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.
Plates, Plumes and Magmas of the African Rift • Visit to the Horniman Museum • The Thames Tideway Project - or how to un-pollute the Thames (again!) • Gas Shale • East of Everest - the Geology and Tectonics of Bhutan • Hornchurch Railway Cutting SSSI • Behind the scenes at the NHM • Re-interpreting the Volcanic Evolution of the Isle of Skye • Sedgewick Museum and Building stones Walk, Cambridge • Mapping Mount Caburn in Lewes • The Anthropocene: A New Epoch of Geological Time? • The Chicxulub Impact Site and the Cretaceous-Palaeogene Mass Extinction • Portugal • The Recent Discovery of a Large Fossil Tree from the Jurassic of Southern England • North Bucks • City of London Cemetery • Crags of Bawdsey Peninsula • GA Geoconservation Conference • How the Solar System was born • Autumn Geowalk around Virginia Water • Bath Stone Mine • Watching new crust form on an active plate boundary: afar Ethiopia • Pas de Calais • Student day to Herne Bay • Members' Evening • Landslip hazards in the Isle of Wight • The History of the London Branch
Lecture : Plates, Plumes and Magmas of the African Rift - by Nick Rogers
5 February 2011
We were delighted to welcome as Speaker following the London Branch AGM Dr Nick Rogers, Head of Department of Earth and Environmental Science at the OU, and author of Block 2 of S339, From Rifting to Drifting, for many of us one of the high points of our studies in Geosciences.
The main thing to take away about the title of this lecture is the plural form of the first three nouns. Showing us a range of scenery, topography and environments, he started by demonstrating that Africa is rifting, from the Afar depression in the north, with its new crust, through extensional faulting and flood basalts in Ethiopia, down through the Eastern Rift in Kenya, and in the Western = Rift down to Malawi. He discussed why Africa is rifting, why it is rifting where it is, and why the difference between the two rifts. He answered the first question with reference to an uplifted plateau, and the stress regime tending to pull the continent apart, with much of Africa surrounded by old, dense, sinking lithosphere. Seismic tomography reveals a large anomaly under East Africa: evidence for a mantle plume? The rifting and the volcanism mainly cuts through Later Proterozoic lithosphere, avoiding the Tanzanian craton core.
The different basalts inform us of the thermal structure of the Mantle through the study of their volume and rates of production, major and trace elements, isotope ratios etc. In Ethiopia to the north, the single rift is older. To the south, the basalts of the Western Rift, with big volcanoes, differ in potassium content from the Kenyan rift and come from an ancient and enriched source region. Helium isotope data, giving geochemical evidence for mantle plumes, and Sr-Nd signatures indicate that the deep-seated sources of these two rifts are distinct.
The Red Sea / Ethiopian segment is witnessing uplift and splitting. Citing O’ Connor et al, EPSL, 1999,* he showed that there is a gradual southward migration of the onset of volcanic activity at a rate of 25mmyr-1, consistent with the anticlockwise rotation of the African Plate. With reference to Chang et al, GRL, February 2011,** on the use of sea mount evidence to determine African Plate motion, then unpublished, he concluded that there was geochemical and geophysical evidence for two mantle plumes beneath the African Rift.
Thus, despite his openly expressed fears at the beginning that we should normally be asleep in front of the telly on a Saturday afternoon(!), he kept us wide awake.
* Earth and Planetary Science Letters
Visit to the Horniman Museum
17 February 2011
Part 1, the morning walk from West Dulwich to Horniman Museum
Yvonne and Michael Brett live at West Dulwich and their local museum is the Horniman which they know well. They kindly offered to lead a walking party the two miles from West Dulwich station to the Horniman. Five of us took up the offer and it was a delight with green space nearly all the way. We began in Belair Park where the only substantial part of the River Effra can still be seen above ground.
For most of its length in Belair Park it has been made into a very attractive elongated pond inhabited by a variety of ducks and geese. We followed it along the west bank from where we had tantalising glimpses of Belair Mansion, built in 1785 and currently a restaurant. The park is substantial and open, mostly now used as playing fields which Yvonne and Michael told us are frequently under water as the water table is so close to the surface. Luckily for us, February had been comparatively rain free.
I was particularly interested in this part of the walk as I have a project to walk all of London’s Lost Rivers to look for evidence of the former routes they took. The Effra rises from springs flowing north from Sydenham Hill, in the Norwood area, makes its way through these ponds, continue through Brixton and under the Oval cricket ground to join the Thames on the south side of Vauxhall Bridge.
There is a second Effra Storm Relief sewer on the north side of the bridge which flows under the M16 building. For those who also came to the evening talk by Jackie Skipper on the Thames Tunnel, these two storm relief sewers will be included with the majority that will be linked with the proposed new large tunnel under the Thames. For those interested in the Effra, there are a couple of good websites: http://lndn.blogspot.com/2010_06_01_lndn_archive.html and http://hatmandu.net/index.php?s=effra
The next open space was the gardens of the Dulwich Picture Gallery where we were delighted to come upon a newly-acquired
sculpture by Peter Randall-Page. This had been presented to the Dulwich Picture Gallery by the Arts Trust to mark their
bicentenary. Peter Randall-Page bases many of his sculptures on glacial erratics selected from Scotland and Scandinavia which he
leaves in their natural shapes and cleans up. The shape of the stones dictates the geometric designs he superimposes, in this
case he picks up a frieze from the outside of the Dulwich Picture Gallery. Walking the Dog consists of 3 blocks all of the same
granite. The red colour implies they originated in Scandinavia.
Yvonne is a Guide and Volunteer at the Dulwich Picture Gallery and was singing its praises: several of us decided we would make
a return visit. It was founded in 1811 to house the collection of Sir Francis Bourgeois RA, and built by the architect Sir John
From Dulwich Park we had rather murky views to the south of Sydenham Hill with the distinctive television mast. Outside the park an excavation on the road side confirmed that the flat terrain we had been walking over was underlain by London Clay. A short stretch of road brought us to our next green space, Cox’s Walk, part of the Green Chain Walk. We now left the flat and started climbing up the northern extremity of Sydenham Hill. This pleasant path through woodland first took us over a disused portion of railway that originally went to Crystal Palace.
Although it was considerably drier underfoot than it had been when Michael and Yvonne came on a recce, the mud told us clearly
that we were still on London Clay. At the top of Sydenham Hill, the London Clay reaches its full depth with the sandier Claygate
Beds at the top, capped by pre-Anglian gravels that were probably deposited by an ancient river from the Weald, crossing what is
now the Thames Valley, to join the Thames further north on its former course to the coast of East Anglia.
Part 2: Horniman Museum geological collections
After lunch, we were met by Paolo Viscardi, Deputy Keeper of Natural History at the Horniman Museum, who had very kindly offered to introduce us to the geological collections at the museum.
The Horniman Museum in Forest Hill, South London, was founded by a Victorian tea trader, Frederick Horniman, to contain his specialised collections of anthropology, natural history and musical instruments. It is a small, family friendly museum, welcoming school visits, and producing many exhibitions, events and activities, and is involved in continuing research and field visits.
Paolo began our visit on the ground floor of the museum beside a representative case containing some exceptionally well preserved specimens from the Burgess Shale of the Canadian Rockies.
The main geological displays were around the upper balcony but, in total, the museum has approximately 120,000 geological
specimens in store. In a side room, Paolo had laid out a representational collection for us to examine.
We then continued our visit by following the geological displays around the balcony, starting with an “Introduction to Fossilisation”, continuing in chronological order of the geology of the UK from Cambrian to Pleistocene (therefore, where the rocks are not visible at the surface, no specimens were collected), and ending with specimens from the Arthur Wyatt collection.
We thanked Paolo for a most interesting visit and many of us then remained at the museum to continue our tour of some of the other fascinating displays.
The Thames Tideway Project - or how to un-pollute the Thames (again!) - by Dr. Jacqueline Skipper
17 February 2011
The two lectures in February illustrate once again the wide range of subjects covered by the London branch programme, from geophysical research to engineering and environment. Jackie works with the Natural History Museum as a Consultancy Leader in the department of Palaeontology. Although she apologised for the fact that that much of the lecture was not specifically concerned with geology, it was of considerable interest to everyone. She divided the talk into sections about the causes and history of the pollution of the Thames and efforts to combat it, of which the Thames Tideway Project is the latest.
Lots of interesting facts emerged en route, not least, to me, the fact that the introduction of the Water Closet doubled water use in six years(!), that the permission to connect cesspools to sewers in 1815 was followed by a massive outbreak of cholera and in 1849 the flushing of sewerage into the Thames was permitted. The year of the ‘Great stink’ 1858 led to Joseph Bazalgette’s new sewerage system for London and the reclaiming of land from the river for the Victoria and Albert embankments. Nevertheless in 1878 the crash of a pleasure steamer in the Thames caused the death of 640 people due to the polluted river water.
Although the Thames is now the cleanest major river in Europe, even moderate rainfall overwhelms the system sixty times a year, and effluent is discharged into the river: hence the Thames Tideway Project, a tunnel, diameter 7.2m, depth up to 75m, from Hammersmith to Leigh. This section of the talk stressed the difficulties caused by the geology of the London Basin, with a lot of the different beds in and around the river being fault-controlled, but gradually we are gaining a comprehensive new understanding of the geology and structure of London. Other difficulties of course arise from people who will be affected while the tunnel and treatment plants are being constructed.
In answer to a question, she informed us that after the completion of the project, when the overflow is eventually discharged downstream into the Thames it will be of drinking water quality!
Gas Shale - in Parliament and in Sussex - Coming soon near you?
Article in LP April 2011
In March I had the opportunity to attend the All-Party Parliamentary Group for Earth and Environmental Sciences in the Houses of Parliament, as the OUGS were invited as guests to listen to this.
There were two presentations; the first by Professors Richard Selley rather dramatically entitled "Shale Gas: An Energy Bonanza for the UK or the Breath of Beezlebub?" And the second talk was on "UK Gas Shale Potential" by Professor Mike Stephenson (BGS and imperial College).
A private company, Cuadrilla, are currently drilling an exploratory borehole at Lower Stumble, south of Balcombe in West Sussex. Their next borehole will be east of Blackpool, and the meeting was to provide background information on this to the Parliament. The aim of the boreholes is to sample the shales, and thus to establish if there is sufficient methane gas available to power a small electricity generating plant. The gas would be used near the extraction site, avoiding the need to clean and compress it before piping it for use elsewhere - this reduces the energy and infrastructure requirements and thus costs – and also the environmental impact.
In the USA the amount of gas harvested from shales is now sufficiently large to depress the global gas price, as they have extensive areas of suitable shales with a large extraction industry. Historically it was used to power individual farms or public buildings (for example hospitals), thus it was generally used locally to the extraction site - and was available to anyone who drilled a hole in the right place. The new development is directional drilling and “fracking” - the injection of water and sand at pressure to fracture and locally crack the gas bearing shales, thus connecting the pore spaces to allow the gas to escape and be collected.
The major concerns raised were:
Other, more contentious, concerns also exist. Though some reported examples were due to other causes, such as modern organic methane, or other local activities. These depend upon the actual location, pressure applied, depth and local strata.
Professor Selley mentioned previous gas shale exploration in Sussex, at Netherfield in the nineteenth century (this was part funded by Charles Darwin). Shale gas was also extracted at Heathfield, East Sussex (and used to power the railway station lighting) - having been discovered in a well dug for water.
For those interested, the Department for Energy and Climate produced a report in 2010 on the UK potential for gas shales; THE UNCONVENTIONAL HYDROCARBON RESOURCES OF BRITAIN’S ONSHORE BASINS - SHALE GAS, and available to download on their internet site. They state that "by analogy with similar producing shale gas plays in America, the UK shale gas reserve potential could be as large as 150 bcm – very large compared with the 2-6 bcm estimates of undiscovered gas resources for onshore conventional petroleum." (One bcm is one billion cubic metres).
They go on to say that the lowest risk (i.e. most likely to be economically successful) shale gas exploration is where shale
gas prospects are associated with conventional hydrocarbon fields.
The 14th round of UK Petroleum Exploration and Development licences are due to be announced this year, and successful bidders than then explore for oil and gas in the areas they won. These do not cover underground coal gasification - and gas shales are nothing to do with tar sands, but these have certainly sensitised the issue.
It should be said that the two big constraints to proceeding commercially with this are; firstly, gaining planning approval, and, secondly, economics, i.e. the current price of gas and electricity. Always assuming the successful location of shales containing sufficient gas to be viable, of course.
