of fauna, invertebrates and vertebrates, and flora, accumulated over 5 million years.
In these calm, lagoonal conditions, the remains were preserved in minute detail, and we were shown examples of plants such as gingko and cycads, plus algae and cyanobacteria. Then came the invertebrates: worms with even the gut visible, sponges, molluscs, ammonites, arthropods and even crab larvae. There were insects, wings and all, trace fossils, and coprolites.
Finally the vertebrates: fish with individual muscles, intestine and all, and reptiles, though very few dinosaurs. What the site is most famous for, however, is the first fossil of the first bird, Archaeopteryx, amazingly complete, which was promptly acquired for the British Museum in 1861 for the fabulous sum of £700.
For those who couldn’t attend, there is a good description on: www.fossilmuseum.net/Fossil_Sites/solnhofen/
Kensworth Quarry, Dunstable Downs & Totternhoe Quarry - led by Haydon Bailey
21 April 2012
The two chalk quarries on the itinerary for this trip are both near Dunstable, Beds and currently active. The day was led by Dr Haydon Bailey and his colleague Dr Liam Gallagher, both of Network Stratigraphic Consulting Ltd. We met in the National Trust car park for Totternhoe Knolls approximately 3 km west of Dunstable town centre (SP98605 21760). Di Clements (LOUGS), trip organiser, set out the programme for the day and verified that we all had appropriate safety gear – hi-vis jackets or vests, eye protection, gloves and hard hats.
Dr Bailey then explained the aims of the trip, the principal ones being to look at the Totternhoe Stone and the Kensworth Member of the Chalk in their type locations and to understand more about Chalk stratigraphy with its lateral consistencies and variations and to try and explain these features. We were all given copies of the Field Guide prepared by Dr Bailey, which proved to be very comprehensive and informative.
We then walked approximately 1 km to Totternhoe Quarry (Figure 1). This is now operated by Angus J Clarke, successor to H G Clarke and Son who ran the quarry in the 1920s. Historically the quarry and its associated mines go back some 2000 years. It remained closed for some time but reopened in the 1970s as a source of rock for repairs to Woburn Abbey in which it was originally used and is now an SSSI called Totternhoe Stone Pit as a Geological Conservation Review Site. The stone, although a chalk, is soft when first extracted but becomes unusually hard after drying out, due to partial cementing with silica. Thus it is easily worked initially but makes for a usable but not high quality building stone. It was also used in Totternhoe church, Luton parish church, St Albans Abbey, St Pauls Cathedral and various other sites.
Stratigraphically the Totternhoe Stone is mid Cenomanian (96 – 94 Ma). At the entrance to the quarry we were on the middle of the Grey Stone or Grey Chalk which is extracted but although rare is not used. This passes down through the Zig-Zag Chalk Formation which in turn leads down to the Totternhoe Stone, a very important marker band, some 7 m thick here but reducing laterally east and west to approximately 1 m. The stone was deposited in a channel, which accounts for its localized thickness, it cuts down into the underlying Westbury Marly Chalk (Chalk Marl) to beneath the underlying Doolittle Limestone (named for the nearby Doolittle Farm), some 18 m down from the Grey Chalk.
Dr Bailey invited discussion on the possible reasons for the formation of a channel. It seemed clear that it had exploited a fault, which we were told was an active deep fault during the Cretaceous, known as the Lilybottom Fault. The Chalk was deposited on the Midlands Micro-Craton and the channel cut down into the contact with the adjacent country rock during the mid-Cenomanian (96 – 97 Ma) when sea levels were very low. The return of sea levels led to re-deposition of the chalk into the channel along with the phosphates and ironstone nodules which formed during the break in deposition when O2 levels were high. Fossils to be found include gritty shell fragments of the bivalve Inoceramus, trace fossils Thalassinoides and Planolites, sharks' teeth and ammonites. Some time was given to individual investigations and fossil collecting before leaving the quarry for the lunch-time venue.
After lunch at the National Trust Centre on Dunstable Downs we re-convened at the car park in Kensworth Chalk Pit nearby (TL01392 19726). Kensworth is operated by Cemex Ltd and is the largest active chalk quarry in the UK. 8,000 tonnes of chalk per day are transferred as a slurry via a 92 km underground pipeline to the Cemex cement works at Rugby. The pit is approximately 1 km long, 0.5 km wide and 40 m deep, exposing an uninterrupted stratigraphic record of the Chalk. In spite of its size it is completely invisible from the surrounding area and in spite of being an active quarry is an SSSI. It is not practical to effectively photograph the pit from ground level and I therefore recommend viewing it on Google Earth.
It descends from the top level via 5 or 6 roadways (benches) each large enough to carry the quarrying machinery which can also pass from one bench to another via ramps.
After signing in individually at the security hut we walked approximately 1 km to the far end along the top level of the quarry. Dr Bailey gave us a summary of the stratigraphy of the Chalk, which was laid down in the late Turonian (c90 Ma). Kensworth was described to us as a 'fossil sea floor' with very low sedimentation rates during a period of global high temperatures and very high CO2 levels resulting in a 'carbonate factory'. The Chalk is mostly formed from the algal remains of coccoliths.
Marl seams up to 6 cm in thickness are laid down within the Chalk and within these seams there are volcanic deposits whose geochemistry indicates spreading ocean ridge sources such as Iceland or Surtsey. Further, within individual seams the geochemistry is specific and can be correlated laterally as far afield as Germany. Phosphates and glauconite are also deposited within the Chalk.
Our starting point at the top of the Pit found us on the Main Chalk Rock and we saw ample evidence of the intensive burrowing which had also been described to us. The burrows were up to several cm in thickness and are now hollow, in some cases penetrating metre-scale blocks the original sediment infill having washed out (Figure 3). It was very interesting to see the burrow shapes not filled by flint which is much more familiar to us. The chalk is rich in fossils, including large numbers of gastropods and ammonites. Dr Bailey considered the evidence from which the depth of the Turonian sea could be deduced. Algae indicate the photic zone therefore between 50 – 80 m deep but with no evidence of a storm wavebase 80 m is more likely. Some time was again given to 'fossiling' and general examination of the burrows etc.
On the bench below the top level a marl seam was pointed out to us, the Caburn Marl, only 4 cm thick. Iron-staining along the top margin of this seam was attributed to dead sponges. Another bench down and we saw the Southerham Marl and Finger Flints, irregular branching fingers of flint 2 – 3 cm thick and up to 20 cm long formed within burrows in the Chalk. Also on this bench and several lower ones we saw bands of sheet flint, remobilized silica forming horizontal seams up to 10 cm thick (Figure 3). In some places this flint had also exploited angular fault planes approximately 30o from the horizontal within the Chalk. Apart from these minor faults the Chalk in this area shows little evidence of seismicity. Dr Bailey suggested that he could give an explanation for the formation of flint as researched by Chris Clayton for his PhD thesis between 1979 and 1982 (unpublished) and was pressed to do so by Di Clements.
The work was based on Paramoudra Flints (vertical columnar or barrel shaped structures up to 1.5 m high with a cemented chalk core). Animals burrowed in the original chalk in a circular manner and this biogenic activity produced H2S which is anaerobic. The sea water above the burrow contained O2 so a redox boundary formed with the result that the silica (sponge spicules etc.) came out of solution (due to reduced pH) whilst the sediment was still soft, to form solid silica – flint (Clayton 1986). In the case of increased clay deposition flint production reduced because the silica was absorbed onto the clay minerals.
Time demanded that we commence the long walk back to the office area to sign out and leave, having expressed our gratitude to our leaders for the day. Acknowledgements and grateful thanks are due to Dr Bailey and Dr Liam Gallagher for giving us their time and answering our questions, to Angus Clarke and Cemex Ltd for allowing us access to their respective quarries and to Di Clements for organising the day.
Bryan J Carter
The Thames Barrier - with South East Branch
9 May 2012
It was entirely appropriate that stepping out of North Greenwich Station and seeing newly built concrete apartment blocks, trimmed greenery, cycle tracks and constant drizzle reminded me significantly of the Netherlands. Despite the Thames being recorded as "overflowing" as early as 1236, it was the flood of 1953, in which over 300 people in the Southeast, and over 2000 in the Netherlands lost their lives, which ultimately prompted the building of the Thames Barrier.
The Thames has been gradually more constrained over the years. It is reported that in Roman Times at Westminster it was over three times its present width. Most of the South Bank stands on reclaimed marshland, and to the North, the Embankment has narrowed the river by ~80 more metres. Arriving early for our visit, I spent some time looking around the site. Along the side of the tunnel giving public access to the Thames Path, I found marked mean sea level, and a representation of the height of the river along its course. I was surprised to notice that at its source, Thameshead, the river starts only 105m above sea level. In conversation later in the day, we remarked how the Thames is not a particularly large or fast-flowing river.
We settled in to view a short film on the Barrier. The main threat of flooding is from surge tides, something anyone who attended last year's Members Evening should know all about. While the largest surge tides come round the North of Scotland, capable of adding up to 4m to a high tide, lesser surges also come up the Channel. Their threat is not only increasing as sea levels rise, but as the great weight of the city settles into the clay, and the general trend of the Southeast to be tilting downwards. Tides here are rising by ~60cm every 100 years.
The design brief was for a barrier which would handle a large tidal range, remain generally open to river traffic, and also be aesthetically pleasing. Construction of the Barrier began in 1974 and took 8 years to complete. It comprises of 4 large Rising Sector Gates and 2 smaller ones, plus 4 Falling Radial Gates. The location was decided by geological suitability (alluvial deposits and Thanet sands overlie the relatively shallow-lying Chalk); the river at Woolwich Reach is straight, good for river traffic approaching the Barrier; and not unimportantly, the land here was available to be bought.
The Barrier is only part of a larger system of defences. There are further barriers at Barking and George V Lock, and over 135km of steel plated flood walls. Many of these walls are approaching the end of their lives, but the Barrier itself is still going strong. Modifications could add a further 1.2m to its present protection of 7.2m a.s.l, and with recent estimates of future sea levels, it is expected to be in operation until at least 2070.
Although routinely tested for multiple fail scenarios, the Barrier is needed on average only 3-4 times a year, although not at all in some years and 19 times in another. The cluster of 19 closures was to alleviate an increasing threat from upriver flooding, something which the Barrier was not designed for, but manages well. As climate becomes stormier and heavy rainfall more frequent, along with the concreting over of the Southeast, storm runoff into the Thames has been increasing. When the Barrier is closed at low tide, it creates a large basin to catch the increased river flow and keep it from meeting the incoming tide head on, but dissipating it slowly with the falling tide.
We were taken down stairways and into tunnels under the river into one of the piers where some of us noticed the Barrier had closed while we were watching the film. For those who hadn't stopped to notice, our attention was drawn to how tidy and clean everything was. Although the maintenance program is extensive, we were reminded that most of the time, the Barrier is not in use, and, as our guide Melvin French reported, "What isn't done today, can be done tomorrow". Problems can be investigated, and repairs can be done. It is partly due to this care and maintenance that the barrier's life will be so long. We were also shown there is a lot of redundancy in the system - each gate can be operated by any of 6 pumps, or even manually if necessary, and can draw on any of three electricity supplies.
We stepped out onto a deck on the upriver side of the pier, and less than a minute later, the gate next to us started to open. Apparently this was entirely good timing, scheduled testing was taking place that day. Personally I suspect showmanship, but it was nevertheless a treat to see the Barrier in operation. The five or so minutes it took the gate to open was carefully controlled - it can be opened quicker, but sudden river level changes elicit complaints from houseboat residents.
The visit was very worthwhile, I'd like to thank Southeast branch for organising it, and the Environment Agency for having us. We did not visit the Information Centre, but many of the group headed on to examine the building stones of Lesnes Abbey. I went to Gilbert's Pit, that I have heard so much about. It was a worthy detour to see some of the strata under London in a sheer face, with Blackheath pebbles tumbling down the front. There will be more on these locations later, as they lie on the newly researched Green Chain Walk. The committee is currently planning a full day on the walk as an event for 2013.
Storm Surges in the Thames Estuary - by Richard Trounson
A 2010 BSc Geosciences SXG390 Project
At the October 2011 Members' Evening, Richard Trounson gave a presentation on the project he had carried out in 2010 for SXG 390, which had been on storm surges in the Thames Estuary.
He introduced this by briefly discussing the scope and purpose of the module, specifically in relation to projects on geohazards, which were required to focus on the causes, effects and mitigation of natural geohazards. He said that the module provided a very useful introduction to the understanding, and possible future writing, of articles for peer-reviewed journals (which formed a key part of the required source materials for the project), as well as devel- oping more general project management skills. He stressed, however, that work on the module could be very time consuming, particularly because most of the work had to be carried out in front of a computer, or in academic libraries. It could not readily be carried out at odd moments of leisure, or on trains, as with work on other OU modules.
He explained that, following an initial interest in the topic of tsunamis potentially affecting the UK, he had been encouraged to turn to this particular subject by his tutor, who turned out to live in a part of the Thames Estuary which was particularly vulnerable to storm surges!
He said that flooding by North Sea storm surges was a significant geohazard affecting the Estuary and London, which had taken many lives and damaged considerable amounts of property in the past, notably during the East Coast Big Flood event of 1953. Richard showed on a map the track of this storm, which killed 307 persons in the Eastern counties of England, together with 133 lost in the sinking of a ferry off Northern Ireland, but took some 2,000 lives in the Netherlands.
The cause of North Sea storm surges was best seen as the result of the reinforcement of high tides through the raising of sea-level by low atmospheric pressure in severe mid- latitude depressions, which quite commonly affected the area, and by consequent high onshore winds. High tides resulted from an astronomical signal reinforced by hydro- logical effects in shallow basins and estuaries. Local contributory factors to surges included estuary geomorphology, and high river levels from rain.
Some researchers argue, on the basis of data collected at tidal gauge stations, that the tidal wave always advances the surge "residual" in the Thames Estuary, so that the highest point of the surge will not coincide with High Water, and that engineers need not therefore plan for the coincidence of surges and highest tides. However this view should be treated with caution, due to the difficulty of predicting the effects at potential flooding sites, as opposed to tidal gauge stations.
Richard said that the potential flood risk in the area would be significantly increased by new development in the floodplain, particularly as envisaged by the Thames Gateway Project, which included plans for new residential developments as well as for large infrastructure projects. It could be argued that current plans to mitigate the risk up to 2100, such as the Environment Agency's TE 2100 Plan, focus excessively on climate change, which might not be the main risk factor in this timescale. The difficulty of predicting the incidence and extent of actual flood events, especially in less protected areas downstream of the Thames Barrier, might be more relevant. Considerable uncertainties attach to this, in particular because the estimation of future flood events depends on extrapolating from the comparatively limited more recent part of the historical record which contains useful quantitative data.
Richard said that it might be prudent to restrict residential development in the less protected areas, at least pending further investigation of the tidal contribution to the risks, and improvements in the effectiveness of defences. This is because residential development in such areas posed greater risks to life, which were also difficult to mitigate by evacuating vulnerable people in an emergency. Residential development there should be restricted to special flood- proof properties, which could rise with the tide, and industrial developments (the sites of which could more readily be evacuated in an emergency), should be preferred.
He suggested that public authorities should focus on managed realignment, involving the creation of areas for environmental and leisure use which could be flooded in a storm event, and on emergency evacuation plans.
The Green Chain Walk - with South East Branch
9 May 2012
Following on from the visit to the Thames Barrier on 9 May (see Gavin Mair’s report in the June issue of LP), South East Branch advertised an afternoon following the Green Chain Walk Geotrail. I was initially curious why they didn’t do the first stops starting at the Thames Barrier but instead had decided to start at the far end at Stop 12, Lesnes Abbey. All became clear as, of course, Geoff Downer is an expert on the stones of such edifices see also the review of his Stones of Reculver in October 2012's LP).
I was involved with putting together the Geotrail as part of the London Geodiversity Partnership (LGP)/Green Chain Walk (GCW) initiative. Initially we had intended to finish at Lesnes Abbey with a discussion on the building stones but on our recce it soon became apparent that we needed to refine our ideas and we cut out some of our initial suggestions. Details of the building stones here and elsewhere were removed in order to concentrate on the local geology and its impact on development. At Lesnes Abbey we mainly tell the tale of flooding although a small reference that the building stones are ‘mainly from Normandy’ remained.
After a short general introduction in the Visitor Centre in the dry, Geoff first took us to see the ‘stones from Normandy’. These were pillars attached to the highest wall in the interior of the ruins. The Caen Stone was much loved by the Normans and was imported from north France as a primary building stone in many of their castles and churches in England. It is a cream-coloured Jurassic limestone that is durable yet relatively easy to carve and indeed we found small carvings at the base of the pillars (Figure 1). But that was about it for the Caen Stone and clearly we will need to revise the GCW Geotrail text at the next opportunity. One or two of the blocks had been replaced by the pale green Reigate Stone. This weathers badly and we were able to see the typical concave weathering expected. In September London Branch are visiting Reigate Mines where we will have an opportunity to learn much more about this Early Cretaceous stone from the Upper Greensand.
Next we examined the main building stones used in the walls of the ruined Abbey. We looked at the better-preserved internal walls of ‘rubble’ which mainly consisted of Lower Greensand blocks of irregular size. Many of these were chert and others were from the Kentish Ragstone, most likely from the Maidstone area. This is a stone much used in Roman London and right through to 19th Century churches in Greater London as it is the closest and most easily transported building stone to London. There were occasional large flints which may have a much more local origin in the small quarry in Chalky Dell in Lesnes Wood to be seen later in the afternoon. In one place chalk itself was also used and although totally unsuitable externally it did not have far to travel and survives in the interior of the wall.
Abbey Wood has put in a bid for Heritage Lottery Funding to build a new visitor Centre and the LGP will aim to work with them to add details of the building stones as part of the visitor and educational offer. We hope that Geoff will help make this a reality.
Lesnes Abbey is on a slope on the south bank of the Thames with a much more gentle slope than the Chalk ‘cliff’ further west behind the Thames Barrier at Gilbert’s Pit, Maryon Park. Following the examination of the building stones we discussed flooding in the area in both historical and recent times. The disastrous tidal surge in 1953 that prompted the building of the Thames Barrier caused the houses in front of us to be flooded too. In some of the streets the houses are built with above-ground windowless, solid ‘basements’ with tall steps up to the living rooms which are situated on what would normally be the 1st floor.
Continuing up the slope we came to the fossil enclosure that is opened up each year by the Tertiary Research Group for the extraction of sediment from the Lessness Shell Bed [sic]. The site is an SSSI for the rare mammal bones found amongst the shells. These have been described by Jerry Hooker and are stored in the Natural History Museum. From the tiny teeth, occasional jaw fragments and other bones comparisons have been made with the much better-preserved early Eocene material from Big Horn in America and Messel in Germany.
Reconstructions have been made depicting an equatorial environment with dog-sized ancestors of horses, an early primate and some animals completely unknown to us. The Lessness Shell Bed lies at the base of the Blackheath Beds of the Harwich Formation, beneath the London Clay. As with the London Clay the animal bones will have been washed into the sea from the land. The shell bed contains both marine and brackish water species and also a large number of sharks’ teeth – a big attraction during the annual dig when the sediment is wet-sieved on site with water transported up the hill by Bexley Borough Council.
The hole excavated is always back-filled the same day but for the persistent, residual shells and sharks’ teeth can be found dry-sieving the loose sand at the surface. Digging is not permitted.
Back on the path up the hill, the more familiar facies of the Blackheath Beds was ubiquitous. This consists of the rounded black pebbles described by Gavin in his report of Gilbert’s Pit, Charlton (Part 1, June issue). The Blackheath Beds extend right along the plateau from Blackheath in the west (so named for the pebbles) to Lesnes Abbey Woods in the east. We then took a turning back down the slope, past a beautiful carpet of bluebells, to the fenced quarry that is Chalky Dell.
The quarry may go back a long way but no activity has been recorded since at least 1925. The chalk was probably primarily used on fields, possibly owned by the Abbey, and as we have seen, flints and chalk were incorporated into the fabric. Near the top of the old quarry the junction with the overlying Thanet Sands was photographed by Marriott in 1925 (Figure 2) and pictured the large glauconite-covered unweathered flints typical of the ‘Bullhead Beds’ (so-named for their appearance).
We had to content ourselves with a small exposure of chalk with large flints at the base of the quarry. This is a site the LGP wish to conserve, possibly as part of the HLF bid, by re-exposing this junction and making rudimentary steps up the steep scree slope to allow viewing. Maybe this is something that members of LOUGS could get in involved with in due course in conjunction with the Friends of Abbey Wood? For the moment though, a fallen tree has made it impossible to even see the non-vegetated part of the scree slope.
Fossil Sponge Reefs and Jurassic Bahamas in Oxfordshire: Faringdon, Abingdon and the Oxford University Museum - Led by Graham Williams
12 May 2012
Roughly 20 members of the OUGS (London branch) met in Oxfordshire on a warm and sunny day for the field trip to learn about sponges.
Graham started the day by telling everyone about sponges and allowing them to view (and handle) a collection of fossil sponges. Sponges are the simplest of multi-cellular organisms; roughly speaking, there are 5,000 living species of sponges (only some fossil sponges exist as soft bodied sponges have not been preserved). They date back approximately from the late Precambrian. Sponges do not have nervous, digestive or circulatory systems. They rely on a constant flow of water through their bodies to obtain food, oxygen and remove wastes. Sponges are classified in the phylum Porifera and are categorized into three main classes: Calcarea, Hexactinellid (often referred to as glass sponges) and Demospongia.
The skeletons of sponges are made of calcium carbonate in the form of calcite, or are siliceous. The structural elements found in most sponges are called spicules which provide structural support and deter predators. Some are visible to the naked eye (megascleres) while others are microscopic (microscleres). Fossil sponges live in marine conditions and can give limited evidence of water depth. Porifera were major reef builders throughout the Phanerozoic, much more so than corals.
The rocks at Little Coxwell Pit are the Faringdon Sponge Gravels which consist of the Cretaceous, Aptian, Lower Greensand Gravels; they were deposited in a near-shore shallow marine sedimentary environment. Fossil sponges, ammonites, echinoids, brachiopods and bryozoa have been found at Little Coxwell Pit. Other finds include some dinosaur and plesiosaur remains. Everybody had an opportunity to look for fossils.
The OUGS next visited a Site of Special Scientific Interest (SSSI) in a former quarry where houses have now been built. The group were able to find both the biogenic layers and the mineral layers. In places the pebbles were aligned and in other areas the arrangement of particles were “chaotic” suggesting a difference in flow (turbulence). There is bedding, each one representing a separate event.
The group then visited Dry Sandford nature reserve near Abingdon. Here an old quarry exposes richly fossiliferous Corallian Beds. The sediments were deposited in shallow coastal waters close to coral reefs. The succession includes the Lower Calcareous Grit, Trigonia Beds, and the Urchin Marl and Coral Rag of the Osmington Oolite formation, with brachiopods, ammonites and corals. Bees and wasps live in the sandy layers.
The Corallian is often described as a sequence of sandstone and limestone. Graham made us look at the grains with a hand lens to show that most of the layers were actually composed of sand grains. The quartz sand is cemented by calcareous mixture. The Corallian bed derives from the sponges. The Coral Rag is a micritic oolitic limestone with embedded corals and shell fragments. The sandy base is bioturbated and crossbedded with pebble sand horizons with a pebble base. The sandy layers are made of quartz (silica) grains that were deposited as beaches and sand dunes.
When the Faringdon Sponge Reefs formed during the Lower Cretaceous, the area was a sea bed and consisted predominately of eroded Corallian sandstone and limestone beds (Oxfordian Stage) and Kimmeridge Clay (Kimmeridgian Stage). Corallian sediments provided a hard substrate, a suitable surface upon which the sponge reefs could grow. In Oxfordian times (159 - 154 Ma, Upper Jurassic), southern England was at about 42° N and resembled the Bahamas with warm seas, coral patch reefs and lagoons.
The day finished with a visit to the Oxford Museum of Natural History where a fascinating array of sponges were on display. Further information about individual sponges and the different classes for the phylum Porifera was provided: Archaeocyathida, Calcarea, Demospongia, Hexactinellida, Sphinctozoans (sponges that grow as a series of chambers, one on top of the next) and Stromatoporoids (a class of aquatic invertebrates common in the fossil record, now classified in the phylum Porifera) were all discussed.
Many thanks to Graham for another most interesting, well researched field trip on a fascinating topic.
Kebabs, Carbonates and Baggy Trousers – Adventures of a Geologist in Kurdistan - by Paul Logan
17 May 2012
Paul, as Chief Geologist with Heritage Oil with 30 years’ experience in the industry, gave a fascinating lecture, providing insight into the whole process of oil and gas exploration and exploitation with particular reference to recent developments in Kurdistan. One of the first western companies to go into Kurdistan, they have found a very large gas field containing 10 trillion cubic feet of gas.
First the location: Kurdistan is an autonomous region in Iraq, capital Erbil, population 5 million. It has hot, dry plains and cooler mountains; the Miran block is part of the Zagros Fold Belt. It lies in a high folded zone with a big thrust fault.
The stratigraphy comprises heavily fractured Tertiary Limestone. Maastrichtian deposition was influenced by two factors; turbidites from the obduction of the southern neo-Tethys Ocean, and the development of a carbonate ramp. The turbidites are mainly sandstone with some shale.
Many of us, students or past students of S369, were fascinated by the application of sequence stratigraphy, invented apparently by Esso. The Triassic of NE Iraq is dominated by shallow marine evaporites which are impervious and act as a seal on the hydrocarbons below.
There was a discussion of methods and tools: bombardment of rocks with neutrons, electrical tools to test resistivity, radioactive spectrometry, gamma rays; a relatively new technique of directional drilling is proving particularly useful in fracture zones.
A passage on reservoir geology: more than 60% of the world’s oil and 40% of the gas are found in muddy carbonates, with
fracture corridors providing storage. Minor changes in clay content can provide a seal.
Finally we were given a glimpse of the local civilisation. Erbil with 8,000 years’ continuous habitation, villas built of anhydrite, the British Military Road built in World War I, … and the unveiling of the statue of a donkey by the Donkeys’ Party, created in 2005 to symbolise the armed struggle of the Kurdish people. Life in the oil industry can be eventful!
Boxgrove and Bracklesham - Led by David Bone
3 June 2012
We assembled in the car park of Boxgrove Priory at midday on the Sunday morning of the Diamond Jubilee Bank Holiday weekend. It had been raining earlier in the day, but the weather was now dry but overcast.
David Bone, who was leading us, said that we would look at the building stones of the Priory before lunch. He would then give a short display over lunch of the characteristic fossils which we would be looking for on the beach at Bracklesham Bay, when we moved there in convoy after lunch.
David pointed out that, contrary to the impression that might be given by the English Heritage sign-posting at the car park, the Priory was much more than the ruined guesthouse conspicuous from that location. It had of course also included the church, and the land in between, on which had stood the cloisters, refectory, dormitory and chapter house. These had been demolished following the Dissolution of the Monasteries. The church had been much larger prior to the Dissolution, the now demolished buildings of the nave to the west having up to that time served as the parish church. The chancel, which together with the crossing, the transepts, and a small monastic part of the nave, had up to then served as the monastic church to the east, then became the new parish church.
After this explanation, we followed approximately the course of a walk round the Priory set out in a booklet on its building stones which David had written 1. We started at the south porch and walked along the southern wall of the church towards the east end. The walls were made largely of flint rubble, with quoins and window frames of dressed stone. The flint was set in lime mortar. David pointed out that the walls had been built in stages, as was shown by the fact that the rubble work was divided into horizontal sections. This was because the lime mortar hardens by drying, as opposed to setting in a chemical reaction, as does Portland Cement. Consequently it could not take any significant weight until dried, so the walls were erected in stages, with a pause before the next stage, to allow the mortar to harden.
The flint work had been galleted, i.e. the mortar between the knapped flints had been filled with protruding flakes of flint, partly as decoration, but also to protect the mortar. The flints were of different varieties, their heterogeneous origin reflecting the different "job-lot" contracts for flint and other building stones, which had been entered into as the church was built. There were black flints with a white cortex, generally knapped, which had been taken fresh out of the chalk. Completely white flints were weathered, and represented "field brash". Brown flints, iron-stained by burial, came from Quaternary deposits in local gravel pits and raised beaches on the coast.
There were also the occasional rounded beach cobbles in the rubble, re-used pieces of stone from elsewhere, such as the Caen Stone used in the dressings, and Malmstone, a stone from the Upper Greensand north of the Downs.
Some exotic stones, including pieces of igneous and metamorphic rocks, were also to be found. There were two possible methods by which these had come to the area. The first was anthropogenic, the stones having been brought to the area as ship's ballast. The second was geological in nature, and more difficult to be certain about. Many of the rocks were associated with the Channel Islands, but the stones appeared to have been derived from boulders which could not have been brought from there by long-shore drift, or even by storm wave action. The most likely candidate for their transport was ice-rafting, the ice-rafts having previously been detached by the action of a tsunami.
The ashlar dressings in the walls of the church were also of heterogeneous origin.The stones employed included Caen Stone, a creamy-yellow granular Jurassic, non- fossiliferous, limestone from an area close to the river Orne in Normandy. It had been imported into the country in Roman times, but there was a big campaign in terms of its use after the Norman Conquest. The quarries were closed in Medieval times, but were re-opened in the Victorian period to obtain stone for restoration work. However the quality of the residual stone used had not been as good, and it may not have come from the original workings. It was softer and weathered badly. Accordingly, many of the windows had been subsequently restored in Victorian times using Bath Stone, an oolitic limestone from the Cotswolds.
Sandstone from the Hythe Formation was also used. This was the best local building stone, locally called Pulborough or Midhurst Stone, and had been worked both in Roman and Medieval times. It was however not of comparable quality with those just mentioned, and had a 50% wastage rate.