Gas Shale at Heathfield? A response to John Lonergan’s article
I read with interest John Lonergan’s article on the prospect of retrieving gas from ‘gas shales’. In his article John mentions early gas extraction from a borehole sunk to extract water for the locomotives at Heathfield Station in Sussex. I was brought up in Heathfield and remember the gas-lit station before the Dr. Beeching axe cut the ‘Cuckoo Line’ in 1965. Later, in the 1990s during my OU studies, I discovered that Heathfield was the first UK location to extract natural gas and I became interested in finding out more about it.
The story is an exciting one. Initially, in 1896, a smell of gas provoked one of the station workers to put a match to it and a great flare of gas rose from the platform (Fig. 1). Geologists of the day quickly saw the potential and rushed to market the gas. Thus the Natural Gas Fields of England Ltd. was launched in 1902 amid much celebration (Fig. 2). Charles Dawson was one of the geologists involved, now infamous as the likely perpetrator for the Piltdown fraud, but at the time, much respected. He described the discovery in the Quarterly Proceedings of the Geological Society in 1898. It was assumed that the gas underlay most of the Weald (although not specified at the time, the source is thought to be the Late Jurassic Kimmeridge Clay). More flamboyantly the Natural Gas Fields of England Ltd. claimed that they would ‘control the supply and furnish light and power to the whole of central and southern England’ (Buckman, 1976).
The reality was that the gas only ever lit 70-80 houses in Heathfield, the Heathfield Hotel, close to the station, being the largest. It lit the station and was probably also used to light the streets. After only a few months the pressure of the gas began to dwindle and to boost the pressure a coal gas generating plant was installed in the nearby ‘Gasworks Road’ (now Marshlands Lane). It is not surprising that the Natural Gas Fields of England Ltd. was dissolved in 1904 after only 2 years, but what is very surprising is that a new company, the South of England Natural Gas and Petroleum Company was formed in 1909, and again, Charles Dawson was called in as consulting geologist.
The company limped from one financial crisis to another ending with a court case
seven years after its formation where the nature of the gas was revealed by Beeby-Thompson in his report:
It is not revealed who was behind this deception and one day I may try to turn up the original court documents to see if Dawson is directly implicated and make a more thorough search into the whole saga. As a child I was always led to believe that the station was lit by natural gas but I find now that was a myth too: the natural gas supply was turned off in 1934 and the station was lit by town gas until its demise. Only the leaking pipe marking the borehole in Ghyll Road continued to provide power for a further 20 years to the ‘Tramps Kitchen’ where the local tramps congregated to boil water until rusting pipes put an end to that too.
John reports that new techniques of ‘fracking’ have been developed allowing the gas to escape by fracturing the shales to connect the pore spaces. Clearly there is a possibility that the Kimmeridge Clay in the Weald could provide at least a useful resource, but I would recommend to any potential developer that they look to the failed Heathfield Bonanza as a Cautionary Tale before committing too heavily.
East of Everest - the Geology and Tectonics of Bhutan - by Clare Warren
17 March 2011
Clare, who holds a NERC post-doctoral fellowship works in CEPSAR (Centre for Earth, Planetary, Space and Astronomical Research) at the OU, and is interested in the timing and grade of metamorphism across the Himalayan belt.
She started by giving us a resume of the timing of the India – Asia collision as at present understood from 195 Ma with India part of Proto-Gondwana to 14 Ma when collision was complete. There is surprising along strike continuity from Khagan in the west with high-pressure metamorphic rocks, eclogites characteristic of subduction zones, and very rapid exhumation from 200 km depth at 53-46 Ma, to Nepal and the Great Himalayan Sequence with Mount Everest and Kanchenjunga where erosion from the front led to rapid exhumation at 30-23 Ma.
But east of Everest the story is different. In Sikkim and Bhutan and the Eastern Himalayas, there is no evidence of a subduction zone, but there is high pressure, high temperature metamorphism with mafic granulites, eclogite facies and amphibolite facies. The research centred on Laya in Bhutan, and using Monazite-dating techniques, they found pellitic granulates at 21 and 18 Ma south of Laya, whereas to the north they were at 15-13 Ma with cooling at 10 Ma. Exhumation by channel flow does not fully answer the question of how the eclogites were formed. The answer may lie in a crustal ramp, tapping into deeper, hotter material with exhumation of the lower crust at 12-10 Ma and the ramp still continuing afterwards. In conclusion Clare left us with the fact that the timing of peak Himalayan metamorphism youngs towards the east: the result of diachronous collision or stronger erosion in the east? The research continues.
For an account of the expedition see Geoscientist, April 2011:
Hornchurch Railway Cutting SSSI - Visit arranged by Natural England
23 March 2011
I imagine that not many members of the LOUGS have had the opportunity of visiting the geological SSSI in the Hornchurch railway cutting. For obvious reasons it has been off limits for most of the time since its initial exposure in 1892. In 2010 Network Rail decided to clear the vegetation along this stretch of railway which offered a rare opportunity for Natural England to work with interested parties to re-open the site.
Conservation work was undertaken last summer which resulted in a television programme about this site and Greenfield Pit in Purfleet (visited by LOUGS in 2005) with Professor Danielle Schreve, explaining the importance of both sites to Tony Robinson. Both locations were included as an episode on Ice in his series The Birth of Britain shown on Channel 4 in January this year. Following the excavation there was pressure from a number of groups and individuals to view the newly-conserved section on the railway line.
I tried to organise a special OU day but that was not possible, instead we were invited, very last minute, to take up places still available on the viewing day organised in March. Gavin Mair, our webmaster, tried to contact as many LOUGS members as possible but in the end I think I was the only one who was able to take up this mid-week offer.
Hornchurch railway cutting was given SSSI status because it is the southern-most limit of the Anglian ice advance that covered most of Britain 450,000 years ago. The till lies directly on top of the London Clay, in a lobe that is thought to be a channel cut into the clay, and is overlain by the earliest of the Lower Thames gravels. The site is therefore key to our understanding of how the glacier blocked the earlier course of the Thames, through the Vale of St. Albans to the coast of East Anglia, and pushed it into its current position.
It is also the type section for the Hornchurch Till, having fewer and smaller clasts than the Lowestoft Till described from East Anglia, although relating to the same period of glaciation.
The site was discovered when the Romford to Upminster branch line was constructed through a ridge of gravel-capped land and was first described by T.V. Holmes in 1893. A section wasn’t opened up again until nearly a century later when Colin Whiteman and David Bridgland began respectively to study till genesis and fluvial history in the area. It obtained SSSI status in 1988.
We were fortunate to have Colin Whiteman on the trip who interpreted the till for us. The stiff grey clay has apparently been intensively deformed with the clasts showing a surprising east-west orientation probably relating to the terminal position of the ice at this point pushing against the tip of the channel it has carved. Colin had previously recorded a number of exotic clasts including Carboniferous limestone and igneous and metamorphic fragments.
Peter Allen explained the overlying Orsett Heath Gravel. This is the earliest of the Thames post-Anglian gravels and equivalent in age to the Black Park Gravel found on Richmond and Wimbledon Commons and other high ground further west. It is dated as Marine Isotope Stage 12/11, deposited as the Anglian glacier retreated some 400,000 years ago. The gravel is predominantly flint, often rounded, and with a small amount of Greensand Chert thought to have come from the Weald as well as a few of the exotics found in the till. The exposure at Greenfield Pit relates to a younger interglacial (MIS 9).
As we examined the section a student was busy collecting samples for further study. The intention is to work with Network Rail to keep the site cleared of heavy vegetation and available for future researchers.
My thanks to Emily Dresner of Natural England for organising both the excavation and the trip and to Network Rail for guiding us onto the site. I am most grateful to Peter Allen and Colin Whiteman for interpreting the site for us.
Further details: Essex Fieldclub.
Behind the scenes at the NHM - AGS Event
20 April 2011
On a Thursday afternoon 12 AGS members met at the Natural History Museum for a "behind the scenes" tour on meteorites. The lead organiser at the NHM was, unfortunately, no longer available but her assistant had arranged for the tour to be split into three sections: meteorites; rocks; and minerals.
Meteorites - Deborah Cassey (assistant Meteorite Curator)
The NHM holds 2000 meteorite specimens and 5000 meteorite fragments, has the best collection worldwide of 'finds' and the only
larger collections of 'falls' are those held by the Smithsonian and NASA.
Deborah also explained the importance of meteorites in understanding Earth science, particularly iron meteorites believed to have formed from planet (or asteroid) cores and stony-irons from core-mantle boundaries, as we cannot examine either of these on our own planet. Deborah then passed round (all in plastic bags or containers — so we didn't contaminate the specimens!) some special pieces — a small sample of the Moon (fine grained breccia of mostly feldspar with a small amount of olivine), a fragment of Mars (mostly olivine but also containing hydrous minerals) the origins of both specimens having been determined by their isotopic composition, and some CAIs (because they would be the oldest "things" we would ever handle). Deborah then handed us over to Dave Smith.
Rocks - Dave Smith (Petrology Curator)
Dave began by explaining the petrology collection was organised by the date the specimens were acquired by the NHM and the collection ran in a time line from the mid 1700s to the 1930s. We first looked at the Montecelli collection containing specimens from the Vesuvius area. In addition to being diverse, the time and location of the collection was well documented by Montecelli, who collected rock specimens and volcanic ash following any eruption of Vesuvius.
We also looked at the collection donated by Sir William Hamilton. Sir William, the British envoy to Naples from 1764 to 1800,
was fascinated by Vesuvius following its 1765 eruption. He monitored volcanic activity, collected samples (mainly dolomites) which
he had cut into bricks and polished, and commissioned watercolours and illustrations of the eruptions.
Minerals - Mike Rumsey (Minerals Curator)
Mike took us through the main minerals gallery informing us that about 14000 mineral specimens (representing 2000 mineral species) are on display from the NHM collection of more than 180 000 minerals specimens. We stopped briefly at the topaz collection and Mike showed us a few of the larger topaz specimens (kept locked up beneath the display cases) explaining these need to be kept in the dark to retain their colour - displaying them would change the colour, of the edges if not the entire crystal, from brown to blue.
We moved into the vault at the end of the minerals gallery where Mike pointed out the rarely seen cubic crystals of the gold Latrobe nugget and the Cora 'sun drop' a 110 carat yellow diamond on loan to the NHM for about six months. Mike then unlocked a door in the vault and we went upstairs to the Russell Room—this room contains a collection of (mostly) British minerals. We stopped to look at several minerals including some of the more unusual fluorites from this collection — before
moving onto some of the latest NHM acquisitions — including a quartz with iron and titanium rutile showing perfect six fold symmetry and a mordenite (found in veins and amygdaloids in igneous rocks).
We ended the mineral tour looking at "rickturnerite" the newest mineral to be officially named.
This was a very enjoyable albeit whirlwind tour — three sections in two hours — and far more interesting information and facts were mentioned and specimens seen than I had time to write down (far less photograph) thanks to our three very knowledgeable and enthusiastic presenters. Kris Palubicki
Lecture : Re-interpreting the Volcanic Evolution of the Isle of Skye - by Simon Drake
21 April 2011
Simon is a PhD research student at Birkbeck College and he talked to us with great enthusiasm about his current research working with David Brown and Andrew Beard.
He gave an outline of his talk together with a photo in the April edition of London Platform, but here he conveyed to us the excitement of doing research and reaching new conclusions. However we all know that in mountains it rains more often than not, so determination is needed as well. After discussing the various ways ignimbrites are produced, and the range of different ignimbrites found on Skye, part of the North Atlantic Igneous Province, he stated his aim of mapping and logging the Cuillin Central Complex, which seems to have evolved through a basic lava flow, early dykes, the formation of the granitic Eastern and Western Red Hills and finally, later dykes.
The deposits are chemically zoned, so the question is whether we are dealing with one magma chamber with reverse tapping or two separate magma chambers. The whole of the stratigraphic column, apart from Lewisian gneiss, is contained in these ignimbrites. The early volcanism predates the gabbroic Cuillins, but the Kilchrist Vent shows fine-textured acid rocks within the Tertiary basaltic agglomerates, comprising flow-layered rhyolite and ignimbrite, suggesting ‘two distinct eruptive waves’. (Ray 1966). Blocks of rhyolite and ignimbrite within the basalt at Dun Caan Raasay suggest large eruptions. A NERC grant will enable accurate dating of the zircons within the ignimbrites to establish the frequency of these events.