There was also some Quarr Stone, from the Bembridge Limestone sequence on the Isle of Wight. It was called after the locality of Quarr Abbey (pronounced "quore") which had had a monastery in Medieval times, but was also the site of a modern abbey, built in the early 20th century of brick. The grey-coloured stone made a very strong building stone, but had been worked out by about 1400. It appears then to have been replaced at Boxgrove by Lavant Stone.
Lavant Stone is a fossiliferous, gritty (and therefore hard), chalk from the Upper Chalk, which was been formed in scour troughs in the chalk sea. It comes in two varieties, one greyish-white, which is full of sponge spicules, and a brown variety, full of phosphate minerals. It was worked in the Chichester area, used by the Romans, and again in the Medieval period. It was used in high quality ashlar work as well as for rubble.
During our perambulation of the church we noticed on the south side a stone turret with a quoin made of a mixture of Caen, Midhurst, Lavant and Quarr Stone. Also on the south side were a filled-in door, and a buttress with a crouching figure, suggested to be a cat, with sundials down below it.
Moving round to the north side of the church, we saw a buttress made of the two types of Lavant Stone, in a corner of the former cloisters to the northwest.
We then took the opportunity to go inside the church, the Sunday morning service, and a following baptism, having by now finished. At the west end of the present church there was a crossing supporting the tower, mostly of Lavant Stone, with some repairs in Greensand. This was part of the 12th Century nave of the original priory church. The floor had evidently been lavishly restored very recently, in contrasting pale and dark Purbeck Marble set out in a labrynthine pattern. The church clearly benefitted from the patronage of a number of very well-off parishioners, and this was confirmed by the monuments in the church, which included one to a twentieth century Duke of Richmond and Gordon, head of the most prominent local family, the church being very close to Goodwood.
Further to the east, Caen Stone predominated, but with Purbeck Marble being also used for the columns at the east end. One of the columns in the north aisle had a base in Sussex Marble, a somewhat similar paludina limestone, but with larger paludina shells.
In the south eastern corner of the church, one of the columns was leaning at an angle, as a result of weight applied to it by subsidence, and was being supported by iron straps.
There was a modern free-standing altar with two candles, on a platform of polished Purbeck Marble, which appeared also to date from the recent restoration work.
A local informant talked to us briefly about the history of the priory church, further information on which could be found from a booklet on sale there 2. The monastery had been founded as an offshoot of the Norman Abbey of Lessay, but became independent, and prospered, owing to its close connection with the local family that had founded it, and with successive Bishops of Chichester. The close connection with the local family was evidenced by the Lord de la Warr Chantry Chapel, an intricately carved little building within the church which was constructed of Caen Stone.
It was built in 1537 by the last representative of the founding family, succession to the estates of which had passed on many occasions through the female line, and the influence of this nobleman had been instrumental in ensuring that the church was not entirely demolished at the Dissolution. However, the chapel was never used for burial, because shortly after the Dissolution the family fell out of favour politically, and the Crown acquired their local estates.
We returned to the English Heritage car park for lunch. Over lunch, David told us about one type of building stone we had not yet seen at Boxgrove, though some members would have seen it on an earlier visit to Selsey. This was the Mixon Stone, used in the construction of the monastic guesthouse, which we did not have time to visit.
Mixon Stone is an orange or pale grey glauconitic limestone, rich in Eocene foraminifera, found on an offshore reef south of Selsey Bill. The stone was quarried at low tide as early as Roman times, and was extensively used in the Selsey locality as the Admiralty that this was necessary to protect the reef, which provided a sheltered anchorage to the North East off Selsey Bill.
In telling this story, David referred to the work of Edward Heron-Allen, a solicitor and polymath who translated Omar Khayyam from Farsi, was a writer on violin-making, and had been elected a Fellow of the Royal Society for his internationally recognised work on foraminifera. Having retired from the practice of the law, he had moved to the Selsey area. His interest in foraminifera and local history led him to attempts to unearth the history of the prohibition of the quarrying of Mixon Stone. However, according to David, these attempts had not been very successful, owing to a lack of official enthusiasm then, perhaps understandable in the World War I period, for Freedom of Information.
David took the opportunity of the space provided by the car park to lay out the promised display, in advance of the afternoon's visit to Bracklesham, of a representative array of the Eocene fossils to be found there.
1 Bone, D: "The Stones of Boxgrove Priory", Limanda Publishing, Chichester 2010
The day started with rain and shades of our visit to Bignor earlier in the year haunted us on the drive down from London but things improved and at Boxgrove Priory we were able to sit on the grass to enjoy our sandwiches. While we were all gathered, with his car close at hand, this was an appropriate opportunity for David to tell us something about the fossil SSSI in Bracklesham Bay and pass round some of the fossils that we might find. He even brought a shark’s jaw to help explain that one of the common fossils is sharks’ teeth. Others were the turreted gastropod Turritella (or more correctly Ispharina and Haustator), the ubiquitous bivalve Venericor planicosta which is commonly 10 cm across, and the single-celled foraminifera Nummulites (Latin for ‘little coin’) laevigatus. He also showed us an impressive palate of a ray, used to grind food, fragments of which are not uncommon.
So we set off for the beach. Actually the early bad start to the day was in our favour. I spoke to David earlier in the week and he was very anxious that the glorious weather we were having then might cause us huge hold-ups as it did around Chichester the previous weekend; we need not have worried.
We had been warned that where we were going to look at fossils was now a designated kite surfing zone and when we arrived on the beach the kites started to go up in quick succession; like us they needed to wait for the low tide. I was particularly anxious that they might disrupt our trip so we spent some time discussing potential hazards and how to mitigate them. I offered to be on kite watch as I have been to Bracklesham on a number of occasions and already have a fossil collection from there and it seemed that everyone would have their eyes down and may not be aware of approaching kite surfers. John Lonergan, David and I all carried whistles but in the event did not need to use them. I am not sure how well they would have been heard in the wind.
As soon as we reached the sand people started finding fossils, particularly our two junior visitors. Young eyes always find the best fossils and they finished the day with a very worthy haul including the much-coveted sharks’ teeth and I think someone even found a fragment of a ray palate. These were fossils washed up on the strand line, the exposures for these beds were under the sea at this point and the shells had been eroded and brought to the beach with the waves. We walked as far as the first potential shoreline exposure but it was covered by sand. This bed, the Cardita Bed (named after the large bivalve) also has a cemented area which we reached and were able to view but because of the cementation it is not possible to collect.
SSSI status for the Eocene fossil beds at Bracklesham Bay is designated for the complete succession of the Bracklesham Group which runs from West Wittering to the northwest of Bracklesham to Selsey Bill to the southeast. This overlies the London Clay Formation to the northeast and includes the Wittering, Earnley and Selsey Formations, each of which are divided into a number of different beds recognised for their fossils. For much of the time many of the beds are covered by sand but nevertheless palaeontologists have managed to map out the beds in some detail over the years and about 600 different species of fossil have been found. David has written an excellent booklet Fossil hunting at Bracklesham & Selsey which includes a map of the section and pointers to the most productive beds.
When we arrived at the kite surfing zone I began my duty of kite watch but actually, at this spot, there were no kite surfers to be seen; by now they were all surfing in the sea, running parallel with the shore and not posing any obvious danger. I counted 11 of them including a couple quite far out who were attempting to ‘fly’. They were well away from the other kite surfers and again posed no threat. After a while some of them began to return to the beach and at this point I began to take action by warning groups close by to look out for them and wait until they had crossed their path before continuing along the beach. By now we were quite a way beyond the kite surfing zone and no attempt was made by the surfers to get back to it before leaving the sea even though it would be less far to carry their gear. Actually the kites were very pretty and I enjoyed watching them.
There seemed no problem about both fossil collecting and kite surfing co-existing and indeed, since our visit the kite surfing zone has apparently been abandoned in favour of drawing up a Code of Conduct for all beach users. We welcome this development and both the Geologists’ Association and Natural England have been invited to contribute. David will be critical in providing the local information. We finished beside the Cardita Reef and David told us of the Quaternary fossils that are also found on the beach and we speculated on the origin of the enigmatic erratics that are found along this stretch of coastline.
A much more worrying development than the kite surfing zone is the ‘coastal defence’ scheme currently nearing completion in front of the Bunn Leisure complex at Selsey. Groins of larvikite are already built over the fossil exposures but the more damaging aspect is the recharge of shingle and sand which will fill the 600 m between the groins and will, it seems, cover the fossil beds completely along this stretch. Natural England managed to reduce the scale of the original plans but it is unfortunate that local geology groups had no knowledge of the development or they could have perhaps reduced the scale of the recharge element further. David told us of another development to the east. This is a Managed Retreat scheme involving the Environment Agency. This has been carried out with full discussions with the local geological community.
‘Safe’ areas for bringing the rock ashore were mutually agreed to avoid damage to important fossil-bearing beds and discussions have continued throughout the progress of the scheme. I urge anyone who knows of planning schemes that pose a threat to geological exposures here or elsewhere to be in touch with the Geologists’ Association and Natural England to try and ensure that our geological heritage is protected for future generations.
We had an excellent afternoon down on the beach and we want others to share our pleasure. Many thanks David for your time and expertise once again and thanks also to John Lonergan for organising the trip.
Lecture : Carbon Capture and Storage - by Martin Blunt
21 June 2012
Dr Martin Blunt, Head of the department of Earth Sciences and Engineering at Imperial College, had a message that was much more vital than the dry title of this talk might suggest. Pointing out that earlier alarmist forecasts that the supply of fossil fuels would shortly dry up were based on what was economically recoverable at the time of the forecasts, he showed that in reality the contemporary extremely high price of fossil fuel, particularly oil and gas, has the effect of making it profitable to exploit reserves hitherto considered unviable. The danger posed by the consequent increased percentage of atmospheric CO2 will inevitably pose a threat to the environment, in the form of rising sea levels and extreme weather, unless some means is found of sequestering it.
With a rising world population, and improvements in standards of living in emerging economies, demand for energy is bound to increase, while there are large fossil fuel reserves of coal and shale gas to be exploited, both are highly polluting. The solution is to bury the CO2 by injecting it deep underground at supercritical depths. This poses a big challenge. One possibility being explored is to store it in underground oil and gas reservoirs under the North Sea, though this raises the problem of possible escape. Current projects include pore-scale CO2 trapping.
He concluded that carbon capture and storage is a key technology. The problem is, as always, money. Governments and oil companies have to be persuaded to invest in further experiments towards ensuring that such an approach can be safe and profitable if disastrous climate change is to be avoided.
Ramsgate - Led by Geoff Downer
15 July 2012
"The time has come", our leader said, "to talk of many things,
The only in situ rock we saw was chalk, behind a high fence. This was the blocked-off Westcliff Harbour entrance to the "ARP Deep Tunnel Shelter" boasting some impressive statistics – 4 miles in length, 67' average depth, 23 entrances. These World War 2 shelters were enlarged Victorian railway tunnels which had been decommissioned in 1926. They could accommodate 35,000, the entire population of Ramsgate. A Zeppelin airship had bombed the harbour in World War 1; the fishing fleet was relocated to Brixham in 1915.
However Geoff showed us 7 examples of sedimentary rock, along with 4 igneous, 3 metamorphic and a fake rock called Pulhamite used as building stones. Many were in the shopping centre in Ramsgate Town and will not be described further.
Once upon an Iron Age time, there was the Wantsum Channel. The River Stour flowed around the Isle of Thanet leading to the port of Sandwich. Richborough Roman Fort protected the coastal end, while Reculver guarded the Thames Estuary entrance (see map at www.ecastles.co.uk/wantsum.html). Unfortunately the Wantsum had been silting up since Roman times. By 1700, Sandwich was being cut off; streams were diverted and their courses straightened to increase the flow of the Stour, to try to flush out the sediment. This failed and the river channel was no longer navigable by 1749.
By contrast, coastal Ramsgate was the only Port of Refuge for much of the English Channel, between Portsmouth and Chatham. The nearby Goodwin Sands protected shipping from all storms - except gales from the south. The Goodwins are sandbars 20 km long east-west by 7 km wide. At Spring low tide, the 30 m high sand banks can rise 6' above the sea (See the 'Goodwins Sands map' www.whitecliffscountry.org.uk/images/goodwins.gif).
In 1840 a game of cricket was played on the Goodwin Sands at low tide and painted in 1828-1830 by the Impressionist JMW Turner (Google Image). Definitely high risk cricket, since the sand grains are floated apart and become quicksand before the tide covers the sandbar. Neil Oliver and a BBC TV crew filming the series "Coast" in December 2006 had to be rescued by two lifeboats when they recreated this stunt (Daily Mail Article)..
Between the Kent/Sussex coast and the Goodwins is The Downs, an area of sheltered deep water channels. In the 18th Century, hundreds of square riggers might wait for weeks anchored in The Downs, awaiting an easterly wind to sail up the Thames to London. Small boats from Ramsgate serviced the anchored ships, termed 'hovelling' or 'foying'.
However the Goodwins are called The Ship Swallower, estimated site of a thousand sinkings (more info). Geoff told us that in 1703, 160 ships were anchored in The Downs. A storm from the south sank 90 vessels, including 4 warships with the loss of 1100 sailors. In 1748 there was another terrible storm but some ships saved themselves by cutting their anchor chains and running before the storm. They sheltered behind Ramsgate's wooden pier, hence the phrase "to cut and run ".
In 1749, an Act of Parliament allowed ships sailing east-west between Ramsgate and the Goodwins to be taxed. A Board of Trustees was appointed to commission the building of stone harbour walls. Like most committees, they dithered then made stupid decisions, which required further outlays to correct. For example, the dry dock was built of Jurassic Portland Stone limestone from Dorset, with a stone floor rather than the wooden one specified by the architect John Smeaton, the Town Engineer from 1774 -1772. It was constructed over a natural spring which actually caused the stone blocks to shift and was only used for a couple of decades (click here for more info).
This picture is actually of a 19th Century gunpowder store, with a nearby harbourside icehouse where shiploads of Norway ice was stored to keep the 18th Century fish catches fresh. Fossiliferous Portland Stone was used to replace a crumbling wooden deathtrap between the harbour and town called Jacob's Ladder with large well-constructed steps. Consequently, the harbour building costs from 1750 to the 1840's reached a (then) colossal £1,700,000.
Ramsgate Harbour is located in a submerged Ice Age dry valley lacking a modern river. With a 6m tidal range, it was thought to be "clean" i.e. free of silt, unlike Sandwich. The dry valley meant deep water close to the coast for large ships. The chalk wave-cut platform formed a solid base for stone foundations. The first two harbour arms were constructed during 1749/50, with a lighthouse at the end.
By 1770 Ramsgate Harbour was also silting up, due to longshore drift being interrupted by the new harbour walls. John Smeaton the Town Engineer calculated that 268,700 cubic yards of sullage i.e. silt had already accumulated. With the spurious accuracy of early calculations – Darwin's worms-per-acre for example - Smeaton calculated this would require 12 years to clear by the methods then used - raking and shovelling. In 1774 he proposed that a cross-wall be built forming an enclosed inner harbour, so that the water could be impounded at high tide and released at low tide via sluices to flush through the outer harbour.