He concluded by showing us the spot where his wedding would take place the following week. It only remains to wish them joy.
The Sedgwick Museum and Building Stones Walk, Cambridge - Led by Diana Smith
30 April 2011
Today's visit to the Sedgwick Museum took place in the morning, Saturday 30 April 2011, and was followed by an afternoon building stones walk around Cambridge.
Morning — The Sedgwick Museum (Downing Street, Cambridge CB3 2EQ, www.sedgwickmuseum.org)
The London OUGS were welcomed by Dan Pemberton, The Collections Manager for the Palaentology Collection as well as Mineralogy
and Petrology. He gave a very interesting talk about the history of the museum in the main lecture hall.
Adam Sedgwick FRS (1785-1873) graduated in 1808 with distinction in mathematics. Sedgwick built up a major geological school at Cambridge and inherited a geological museum hardly expanded from John Woodward’s original 17th century collection. By 1841 Sedgwick had persuaded the University to establish a proper geological museum in the Cockerel Building. The museum has moved several times since to accommodate a larger collections.
In the Whewell Gallery a collection of minerals are displayed according to their chemical group along with various manmade artefacts. Displays include a gallery of minerals and gemstones, rocks collected by Charles Darwin on the 'Voyage of the Bea- gle' (1831 – 1836) with his personal field notes, dinosaurs from the Jurassic and Triassic, and fossils from the local area including a hippopotamus from the nearby Burlington gravel pits.
John Watson (1842–1918) presented his collection of more than 300 building-stones, ornamental marbles, and other materials connected with buildings to the Museum of Economic Geology in 1905. In 1908 he started the first catalogue, British and Foreign Building Stones, published in 1911. The second, on British and Foreign Marbles and other Ornamental Stones, was published in 1916. These are still the main catalogues in use, with his original annotations. The John Watson Building Stone Collection occupies what was originally the Museum of Economic Geology situated on the ground floor of the Sedgwick Museum, now the common room. John Watson died on 3 July 1918 after falling from a low ladder whilst trimming a fig tree in his garden.
As can be seen behind the ornamental table, the collection is housed in cabinets. The first half along the left hand side facing the room are British Building Stones, and on the right, Colonial and Foreign. Each specimen is a 4.5” cube, one face left rough, one polished, and another dressed. The labels are also ordered in a particular way: Number, Name, Geological name, Donor.
The collection included slates, marbles, paving slabs, cement and artificial stones, and a thin sections collection. This will
include igneous, metamorphic and sedimentary rocks. At the moment these collections are available only for professionals and
enthusiasts to see, but the museum would like to open it up for general interest.
After thanking Dan, who refused to accept payment for the coffee and biscuits(!), and gave up his morning for us, we went our
separate ways for lunch and reconvened at the Museum entrance for the afternoon’s continuation.
Afternoon — Cambridge City Centre Building Stones Walk
Cambridge town centre on the sunny Bank Holiday after-noon of Saturday 30th April was packed with people. The Royal Newlyweds had just been made Duke and Duchess of Cambridge, which attracted the tourists, plus there was Ye Olde Protest March against council cuts with police escort and the Cambridge University graduates in their robes were progressing through the streets. We joined the throngs for the London Branch building stones walk led by Di Smith.
Di led us to at least 7 colleges, including the oldest, Peterhouse, begun in 1209, which was mostly constructed using local
Jurassic oolitic limestone (for a map of the quarries used see: http://wserv2.esc.cam.ac.uk/teaching/geological-sciences/
We viewed street furniture, shops, houses, museums and churches, so only a few highlights are included, for exam-ple, the Great Cambridge Unconformity at King's College which (because the Wars of the Roses interrupted its construction and emptied the Exchequer) took 80 years to complete. Henry VI commenced construction using expensive Permian magnesian limestone but ended with cheaper local Clipsham and Weldon limestone.
Di explained that building can be grouped into three phases, the first prior to the 1500s, when transport was difficult and only local stone was used. "Clunch", a Cretaceous chalk which weathers rapidly and is a poor building stone, was nearest and was used in the oldest constructions for example round the windows of Little St Mary Church, which also featured glacial cobbles and Ancaster limestone with a characteristic "streaky bacon" yellowish striped look .
By the second stage around the 1460s, Barnack limestone, the best local stone, had already been used up. After the dissolution
of the monasteries by Henry VII, dressed stones from these buildings were reused along with Tudor red bricks, for example in the
construction of Trinity Chapel.
Collyweston fissile micaceous limestone made roofing "slates" during the first and second building phases. The quarried rock was hewn from adits (horizontal short tunnels cut into hillsides) and left outside over the winter because it split more easily after being weathered. Other limestones would also be "green" meaning the rock was soft when first cut and developed a harder crust after being left over winter before being used. Ketton oolitic limestone was used with Collyweston roofing for Christ's College, founded by Margaret Beaufort, the grandmother of Henry VIII, in 1505.
The third building stage came with the railways in 1845, when stones were imported from all over the UK. Ordovician Welsh
slate replaced Colleyweston limestone for roofs. Smooth white Jurassic Portland limestone from Dorset was used to face the
The Grand Arcade shopping centre again featured Jurassic limestone, but from Bavaria with big ammonite and belemnite
fossils in Treutchlinger limestone flooring.
Mapping Mount Caburn in Lewes - Led by Rory Mortimer
7 May 2011
This one-day field trip to the area around Lewes was led by Professor Rory Mortimore. It was a fascinating trip. I’ve heard it said that what Rory doesn’t know about the chalk – could be written on the back of a coccolith (that would be the concave side of course) and by the end of the day we could wholeheartedly verify this.
So, we were in very good hands and, additionally, were provided with a comprehensive field guide to the Mt. Caburn group of
chalk pits that we would be visiting. We learned that this type of mapping is best done in autumn and early spring when the
vegetation is down and the geological clues across the landscape begin to reveal themselves.
Later, from a few spectacular viewpoints, we would be able to see the bigger geological picture and structure of the area and begin to give the past geological events some order.
The day was split into two parts. Unfortunately I was only able to attend the first leg so my thanks to Geoff West who helped
fill in the gaps here for the second set of locations.
We then reached the (Middle and Upper Turonian) Bridgewick Pit (TQ431113). The former workings of this chalk pit are listed as a geological and biological SSSI and are now part of Malling Down Nature Park. The Holywell Nodular Chalk Formation was not exposed on the way to the pit – the quarry floor being the top of the New Pit Chalk Formation with the Bridgewick Marls (this is the type section) and Lewes Marls (part of the Lewes Nodular Chalk) visible above in the cliff face. It is incredible to imagine the environment when this chalk was formed – with the ocean surface 300m above our heads and not a coastline in sight. We spent a good hour or two here (lunch stop included) rummaging around on the scree of chalk blocks that had fallen from various levels.
The chalk was a stunning bright white and yielded many fossil finds, evidence of faults (slickensides) as well as beautiful
nodular horn-flints and Lewes Tubular Flints. The ubiquitous Bridgewick Flints here are a marker bed and type locality – the
same flint horizons as those once mined at the Neolithic Grimes Graves in Norfolk. (If you’ve never been to Grimes Graves – it is
certainly worth a visit).
Moving on after lunch and following the track around the pit we came to New Pit Depot. We weren’t able to access the pit so it was out with the binoculars to view the grey bands of the volcanic ash-bed Bridgewick Marls and the boundary between the New Pit Chalk and the overlying Lewes Nodular Chalk. This is also the type locality for the New Pit Chalk Formation containing marker marls and marker flint beds.
Continuing up the track we came to the Malling Hill Track Pit on our left. White chalk with an absence of flint and slabby as
opposed to nodular meant we were looking at the top end of the New Pit Chalk with its characteristic bentonitic ash marl.
This was where I departed for London as the others took the vehicles over to Week Lane and the Glynde- bourne Pits (1 and 2). Stepping back in time as it were….the first outcrops up the track to the pits revealed the Zig Zag Chalk Formation. The changes in the track slope and heavy rutting by tractor tyres were evidence of the softer, clayey, shaley Plenus Marls (end of the Cenomanian) overlying the Zig Zag Chalk. The track levelled out as we came upon the harder Melbourn Rock.
Glyndebourne Small Pit (No. 2) is smaller than the Bridgewick Pit but of a similar morphology with a scree slope at the base of
the quarry face.
On to Glyndebourne Pit (No. 1) which has two main exposures. Bigger than Glyndebourne Pit 2, it also has a scree slope to be rummaged upon. This pit is excavated into the Holywell and New Pit Formations. Rory was able to point out to the group some sediment filled channels in the rock face and showed us phosphatic chalk that he collected from further up the face. Iain Fletcher found a fossil in the scree that should not have been there so Rory took it off for further investigation!
So ended our foray across a few million years of the Earth’s history. Many thanks to Rory for guiding us through an excellent day of elementary mapping.
Conference review : The Anthropocene: A New Epoch of Geological Time?
11 May 2011
The Anthropocene is the proposed new Epoch, delineating the changes to the planet solely attributable to human impact. The term "Anthropocene" is used today by some scientists and the media and opinions on its usage varied between it being just a new buzzword (replacing sustainability) to being a necessity that could influence global law/policy makers and serve to warn the general public.
The decision of whether, or not, we are now in the Anthropocene is, however, not a social option but an evidentiary one based entirely on the global stratigraphic record that the changes wrought by human impact have left a permanent, irrefutable record. If formally adopted, the next question will be the date the Anthropocene began: when humans first started using fire to clear land? the commencement of agriculture? the burning of fossil fuels? or when industrialisation accelerated?
The themes of this one day (GeolSoc/BGS) conference were: life and its diversity; humans and geology and socio-economic issues.
Following a welcome by Mike Ellis (BGS), Andrew Revkin (Dot Earth Blog) gave a brief introduction to the conference including the
suggestion that humans were analogous to cultured microbes that had reached the edge of their petrie dish and that we too are
running out of agar. He emphasised the impact of visualisation to reinforce warnings and information and, as an example, put up
a slide of 'Global Water and Air Volume Compare to Earth', viewable at:
Dennis Dimick (National Geographic) continued with the visual theme—based on a current population of 7 billion. He said space is not the problem, 7 billion would fit (standing shoulder to shoulder) in the City of Los Angeles — it is the support system humans need that has such a great impact (see: http://ngm.nationalgeographic.com/7-billion for more information/photographs/video. The current rate of population increase is 3 people per second (5 are born and 2 die every second). He explained we would need four planets just to support the current population to the US standard way of life. He also highlighted the disconnect of the electric world — the have and the have nots, the finite nature of resources and that none of us are immune (for example wheat /crop production is already in decline).
Will Steffen (ANU) described the Anthropocene as having distinct stages:
Will referred us to a paper describing and explaining the concept of planetary boundaries (which he has co-authored):
The conference continued with a look at geological evidence. Erle Ellis (Maryland) contends that land use change has already
irreversibly altered the terrestrial biosphere to an extent that differentiates it from the Holocene in the geological record.
Erle said we have so transformed terrestrial biomes that classical biomes (e.g. temperate forest) are no longer applicable and
suggests new anthromes for example wildland (areas too cold and dry to be of any interest to humans) and semi-wild (managed forests).
Further information can be found in a paper co-authored by Erle on "Anthropogenic transformation of the biomes, 1700 to 2000" at
James Syvitski (Colorado) also believes the geological evidence is sufficient to warrant a change in Epoch, from the perspective
of sediment flux. He detailed the impacts humans have had: intervened against gravity; decelerated and accelerated natural
processes; refocused energy; and altered or destroyed ecosystems.
Dorothy Merritts (Franklin & Marshall College) also spoke about sediment flux, particularly the importance of understanding the source of sediment in streams. Her research has shown that anthropogenic influence had a strong impact earlier than originally thought and the number of milldams (for water-powered millstones) has changed the landscape from spring-fed wetlands to land with discrete streams (that have, until recently been considered the "natural" landscape). Breaches in these ancient, disused milldams result in local base level drops leading to stream channel incision, bank erosion and increased sediment loads and the black hydric soils of these wetlands can be seen as a distinct stratigraphic layer at the base of incised stream valleys. (Merritts et. al., 2011, "Anthropocene streams and base-level controls from historic dams in the unglaciated mid-Atlantic region, USA", Phil.Trans.R.Soc.A., 369, 1938, pp 976-1009).
Toby Tyrrell (NOCS) then spoke about human impacts on the ocean (ocean warming, sea level rise, slowing down of circulation, loss of Arctic ice and ocean acidification). The information on circulation was particular interesting: all models predict the slowing down of circulation as decreased density will impede deep water formation, however, there is no evidence of this happening. The need for tidal energy to dissipate and the role of, or interaction with, atmospheric winds have not been modelled. Toby also gave us a detailed explanation about ocean acidification — exactly what it is, evidence of occurrence and effect, again a very interesting, albeit complex, topic.
Davor Vidas (Fridtjot Nansens Institut) then spoke about the Anthropocene and the Law of the Oceans.
Lecture : The Chicxulub Impact Site and the Cretaceous-Palaeogene Mass Extinction - By Joanna Morgan
19 May 2011
Jo, Professor of Geophysics at Imperial College, London, started by filling in the historical background to the debate between proponents of the impact theory as the trigger of the end-Cretaceous mass extinction and those who believe that the environmental effect of the massive volcanism caused by the Deccan Traps was the cause. Or were there multiple causes?
At 65 Ma 60% of all species, particularly the larger ones, became extinct and the evidence currently is that the likely main cause is the impact. The only two known larger impact craters are Vredefort (South Africa) at more than 2 billion years, and Sudbury (North America) about 300 Ma younger.
The evidence cited include the high levels of iridium in the K- Pg boundary clay, a mineral found in extra-terrestrial bodies, and shocked minerals, in particular quartz, with a double layer found only close to the impact site.
The crater was discovered in 1991 as a result of a circular gravity anomaly and a magnetic anomaly. The impact, half onshore and half off-shore, released the energy of one billion Hiroshimas, and gave rise to 3000 km3 of ejecta. All planetary bodies have been regularly hit by asteroids, and Chicxulub is the largest and most recent known and has the only known global impact layer.
One of the major arguments is that particle size decreases away from Chicxulub. Its effects were instantaneous: mountains the
size of the Himalayas were flung up in two minutes; an unusual oil reservoir resulted (more oil than in the entire North Sea).
28 May - 4 June 2011
Saturday 28 May (Arrival)
The aim of the field trip was to study igneous and metamorphic rocks in the Central Iberican Zone, consisting of granites, pegmatites and quartzites, and our centre for the week was Guarda, the highest city in Portugal at 1,056 m, situated on the slopes of the Serra da Estrela.
Setting off southwards from Porto airport pm, pleased at the size and comfort of the coach, we feared the worst for our week when a very violent thunder storm with huge hailstones arose just before we turned inland, however it cleared up as we turned east and climbed to Guarda to see our hotel sticking up from the hillside like a thumb!
We met in the bar before dinner for Lesley to give us a briefing on the week’s programme. We would start on Sunday with a visit to the Geopark at Penha Garcia where there are quartzites dated to the Hercynian Orogeny, the same age as those in Scotland, and would be joined by a geologist from the Geopark. We would see trilobite tracks, and visit three localities, showing metamorphic contacts and metamorphic folding.
As we shall see (Monday), this plan was to be slightly modified.
Sunday 29 May (Day 1)
Penha Garcia - Part 1
Our first day began with a scenic drive through hills formed in the Variscan Orogeny, to the Ichnological park of Penha Garcia in the north-east of the Geopark Naturtejo (a UNESCO European and Global Geopark). The landscape was very reminiscent of Dartmoor.
The main feature at Penha Garcia is a scenic gorge in Ordovician quartzite formed in the last 2 million years. The quartzite was metamorphosed in the Hercynian Orogeny. There are many such quartzite ridges in north eastern Portugal all having a strong NW/SE alignment. This one is formed from Arenig stage sandstones and is famous for its fossils. We followed the ‘Fossil route’ around the gorge, a path well named. The ridge begins here and is thought to be 480 Ma old. Rocks at the southern edge are the same age as appenine quartzite.
Having walked up many steps admiring fossils we arrived at our highest point, a castle built on a smuggling route for tin and tungsten. One of a series of castles it was thought to have been to prevent smuggling, although there was some debate, the Moors were mentioned. The views were extensive and it was good for bird watching – storks and Griffin vultures.
A fault runs along the gorge which starts in Castelo Branco and continues to the geothermal area of Fermontina where water temperatures can reach 29oC. In the Palaeozoic it was strike-slip and in the Cenozoic it was reactivated as a reverse fault. We saw a large area of slickensides on a rock face, the fault plane, just before descending to the valley floor; on the opposite side of the valley there was a drag fold.
At the NE end there is a dam from which a small stream flows. Historically there was enough water to power a water mill and the
small mill house and some of the workings can still be seen. Old grinding stones of granite and limestone could be seen and the
water wheel houses.
After a pleasant walk back and very welcome beer we boarded the bus for our trip to Monsanto, not the chemical works I associate with the name but a granite iselberg. On the way we stopped to look for the contact between the schist bedrock and the granite. This was identified by the presence of the metamorphic minerals cordierite and andalucite. There followed many debates as to which we were looking at at any one time and the conclusion was you really needed a thin section to tell. However it was decided that black indistinct crystals were cordierite and the more elongate obvious crystals were andalusite. Telling the difference was a problem. Our Guide from the Geopark, Margarite, said that when you looked in thin section you saw more andalusite than with the naked eye. Cordierite is a ferromagnesian silicate and andalusite is an aluminium silicate so which you get depends on the chemistry of the original rock.
The aureole is not found consistently being more defined in this area. Could be that the outcropping is poor or Margarite
thought it could be because the granite is tilted. Can find granite contact but not schist.
Monsanto is 758 m high and a granite mound known as an iselberg. This attractive village won the silver cock for being the most Portuguese village in Portugal, a copy of which is proudly displayed as a part of a wind vane. Monsanto was formed by the intrusion of a granitic pluton which was hydrothermally altered deep in the crust by percolating rainwater. Subsequent erosion of the overlying layers exposed the granite which rotted (tertiary). The granite here is leucogranite, evidence of being near the top of the magma chamber. As the boulders of granite are too large to move the houses have been built around them leading to some bizarre sights of huge boulders sticking out of roofs and walls (Figure 1).
Part 2, post-script on the ‘Fossil Route’
OUGS students who have done the Geology Course will be familiar with Cruziana the trace fossil made by trilobites. We were promised a high spot on our first location at Penha Garcia and we certainly got it. The fossil Cruziana trail is Stop 1 on the Geotrail for the Geopark Naturtejo. Immediately trilobite tracks jumped out at us on the steps up to the castle, in the walls and boulders at the side of the trail; once we got our eye in they were everywhere, and they were all different (Figure 2).
Many were multiple tracks that crossed over each other and some blocks had more than one species. There was only one face that we saw that was in situ and that was on the under slab of the sheeted quartzite that was the host rock. Some tracks stood very proud and probably all were proud to some extent. Grooves in soft sediment made by the trilobites were rapidly filled and buried by sand and it was this counterpart that we were seeing. In most cases a distinct V was observable: the animal moved in the opposite direction to the point. Only at one spot were we able to make out the thorax of a trilobite, and fossils of the creatures themselves are apparently extremely rare.
At the bottom of the gorge was a small display and the custodian brought out 4 specimens that I was privileged to see which have subsequently been identified as Neseuretus. Some of the traces were conveniently labelled but some of the identifications seem decidedly suspect. Cruziana rugosa and C. forcifera were certainly represented. The tracks were dated as Ordovician in age, Arenig stage, and the sediments were metamorphosed during the Hercynian Orogeny that was also responsible for the emplacement of the ubiquitous granite that dominated this part of Portugal.
Monday 30 May (Day 2)
The theme for the second day was the contact between the granite and the schist. The granite was late-stage porphyritic monzonite just under 300 Ma, altered by hydrothermal fluids and weathering. The coach took us to the N183 road about 10 km SW of Sabugal.
First stop was at an abandoned roadside quarry where much of the feldspar had been altered to kaolin. Lesley explained that the hydrothermal fluids contained chemicals which might be compatible with different feldspars. So orthoclase might be altered while plagioclase is not, or vice-versa. Feldspar could also be altered to sericite, a fine-grained mica with a greenish appearance. Fluids circulating through joints affected the granite to a depth of 10 cm or more, producing more mafic minerals, biotite rather than muscovite, and making the feldspars pink with iron. Fluids rich in boron could alter mica or sometimes quartz to tourmaline, small examples of which were seen. The horizontal alignment of feldspar crystals suggested that this material was formed close to the top of the pluton. Pegmatite veins and xenoliths of schist were also seen at this exposure.
The second stop was about 1 km down the road at the contact between the granite and the schist. No clear contact line could be seen in the exposure at the road cutting: rather a zone a couple of metres wide separating the severely weathered granite and the schist (Figure 3).
The original Ordovician mudstones had been metamorphosed and folded before the intrusion of the granite, which would have been emplaced passively and without a great temperature difference, as no hornfels was seen. Granite veins could be seen extending into the schist in places. The mudstones must have been rich in iron to produce the strong colouration of the schist with cordierite rather than andalusite being present.
A short walk down the road brought us to a side track leading the an abandoned mine entrance. This had been a 19th Century adit mine into the hill behind, looking for iron minerals especially in pegmatite veins. Tin and tungsten minerals had been extracted as well as haematite, but not in substantial or commercial quantities.
Lunch was taken beside a reservoir dam near Meimoa, a pleasant spot in the warm but occasionally cloudy weather. Behind the picnic place was a large cliff of schist which had been cleared for the road replacing the one up the valley submerged by the lake. The schist here was sufficiently far from the granite to have suffered no further contact metamorphism during the emplacement. This was Ordovician/Silurian (it’s impossible to be more precise without dateable fossils) sediment metamorphosed during the Hercynian Orogeny. Although referred to as schist, it was not as highly metamorphosed as this suggests and appeared to be a phyllite with wavy mica-rich surfaces. Lesley called it a meta-pelite, or metamorphosed pelitic rock. Original clay beds were more highly altered than the sandstones, and it was possible to identify cross laminations in some of the sandstones (Figure 4), and to see that the cleavage direction was almost the same as the bedding, running practically vertical.
The coach which was supposed to pick us up from this site after lunch was severely delayed by a medical emergency for the driver. We didn’t know exactly what had happened because of the language and poor mobile phone reception, and we spent the afternoon studying a herd of goats and a couple of elderly Portuguese peasants. It’s a very slow pace of life in rural Portugal! Eventually Julio, our driver, returned none the worse from his trip to hospital. We had, however, been forced to miss the planned afternoon visit to Sortelha and had to return directly to Guarda as it was then so late. Lesley was able to re-schedule Sortelha for the Friday afternoon so nothing was missed as a consequence.
Tuesday 31 May (Day 3)
Site 1—Lajeosa do Mondego
A cloudy day saw us visit a recently active quarry in porphyritic peraluminous ‘S’ type granite near the Mondego River. The post Variscan granites in this area are grouped into a late, intermediate and early series: this site being intermediate; about 300 Ma old. The initial appearance was one of a uniform coarse grained granite, rich in biotite with rounded quartz grains, long twinned orthoclase crystals, smaller plagioclase grains, a paucity of muscovite together with large dark finely granular xenoliths.
However further inspection revealed:-
There has been hydrothermal fluid change here and the site is one of metasomatism rather than metamorphism.
At the end of the 19th Century local people noticed exteriorised veins of quartz in this beautiful wooded valley. These were found to contain ores of tin and tungsten and the veins were followed from adits. Regrettably insufficient ore was found and the scheme was abandoned.
Site 3—Seixo Amarelo-Gonzalo
Unfortunately our access to this enormous quarry which has yielded rare earth elements was restricted but large representative fallen blocks were examined. The granite here is mainly porphyritic and biotitic though a number of different types are found of the late and intermediate series. Lithium is found not only in the ground mass and veins but also, and notably, in the pegmatites which are of a beautiful purple colour. The lepidolite, (a lithium mica), of the veins has been potassium-argon dated at 270-277 Ma and the pegmatites contain up to 230 ppm of lithium. Ores of many metals including tin and manganese are found here together with minerals such as beryl, and the granite is mined for aggregate.
Wednesday 1 June (Day 4)
On another fine morning, we set off south from Guarda along the A23 motorway, turning off at the Corvilhã South exit to the West.