The Trustees adopted a cheaper design in 1775, by Thomas Preston the previous Town Engineer 1755-1774. It was so powerful that it ripped up pieces of the chalk bedrock. The sluices undermined and damaged the harbour wall, so that Rennie, a later Town Engineer, had repairs made using a diving bell. The lighthouse was rebuilt using Cornish granite; its Latin inscription means "Protection for the wretched". Pale grey Cornish Granite was used to face the harbour wall. The sluices were last used in 1985; the harbour is now dredged .
Another famous Ramsgate architect was Augustus Pugin, who had his residence 'The Grange' constructed of oolitic limestone, probably Jurassic Bath Stone. This weathered badly in the salt winds. We met up at the end of Grange Road nearby. Pugin used fine grained "Whitby Stone" i.e. stone shipped from Whitby for the window and door surrounds of his knapped flint St. Augustine's Roman Catholic church. This is actually Aislaby deltaic Middle Jurassic sandstone and we saw a coarser grained version used in the inner harbour wall.
Ramsgate sent money to the Irish victims of the potato famine and was gifted with granite. This was used to make an obelisk dedicated to King George IV outside the Maritime Museum using either Whitlow or Leinster Granite. This unpopular monarch got a rousing send-off from Ramsgate to Hanover in 1820; he gratefully decreed Ramsgate the only mainland Royal Port (click here for more info).
After lunch we visited the Pulhamite fake rocks. These were manufactured by four generations of James Pulhams at Broxbourne. Cement or terracotta render covered a brick or rubble core and were intended to provide a background for waterfalls and flowers. A promenade area called Madeira Walk was built in 1891 to attract the right sort of tourist i.e. posh ones with money. This grandiose project cost so much the waterfall was dubbed "Rate-payers Tears.
Geoff showed us lots of mostly igneous and metamorphic foreign building stones in the town – orbicular granite, 'Baltic Brown' Rapakavi Granite from Finland, Norwegian 'Black Lace Schist ' from Otta (near Oslo), Alpine brecciated serpentinite.
Then we were back at Grange Road and departing, after a rare dry and mostly sunny day at the seaside. Many thanks to Geoff for an interesting and informative day in Ramsgate.
Shetland - with Allen Fraser
31 July - 7 August 2012
Tuesday 31 July (Day 1—Arrival)
We arrived on the overnight ferry from Aberdeen and had breakfast on board before meeting Allen, who was there with his van and the second mini-bus in the car park. Before we set off we were free to wander around Lerwick for a couple of hours. After looking around the shops, particularly at the knitwear, I went to the museum where the group were to meet for lunch. The museum is only five years old and it is really well set up. The history of Shetland in rocks and the people is set out over two floors.
The trip started out sunny but as we sat down to lunch the drizzle was falling outside the window. What was the weather going to be for the rest of our stay? The next stage was the start of our trip – two ferries later, one to take us to Yell, where there is very little interesting geology, and the other to take us across to Unst where there is lots of interesting geology. We drove to Saxa Vord, a decommissioned RAF station where the Officers mess had been turned into a hostel where we were to stay for the next three nights. Once we got used to it not being a hotel it was comfortable enough and the staff did feed us and look after us well.
Wednesday 1 August (Day 2)
To explore the geology of the Shetland Islands is to explore the floor of an ancient ocean and try to understand the processes which have led to the preservation of the ophiolite sequences on Unst and Fetlar. The whole story is long and complex and would take a lot of words to describe in detail. I want to give you a feel for what it is like to be there, on the ground and looking for the evidence of the story for yourself. Good places to start your own investigation into the story are www.visit.shetland.org from where you can find leaflets and information and www.geoparkshetland.org.uk on which you can find information about the geology of Shetland. Geopark Shetland has also produced some fabulous geowalls and information boards to help your understanding. A lot of the information that is available about the ophiolite sequences is thanks to the studies undertaken by the late Professor Derek Flinn. A list of his works can be found at www.landforms.eu/shetland/bibiography.htm.
Our trip was to take us around the Island of Unst looking at the processes, products and effects of the closing of the ‘Iapetus’ Ocean. We first stopped at Saxa Vord hill to get an overview of the geology. We were able to see Muckle Flugga lighthouse and the steep dip of the rocks that it was standing on, evidence of what we were to see later in close up. Allen gave us an overview of the story. An ophiolite sequence is a slice of ocean crust that has been preserved rather than being subducted. This is unusual because the density of the oceanic crust compared to the continental crust should result in the oceanic crust being subducted, melted and recycled. In Shetland this did not happen and the thick slab of oceanic crust emplaced and preserved on Unst and Fetlar has given geologists an excellent opportunity to study the mineralogy of ocean floor rocks.
On Unst and Fetlar the ophiolite sequence is found on the eastern side of the islands with the continental crust on the western side of the islands. The two groups of rocks, separated by the thrust zones, formed during the closing of the ‘Iapetus’ ocean 420 million years ago. On Unst the thrust zones generally run NNE – SSW through the middle of the island from Norwick to Belmont. Moving on to our next geological stop led us to the spectacular sight of the Unst bus shelter, the most northerly decorated bus shelter in Unst with its own website www.unstbusshelter.shetland.co.uk. We were privileged to see it in its Jubilee and Olympic colours. Sue Vernon left a message in the visitors book on our behalf.
Our second geological stop was the Keen of Hamar National Nature Reserve. Map reference HP639 to HP645 098. Here we were to see the effects of weathering of the olivine formed in the lowest layers of the ophiolite sequences. The olivine has weathered to serpentine to form the rock serpentinite. The serpentinite itself, in the Keen of Hamar has weathered to form an extensive area of serpentine debris. The effect on the ecology is to have produced a heathland area with soil that is very free draining in which drought tolerant plants have evolved. The most famous of these is the Edmondston’s chickweed (Figure 1) which is endemic to Unst.
There were some strips of more fertile soil, with a greater abundance of plants, transported to the Keen during the Ice Age and some stripes of stones of different sizes. The difference is size is caused by continuing episodes of freeze- thaw activity and the movement of the stones down the hill as there is no soil to hold them together. These are evidence of the processes that still shape our landscapes.
From the Keen we were able to look across the valley at the rocks on the other side. There was a difference in the colour of the rocks, pale ochre coloured at the top of the hill and rocks of a yellower hue lower down the hill. This we were to investigate later.
But first we visited the restored Hagdale crushing circle also called the Hagdale Horse Mill. As before when we visited the Horse Mill there was much discussion about the way that the stone had been set up. We could not see how the mill stone could be moved around in the small pit in which it was stood. But what was if for? Why was it here? It is all to do with the proximity of the dunite rock across the valley. The dunite represents the rocks at the bottom of the ophiolite sequence, the bottom of the magma chamber. Chromite is one of the minerals that formed crystals early on in the formation of the magma and formed pods and streaks at the bottom of the magma chamber interleaved with the olivine. The mineral used to be mined and processed between the 1870s and 1920s, hence the placement of the Horse Mill.
We followed the trail of the miners up the hill to find the chromite; heavy black speckled rock. As we moved up the hill towards the highest quarry we were moving towards the bottom of the magma chamber where the best pods of chromite could be found. This was an energetic but amazing walk all the way to the Moho under bright skies and sunshine.
Lunch was taken at the Unst Heritage Centre. Again somewhere that is well worth a visit. There is a story of rocks in the fabulous geo wall outside (Figure 2) with tea and coffee making facilities and the story of crofting, knitting and lace making inside.
The afternoon was spent in the North East corner of Unst in the bay of Nor Wick on the beach. We first looked at the Skaw Granite (Figure 3), a porphyritic biotite monzogranite with elongate tabular phenocrysts of microcline feldspar. To look at they are a very mixed set of granites with some very pale and some very pink with the large red feldspar crystals standing out up to 8 cm long. There was convincing evidence of alteration with some minerals showing clear lineation, almost tending towards Gneiss like configuration. They are like no other granites that I have seen before and were new to other people in the group too. There was much heated discussion about their origin and the association of these sets of granites with episodes both in England and Shetland. Current dating information suggests a date of around 460 Ma. The only answer is to go and visit them yourself and see what you think.
We stood with one foot on the continental crust of Ancient America and one foot on the Oceanic crust of the Iapetus ocean. Who would have thought that we could visit the Moho in the morning and such an important junction in the afternoon? The other highlight of the locality was the wonderful views of turnstones in full summer plumage, skuas flying overhead, arctic terns fishing and best of all a lone puffin fishing close to the shore. I hoped that this was not the only view of puffins that I was going to see.
Back to the geology. The final visit of the day was a walk past the talc quarry at Clibberswick (Figure 4) down to the beach. The quarry, a large white hole in the ground with industrial buildings and machines ready to work and then the beach. Spectacular soapstone cliffs and serpentinite pebbles on the beach all wet from the sea and shiny from the sun. We trudged back up the hill off the beach back to the hotel ready for supper. A brilliant day with excellent geology telling the story of the Islands and the chance to see interesting flora and fauna too. And the bonus – all done in the sunshine.
Thursday 2 August (Day 3)
The weather was misty and overcast with a moderate rain, the visibility being limited to just a few hundred yards. As a result, our first planned walk along the beach at Mu Ness and Ham Ness to see layered gabbros and sheeted dykes had to be abandoned: “distinguishing grey rocks covered by grey algae on a grey day” on a wet beach was considered too difficult, the contacts between the rocks being impossible to find. Instead, the morning was largely spent on the study of the history of Unst.
We stopped along the road to Muness to examine a standing stone, covered in lichen, which at first glance appear to be a rotting piece of wood (Figure 5). On closer inspection, the stone was made of the local phyllite, its wood-like appearance due to the typical foliation of the rock. This stone, possibly dating to the bronze age but could easily be much more recent, is known as the Clivocast Stone.
Our next historic visit was to Muness Castle in the south of Unst, which is the most northerly castle in Great Britain. It was built in 1598 by Laurence Bruce, a petty tyrant, who ruled Orkney and Shetland as sheriff. His cruel and corrupt rule in Shetland is known as “the oppression”. The castle, really a fortified house, was built using forced labour, with locally sourced stone. This was mainly the local Unst phyllite, but lintels and carved, decorative pieces are of sandstone. Some recent renovation work (1992) also used sandstone. The castle was only occupied for a few years, Bruce dying at Muness castle in 1617.
From the castle, we drove north again to Haroldswick, to visit the Viking longhouse and Viking boat. The longhouse is under construction and not strictly open to visitors. However, as Robina had once been involved in the project and the builders were on tea-break and were shown around. The longhouse replica is being built with local sandstone stone from a dismantled croft from the island of Yell. The timber is from Scotland, imported from Caithness. We were informed that the remains of over 50 Viking longhouses have been found on Unst.
At the same site, there is a reproduction Viking long ship, built in 1974 in an attempt to sail from Scandinavia to North America in emulation of the voyage of Leif Eriksson. The attempt failed when the crew “mutinied” and the boat remains in Shetland at the centre. It is intended that the house and boat, together with a small display of replica Viking artefacts many of which are made of soapstone, will become a full-scale tourist attraction by 2013.
Leaving the Viking longhouse and ship we went south again, to the Belmont Ferry terminal. While waiting for the ferry to arrive to take us to our next island, Fetlar, we visited a disused quarry located to the rear of the ferry terminal. The quarry, semicircular with a boggy ditch at the base of the quarry face, contains exposures of a serpentinised harzburgite thrust over phyllite. This thrusting represents the second thrust of mantle rocks over continental rocks to take place in the orogeny. In places in the quarry, the harzburgite has degraded by serpentinisation into steatite and powdery talc. This steatite, or soap stone, was used by the Vikings (and no doubt other occupants) for carving small objects replicas of which we had seen on display at the Viking longhouse. Only one of us fell into the soggy quarry ditch!
After about half an hour in the quarry, it was now time to take the ferry from Unst to our afternoon destination on the island of Fetlar. The passenger deck of the ferry was beneath the car deck, and we took lunch there. As there were only three vehicles on the ferry for the 25 minutes trip –ours - there was plenty of space for the birders of the party to scan the sea for the local seabirds. The weather had lifted and the rain ceased when we arrived at Fetlar.
It is on eastern Fetlar that the ophiolite sequence is most visible, since it is the most easterly of the major islands, so we drove across the undulating landscape to the other side of Fetlar across the Dalradian continental crust to the Loch of Funzie, where a Geowall demonstrates the relationship between the Upper Nappe and the Lower Nappe over which it was thrust (Figure 6). This image of the geowall shows, moving from Left to Right, the interbedded pelites and semi- pelites of the Continental crust, separated by faults from interbedded greenstone and serpentinite Oceanic rocks, and finally the phyllite and the Funzie conglomerate.
The Lower Nappe, the eroded product of part of the original mountain chain, consists in part of the ‘Funzie Conglomerate’ (Figure 7), the remains of an alluvial fan, consisting of boulders and cobbles of quartzite and granite in a phyllite matrix which have been flattened and elongated by the contact with the overriding Upper Nappe, obducted from an island arc as the Iapetus Ocean closed.
We went down to the beach to have a closer look and then the majority of the group walked up the hill and round Funzie Ness to a height of c100 m, travelling up the sequence to Staves Geo (inlet) where the conglomerate gives way to phyllite. It was apparent from the cliffs during the walk that the conglomerate was bedded and tilted at about 30° to the west. The contact is exposed beyond Staves Geo, where a ravine 3 m wide revealed conglomerate dipping down and forming the floor of the ravine. At the far side phyllite can be seen below a wall of gabbro with some iron staining. The gabbro formed the rest of the headland, perhaps 100 m to where it dropped to an inlet, Litlaland Geo, beyond which was a cliff of harzburgite and serpentinite, the base of the Upper Nappe. We then returned over the headland to the geowall and the waiting minibus.
The smaller group visited the Heritage Centre, learning about the history and culture of Shetland. Fetlar is the most fertile of the islands and a large part of it is a bird sanctuary. There is a profusion of flowers and seabirds, including the whimbrel, a topic much discussed on the way back. The centre houses a museum of Shetland traditions, farming and crafts.
Back to the Ferry where we had time to look at the final quarry of the day. Very close to the junction between the obducted Oceanic and the original metamorphosed Continental crust, it displays a range of rocks: gabbro, chromate, pentonite and serpentinite. There were long discussions about the precise location of the contact between gabbro on one side and harzburgite on the other before we crossed back in the gathering gloom to Unst – and in my case at least a rather large dinner of Shetland lamb and sticky toffee pudding!
Friday 3 August (Day 4)
On Friday, we returned to Mainland Shetland to look at the igneous and metamorphic rocks in Northmaven – a day full of interest, with intrusions, metamorphism, faulting and even a tsunami.
Our first stop was for a triple tombolo at the Ayres of Swinnister – sand bars are common in Shetland because, unlike the rest of the UK, isostatic readjustment after the end of the Ice Age here has led to the drowning of the coastline. Near Sullom Voe oil terminal, we saw a layer of sand, running through peat at around 9 m above present sea level and ~20-25 m above sea level ~7000 years ago, when it was laid down. At this time, a huge submarine slide occurred in the sea off Norway causing the Storrega tsunami, which was responsible for this sand deposit (Figure 8), which contained bark, twigs and rip-up clasts. It was at this location that we had extended discussions with Theya about the differences between Carbon 14 years and calendar years and the need for the Carbon 14 data to be calibrated.
At Mavis Grind, we could see the North Sea and the Atlantic only a few metres from each other, making it a useful short cut for boats small enough to be dragged across it ( ‘Maven Aid’ = ‘narrow short cut’). Allen was explaining the features of the area on his large map - held onto the van with some beautiful magnets, the backs of which looked like polished rocks, provided by Robina - and didn’t notice three fulmars on the cliff above taking note and discussing his briefing.
At Verdins Quarry nearby, we could see how red Eastern Granite and grey basic Mangaster Voe Intrusion magmas had roughly mixed together. Later these had been intruded by red Ronas Hill Granite and white apatite and also faulted (Figure 9).
Our next stop was Haggrister Quarry, where the Eastern Granite was crossed by impressive veins of black ultrabasic rock and white scapolite (Figure 10). The black dyke was very impressive and the white scapolite could almost have been mistaken for chalk when seen from a distance.
The final treat was an exploration of the Walls Boundary Fault – a strike slip fault, which is a probable extension of the Great Glen Fault. Here we could see Graven Complex Granite next to Dalradian metasediments with a thick layer of fault gouge between them. We walked along the line of he fault from Ollaberry and then down the fault plane (to Back Sand, where we could see rocks folded and contorted at different scales, together with impressive boudins.
Only day 4 on the trip, but already great skuas (bonxies) were no longer exciting (compared with 2 or 3 days before) and we were getting blasé about gannets. However we were still looking out keenly in the hope of getting good views of red-throated divers and puffins.
Dave and Kate Smith
Saturday 4 August (Day 5)
Central Mainland Traverse - Our first traverse was east-west down succession from Upper to Lower Dalradian across metamorphic limestone rocks (marbles) that were originally marine sediments (c 650-550 Ma). We also looked at the Aith-Spiggie Igneous Complex that was intruded into Moine and Dalradian rocks east of the Walls Boundary Fault.
Our second traverse was west-east across the Moine and Lower Dalradian rock sequences repeated by the Nesting Fault. These rocks represent part of the eroded core of the Caledonian mountain chain. Moine rocks were originally sediments laid down in a deep trough and metamorphosed on the edge of the eroding Lewisian continent. These were later caught up in the Caledonian Orogeny and thrust north-west over the Lewisian basement. The Yell Sound Division is about 10 km thick and is made up mainly of psammites (originally laid down as arkose and feldspathic sandstones) which have experienced various degrees of gneissification. Quartzites and mica-schists also occur within this division, as do lenses of orthogneiss and inliers of Lewisian gneiss. Garnet -hornblende-schists within the division have been correlated with the Glenfinnan/Loch Eli Division of the Moine in Scotland.
Locality 1 - Scalloway viewpoint - The rocks of the Clift Hills Division make up the long ridge of hills, part of the ‘spine’ of Shetland running from Laxfirth, north of Lerwick, to Scousburgh and Fitful Head near Sumburgh. The large quarry above Scalloway is Shetland’s major roadstone quarry and shows colour variations in the Clift Hills Phyllite.
Locality 2 - Quoyness, Loch of Strom Whiteness Limestone “Cap Carbonates” - Cap carbonates form part of an unusual sequence of limestone that formed directly on top of (the ‘cap’) glacial deposits. These are postulated to have formed under tropical conditions and record the end of episodes of ‘Snowball Earth’ global scale glaciations during the Neoproterozoic (1000-543 Ma). Shetland’s Dalradian limestones (and in particular the Whiteness and Girlsta Limestones), although altered to a calcite marble, record evidence of this.
Location 3 - Viewpoint (Figure 17) - Scord of Sound Whiteness and Weisdale Voes have been cut into crystalline limestone marble that is less resistant to erosion than the ridges of hills on either side (gneisses, schists and quartzites). The difference in the nature of the vegetation between the low lying ground around the voes and on the hillsides is also due to the difference of the underlying rock type. On the north side of the shore of Weisdale Voe the over-steepened hillside above the road is evidence of excavation by ice. These prominent topographic ridges dominate the skyline north to Dales Voe and make up the spine of central Mainland.
Location 4 - Ward of Tumblin Quarry, Monzanite of the Aith- Spiggie Intrusive Complex. The quarry is cut into foliated monzonite, an earlier more basic stage of the complex. The monzonite is coarse grained and is composed of feldspar, quartz and pyroxene. Foliation of the rock indicated by the alignment of the pyroxene crystals suggest tectonic activity at the time of emplacement, i.e. unilaterally directed pressure (stress) has caused the pyroxene crystals to line up in the same direction so they lie at right angles to the stress direction.
Location 5 - Hill of Lee, Valayre Gneiss, junction between Moine rocks and the Boundary Zone - Excavations in the west (left) side of the quarry have revealed narrow bands of the Valayre augen-gneiss (Figure 15) in the hornblende schists and psammites of the Yell Sound Division. Here large crystals (megacrysts) of microcline feldspar have grown in a schistose matrix. Exposed in the east side of the quarry are granulites and gneisses of the Boundary Zone.
Locality 6 - Kirkhouse, Weisdale Limestone Quarry (Figure 14) . Boudinage in Psammite (found in Burn) (Figure 16) - The quarry is cut into the Weisdale Limestone that forms the base of the Dalradian Whiteness Division. It is an excellent exposure of one of Shetlands ‘limestones’ which is crystallised limestone or calcite marble. Like the younger Whiteness Limestone this too appears to be a ‘cap carbonate’ and in places displays banding (preserved rhythmic sedimentation) indicative of oscillations in the climate.
In the Burn of Kirkhouse there is a text-book example of boudinage of a quartz vein that cuts folded calcareous rocks that also display ‘ribbing’ of differential erosion.
Locality 7 - Laxo Shore - Dalradian inter-banded calc-silicate and semipelitic granulites. Nesting Fault. Inclusion Granite. Moine psammites. The Wadbister Ness Sub-group rocks are steeply dipping banded calc-silicate and semipelitic rocks. They are cut by the Nesting Fault and lie west of this fault from Swining Voe south through Laxo Voe, Catfirth and Wadbister. Close to the fault these display extreme conditions of deformation and melting.
Locality 8 - Grut Wick Quarry, Valayre Gneiss - To access the quarry you have to cross outcrops of banded gneisses and schists of the Yell Sound Division, through a glacially moulded knock and lochan landscape. The Valayre Gneiss marks the base of the tectonic Boundary Zone between Moine and Dalradian rocks on Shetland. It outcrops intermittently and can be traced from near Aith, through its offset by the Nesting Fault, to Cullivoe in north Yell.
The quarry has been cut into a wide outcrop of Valayre Gneiss that can be seen in three dimensions in the quarry walls and on loose blocks. This gneiss has already been seen previously in the day at the Hill of Lee (locality 5).
Sue Vernon (with help of Allen Fraser’s field notes)
Sunday 5 August (Day 6)
Sedimentary Rocks of the South-East Shetland Basin
Formed during the arid Devonian period, the South-East Shetland Basin was an elongate north-northwest – south- southeast orientated “pull apart” basin, probably a failed rift valley, surrounded by the rapidly eroding Caledonian mountains. Allen showed us a “Devonian satellite image” (actually a modern-day image of the Himalaya) illustrating the different environments that we would see during the day. Scree slopes formed at the base of the mountains as the metamorphic mountain rocks were eroded. Sediments washed down into the basin were deposited in alluvial fans, braided rivers, lakes and dune systems. We visited a series of localities to observe the sedimentary facies representing these different environments.
Rova Head (Alluvial Fan Deposits)
The conglomerates represent flash floods which rapidly transported material from the mountains onto the edge of the relatively flat lying basin where the larger, heavier sediments (including pebbles, cobbles and boulders) were deposited in alluvial fans. Larger cobbles and boulders had become rounded during transport but smaller clasts were angular. Within the sandstone beds the angular nature of the finer sand grains and the presence of feldspar crystals also indicate rapid transport together with “sheet flood deposition”.
Quarff (Scree Deposits and Underlying Metamorphic Rocks)
To the west of Quarff a deep, steep sided valley cuts through the Clift Hills. This prominent feature was even visible from the ferry when we arrived in Shetland earlier in the week. Thought to be a ‘pre-glacial’ river valley, this east-west valley may represent a Devonian river.
Travelling further south, we had time for a Shetland pony photo opportunity and a quick roadside stop at the Dalsetter Erratic before lunch. This large igneous rock – tönsbergite (a red larvikite found in Norway) – was identified by a BGS geologist in the 1920s. It is thought to have been transported from Norway during a glaciation 100,000 years ago (the most recent glaciation, 25,000 years ago, resulted only in a local icecap).
Our lunch stop was at the RSPB’s Sumburgh Head reserve which gave the bird watchers amongst the group the opportunity to see the seabird colonies. It was wonderful to watch the antics of the puffins on the cliffs (Figure 18) and see the fulmars gliding past.
Ness of Burgi (Braided River Deposits)
The headland was a faulted block composed of conglomerate, separated by two inlets (“geos”) from the sandstone exposures to either side. Channel lag deposits are represented by the conglomerate whilst the finer-grained sandstones represent deposition from suspension. Within the sandstones, the sand grains were more rounded than those that we had seen earlier in the day and there was less feldspar indicating more mature sandstone further from the mountain source (more distal).
Exnaboe (Lacustrine Deposits and Dune Deposits)
Finely laminated grey mudstones and limestones containing fish fossils (the “fish beds”) represent lacustrine (lake) deposits. The general lack of sedimentary structures suggests deeper waters beyond the influence of wave action and the fine laminations are thought to represent seasonal cycles of sedimentation associated with rises and falls in water levels in the lake due to higher rainfall during the winter and evaporation during the summer.
Anoxic conditions at the lake bottom, thought to have resulted from a seasonal thermocline due to high summer evaporation, helped to preserve the fish fossils. Allen told us how on a previous visit a well preserved fish fossil had been named Bob (Figure 19a) by a little girl and a less well preserved example had been named Blob! (Figure 19b).
As water levels fluctuated with changes in rainfall, the lake expanded and contracted and beds of terrestrial sediments were deposited between the beds of lacustrine sediments. Fossil plant material washed into the lake indicated the close proximity of the shoreline. In addition to the plant fossils we also found occasional ripple marks where wave action reworked the sediments and fossil raindrop impressions.
Further north along the coast large-scale cross-stratified sandstones represent sand dunes formed during the late Devonian by prevailing northerly winds.
Altogether the day provided a fascinating overview of the different sedimentary environments of Devonian in Shetland.
Monday 6 August (Day 7)
Esha Ness Peninsula – Volcano Trail (Figure 20)
Starting at the lighthouse, we walked along cliffs through the side of a volcano. The volcanic province has been dated at 390 Ma. The existing Old Red Sandstone of the Melby Basin is late Devonian. Possibly this volcanism was caused by the subduction of some oceanic crust from the last vestiges of the Iapetus Ocean.
First we looked, with the eye of faith, at the pillow lavas in the cliff side. At the entrance to Calder's Geo (Figure 21), the pillows shapes were easier to see. Are these stacking lava tubes flowing through wet sediment?
Between Drid Geo and Moo Stack Mugearitic-basalt (contains oligoclase) lava and tuffs appear. At Blackhead the lavas are thin and separated from the underlying thicker flows by a band of soft, pale green material which has caused the cliffs to be cut back.
The Grind (gate) of Navir. Here we found rhyolitic ignimbrite, formed by nuees-ardentes sweeping down a steep slope. Could this ignimbrite be an indication of a caldera?
This is the western edge of the Shetlands and only 6° from the Arctic Circle. Deep water starts nearby. In storms, humans have to shelter in their carefully sited stone house. The high cliffs are exposed to the full force of the North Atlantic. Cooling lava left faults which have been exploited by the sea, aided by fast sea level rise and the power of compressed air, to form voes, geos, caves, arches, tunnels and blow holes. Huge boulders, weighing maybe 10 tonnes, litter the cliff tops where they have been lifted 15 m by the sea and carried onto the fields by wind. We had lunch sitting amongst many hundreds of these boulders, deposited there in 1993 and again on Christmas Day 2011 when the sea breached a cliff and threw the resulting rocks 80 metres inland.
After stopping for lunch and exploring the Grind of the Navir we returned to the Lighthouse car park by taking a detour inland towards the Loch of Houlland to visit the Hols O’Scrada. This is a collapsed cave and subterranean passage. The narrow chasm some 30 m deep and 120 m long is joined to the sea by a subterranean passage that must be over 100 m long. The chasm was divided into two by a natural arch until this collapsed in 1873. The burn that drains into the east end of the chasm from the loch once powered three water mills. A ruined broch is situated on a promontory that juts out into the loch near the headwaters of the burn.
On our return to the mini-buses we turned back to Braewick to investigate the coastal section there (Figure 22). At the beach’s eastern end, there are red granite cliffs. The vertical bank of softer orange material containing suspended pebbles, cobbles and boulders was found to be glacial till. This eroded material was deposited here between 20,000-10,000 years ago by a glacier from Shetland’s icecap on Ronas Hill.
To reach the western end of the beach we had to cross Melby fault, this was indicated by the land between each end of the beach appearing flat. At the western end of the beach there is a narrow outcrop of black rock, this is a basalt lava flow forced out from the Eshaness volcano onto wet sand before solidifying. To truly write about the complete cycle events that took place at this location would take a much longer article that can be produced here. The long and eventful day came to a close with a very good dinner being taken at the Braewick Café.
Tuesday 7 August (Day 8—Departure)
Today we were concentrating our travels in the South West Mainland of Shetland to look at some aspects of the Dalradian rocks. These were formed as a result of sea floor spreading as the Iapetus Ocean was forming and its subsequent closure. During these processes the basement sediments became metamorphosed.
Location one - Mail - This was a short jaunt to the beach to look for evidence of pillow lavas. We looked hard and used our collective ‘eyes of faith’ and were able to convince ourselves that there was indeed some evidence of pillow lavas (altered to spilite) (Figure 23). It was easier to see the mudstone metamorphosed to phyllite between the pillow lavas. Having convinced ourselves that there were pillow lavas to be seen we then moved on to our next location.
Location Two – Catpund Quarry and Catpund Burn - This locality, based in a quarry, was to us the chance to look at samples of steatite, talc and serpentinite of a very special type. We parked off the road to walk up a track which had been cut around 1987 with a view to the commercial extraction of talc. Once again in 1997 commercial interest was shown, but the quarry is not currently worked. As we walked up the track it was hard to know what to look at; where our feet were going so that we did not stumble, or at the amazing rocks shown in the cutting wall.
All the rocks within the Dunrossness spilite group that were mentioned in the notes were there – we were able to see the steatite with a thin black layer of meta-sediments of graphitic phyllite and the paler bands of steatite. To touch, the steatite was smooth and silky.