As Lesley explained en route, the principal mine there was a tungsten and tin mine which had bee worked by a British Company, Beralt Tin & Wolfram, named after the old province of Beira Alta. The mine had been a significant source of the strategically important metal tungsten. This had been sold by the company to both sides during the two major conflicts of the 20th Century, and occasionally the same tungsten had been sold simultaneously to both sides.
The mine workings of Panasqueira had exploited the granite intrusions which had formed the Serra da Estrela, but were not sunk in the granite itself but in the neighbouring schists, exploiting quartz veins protruding from the granite into the schist. During the past few decades the mine had only been occasionally worked actively, as its viability depended on a high price of tungsten, and for most of the time the mine had only been operated on a care and maintenance basis.
Nevertheless the Portuguese Government clearly attached importance to the area, as the road to the mine was being improved, the old cobbled way with a low speed limit being now covered in tarmac. Further evidence of this became apparent when, after climbing into the hills through pine woods formerly exploited for resin, and passing relics of terracing with mounds formed by adits for old workings, we paused on a corner on one side of a valley. Mine buildings faced us on the opposite side. Below these, mine spoil (rich in copper and arsenic) spread down the valley wall. On a terrace below us, sappers of the Portuguese army were dumping and bulldozing boulders, supplied by vehicles which drove past us. This was in order to provide a supporting embankment for a new road which would be straighter than the one we were on.
We arrived at Barroca Grande, a settlement dominated by the mine entrance and its processing area (Figure 5).
Beyond it, up the hill lay terraced streets of miners' housing. Lesley told us that much of the housing was now redundant. Despite the industrial location, an attempt had been made some years ago to exploit the panoramic views into the distance, by selling off the housing as holiday homes. This attempt had foundered on the fact that the houses lacked mains water and electricity, which had previously been supplied by the mining company.
We passed a massive lagoon. Travelling on, we arrived at Panasqueira. There we paused for refreshment and a loo stop at the restored Miners' Social Institute, which was located near the original mine adit. We met the President of the Institute, who spoke good English, and talked to us about its history, and the efforts being made by the Portuguese Government to improve the communications of the area. He explained that the army sappers were engaged in the roadworks because, given the dire state of the economy, this was the cheapest way of getting the work done.
In the vicinity buildings bore the letters BTW, standing for Beiralt Tin and Wolfram, the name of the original Company,
which no longer existed. Opposite, extensive reconstruction works were being carried out to a building. A doorway was being
greatly enlarged to provide an entrance way for a motor truck.
Lesley said that the profitability of the mine was governed by the grade of the ore and the prices of tungsten and tin. Typically about 1 tonne of tungsten was recovered from 270 tonnes of waste. This mine was actually quite a good source of tungsten, but only when the price was high.
One or two samples contained crystals of molybdenite, pyrite and arseno-pyrite, and there was also a large quantity of haematite. One crystal might have been cassiterite, as it showed the right colour for it, but the habit seemed to be wrong. Lesley explained that the presence of particular minerals reflected the temperature zoning of hydrothermal activity. Wolframite and cassiterite were associated with high temperatures, and haematite with the lowest temperatures.
We then headed back toward Barroca Grande, but turned up a side road to a wooded area for our lunch stop. This location enjoyed a panoramic view of the mines and the Serra da Estrela. The wooded area contained a monument to Christ the Worker, a tall tower with helmeted miners reaching up to the Christ figure at the top (Figure 6).
This style of monument reflected social Catholicism, and the ideology of the former Estado Novo, or New State of the era, when Portugal had been under the leadership of Dr Salazar. The monument had an inscription at its base dating to 1967, and a tablet recorded its having been visited in October 1971 by Admiral Tomaz, the last President of the Republic before the 1973 Revolution.
The area had obviously been a place of public recreation and entertainment, with picnic tables, kiosks and a band stand. It also contained old mining trucks, transport carriages and locomotives, together with a strange mobile boiler which looked rather like Stephenson's Rocket.
After lunch we had a half-hour stop back at Barroca Grande, and looked down from the road at the processing area of the mine, with its conveyer belt that passed beneath us under the road into the mine, and the washing crushing and grading plant. Lesley said that the equipment was kept on a maintenance basis, to await a possible future rise in the price of tungsten.
We then proceeded uphill to the mine offices. These were situated in a building which was only partly occupied, due to the mine being kept only on a care and maintenance basis. In a sort of safe-room off one of the corridors, there was a minerals shop, an Aladdin's cave of minerals from the area. These included specimens of wolframite, a heavy grey-black mineral, showing tabular prisms with needle striations. A number of us yielded to temptation, and bought specimens. The corridors outside contained a number of geological maps of various Portuguese territories. At the end of one of the corridors, there was a three-dimensional plan, or model, of the workings at Panasqueira. However, before we could give this the attention it deserved, our visit to the establishment was cut short by a power cut, which caused the sudden evacuation of the building.
We then travelled back in the coach to Guarda, stopping on the way at the pleasant small town of Casegas to stretch our legs, and pass time in a café.
Thursday 2 June (Day 5)
This time we travelled slightly northwest along the A25, to Chas de Taveres. We had splendid views of the river and terracing, as we climbed through a granite landscape. We were making for a quarry at Chas de Tevere. We left the coach beside a huge Felmica quarry, but our destination was a smaller quarry Corvaceira on the opposite side of the road (Figure 7). We needed hard hats as one end had recently been reopened for road-building.
We were looking for garnets (Figure 8) and uranium minerals; the former we found in abundance, the latter, pale apple green
crystals of autunite, were found only once. The quarry is in a monazite granite, close to the tonalite which makes up the Guarda
Back into the coach we drove up to Bom Successo, and then climbed up the pilgrimage route to the viewpoint for the panorama (Figure 9). We looked south to the Serra da Estrela, but also north with clearly marked quartzite ridges.
On to Mangualde for a lunch break, and here, waiting for the coach to return, Lesley was contacted by Alexandra Carolino, who to our great delight was to be our guide for the afternoon to the family pegmatite mine. We were taken to her house in the town, where they are developing a small museum and we had a brief history of the mine where there are pegmatite formations, with quartz crystals 1 m high, plus beryllium and uranium minerals.
Back in the coach we drove to the mine, Pegmatitica – Sociedade Mineira de Pegmatites Lda (Mesquitela), examined the piles of beryllium crystals and spoil heaps outside, and then the majority went down a steep spiral staircase to the domed cavern, with contorted bands of biotite mica, beryllium crystals and lithiophyllite. The Mangualde pegmatite is celebrated for its rare minerals, rather than gemstones, but one of us found a perfect crystal of aquamarine lying on the path.
When the last person had emerged, we crossed the road and climbed through pine trees to a lovely old house (photo) where Alexandra’s mother received us, and welcomed us into the house and gardens to see the crystals, which were displayed in the rooms, in the vestibule and among the trees. A delightful end to a fascinating day. Many thanks to the Carolino family from all of us.
Friday 3 June (Day 6)
Friday 3rd June dawned bright and clear. Leaving the hotel we set off to the A23 southbound, heading through the suburbs, passing industrial estates and smallholdings with their neat rows of vegetables, vines and olive trees among the lavender and broom scrub beside the motorway. This gently graded route descends the upper reaches of the broad Zezere Valley drained by its many small south-flowing tributaries, whilst to the west could be clearly seen the steep granite escarpment of the Serra da Estrela, our destination. Our route took us near the town of Belmonte with its castle standing on granites high above intensively farmed smallholdings with fields bordered with vines on the Beira schists.
We continued west towards the glaciated landscape of Serra da Estrela up the ever narrowing, well-rounded boulder strewn river valley towards the source of the Rio Zezere, beside small alluvial rich fields supporting vines, with a few arable, vegetable and hay fields near the small farmhouses.
An artificial ski slope has been built on slopes adjoining a campsite near a granite built village nestling below heavily pine forested slopes; the trees previously utilised for resin collection. A number of small hotels signposted in villages and Manteigas, evidence diversification into tourism in the area and roads have been straightened and widened in places to cope with increased traffic.
Our first stop was at a roadside cutting where an igneous intrusion approximately 1 metre wide, with slightly chilled margins and small inclusions of country rock was observed in the schist (Figure 10). This early formed dyke was folded and showed evidence of later weathering.
Investigation of the rock beside the road revealed pelites and psammites. Iron stained phyllites and quartzites were found, the exact composition dependent on the original sediment, but generally with minimal mineral alteration or banding. The wavy foliation and mica sheen of the phyllite and minor areas of folding within the quartzite characteristic of low grade metamorphism were seen.
Our journey continued upwards through Manteigas, an expanding ski resort with hotels, forest trails, thermal springs, waterfalls and a trout farm before a sharp turn led to the steep, boulder strewn sides of the ‘U’ shaped glacial trough above the town and the route to our destination, the precipitous sides of which were pinned and netted.
The Serra is the remains of a granitic intrusion emplaced during the Hercynian Orogeny about 300 mya at depths of c4-5 km in Upper Cambrian sediments which now form the Beira Schists, altered hydrothermically at temperatures of 700°C at the time of emplacement and then subsequently by circulating fluids at lower temperatures during cooling over millions of years, the granite now forms a fault-bounded horst structure surrounded by schists. The erosion of many kms of overlying country rock and granite, plus glaciation 400,000 years ago, has radically altered the highest region of Portugal (Figures 12 to 14).
Granite boulder fields surround the fortified village of Sortelha. These were exposed following the erosion of c4-5 km of much altered overlying granite and country rock and form huge tors and boulders over very wide areas formed in the same way as those seen earlier in the week at Monsanto (Figures 15 to 19).
Saturday 4 June (Departure)
9 am saw us once more on the coach heading down from the mountains back to Porto, sad to leave this lovely wild landscape but much better informed about the landforms we were passing through.
Most left at midday but a remnant spent a hot and sunny afternoon in Porto admiring, its situation on the gorge of the Douro, the bridges, including one by Eiffel, the splendid architecture with decorative tiling, before catching the evening flight back.
Thanks to Lesley for a really well-led trip.
Lecture : The Recent Discovery of a Large Fossil Tree from the Jurassic of Southern England - By Paul Kenrick
16 June 2011
Paul, Head of Research in the Palaeontology department of the Natural History Museum, working with eight academics and PhD students, gave a fascinating account of the discovery and work in progress on a silicified tree discovered and dug out by John Needham of Tisbury, Wiltshire in 2008 and donated in 2009. It comes from Chicksgrove quarry, near Chilmark and is at the same stratigraphic level as Purbeck limestone.
The tree is petrified and fractured into blocks, with even the knots on the surface visible: it comes from a completely extinct group of conifers, not conforming to the normal monopodal pattern. The stump of the tree is still in the quarry, but the excavated sections measure about 11 m and weigh several tons.
It dates from about 152 Ma, a time when Pangaea was starting to rift, and Britain was about 37º north of the Equator enjoying a warmer climate with higher levels of CO2. A saline lagoon covered most of southern England, with forest of conifers, ginkos, ferns, and cycads all around.
A slice cut through shows the wood to be fractured and crushed, the sapwood and bark are absent. The cones were about 12 mm diameter. Boring in the root and thin sections show well-preserved wood cells. Stromatolites growing round the base of the tree point to a hypersaline lagoon as the likely cause of death.
One important factor in the discovery of this new site is that research can be carried out into the structure of the forest, the spacing of the trees (about every 5 m), by perhaps mapping the surrounding fields.
North Bucks - with Jill Eyers
18 June 2011
We were a mixed group from London and Walton Hall Branches and representatives of Bucks Earth Heritage Group who met in the car park of Thornborough Bridge picnic site to join the trip led by Jill Eyers to Coombs Quarry and Buckingham Sand Pit, both sites conserved by the BEHG. Despite a sudden heavy downpour, we were soon able to set off. Jill led us first into a nearby field where two large mounds had for a long time in the past been thought to be glacial in origin. It was not until 1939 when the Duke of Buckingham organized their excavation that they were discovered to be the burial mounds of high status Britons during the Roman period. The valuable finds can now be seen in the Old Gaol Museum in Buckingham.
En route to Coombs Quarry, we first stopped at the picturesque medieval Thornborough Bridge, which is still in remarkably good condition, despite its long use. It was built from the local Blisworth limestone and some of the blocks show the cross-stratification of current movement. Unfortunately, repairs to the bridge have been made using blocks of Portland limestone which weathers to a very prominent lighter shade.
Following the river, we stopped in a nearby field in which it was still possible to see undulations originating from the
medieval ridge and furrow system of farming using oxen, and on the wider scale the ancient landscape of larger fields and
Following a brief explanation of the sequences visible, we investigated the site for clues in the form of fossils and rock types to determine the ancient environment at the time of deposition of these sediments. The approximately 12 m of Jurassic Blisworth limestone at the base of the quarry is divided into the Ardley Member, containing corals, and the Bladon Member, containing gastropods and bivalves – all indicating a shallow sea environment (similar to the Florida Keys today). This is overlain by 5 or 6 m of Blisworth Clay – thin alternating beds of limestone and clay - which contains only a few rootlets. A dinosaur, theropod, footprint has been found in this clay at a different location, which shows the area at this time was occasionally above sea level and close to land.
A fault through the quarry has brought the Cornbrash limestone down on top of the Blisworth at the far side of the quarry – interestingly making this one of the few sites where Cornbrash can be seen inland. The Cornbrash contains many, mostly broken, specimens of bivalves and brachiopods, indicating a return to marine conditions. William Smith noted different ammonites in the Upper and Lower Cornbrash. Here the Upper Cornbrash is missing as it is overlain by the Kellaways Formation.
Following a final discussion of the various finds and interpretation of the site, we returned to the Thornborough Bridge and our picnic lunch.
After a picnic lunch at Thornborough Bridge we drove round to the Buckingham Sand Pit where sand and gravel had been extracted during the 19th century with work ceasing in the 1920s. The sky was darkening and it looked as if we would, once more, be drenched. It seems that whenever I’m the appointed note taker it rains which usually makes deciphering my notes a bit difficult.
Jill was disappointed that the site had not been kept clear of vegetation but we were still able to see the sides of the quarry, the important bits.
The sediment examined from the western platform can best be described as a mess. It consists of sandy clay containing large pebbles of chalk, flint and quartzite with no sorting. This is a till deposited by the Anglian Ice Sheet some half a million years ago. Jill pointed out a large block of sand obscured by vegetation which had been plucked up by the ice from the frozen ground beneath it. Di Clements also found a small chunk of laminated clay which had survived, as well as a crushed brachiopod and belemnite. The ice sheet spread out from Scandinavia bringing with it igneous rocks including quartzite. Along the way it plucked up chalk and flints from the North Sea. It made land in Norfolk where it deposited some of its material before continuing towards where we were standing in Buckinghamshire. Along the way it also picked up ironstone from Northampton and some of the local Blisworth limestone.
We then descended onto the quarry floor fighting our way through the undergrowth before climbing up to the next exposure on the north side of the quarry. The sediments here were far “busier” than the previous ones. On one side there were well sorted sands and fine gravels with some stratification, probably a braided river bed. On the other side lying above a bed of well sorted sand was a jumble of rounded boulders and large pebbles fining upwards. This points towards a glaciofluvial environment with periods of extremely high energy. In fact what we were looking at was a meltwater channel at the base of a retreating glacier which had flowed sufficiently strongly during the warmer summer months to move these large boulders. As the winter freeze set in the current waned and the boulders settled, eventually choking up the channel.
All this evidence suggests we were looking at the remains of an esker about 20 m wide which had been flowing in a northerly direction. We were told that this is the only esker known outside Norfolk and Scotland. Last year those of us who went on the Norfolk trip had the opportunity to stand on top of the impressive Blakeney esker.
Evening walk : City of London Cemetery, Wanstead Flats - Led by Wendy Kirk and David Cook
21 July 2011
On a miserable, damp summer evening, we livened things up by visiting a cemetery! Our leaders were David Cook and Dr Wendy Kirk of the Aldersbrook Geological Society. We were accompanied throughout by Gary Burks, Cemetery Superintendent and Registrar, a mine of fascinating detail.
The cemetery, at 200 acres one of the largest in Europe, is at Manor Park by Wanstead Flats in Epping Forest. It was opened in 1856 by the City of London Corporation, by whom it is still owned and managed, after three outbreaks of cholera in the city. The designer was Colonel William Haywood, the City Surveyor, who reported that the graveyards of 88 City churches were overcrowded, leading to their closure in 1850. He was also involved in sewerage work with Sir Joseph Bazalgette (whom some of us remember from a recent lecture) after the ‘great stink’ of 1858, and was the architect of Holborn Viaduct. An article by him in the first issue of The Geologist, led to the founding of the G.A. He was also the designer of the first underground toilets, into which you put a penny! His mausoleum (1889) is one of several listed structures within the cemetery.
Others that we saw are the entrance buildings, the Dissenters’ Chapel, the Church of England Chapel and the Columbarium, constructed by draining a fishpond. Our tour started out at the mausoleum, built of Portland stone and Kentish rag, renovated in 1909, with fine gates. We had a general introduction to the types of stone used in the memorials, the older ones familiar, such as granites from Scotland or Ireland and marble from Carrara. Newer gravestones used materials from all over the world such as larvikite from Norway, labradorite from Canada or Norway and exotic metamorphic rocks from the Himalayas.
The layout was designed for train transportation, but the branch line and station were never built. There isn’t room to mention everything we saw, but a few highlights are worth mentioning. A large tomb of different grey and pink granites with large phenocrysts (called ‘Heathens’ by the quarrymen) and marble showed the effect of weathering on the marble. Lead lettering is fixed to the marble with pegs; the marble weathers at a rate of 1 mm per 100 years, so the lettering sits proud. We saw the effects of the orientation of the graves in sandstones and marble on weathering.
Near the Church, the monuments are chipped because of bomb damage in WWII, when the church window was destroyed, since replaced with Caen stone.
Within the cemetery are memorials to 38 different City churches, whose interments were moved out of the City and re-interred here; those of St Andrews, for instance, originally when Holborn Viaduct was built; but as late as 2002 the lead-lined coffins from the crypt were transferred here.
A fine Memorial Chapel and a beautiful sunken garden, recently restored to its original form, cater for cremation, which started in the 1880s and by 1968 had overtaken interment.
I’ve only given a small taste of the hugely interesting expedition. Many thanks to our leaders and to Gary. For those who couldn’t manage to come, there are guided walks arranged by the Cemetery: details on their website. A pdf of David and Wendy’s booklet "A Geological Walk in the City of London Cemetery" can be found on the webpage of the Aldersbrook Geological Society.
(Photographs © Gavin Mair)
Crags of Bawdsey Peninsula - With Roger Dixon
23 July 2011
Bawdsey Peninsula is located south of Orford Ness National Nature Reserve and to the north of Felixstowe (and between the River Deben and the Alde-Ore estuary). Greater Gabbard wind farm can be seen on the sandbanks being constructed 23 kilometres off the coast of Suffolk. Woodbridge and Ipswich are the nearest towns.
A few years ago a huge quantity of Norwegian granite boulders were brought over to shore up the crumbling cliff by the Martello Tower. All the cliffs have been badly eroded over the past decade and, if continued, would affect the nearby villages of Bawdsey, Hollesley and Alderton. Locally-owned farmland (donated) was sold off to developers for housing to help pay for new sea defences and control the movement of the shingle coast between Aldeburgh and Felixstowe.
While the top part of a cliff is weakened by the weather the sea affects the base of the cliff forming a wave-cut notch. The cliff eventually collapses and the backwash carries the rubble towards the sea leaving a wave-cut platform. As this process is repeated the cliff retreats.
The erosion has worn away the soft crag cliffs at Bawdsey (Figure 1) exposing dark underlying London clay. Above the London Clay (late Harwich formation) is sand and shingle.
Red Crag has a rusty brown colour as it generally stained red by ferruginous compounds. It was deposited about 2 to 3 million years ago and contains many fossil shells, including types familiar today such as cockles, oysters and whelks. At the north end of Bawdsey many of these can be found including molluscs, shart teeth, fish remains, shells. Pyritised wood can also be found. Caroline Markham found a large tooth of a Carcharadon shar (Fig. 2)
All pits visited showed ripples in their strata. The height of the set tells us the depth of the water and, from the size of the grain, the strength of the current. These can be measured as small, medium or large.
Alderton House Pit has an 'old' face and a 'new' face. The first face contains Red Crag (rusty coloured from iron) with a shelly band. Some of the fossils found were thought to live in brackish conditions indicating that there was land nearby. The second face is very similar in structure to the first face. Clams and cockles are very common in both faces (Figure 3). In this pit there is evidence of a larger ripple (cross bedding) with mud drapes and foresets which occur in several shallow-marine sand formations attributed to tidal sand waves.
During lunch we looked at All Saints Church, Ramsholt which was built from a wide variety of building stone including boxstones, London Clay septaria and a variety of metamorphic rocks. It is described as being “One of only two round towered churches that have buttresses, the tower is rather more oval than round”.
The third pit we visited is known as Tarrant's Wood Pit which shows small/medium ripples (Figure 4). As well as shells found here there are phosphatic nodules. The bivalve Glycymeris modesta was found here convex up. This pit is in a trough zone and shows an accumulated sand wave. The ripples show a cross-cutting, typical SW and inter-tidal effect.
Sutton Knoll (also known as Rockhall Wood Geological SSSI) shows an aspect of the Neogene Crags. Coralline Crag (about 3.75 Ma in age) forms an upstanding hill, while the later Red Crag (about 2.5 Ma in age) is found around the sides of the inlier (Figure 5). A geological deposit unique to East Anglia and of Pliocene age, its myriad fossil shells include some of the direct ancestors of our living fauna as well as species that are still living.
We were taken round Sutton Knoll and shown two pits:
GeoSuffolk purchased some land and planted a 'Pliocene Forest' using a grant from the Geologists’ Association Curry Fund. The ‘Pliocene Forest’ is an interpretation project using living relatives of our extinct flora. Four trees representative of genera identified from pollen in Pliocene deposits at nearby Orfordness have been planted. These trees are the conifer, Sciadopitys verticillata (Sciadopitysaceae) [also known as the Japanese umbrella pine], Tsuga heterophylla and Tsuga canadensis (Pinaceae) and Liquidamber styraciflua (Hammamelidaceae) [or Sweet Gum]. More trees of this genus have been planted such as the Sciadopytes (Japanese Umbrella Pine) and the Sequoia (Redwood).
Di Clements had previously sponsored one of the trees in the Pliocene Forest and one or two of our members got out their cheque books to sponsor others on this occasion.
Thanks to Roger Dixon and GeoSuffolk for a most interesting and informative day.
GA Conference : Geoconservation for Science and Society : An agenda for the 21st Century
9-10 September 2011 - Worcester University
The GA conference on Geoconservation was an eyeopener. Conservation is not just about auditing and maintaining sites, it is also very much about involving the local community, geological and otherwise, to understand and appreciate why the geology is important and involving them with the upkeep and interpretation of sites. In addition it is necessary to get local politicians involved and to provide a balanced approach. Some excellent examples of good practice were shown to us, most notably from Lewes in East Sussex (Rory Mortimore) where the local politician became an important link, and in particular from Dudley (Graham Worton) where whole communities were involved with a variety of projects. Both areas are on the edge (or in the case of the Wren’s Nest) in the middle of urban communities where the challenges of vandalism are that much greater. On the day of talks Graham told us about their Ripples through Time project and how local kids had staged a presentation about the Seven Sisters (the name of the local caverns). On the field trip he explained how local initiatives are being converted into action. Sadly for us we arrived a week or two too early to see some of the innovative ideas installed but we learnt how the conflicting needs of accessible geology, public safety, bat roosts and education were being met at this very famous, fossiliferous, Silurian reef.
The other location visited on the field trip was equally inspiring. A quarry in Ordovician Quartzite in the Lickey Hills. Herefordshire and Worcestershire Earth Heritage Trust attracted large grants which have enabled them to train local Champions to look after the section and to guide visitors around the site. We were very ably led by one of them, Julia, who told us how the Fire Brigade became involved: they used the face as a training exercise and in the process cleaned off most of the dust and debris from the extensive clean-up of the site. Incidentally, it was good to learn that the current Chairman of the HWEHT is Sue Hay, a former LOUGS committee member.
As well as talks and field trips there were a large number of posters and exhibits from researchers and geoconservation groups up and down the country. Many of the interpretation guides (walks, booklets, posters, CDs, Apps) were inspirational and it was a bonus to find David Williams and Dee Edwards there (both of whom formerly worked for the OU at Walton Hall) displaying casts of some of the Silurian fossils from the Dudley Wren’s Nest (more details are on the GA website - www.geologistsassociation.org.uk/2011_Geoconservation_conferences.html
Formal geoconservation in London is relatively new. In 2009 the Greater London Authority (GLA) published London’s Foundations which launched the London Geodiversity Partnership (LGP) – a group of interested parties including local authority representatives, local geological societies (including the London Branch of OUGS), Natural England, English Heritage, the Environment Agency, GLA and several museums and university departments. The local geological societies include South London RIGS and Harrow Geological Society, both of which had been involved with geoconservation before the existence of the Partnership. Laurie Baker is the rep for LOUGS and I am the Geologists’ Association rep.