The quarry, when we reached it, was small and it was difficult to see how it could have been exploited in any commercial way but the rocks were very interesting. We could examine them up close and came to the conclusion they were an example of the very special rock that we had been looking for. We could see its brecciated nature, originating at the time of its formation, and the random spinifex texture of the serpentinite crystals without the aid of the ‘eye of faith’ and almost without hand lenses; (Figure 24) we spent a long time looking and marvelling at these rocks.
The observations are so special because, taken together they are indications that the rock was originally laid down as a sequence of komatiite lavas. Allen’s notes gave us a very succinct explanation of komatiite lavas – ‘these represent ultra-basic, olivine rich lava flows that were erupted at very high temperatures (near 16000C) from deep within the mantle’. The extraordinary thing about this occurrence of the rocks and the cause of much discussion among us, and the experts too, is the fact that they are extremely rare in rocks as young as Lower Ordovician. There is a very good explanation about why they might have occurred here in Allen’s notes which I do not have the space to include. As we moved on up the track we saw phyllite, serpentinite and soapstone.
Robina went ‘rock climbing’ to point out layers of calcite on planes of movement, supported by evidence of slickensides (Figure 25). Around the corner we saw larger crystals of calcite twinkling on the rock face. We were very interested in these rocks, but had to carry on up the track to go onto the next site.
The next stage of the visit involved quite a scramble up to the Burn of Catpund to see evidence of exploitation of the rock that had been successful rather earlier than the 20th century. This was a fascinating site in which we could see the old spoil heaps and overgrown quarry workings covering the whole area. On closer inspection we could see chisel marks on the exposed rocks and hollows in the ground where the steatite (also known as talc-magnesite, soapstone or ‘klebber’ in Shetland) had been worked by the Norse people who lived in the islands after the Iron Age. Norse people used the steatite to make cooking vessels, lamps and all manner of other useful and decorative objects.
It was a treat to visit this site and appreciate how the lives of the people were so closely linked to the geology. When asked when were the Vikings here? Allen replied ‘They still are’. Note to self – many of the Shetland people have a different genealogy to those people originating from the Mainland.
Location Three – Maywick-Taing - We walked along the base of the cliffs to study the phyllite, more schistose here than we had seen before. Back across the burn to the cliffs of the Taing on Maywick on the left side of the Maywick fault. There were Bigton grits to look at along with swathes of iron rich sandstone. This again led to lively discussion about the origin of the iron itself. We did not come to any conclusions but discussion is always good to learn new things from other people and, on occasion, surprise yourself about how much you can add to the discussion.
Location Four - St Ninian’s Isle Tombolo - We stopped in a rubbly lay-by at the side of the road near Bigton towards the South end of the Island, not a very promising start. We then walked about 50 metres to the edge of the cliff and looked down on the tombolo between the mainland and St Ninian’s Isle (Figure 26).
This was much more impressive - it is the largest active sand tombolo in Britain with a beach length of 500 metres. What you see is a strip of sand curved symmetrically on both sides, looking in profile like a concave lens, which links the mainland with the Island. This particular tombolo has been in existence since at least the dark ages and it is thought owes its stability to the sand overlying a cobble base. Even though the tombolo remains the sand on its surface changes its profile over relatively short periods of time dependant on weather and tidal conditions. The time that we visited the weather had not been stormy so it was topped with sand with no sign of the cobble base. We caught glimpses of other tombolos during the week, but none as impressive as this one which was well worth the stop and, as stated in the notes, a fantastic photo opportunity and incidentally a very good place to stop for lunch.
Location Five – Garths Ness - The next location is hard to describe in words, but once again “spectacular” is a good starting point. We drove past Quendale and then up a rough track and parked near some derelict and scruffy MOD buildings - just left as they were when the last of the men went away – an even less promising view than the last stop. We walked along the cliff top across the ness looking at the rocks as we went. The rocks, in the Clift Hills Division, are mainly striped hornblende schists containing epidote, plagioclase, quartz, chlorite and biotite. Allen explained that these rocks were most likely to be Dalradian in age originating from an Island Arc environment.
What we went to see was a massive zone of sulphide mineralisation approximately 32 metres long and 4.5 metres wide within the schists (Figure 27).
This was exposed due to the entrepreneurial attempts to extract copper by Alexander Chrighton and Andrew Grierson, Laird of Quendale around 1800. What we saw was a large oblong, brownish body of rock with a curved surface bounded by straight lines delineating the adits that had been dug to mine the copper. It must have been a backbreaking and time consuming job to shift so much rock to obtain copper in economically viable quantities. The men working this area seem to have been paid well for their troubles because, in order to improve the assay quality of the ore, they secreted copper pennies in the assay samples. Eventually they were found out when the ore that was sent for processing did not match the quality of that shown in the assay samples and the mine was closed down as being uneconomic.
The following information from Allen’s notes gives another indication of the tectonic activity that has gone into the building of Shetland. “This ore deposit consists mainly of pyrrhotite with only very small amounts of pyrite and chalcopyrite. The origin of the sulphide deposition is thought to be from a mixture of magmatic water, derived from a cooling volcanic dome and large volumes of sea water, conditions associated with young Island arcs. These deposits can often thicken and evolve to contain economic concentrations of copper, zinc, lead, barite and sometimes gold.” This was a fascinating site, and like so many of the other locations, it showed the close link between geology and people’s lives.
Location Six - Scatness Iron Age Village and Broch www.shetland-heritage.co.uk/scatness. - What a fabulous way to end our stay in Shetland. You really do need to see it to appreciate the significance of the site to the understanding of the development of a way of life in Shetland. I could write so much about it but it is much better for you to read about it for yourselves. Go to the Shetland heritage website where you will find excellent information. It will not be as good as the tour guides that we had to show us around and you will not be able to sense the atmosphere and smell the smoke, and touch the materials but it will give you the information that will help you to understand the site.
Overall a brilliant trip with an excellent leader supported by good organisation and above all good company.
All photographs are copyright to the author(s) of that day or part day.
Northampton Building Stones Walk - Led by Diana Sutherland (OUGS Symposium 2012
19 Aug 2012
Northampton, almost unknown to me, is a fine city, dominated by the local sandstone whose colour ranges from ochre to a deep reddish brown. Of the great Norman castle, where Thomas a Becket was tried in 1164, important during the Civil War, nothing remains but a bit of retaining wall on the edge of the station car park!
Our first stop was a major disappointment: we arrived at St Peter’s to find the gate closed! Two reasons for our sorrow:
This church, being down the hill, escaped the Great Fire of Northampton, 1675, which destroyed much of the town centre, sparing the 14th century tower of All Saints’ Church, rebuilt 1680 and Welsh House, 1595. Most of the public buildings are 18th and 19th century, including the Guildhall (Figure 3), with a plinth of oolitic Lincolnshire limestone, walls of many-coloured midlands sandstone, and sculptures depicting the town’s historic importance.
Our final church, The Church of the Holy Sepulchre (Figure 4), was built in 1100 by Senlis on his return from the First Crusade on the model of the Holy Sepulchre in Jerusalem. It is one of only 6 round churches in the country, and retains the original round nave with eight massive Norman pillars. Enlarged in the 13th century, tower and spire added in the 14th, wooden corbels with musical instruments in the 15th, it was restored in the late 20th century and serves as the regimental chapel of the Northamptonshire Regiment.
Lecture : The Basement of the London Basin and its Impact on the Geology of Southern England - by John Cosgrove
20 Sept 2012
John is Professor of Structural Geology at Imperial College, London and a member of the London Basin Forum, set up in 2008 after the 9th Glossop Lecture as a Working Party of the Engineering Group of the Geological Society of London under the chairmanship of Michael de Freitas, with the aim of collating information on the effect of Basin structure on cover structure and producing an Atlas. (More about the Forum can be found at its website: www.bgs.ac.uk/LondonBasinForum.)
His first consideration was: What is the Basin? The origin lies in the Hercynian orogeny, a mountain belt thrown up by the coming together of Laurentia and Gondwana to form Pangaea. Mountain belts are isostatically unstable and at the break-up of Pangaea, England’s position on the margins of Europe was crucial. The subsequent Alpine orogeny led to two types of brittle failure – compression and extension producing fractures and joints.
There followed a discussion of different types of fault with diagrams and the observation that the same classification applied at depth, producing a series of east-west trending south-dipping faults.
There was an earlier Caledonian trend in the north, but Hercynian/Variscan in the south with strike-slip faults affecting the whole of southern England. An increasingly rapid run through the links between stratigraphy and tectonics; the effect of early fractures on later ones, stress fields; and why the Paris Basin, between two important Variscan structures, is not fractured, led to a demonstration of the way a variety of Basins all relate to mechanical features of the Basement, here and elsewhere, a point illustrated with reference to the Persian Gulf and the way faults allow the development of gas and oil reservoirs.
I hope I’ve conveyed a feeling, however over-simplified, of the range of this lecture. It left one person at least feeling that she’d run a marathon!
Reigate and Godstone Caves - With Andy Belcher
23 September 2012
A lay-by on the A22 just north of the M25 would not normally be thought of as the most auspicious location to gather for a trip to a mine but looks can be deceiving.
A group of us met on a Sunday morning and then made the short walk to the mine entrance. After a detailed safety briefing from Andy our guide we descended, via a former ventilation shaft into the mine.
The mine itself is at the base of the scarp slope of the North Downs just north of the village of Godstone. The primary reason for the mine is to access the deposits of the Upper Greensand formation for a building stone locally known as fire stone. Above the seam is the chalk of the North Downs and below is the Gault Clay. The main seam itself is in the order of about 3 to 4 metres and is relatively continuous below the chalk. The mine shafts are relatively extensive, with a total combined length of about 2 miles. There is a general slight dip to the north of around 10 degrees and it is the dip that limits the extent of the mining as the more northerly part of the seam is below the water table.
The Upper Greensand and Gault Clay formations is Mesozoic in era and is consistent with a shallow coastal depositional location. Ammonite fossils are relatively common in the Gault and Lower Greensand formations. (British Geological Survey – The Wealden District, 4th edition).
While in the mine Andy showed us the fossil remains of a plesiosaur (a crocodile-like creature), unfortunately because of the angle at which the fossil is exposed it is difficult to make a more exact identification of the fossil.
Method of mining: The main seam has in general three main beds within it, each broadly about a metre in height. Increasing silication (most likely as a result of chemical action from the water percolating through chalk above but possibly depositional) caused increasing hardness towards the base of each bed. The hard base of each bed is immediately in contact with the top of the initially softer top of the bed below, however this does not appear to be an unconformity. The material itself is a relatively compact type of fine-grained sandstone and is grey in colour. Using this difference of hardness slabs of material could be removed with the softer material initially being discarded.
History: Mining has existed in the area since Roman times, however the this was mainly because the stone was the only usable building material in the area and given the relatively light population and the physical barrier of the North Downs it was not economic to transport it from elsewhere. Hence the quarries were relatively small and basic. Some Reigate Stone (a type of fire stone) is to be found in the Tower of London.
The earliest record for the Godstone mine is a licence granted in 1670. Population growth after this time provided fairly regular demand for the building stone. The earliest date marked within the mine is 1748. By the early 19th century demand for the building stone was falling. Nearby sources of clay could be used for brick manufacture, however this was also to provide the mine with another lease of life. The new brick houses needed hearth stones for their fireplaces and the firestone was ideal. As an additional advantage the individual pieces of stone required were relatively small and a lot of the earlier waste material made up of stone too small for building could be used. This did not require much additional mining and therefore the operation could be relatively inexpensive. The method used was a combination of backfill and infill of earlier chambers and this continued until around 1880.
The world’s first public railway, the Surrey Iron Railway, incorporated in 1803, devised as a means of crossing the North Downs from nearby Merstham to Croydon gave better access for the mine, however the organisation of the line, with individuals hiring wagons as required left much to be desired and the line itself was also one of the earliest failures of a railway, closing by the 1840s. Much of the trackbed was used as part of the West Croydon-Wimbledon line and other parts were incorporated into what is now the A23 London-Brighton road. It is significant however that when this line closed a lot of the track was relaid in the mine, albeit to a smaller track gauge. We were able to visit a section of this line within the mine.
Later History: The mine lay abandoned but had a few more years of use by local mushroom growers for a few years in the early 20th century until 1917. We able to view some, by now very well rotted, mushroom beds within some of the larger chambers. The north trending dip was however to prove the mine’s final demise. A serious flood in 1936 resulted in the deeper chambers becoming unusable.
The mine was considered as an air-raid shelter in World War II, however the earlier flooding meant that this option was never taken up.
A short walk and crawl out of the mine brought us back to a very wet surface with pouring rain!
We thanked Andy for a very interesting tour. We would also like to extend our thanks to the other members of the Wealden Cave and Mine Society who look after this site.
Autumn Geowalk: Wander up the Wandle
7 October 2012
Geology of the Wandle
The Wandle is a nice example of what some of the larger 'Lost Rivers' might have been like before they became polluted and were covered over. Through Morden Hall Park and elsewhere much of the area has been left as wetlands. Elsewhere it provides a green route through an urban environment. It has been described as Britain's 'hardest working river' with many former mills along its banks. The source of the Wandle is from springs coming from the Chalk of the North Downs at Wadden and Carshalton Ponds and the 11 miles from there to the Thames is one of the fastest-flowing of the Thames tributaries which is probably why it was the river of choice for so many mill owners.
As with the Thames, a series of terraces developed on either side of the Wandle on its route north. These are arranged with the oldest at the highest level and relate to the different Ice Ages that succeeded the great Anglian glaciation that ended about 400,000 years ago.
Before that the drainage was very different. The course of the Thames was further north, through the Vale of St Albans reaching the North Sea in the Clacton area. Its course can be traced by the gravels deposited and geologists have also identified a number of tributaries flowing from the south to join the Thames on its former route. In north London the Dollis Hill Gravel has been recognised as one of these tributaries as it contains clasts of Lower Greensand chert. These can only have come from the Weald, to the south of the North Downs. The Anglian ice sheet forced the Thames into more or less its current route by the mass of melt water pouring from the glacier as it retreated. The Wandle as we know it also dates from this time.
Looking at a topographical map in the Croydon area it is clear that there is a large gap in the North Downs. It has long been realised that the proto Wandle was probably therefore much more extensive with its route through this gap and source in the Weald. After the gap the river is thought to have joined the Mole and Wey, also from the Weald, to deposit their bedloads on what is now the north side of the Thames but what was then a continuous slope down to the proto Thames still further to the north.
Mills of the Wandle
Even though the Wandle is only 19 km (11 miles) long, it has supported many industries. In its heyday the river boasted between 49 and 51 mills.
Our first location was Wandle Park once the site of Wandlebank House, demolished in 1962. The house was built in 1791 by James Perry. Perry owned the corn mill next door. This mill was taken over by the Connolly brothers in about 1919. The building seen here was then converted to residential in the late 1990s.
The area now covered by Sainsbury's once hosted a number of important mills. One of these was the old William Morris Works. The William Morris business carried on into the 20th Century, however, the directors decided to close down the works in May 1940. After this, the site was taken over by a paper milling business that also ran the Amery Mills who demolished the existing buildings.