The London Geodiversity Partnership is still in its infancy. In the 2009 edition of London’s Foundations 36 geological sites were audited, including the 9 SSSIs (Sites of Special Scientific Interest) within the Greater London area and the Partnership is now in the process of obtaining formal recognition from the local authorities of their importance as Regionally or Locally Important Geological Sites (RIGS and LIGS). Here Laurie is invaluable as he has professional experience of the workings of local planning authorities. During the past year we have audited a further 21 sites for inclusion in the 2012 updated version of London’s Foundations.
The conservation aspect has proved more problematical so far as there is very little spare money around in local authority or Natural England coffers and the demise of the Aggregates Levy Fund as a source of funding for geoconservation projects has hit all geoconservation groups. The priorities identified by the LGP are Gilbert’s Pit Charlton and Harefield Pit, both SSSIs and visited within the last 10 years by London Branch.
The take-home lesson from the conference is the way to attract funding (such as Heritage Lottery Fund) is to involve the wider community. This has the wider advantages of communicating the geology and providing a sustainable maintenance programme for the sites targeted.
There are very few working quarries left in the Greater London area. I visited the remaining three in northeast London during the summer and there are some in the Colne Valley still to be audited but all of them are working the Thames Gravels so if we wish to preserve some of the solid geology, we (as a group and as individuals) need to help protect existing sites. By so doing we learn firsthand about our local geology as well as helping to conserve it for future generations. As a London-based group, we hope that this is something LOUGS might wish to get involved with in due course. In the meantime keep an eye on the LGP fledgling website: www.londongeopartnership.org.uk where we will publish our initiatives which at present include a Thames Path Geotrail and the new Green Chain Walk Geotrail which will be launched on 17th March (see advertisement on this page).
Lecture : How the Solar System was born - by Sarah Russell
15 September 2011
Dr Sara Russell, Head of Meteorics and Cosmic Mineralogy at the Natural History Museum, came armed with wonderful examples of different meteorites so that we could examine for ourselves these strange objects as old as the Solar System itself, and containing inclusions from even earlier.
She began by giving us a brief history of the different theories, starting with Kant and Laplace in the 18th century, when it was already appreciated that the planets have much more angular momentum than the Sun, and any valid theory had to account for this. By the end of the 20th century the nebular theory, propounded in the 18th century and then discarded, was generally accepted.
Today we can learn more by studying meteorites, the building blocks of the planets, by theoretical modelling and by observing the formation of stars in stellar nurseries such as the Orion nebula. We are helped by recent developments of chronometers measuring isotopes, extinct now, but present in the early solar system. Nevertheless, there are still many questions about the process by which a cloud of dust and gas produces pebbles, then planetesimals and then planets, the biggest step being the first. There is also the ‘metre’ problem; over a kilometre, theoretical models show runaway growth.
We then looked at images of meteorites with chondrules and CAIs (Calcium Aluminium Inclusions) in a matrix of chondrule fragments, metal fragments, organic materials and pre-solar grains. We moved on to the models of the formation of the Moon, with the Impact theory now generally accepted, with core formation now dated to between 4570 and 4510 my, thence to the Outer Planets, and finally to the observational constraints of the hunt for Exoplanets.
This stimulating talk covered a lot of ground, or rather space and time (!), and gave us a lot to think about.
Autumn Geowalk around Virginia Water - Led by Brian Harvey
25 September 2011
On a warm, sunny, early autumn day, Brian Harvey led a LOUGS walk around Virginia Water, which is – as we discovered – adjacent to Windsor Great Park and part of the lovely rolling landscape of Surrey west of Heathrow. Starting at the Valley Gardens Car Park, where there were restrooms and guide pamphlets available, we began by walking down the long Canadian Avenue to the Totem Pole. It was difficult to orient oneself with the sun as the map in the pamphlet is printed upside down without showing north.
Along the Avenue were stands of birch to the left, and on the right pine – possibly the “giant redwoods” (Sequoia sempervirens) mentioned in the pamphlet but nowhere near the size to be seen on the West Coast. Then we came to some small settling ponds surrounded by low bunds, where fluid effluent from the buildings drained through reed beds, thus purifying the water. Discussion entailed how much water could be processed this way, and Brian put paid to the idea of the whole of London processing its effluent through Thames reed beds.
The Canadian Avenue led to a 100-foot tall Totem Pole made from a 600-year old western redcedar (Thuja plicata, British Columbia’s official tree) and erected near the shore of Virginia’s waters. A gift to the Queen by the people of British Columbia, it was carved by Chief Mungo Martin and erected in 1958; in 1985, Mungo Martin’s grandson, Richard Hunt, came to repaint it, but sadly it needs doing again.
Near the totem pole is a small dam and bridge made of sarsen stones, our first look at the geology. Sarsens, Brian tells us, are silcretes, which can form in semi-arid climates where evaporation is nearly equal to rainfall. High precipitation rates leach out all chemicals from sandstone during the rainy months, dissolving silica in the process; in the dry months, silica-saturated groundwater is drawn back to the surface, infusing the sandstone and forming a silica matrix cementing the sand grains together. This process affected the sandstones of south-western Britain in the Palaeogene and is now occurring in Australia northeast of Perth and West Africa.
The sarsens at Virginia Water are probably locally sourced, continuing a long history of sarsen use in the southwest such as at Stonehenge, Avebury and Wayland’s Smithy long barrow (see sarsen in situ at White Hill or scattered on Marlborough Downs, whence came the Stonehenge sarsens). It is known that sarsen stones were cut around Camberley in recent times. In the ground, they were slightly damp and could be cut with a saw; but when exposed to the air, they quickly hardened. Australian sarsens are said to require a hydraulic rock-breaker. Australian experience also says plant roots cannot penetrate a silcrete hardpan, though Brian pointed out to us small holes in the dam sarsens that are thought to be trace fossils of roots.
At Knaphill near Woking, sarsens are known to rest on Bagshot Beds, which belong to the Bagshot Formation within the Bracklesham Group, the most recent Group of the Palaeogene (54-44 Ma). We were able to see some Bagshot sands opposite the Virginia Water Car Park on the small lane leading down to the water. There, shore erosion had exposed the pebbly alluvium overlying yellow sand streaked with black staining. Several hand lenses were produced to see if the black bits were glauconite – without resolution. It seemed there were some water-escape structures, causing deformation of the fairly horizontal linear black staining to erupt towards the surface.
The next feature we encountered walking clockwise around the lake on the southern shore was The Cascade. This is an artificial cascade constructed to drain the lake at its south-eastern elbow, first built in the 1750s and rebuilt thirty years later. Sarsens have been arranged to form the dam and its bulwarks. A new observation platform has been installed for viewing, but this doesn’t stop people from climbing up to the top (but the view from there isn’t nearly so interesting). Here we discovered that our pamphlet on The Valley Gardens doesn’t deal with the southern shore. Brian informs us that there is evidence for the cavities and depressions in the sarsens here representing growth structures rather than water erosion.
Between The Cascade and the next feature, the Leptis Magna Ruins, Brian instructed us to pick up as many unusual pebbles as we could to inspect over lunch at the Ruins. Having received those instructions, few picked up the ubiquitous misshapen flints, instead focussing on dark rounded pebble flints of “Tertiary” origin. One quartz vein pebble was in evidence, from somewhere far to the west, as was a single small clast of Lower Greensand Chert from the Weald and a piece of coconut shell; but the red pebbles Brian wanted were nowhere to be seen.
The Leptis Magna Ruins consisted of “22 granitic columns, 15 marble columns, 10 capitals, 25 pedestals, 7 loose slabs, 10 pieces of cornice, 5 inscribed slabs and various fragments of figure sculpture, some of grey limestone” a signboard said; they were shipped from the Leptis Magna Roman capital in Libya to Britain in 1816 as a gift from the local governor (Basham of Tripoli) to the Prince Regent (later King George IV). The ‘marble’ columns must be the ones in the grotto beyond the bridge, which Brian designated as ‘metamorphic’. Most of the capitals, lintels, and cornice were made of carved limestone, and one of the miscellaneous blocks lying around had evidence of bioturbation on its surface. Brian cites the OU’s Olwen Williams-Thorpe’s research, demonstrating that most columns originate from quarries at Troad and Kozak Dag in Turkey.
The Bagshot Formation surrounds the eastern end of Virginia Water, but as one moves west beyond the Leptis Magna Ruins, London Clay is exposed along the shoreline. The London Clay Formation, the newest in the Thames Group, consists of “brown to dark grey clay and silt; sandy in places; and scattered septarian nodules, pebble beds and selenite”, according to Brian’s handout. We saw none of these, just very muddy sides off the path.
Winding around the southwest end of the lake, we crossed the Five Arch Bridge, built originally in 1827 of limestone. In fact, oolitic limestone, as we could see the tiny holes left on the surface from departed oolites. Thence commenced a hard hike up Heartbreak Hill, to the car park and on to tea at Saville Garden where we celebrated a wonderful day out thanks to Laurie’s excellent organisation and Brian’s superb guiding.
For more about the history, rather than the geology, of Virginia Water see The Royal Landscape website.
Bath Stone Mine
19 October 2011
On 19 October 2011, a number of members of the branch visited the mine operated by the Bath Stone Company at Limpley Stoke, a little way south of Bath, in Wiltshire. The visit was a joint trip by members of three branches, Severnside, Wessex and London. It was arranged by Janet Ashton-Jones of Severnside branch, but the majority came from Wessex.
Bath Stone is a generic term for building stones worked from a series of beds in the Great Oolite Formation in the Bath area. The Great Oolite Formation was laid down in the middle Jurassic Epoch, about 165 million years ago, and its widespread use as a source of building stone in the Bath area and elsewhere has led to its Age being referred to as the Bathonian. Local stone was used in the construction of buildings in the Roman Aquae Sulis, as Bath was then known, but it came to national attention at the height of the city's prominence as a social resort in the 18th Century. The stone was also used in past centuries for grand country houses in the area, such as Longleat.
Initially, the local limestone was worked in the hills in the immediate vicinity of the city, which came from the lower parts of the Great Oolite Formation. This stone, the Box Ground Stone, is particularly well cemented, and therefore has especially good weathering properties, the cement being derived from shell-debris, in which this variety of Bath Stone is particularly rich, and which is mixed in with the ooliths. Later, purer oolites were worked further away from Bath in Wiltshire, and higher up in the succession. These include the Stoke Ground Stone worked at Limpley Stoke. The purer oolites are softer, but still with sufficient shell-debris cement to give very good weathering qualities. Initially the stone was quarried, but mining was resorted to in order to follow the seams into the hillsides.
On arrival at the mine our party was met by the Mine Manager, Mr Matthew Hawker. We were given a full briefing on safety,
issued with helmets and lamps and ear defenders, and instructed in the use of "self-rescuers" (emergency breathing apparatus).
Noise was a particular problem where diesel machinery was passed in the narrow confines of the workings, and could cause permanent damage to hearing, hence the ear defenders. It was explained that, as the mine was a working mine, we would be going underground under the "close personal supervision" of the Manager, in order to meet the requirements of the MASHAM (Management of Safety and Health at Mines) Regulations. Accordingly we were not to wander off on our own devices. If any of us experienced physical or psychological distress, as occasionally happened, we should communicate immediately with the Manager.
On entering the mine, we were shown the system of indicators which showed the safe escape route. The entrance to the mine was surrounded with 20th Century brickwork which dated from the use of the mine for munitions storage during World War 2. We initially descended down a gently-inclined drift, but on the way in-bye (i.e. towards the workings) we acquired a new perspective on faults and the significant adverse implications they pose in terms of working a mine. The Midford fault, a prominent feature of the local geology, required as to embark on a sudden very steep ascent, with a sharp reduction in the height of the roadway. In general, however, the roadways and workings were high, reflecting the fact that there were about 3 m of seams from which the stone was worked.
At one point, were passed by one of the vehicles used for transporting out stone out to the surface and had to use our ear defenders. It was made clear that the commercial constraints on the mine could not allow our visit to stand in the way of the on-going process of mining and bringing product to the surface.