The buildings of Merton Abbey Mills also known as “Libertys”, now the Merton Abbey craft village, are from the time that Libertys used the site for printing silks and cashmeres. The site was investigated for “Time Team” on Channel 4 in 2002. Libertys sold the site in 1972. A fire in 1982 finally finished commercial production at the site. The buildings were saved from demolition, became the craft village in 1989 and contain the only working millwheel on the Wandle.
We visited Morden Hall Park. Morden Hall was built in 1770 for Richard Garth and his family. It had a moat that was fed by the River Wandle. The Morden Hall Mills were used for making snuff and appear to date from the 1740s, and were certainly in use for snuff by the 1750s. They supplied tobacconists' shops in the City of London. The Hatfield family (who later owned Morden Hall) becoming involved in the business in the 1830s. Snuff making appears to have ceased here in the 1920s.
After a short stroll through the park we arrived at Ravensbury Park. Several mills were located in and around the area. In the 17th and 18th centuries the park was part of an important industrial area with Ravensbury Mill located on the river at the western end of the current park and a calico factory to the north, just outside the current park boundary.
The Ravensbury Mill building is still in existence, the present building dating from 1800. The water wheel was in use up until 1965. Industrial activity ceased in 1994.
The calico printing works was notable for employing both men and women in production.
Our final stop, at the end of the walk was to look at Grove Mill & Crown Mill - These are now housing developments, still under their old names, at the end of a spur of London Road, near to Watermeads, a National Trust property. The mills on this site were put to various uses over the years. The Grove site being in use for snuff making and then in paper making. Crown Mill was used to make jerkins and boots for the British Army during the Crimean War.
Hidden beneath a raised section of Meranton Way (A24) as it passes between the superstore car park and the Abbey Mills complex lie the remains of the mediaeval Chapter House of the once great Augustinian Priory of Merton.
Founded in 1117 AD on Stane Street, with the earlier course of the Wandle to the east, at first with timber church buildings, by Gilbert, Sheriff of Surrey, godson of Matilda, it gained a royal charter from Henry I in 1121. It gradually increased in size and splendour. Thomas Becket was educated here in about 1128, and a century later, Walter de Merton, the founder of Merton College, Oxford. Henry III made it on of his principal staging posts and made over 50 visits during his reign. Valuable as a tax collection point in the 13th and 14th centuries it is celebrated for the Merton Statutes, 1236. It hosted the Royal Sports celebrating Epiphany in the years 1346-49 under Edward III. The Prior had a seat in Parliament.
It was dissolved under Henry VIII in 1538, at which date it housed 14 monks and had a revenue of £957.19s 4½d and the last Prior was made Canon of Windsor. Most of the dressed stone and decorative elements were removed to build the new Nonsuch Palace, now also disappeared. In 1648 the buildings housed parliamentarian troops and in the 17th century it was used for the production of textiles and became known as Merton Abbey.
Thanks to the dedication of members of the Merton Priory Trust, with the participation of the Museum of London, today one can visit the foundations of the Chapter House together with sarcophagi and a few architectural elements. The stones identified were mostly 'local' to London: (flint, chalk, Kentish Ragstone, Reigate Stone (not far down Stane Street) with rather more exotic ones from Purbeck and Caen in Normandy. They have also excavated the mediaeval cemetery (738 inhumations have been excavated) and the other buildings (church, dormitories, workshops, refectory etc) lie buried under concrete. The website www.mertonpriory.org gives more information.
Lecture : Meteorites by Caroline Smith
18 Oct 2012
Caroline wears many hats:
We started with Meteor Crater in Arizona with a diameter of 1.2 km and depth of 400 m, the result of the impact of a meteorite 50 m in diameter, which caused local extinction over an area the size of London. Historically such an event has occurred every 5-10,000 yrs, so since this one is over 49,000 yrs ago we are overdue for another.
After listing the four different types of meteor: enstatite, carbonaceous and ordinary chondrites, HED diogenites, she distinguished between unmelted and melted, of which the former retain primary minerals, some pre-solar. The melted meteorites show evidence of differentiation.
We handled a chondrite with CAI inclusions, formed at temperatures of 2,000°C at 4568.2 Ga in a complex environment, but there is no consensus on a formation model.
The final discussion was of planetary achondrites, known as the SNC meteorites: Shergottites, Nakhlites and Chassignites, named from their discovery location.
Shergottites, mostly of basaltic texture, displaying certain shock phases have crystallisation dates of 180-330 Ma, and were ejected from Mars in the last 20 Ma. Nakhlites are augite-rich fine to medium grain cumulate rocks, probably formed in a lava flow, having crystallised at 1.3 Ga; these were ejected from Mars 11 Ma ago.
Finally the Chassignites, of which only two samples are known, were ejected from Mars at the same time as the Nakhlites, but are of Fe/Mu composition, evidence of planetary scale processing. Having handled a small sample, we have been in touch with Mars!
Isle of Wight - With Rory Mortimore and Steve Tracey
19-21 October 2012
Friday 19 October
We started the weekend with dinner at our hotel, the Ventnor Towers, in Ventnor before gathering in the bar for an introductory talk. Rory and Steve presented an overview of the island’s geology and tectonics, together with some of the hydrogeological and geotechnical/engineering implications.
The general structure of the Isle of Wight is a monocline, as we would observe during the weekend, it dips steeply to the north but more gradually to the south. During the Cretaceous period, Chalk was deposited in basins and major east-west trending faults controlled sedimentation. Subsequent reversal of stress fields resulted in fault inversion and basin uplift. The resulting unconformity between the Chalk and Palaeogene deposits can be identified by a paleo-karst surface. A series of cycles can be in observed in the overlying Palaeogene deposits relating to fluctuations in sea level. Syntectonic events, related to the Alpine orogeny, occurred as pulses of movement with folding controlled by major faults.
We also had the opportunity to look at geological mapping of the island and various geomorphological maps used by the council to assess instability. Ventnor is situated on a landslip, so it was reassuring to note that the hotel in which we were staying was not in one of the high risk areas.
Saturday 20 October
Morning—The Chalk and Palaeogene of Whitecliff Bay
Congregating on Culver Down we had wonderful views north over the Solent towards Portsmouth. Knowing that the tides were not in our favour we set out for Whitecliff Bay with the aim of exploring both the Chalk and Palaeogene deposits in the coastal exposures.
The Culver – Whitecliff section provides excellent Chalk exposures; from the Holywell and New Pit Chalk Formations in the south (towards Sandown), through the Lewes Nodular Chalk, Seaford Chalk and Newhaven Chalk to the Culver and Portsdown Chalk at the northern end. Variations in the properties of the different Chalk formations result in distinctive landscape features.
Due to the tides the southern end of the section was not accessible but we made our way out over the wave-cut platform (Figure 1) to view the Whitecliff (northern) end (Figure 2). At first glance the Chalk may seem like a large expanse of white but once you start looking you find numerous marl seams and flint bands. These ‘marker beds’ are used, together with fossils, for correlation and can be traced across the Anglo-Paris Basin. Nodular flints (formed around burrows) picked out the bedding, whereas sheet flints (formed in conjugate fractures) cut across the bedding. We also recognised tubular flints which are particularly useful for correlation.
Just as impressive as the Chalk sequence is the Palaeogene succession (Figure 3). Working our way up the sequence we considered the changes to the depositional environment represented by the different units.
Red beds of the Reading Formation were the oldest of the Palaeogene deposits that we observed in the cliffs and on the foreshore. Eroded fragments (pellets) of this red mottled material were found in the basal bed of the overlying London Clay Formation. This bed of dark (glauconitic) silt and fine sand also contained worm tubes (Ditrupa plana) and was intensely bioturbated.
Coarsening upward cycles with relatively deep water shelf clays and more sandy shallow water material were noted in the London Clay together with a rafted block of sandstone speculated to have resulted from a submarine landslide. Septarian nodules were also visible at particular beds in the cliff and loose on the beach. Formation of these concretions is associated with changes in chemistry possibly due to increased burrowing activity at particular horizons. These relatively hard nodules have engineering implications, for example causing problems for tunnel boring machines.
A glauconitic pebble bed with an erosional base marks the base of the yellow Bagshot Sands. Cross-bedding observed in Begshot Sands indicates lower shoreface facies. Overlying sands and laminated clays (with lignite and fossil beds) of the Bracklesham Group represent shallowing of sea level.
Exposures of the succeeding Barton Group clays and sands are not so easily accessible so we paused for a quick lunch stop on the beach before looking at the Bembridge Limestone. Whilst the Chalk and the Palaeogene deposits examined in the morning generally dipped to the north at about 70°, the Bembridge Limestone dips much more gently to the north. This freshwater limestone contained land snails indicating emergence.
The sequence we observed is similar to that exposed at the famous Alum Bay at the western end of the island where Victorian tourists filled bottles with layers of the different coloured sands as souvenirs.
Afternoon—Duxmore Farm Chalk Pit (Figure 4)
Rory said that when in a pit, it is always a good idea to look at spoil heaps to get an idea of the pit’s geology. He pointed out a sheet flint, and said that there are only two such sheets in the UK—Portsdown and Newhaven—so it must be one of those. He also showed us a marl seam, and said that it could be traced from France to Portsdown.
Rory explained how flints are formed. Each flint is made up of an outer cortex and an inner black heart. Flints form in burrows made by infaunal organisms. Ultimately, the flint is larger than the burrow it originally formed in because it replaces some of the surrounding chalk. Flint is silica, and it can be precipitated only in an acidic environment. This environment is created when a creature dies in a burrow. As the organic matter decays, hydrogen sulphide (H2S) forms. This reacts with oxygen from the seawater to form sulphuric acid (H2SO4), producing the conditions necessary for flint formation. Flints form early, before much chalk has been deposited on top, typically at a depth of 1-2 metres. Marly chalks have no flint, because the marl has prevented silica diagenesis. Rory showed us a sponge flint, which looked like a sub-spherical pebble.
Evening - Short talk on Chalk in general by Rory
Deposition of the chalk in southeast England began about 100 Ma ago, when the supercontinents were breaking up and creating new oceans. The sea level then was about 300 m higher than it is now. To put that into perspective Rory explained that, if both polar caps melted completely, today’s sea level would rise by about 100 m. Heat expansion caused by higher temperatures would raise the sea by about 20 m, so the total rise would be about 120 m.
Rory asked where the additional 180 m could have come from. He said that hydration of mafic minerals on the seabed produces serpentinite, which rises up and swells the ocean. This could account for at least some of the higher sea level.
In addition to the mystery of where the water came from, it is also not known where the chalk itself came from. One possibility is that, because there were fewer mountain ranges in the Cretaceous, there would have been less freshwater runoff: so, limestone growth would have been less restricted.
He briefly discussed faults, and said that a “shock wave” spreading out from the convergence of the Eurasian Plate and the African Plate was the wrong way to think about what happened during the Alpine orogeny. It was more accurate to think of the orogeny as activating faults so that, as the plates converged, the faults formed and grew.
Sunday 21 October
As the group were not under time pressure to make low tide, we had breakfasted at a leisurely hour; Rory briefed us on the day’s activities. We would visit 3 locations in the morning: Blackgang Gore Cliff, the beach at Hanover Point and the Military Road rising through Compton Down.
Blackgang Gore Cliff
At close on 90 metres height , the view from Gore Point to the NNW was spectacular (Figure 5). In the distance could be seen the Chalk of Freshwater bay, Hanover Point, Atherfield Point and closer, the severe erosion affecting the Wealden deposits was plainly evident. Erosion along this part of the coast could be as much as 1 metre per year. As the sea encroached the area of instability moved inland and several rotational slips could be seen. Rory explained that water drainage here was critical and water ingress into the soft Wealden deposits, particularly the Gault Clay, would destabilise the land. At Gore Point we could see the Upper Greensand sections exposed at a prominent cliff (Figure 6). The top layers consisted of Chert beds (about 15 or more were visible) overlying sandstones. These layers were clearly more stable that the Gault clay to the west but Rory pointed out that these cliffs were nevertheless, still at risk of movement and eventual collapse.
At Hanover Point the margins of the car park had already succumbed to severe erosion. Nevertheless we were able to descent the two sets of steps down to the beach. Here the Wealden deposits represented the start of the Cretaceous period. Dinosaur remains had been exposed here from time to time. A concrete obelisk could be seen offshore in close proximity to the Hanover Point fossil forest. We could not examine any of the fossil trees so our attention switched to the seashore deposits.
Hanover Point Sandstones (Figure 7)
This early part of the Wealden succession was fluvial in character. A braided river system had brought in the sandstones (probably a tributary of the Western Yar predecessor). An eagle-eyed OU staff member interpreted the sandstone patterns, shown in close up in Figure 8, as rising ripples. It was explained that ripples could rise when the sedimentary load exceeded the equilibrium point and the excess deposition would cause the ripple to rise as it moved.
Another interesting feature were small areas where the sand appeared to be black in colour. Viewed through a and lens the sand grains, although not well sorted, were well rounded and glistened. It was thought that the some of the grains were actually of glauconite with a covering of an oxide of iron ( Figure 9).
Military Road Compton Down
Our third location was the Military Road as it climbed over Compton Down. Looking back the severe erosion of the Lower
Greensand Ridge over which we had just travelled was evident (Figure 10). Some 150 metres of land had slipped seaward. The
Military Road itself was under severe threat as the distance from the road to the cliff edge was in parts only 10 metres. The
chalk on either side of the road had been extensively instrumented to give the best warning of an imminent collapse. Rory
pointed out new depressions in the surface between the road and the cliff which were harbingers of a slippage which would
surely take place in the foreseeable future. The cutting itself exposed the base of the Upper Lewes Chalk Formation. This
could be identified from the presence of large numbers of tubular flints. The flint in Figure 11 has been formed in a burrow
and has the characteristic hollow centre filled with chalk. Rory also detected a chalk hard ground within the Lewes Chalk.
This particular hardground was coloured yellow, glauconitised and phosphatic but was partly obscured with vegetation. Fossils
abounded in the Lewes Chalk.
In the afternoon we visited the Cheverton Farm Quarry, the main face is in the Upper Lewes Nodular Chalk Formation of the Upper Coniacian. There is found one of the largest faults currently visible in the chalk, with 20-30 m of the eighty metres total displacement visible (Figures 11 and 12).
There is also a large amount of sub-horizontal movement associated with this fault (Figure 13). Also seen are slickensides, displaced chalk bands and bedding bent in the direction of movement (Figure 14). This fault is the edge of the Hampshire High, an inversion structure, and controls the hydrology and flooding at Calbourne by providing a path for groundwater flows. The open and extensive fracturing found near the fault make this area of the quarry unsafe to work – the nodular chalk being more brittle.
The upper parts of the quarry are in Newpit Chalk, which has no flints, a smoother texture and Einoceramids. This material is used as a road base, and also produces the creamy white slabby brash in fields.
The weekend ended with Rory answering general chalk questions in the upper pit, and the party departed for the ferry as the heavens opened.
All 32 of us enjoyed this excellent weekend, and our thanks go to both of our leaders, Professor Rory Mortimore and Steve Tracey (Steve stood in at very short notice). They chose particularly good field locations (including one new to the leaders and very quickly and efficiently interpreted). They explained the geology very clearly and thoroughly both in the field and in the evening discussions.
Extreme Geology on the Kent Coast: Introductory S276 field trip to Herne Bay
4 November 2012
I’d visited Reculver in April 2011 for a marine flora and fauna recognition course during which I had made some geological observations. To the north lay water which was interesting while to the south lay land which wasn’t but did never-the-less support the pub. Between then and now I discovered the OU, met earth sciences on S104 and been blessed with a field trip to Newhaven and Birling Gap while on the SXR103 residential practical science course, since shamefully (in my view) withdrawn. My OUGS welcome pack arrived on Friday, S276 officially started on Saturday, and there was a chance to revisit Reculver and see live rocks in the wild on Sunday. Perfect.
We met in the pouring rain at a chilly Beltinge car park. 30 or so intrepid people with rucksacks full of the prescribed field kit, many of us with new(ish) hard hats. Rocks can behave unpredictably and attack without warning, especially when given a height advantage. I was impressed by the number of tutors who’d turned up to give us there wisdom and lead the safari.
Off we went down to the shore line where the ‘work’ started with a description of coastal erosion and its prevention by grading (cutting back the cliff slope), surface planting the gentler incline, installing deep and surface drains, and building the concrete cliff footing which doubled as the walkway. We then had a look at the current day beach sediments and its properties. With the rain still coming down, this wasn’t a day for note taking. Any exposed paper was immediately weathered and eroded into beach sediment. Fully gloved, hooded and zipped-up we then split into three groups to look at other rocky things.
From a vantage point above the (now flooded) Bishopstone Glen valley floor we were led gently through the basics of observing geological features. This built up to the description of the exposed layers of (from top down) brickearth (unimaginatively named because it could be used with little further preparation to make bricks), London Clay, Harwich sand formation (with its black pebbly basal layer), Upnor sand formation and finally the Thanet sand formation. These beds seemed to slope very slightly down to the north which would make sense at the northern end of the buckling caused by the alpine orogeny producing the domed and subsequently eroded Weald.
Further to the west a small slip was accessible from a track allowing us to see the junction of the London Clay and Harwich formations at close quarters. Capturing some live rock specimens convinced us that the clay could be rolled into sausages while the sand couldn’t. The junction between the two layers apparently represents an unconformity of several hundred thousand years.
Interestingly the water running out of the flooded Bishopstone Glen had formed a new stream diving under shingle to emerge half-way down the beach and causing a miniature version of the beach fringe cliff geology and a dramatic example of erosion in action. Then it was back in the cars (thanks for the lift Eryl) and over to the Reculver car park for a lunch break and a coffee to warm up in the King Ethelbert.
After lunch Reculver expert Geoff Downer led us round the remains of the southern Roman wall, explaining Roman building techniques and how the facing layer of Kentish Ragstone had been stolen (rock rustling?) for reuse before showing us a section of wall repairs with several stone types as well as some ragstone facing missed by the opportunistic locals. Going round the east side Geoff pointed to where the Wantsum Channel had run, separating Reculver from the Isle of Thanet until around 1800.
Then up onto the Reculver Towers into a biting wind, to examine the stones used in its construction, this is where geology began to really feel extreme exposed as we were exposed to the full force of nature. Weather not-withstanding, Geoff’s exposition of the different stones used was fascinating.
From the towers we next checked out the rock used in beach defences which range from the Kentish rag flags used for the sloping apron below the towers to the armour stone blocks dumped to break-up the waves in front of the car park area. These were primarily Norwegian Larvikite, its large twinned feldspar crystal faces sparkling prettily in the late sun now peaking between scudding clouds, and French partly metamor- phosed limestone blocks. It was apparently cheaper to bring these hard wearing materials by ship from France and Norway, than similar rock would be from the West country.
Three hundred metres or so west of the car park we were able to examine fossil corals in the French limestone, a huge fossil theropod dinosaur (that’s T-Rexy type) footprint in a sandstone block, lots of fossil bivalves in the cliff face Thanet sand as well as ‘doggers’ which I believe are formed when storm action sorts sandy sediment leaving a pure clean deposit which is then cemented and compacted to give areas of more resistant rock within a sandstone formation, seen as sandstone sills projecting from the cliff face, or when they fall out, slabs lying around.
By this time it was four o’clock. My ‘driver’ was waiting in the pub with a hot coffee and a pint of best. I was a bit wet, a bit cold, quite dirty from grubbing around but I’d had a good day. I learnt lots, had plenty of opportunities to ask questions, was never picked on or made to feel silly, and met nice people. Extreme geology in extreme weather? Maybe not but extreme satisfaction – definitely. Can’t wait for the next trip. Maybe in the spring though.
Acknowledgement: Thanks to the London branch of the OUGS for organising this, and especially those tutors who gave up their time when the temptation must have been to turn over in bed as the rain hit the window panes. Special thanks to Geoff Downer whose booklet: (The Geology of Reculver Country Park. 2011. Buckland Press Ltd. ISBN 978-0-956160-1-3) I relied on heavily to remind myself in the absence of notes.
Ian J Kenney
15 November 2012
A Visit to New Brunswick, Canada
My talk concerned my visit to eastern New Brunswick, some of the sites in the Stonehammer Geopark, and areas along the coast of the Bay of Fundy, where the rocks range in age from Pre-Cambrian to Jurassic. The consequences of the opening and closing of Iapetus, plate tectonics, continental breakup and collision, terrane accretion, uplift and later erosion, have all played a part in the geological history of the area. The Bay of Fundy itself is a rift valley formed during the Triassic to early Jurassic when Pangea began to break apart.
After crossing the Bay of Fundy from Nova Scotia, we began our explorations in the area around St John, a city on the east coast of New Brunswick. Here, the St John River runs through a narrow gorge into the Bay of Fundy, and under a bridge across the river the site of a terrane contact between Pre- Cambrian and Cambrian is evident. Here the phenomenon of the reversing falls occurs daily with every high tide when the water in the gorge rises up to 14½ feet above sea level and forces the river’s flow to reverse.
At various sites in and around St John, in the Stonehammer Geopark, and along the Fundy trail we noted Pre-Cambrian marble, Devonian conglomerate, Triassic sandstones forming sea caves and, at the furthest point of the Bay of Fundy at Hopewell Rocks, sandstones formed into “flowerpot” shapes by the tides which can reach up to 52 feet. Features of glaciations were also evident in places from striated boulders and moraines of sand and gravel, now being quarried. The final site visited was Joggins on the eastern side of the Bay of Fundy in Nova Scotia, where Carboniferous rocks have yielded spectacular finds of reptiles and plants.
My presentation about Dartmoor was split into three parts. First, I discussed the origin of Dartmoor, then I showed some photographs of some geological features on the moor. Finally, I presented some photos of archaeological features.
Dartmoor is one outcrop of the Cornubian batholith, a granite intrusion that stretches from the Isles of Scilly to mid-Devon. It has a length of approximately 250 km, and a maximum thickness of about 20 km. It was emplaced at the end of the Variscan Orogeny (late Carboniferous to early Permian). It is characterised by a Bouguer anomaly, a region of reduced gravitational attraction. This is the result of the low density of the granite, in comparison to the average density of continental crust.
I showed some photos of well-known tors, including Haytor, Saddle Tor and Watern Tor. I pointed out various features, including feldspar phenocrysts, jointing, granite litter and the way the rock weathers to give a rounded landscape. I then showed some photos of archaeological features, including a dry stone wall, medieval longhouse, a menhir (standing stone) and two adjacent stone circles.
Geological problems of building the London to Brighton Railway
Please see next article for details of John Lonergan’s talk.
Montserrat and its Volcano - a Flying Visit
From Antigua we flew direct to the East Coast of Montserrat. The entire flying tour of Montserrat was over the “exclusion zone”, which is an area that has been designated off limits to habitation or ground transportation.
At the centre of the exclusion zone is the Soufriere Hills Volcano, rising to 3000 feet above sea level, still active and frequently emitting a combination of ash, smoke and steam. One thousand feet below is the massive Tar River Gorge and Delta, which has formed over the last six years as a result of pyroclastic flows. The helicopter then flew over Plymouth, allowing a view of the former capital of Montserrat which is now uninhabited, and in some places is buried in 40 feet of ash.
We also saw villages trapped in the exclusion zone such as Long Ground, Windy Hill and Spanish Point. On the seaside, was the W.H. Bramble Airport. Closed after being hit by a pyroclastic flow in the fall of 1997, the runway and airport buildings are now just ruins.
After travelling along the East Coast, we left the island of Montserrat behind, taking vivid memories of the devastation of what a natural hazard can do.
Lecture : Geological problems of building the London to Brighton Railway 1835-1841 - by John Lonergan
15 November 2012
Introduction: The London to Brighton railway was constructed across the geological strata of Surrey and Sussex; it was one of the first railways to be built principally for passenger, and particularly, excursion traffic. It is also one of the earliest mainline railways in the UK. There is also a particularly good record of the discussions and decisions from the evidence given to the Parliamentary committee.
The geological aspects can be divided into three:
Choice of route: The London & Brighton Railway commissioned Robert Stephenson to determine the 0ptimum route from the six potential routes proposed. These were divided between two corridors reflecting fundamentally different philosophies aiming to minimise operational or construction costs. The various options are shown below on J. H. Turner’s 1835 map.
The six routes were narrowed down to two, those of Sir John Rennie and George Bidder (worked up by Nicholas Cundy and in Stephenson’s name); one from each corridor. Rennie recommended a direct line between London and Brighton, crossing the grain of the country, he recommended unlined tunnels and 4 in 1 cuttings in chalk, based on inspecting the North Kent quarries, also selling Merstham cutting material as hearthstone and in the weald he assumed slopes of 1 in 1.25-1.5. Bidder favoured a longer route, via Dorking, Horsham and Shoreham, which avoided both steep gradients and the need for tunnels, and included a port and two towns on the main line.
Stephenson preferred Bidder’s longer route, both for construction and operation, but the directors of the Railway Company opted for Rennie’s shorter and more direct route, despite the greater engineering challenges it posed. Stephenson stated that the initial cost of the Dorking route would be lower, and that first costs were more important than subsequent operation and maintenance costs. He also said it is better to go round than over high ground, better a level than an undulating route (and that the Adur is the only gap in the South Downs). “From experience the cheapest line has the lowest gradients, in spite of the additional distance...especially in this case.” Both Stephensons believed in using the power of locomotives to haul heavier loads, not for climbing steeper gradients. He anticipated less than five years to construct Bidder’s route, more for Rennie’s, due to earthworks; Rennie predicted 2-2.5 years.
Extensive records of the competing schemes were recorded in the evidence given to Parliamentary Committees, who then appointed Captain Robert Alderson, R.E in May 1837 to determine the best route. In June 1837 he reported that Bidder’s route was the best engineering solution, but the commercial and public benefits of the direct (Rennie’s) route outweighed the necessity for heavy engineering works. Then in July 1837, permission was granted for Rennie’s proposed route, from the existing London & Croydon Railway at Coulsdon to Brighton.
Construction of the line: Rennie’s proposals used more existing track and thus required the construction of 39 miles of new railway (compared to 49 for Bidder), but provided a longer route overall between London and Brighton. The total length was 49 miles compared to 54 miles on Bidder’s route (journey time predicted by Stephenson at two hours and quicker than the steeper route).
The direct route needed five tunnels, each 25 ft wide and high, at Merstham, Balcombe, Haywards Heath, through the South Downs at Clayton (2266 yards) and Patcham, plus two long viaducts, across the Upper Ouse Valley - the famous Balcombe Viaduct, and the New England Viaduct, just north of Brighton. Rennie’s ruling grade was 1 in 264, compared to 1 in 330 for Bidder’s route. Rennie suggested that chalk excavated from the North and South Downs could be sold to improve the clay of the Weald.
John Rastrick was appointed as Resident Engineer, and building of the line began in 1838. It took 6206 navvies, 960 horses and five locomotives to build the railway, over three years, at an 1841 cost of £2.63 million (over £57,000 per mile). Bad weather in winter of 1840-41 seriously slowed progress, and thus the Shoreham branch line opened first on 11 May 1840, allowing delivery of rails from the port.
Construction problems on the southern section, across the Weald and South Downs, delayed the opening of the main line until a grand ceremony at the new cast-iron-vaulted Brighton station on 21 September 1841. When opened the Clayton, Merstham and Balcombe tunnels had whitewashed brickwork and were lit with gas lamps. Balcombe and Haywards Heath tunnels were lined with corrugated sheeting shortly after opening, to keep passengers in open third class carriages dry.
Post-construction: The original cuttings through the chalk were vertical, but chalk falls soon after opening led to reduction in the angle of the lateral slopes, to 4 in 1 then 2 in 1. There were also landslips in the London Clay at the northern end, which led to a general flattening of side slopes. As the embankments were constructed of tipped, unconsolidated fill there was continual settlement and slipping at Earlswood and Keymer. The Haywards Heath and Balcombe tunnels in particular proved very wet and needed extra lining. In 1841 in Copyhold cutting a slip derailed a train and a serious clay fall near Dartmouth Arms closed the line for 17 days – a forerunner of other slips there in 1841-2 leading to the London & Croydon Railway spending £30,000 flattening cutting slopes.
Conclusion: This line was built in the pioneering era of railway construction, and demonstrated the advantages and disadvantages of designing along strike (by minimising construction over both the worst and best strata). However locomotive power grew and soon overcame the steeper gradients. The direct line was immediately successful and profitable.
Lecture : Dinocochlea the "Terrible Snail" of Hastings - by Paul Taylor
6 December 2012
A social half hour of refreshments for Christmas preceded our last meeting of the year, with a talk given by Dr Paul D Taylor of the Department of Earth Sciences at the Natural History Museum, London.
He had brought along two examples of this puzzling large spiral ‘creature?’, but started his talk with a photograph of himself with another one over 2 m tall, and outlined the proposed scheme: history of the discovery, theories of origin and finally a solution to the mystery.
It was in 1921 that road-builders in Hastings found the first example in the Lower Cretaceous Wadhurst Clay in an area known for dinosaur bones, and contacted museums. Labelled Dinocochlea ingens, it was nicknamed the terrible snail and various theories were advanced to explain it: a large gastropod, a dinosaur coprolite, a burrow, a concretion etc. The gastropod theory didn’t hold up, partly because of the enormous size and irregularities in the structure. In the largest example the coil is dextral while in the others it is sinistral, and there is no trace of shell or of an aperture. Although there are large spiral dinosaur coprolites, there is no trace of faecal matter in the known specimens. The burrow hypothesis is difficult since Dinocochlea is placed horizontally in the sediments.
The new model is that of a concretion nucleated on the horizontal helical burrow of a polychaete worm, greatly enlarging it. Dinocochlea has a concentric internal banded structure consonant with progressive accretion, preserving the original trace fossil. More can be learned, illustrated with drawings and photographs in: Paul D Taylor, Consuelo Sendino: Proceedings of the Geologists’ Association 122 (2011) 492-500.