In the period between 11.30 am when we descended, and 2 pm when we left, we visited three workings. The first was an active cutting face. The second was another face which had been cut. There blocks of rock were being lifted out using modern Italian technology, in which the cut stone was prised up from the bed using a steel bag which was gradually inflated with water. This technique was considerably more efficient and safer than more old fashioned methods. The third face was some distance away, in an older part of the workings, which was being re-developed for further use.
Towards the end of our visit we visited the refuge, a steel hut with an air borehole to the surface. This was equipped with oxygen and was slightly pressurized to keep out any noxious gases from a fire, and protect the workforce awaiting rescue by the specialised mines rescue team. The area was also equipped with sophisticated communications equipment and computer technology to control operations at the mine. We were told that in this mine it was possible to have this type of equipment below ground, which would not be possible in a mine with a potentially explosive atmosphere.
During our visit we learned about the history of the mine. This had not operated in the post-war period due to lack of demand for the product, but had re-opened in the early 1980s in response to the demand for building stone for restoration work in the Bath area and elsewhere, as a result of increasing prosperity, and increasing interest in building heritage and conservation. The principal shareholder in the company was now the widow of the man who had been responsible for re-opening the mine.
In the contemporary environment, the mine benefitted from the fact that its operations, though much more costly than quarrying, were environmentally more acceptable in an area where nature conservation held the highest priority. There was also a steady demand for its products, not only in the Bath area itself, but from further afield, where high quality building stone was required. For example stone from the mine had been used in the restoration of Windsor Castle after the great fire there in 1992.
The company worked two beds of the Stoke Ground Stone: the Top Bed and the Base Bed. The Top Bed stone was more favoured for decorative work, being softer and more readily carved. It also had a particularly high bed height of about 1.5m, which meant that large blocks could be produced from it. It was of a cream colour, which weathered into the golden colour with which Bath Stone is generally associated. The Base Bed stone was greyer and also harder, and so more suitable for areas exposed to weathering.
We also received the benefit of Matthew Hawker's clearly expressed views on a number of topics:
Safety was clearly of the highest priority for the company, with ever present risks from possible roof falls and the hazards of
lifting large blocks of stone. There was also the possibility of water inrushes, and there was some discussion of the recent fatal
accident at an anthracite drift mine in Wales.
We were also told that the company had actively been involved in learning from the methods used in other mines, such as coal
mines, operated by larger companies. It had itself taken the initiative in promoting the development of compulsory qualifications
for management at stone mines, which had not previously been required in the same way as at coal mines.
After thanking Mr Hawker and his colleagues for a most interesting and informative visit, we dispersed, the writer then spending a fascinating couple of hours at the Roman Baths museum in Bath itself before returning to London.
Lecture : Watching new crust form on an active plate boundary: afar Ethiopia - by David Pyle
20 October 2011
David Pyle, Professor of Earth Sciences at the University of Oxford, started by situating his work in the context of the Afar Rift Consortium, a £3 million NERC-funded international collaboration studying: How the Earth’s crust forms at divergent plate margins. The project will end in 2012 with an international conference.
The starting point was a major tectonic fracture, located at the triple junction between the Nubian, Arabian and Somalia Plates, clearly shown on a satellite image. In 2005, preceding the fracture 1,000 fairly shallow earthquakes were registered. A rift formed over about two days with the sides moving apart by 8-10 m over 80 km; the centre dropped 2 m and the sides rose 2 m, pushed apart by the intrusion of 2.5 km3 of magma. The early stages covered a wide zone with the development of fractures and thinning crust, leading to magma rising. The Afar Rift zone is bounded by off-axis volcanism, but the chemical signal differs, indicating another source of magma. At the northern end is the Dabbahu volcanic rift zone. The area is remote and sparsely inhabited, but offers a chance to see the formation of new oceanic crust at the surface. The lavas are silicic – peralkaline rhyolites, and since 2005 eruptions have occurred near the mid-point of the rift, one in 2007 and another in 2009; sensing techniques and satellite measurements are used to keep track of the Earth’s movement.
40Ar-39Ar dating has been used on collected samples, and on the volcanoes on the flanks of the rift, giving dates going back ½ million years.
In summary, we now have a much better idea of how fresh crust forms.
Pas de Calais - with Graham Williams
22-25 October 2011
The aim of the extended weekend was to look at the eastern part of the sediments represented in southern England by those of the Weald. These are predominantly Middle and Late Jurassic and Middle and Late Cretaceous, with some Tertiary. Of particular interest to the participants was the Ferques Inlier, with exposures of Devonian and Carboniferous rocks.
Our base was La Ferme du Vert, an original farmhouse, adapted and extended as a charming, idiosyncratic hotel. The group, about 30 strong, assembled at Embankment Station in London, picking up a few in Folkestone, and travelled in a very comfortable coach via the Channel Tunnel to northern France where the rendezvous arranged with Graham worked perfectly.
Saturday 22 October (Arrival and Day 1)
The first part of the trip was an 8:30 am departure in a coach from the Embankment, reminiscent of Winter weekends of old when a coach trip was the norm. We travelled across South London to the Channel Tunnel at Folkestone. A promising start with clear skies and bright sunshine. After a trouble free journey we arrived in France still under clear skies and bright sunshine.
The first geology we saw were the white cliffs of Dover back across the channel or La Manche (as we were now in France). We met up with Graham Williams our leader who gave us a comprehensive set of field notes which were to prove very useful as we visited various geological locations as we went through the trip.
The first French geology we were introduced to was Equihen Plage south of Boulogne. Here, from the safety of the beach, we saw a cliff face showing a section of the mid Kimmeridgian of the Upper Jurassic (Photo 1). The Crèche Sandstone was at the top of the cliff, the Chatillon Clay beneath it in the face of the cliff and, as it was low tide, the Chatillon Sandstone on the beach. These sediments are indicative of basin margins rather than the deeper water sequences found in the Kimmeridgian of the U.K. These are unusual rocks in the cyclic sequence of the Kimmeridgian with the Chatillon Sandstone, overlain by the Chatillon Clay and then the Crèche Sandstone, three very different rocks.
The Chatillon Sandstone is very important here as it acts a natural reef barrier to protect, to some extent, the cliffs by breaking the power of the waves which otherwise would erode the cliffs at a rate of approximately 1 m per year. There are houses on top of the cliffs which could probably be bought for a very reasonable price! I am not sure that they would be a good investment!
The introduction to the area and the information in the leaflet was the only bit of knowledge that we were given. The how and the why about this section was for us to figure out based on our observations to answer pertinent questions.
First the Lumachelle bank in the limestone. A death assemblage from a life assemblage close by of the little oyster bivalve – Nanogyra striata (Photo 2) which lived in a muddy environment. What would be the depositional environment? How and why was the limestone band left in the clay? Why at this horizon? The questions to think about – What was the speed of sediment deposition? What was the sediment supply? How were the shells preserved?
Looking carefully at the rocks we were able to pick out oscillations in the sand demonstrating emergent sands. We saw what could have been an intertidal area ripple surface with no fixed direction. Could it be at the base of the wave base? Was there a sudden influx of sediment caused by a one off event? Was there another reason for a deposition of sediment that covered the oysters? Even with much discussion we did not come to a conclusion. However we were made to think and came to a better understanding about the process of academic discussions that happen when new information is found.
We walked further along the beach to look at other features such as doggers up in the cliffs which are indications of clean sands with a high silica content. We saw pebble layers indicative of a high energy environment and had an interesting discussion about the relative flow rates of water that could be inferred from the style of layering in the sediment. Our final discussion was about the bioturbated rock layers (Photo 3) that we saw which are indicative of an environment rich in food with a slow rate of sedimentation or one that was paused.
This was the final point of the afternoon. With lots to think about and mull over we went back across the beach and up to the coach to go to the hotel for a shower, a well earned drink in the bar and a very good supper.
Sunday 23 October (Day 2)
Off to the Supermarket to buy lunch then a short drive and a walk along a field edge to a gap in the hedge which ran parallel to a long ridge similar to the Kentish Weald. The gap led into a small clearing and the exposed flank of the ridge. This was the St Godeleine formation of Upper Devonian age within the Ferques Inlier.
The exposure was quite narrow with not enough space for everyone to have a good look but the floor was covered with rock fragments which contained examples of the numerous compressed trace fossils and lamelibrancs for which this deposit is renowned. The east-west running ridge was formed of limestone beds 10 – 50 cms thick separated by thin bands of silt stone. It dipped Northwards at an angle of 45°.
Back to the coach and a drive to the Griset quarries were the Mid Devonian Balcourt formation is a dark grey when viewed
at a distance outside the barriered entrance.
Close to where the coach had parked where some 10 – 15 cubes of limestone each side about one metre long and it was suggested
that we examine two particular blocks.
Lunch was taken sitting in the sunshine on these uncomfortable blocks of limestone. It’s wonderful what a slice of Brie can do to improve a dry salami bread roll.
After a picnic lunch eaten whilst perched on limestone blocks at the side of the road leading to the Stinkel quarry we moved on to the Boulonnais quarry (Photo 5). This was a point of view stop only but nevertheless interesting.
This large quarry, one of several creating a massive hole in the ground, exploits the Carboniferous limestone exposed in the Palaeozoic Ferques inlier. This inlier, a result of Hercynian faulting, reveals Devonian strata resting unconformably on folded Silurian strata. This is overlain by Carboniferous limestone comprising ±200 m of dolomitic limestone overlain by ±270 m of limestone. The limestone sequences are in turn overlain by sandstone and then thin coal beds. The quarry face visible to us had subhorizontal strata of predominantly pale grey limestone of the Lunel formation of the lower Carboniferous. Some of this limestone takes a polish and has been extracted as ornamental stone for centuries.
It is erroneously called marble, one of the best known being Napoleon marble. Columns of this limestone had been placed around the viewing platform. The top half had been polished which contrasted well with the rough finish below. The limestone paving slabs in differing shades of grey contained large areas of sponge spicules and corals. There was disagreement as to whether they were spicules or collapsed evaporite features which had filled with rubble. They also showed natural sedimentary boundaries as well as vertical pressure solution boundaries (stylolites). We thought the vein colouring was possibly MnO2. The information board indicated that this limestone was considered to have good aquifer qualities but we weren’t convinced as there seemed to be insufficient porosity.
We then drove a short distance to Elinghen (Photo 6), erstwhile heart of the Pas de Calais coal mining industry. The coalfields, part of the upper Carboniferous Westphalian coal measures, had been very splintered and must have been a nightmare to work. The mining area was deserted and overgrown and had clearly seen busier and better times.
Our next stop was La Maison du Marbre et de la Geologie – a museum which unfortunately is now closed but some exhibits remained outside for us to examine. They consisted of large blocks of well bedded, fine grained limestone, described as porcellaneous so very hard and therefore brittle. We walked up to an old workface, the top of which was nicely bedded in contrast to underlying areas. There was a discussion on dolomitic limestone which we learned makes a good gas reservoir but not for oil. It’s apparently the secondary dolomitisation which creates the conditions needed by the oil industry.
We studied a fault in the face which we believed was part of a compression fault, resulting from the Hercynian orogeny. Across the road from this disused face was a large working quarry of the Vallée Heureuse where we could see more of the Lunel formation.
Back outside the museum we were assigned limestone blocks and set the task of deciding which was the right way up. Geopetals
and gaps between algal growths filled with sediment gave us the evidence we needed to make a decision.
Monday 24 October (Day 3)
Location 1 – Pointe du Nid de Corbet – Beach north of Audresselles (Photo 7)
We started the day in sunny weather with a cold wind coming in from the sea. The outcrops seen in the cliff were of, from the base upwards, Chatillon Sandstone, Chatillon Clay and Crèche Sandstone. These are of Upper Jurassic age and are equivalent to the Kimmeridge Clay series in England.
The Chatillon Clay is dark grey laminated mudstone with thin limestone beds. The bed contained many small sandy lenses which exhibited cross bedding. These were interpreted as tempestites – storm deposits below fairweather wave base. We also observed shelly beds, not in life position, cemented in the clay, which also suggested short periods of high energy currents.
The Crèche Sandstone displayed evidence of ripple structures. It is a well-cemented medium grained sandstone.
Given the palaeogeography of the Jurassic, it is likely that the source of the sediments was the emergent Anglo-Brabant landmass to the north-east.
After exploring the beach, we took a coffee break in the pretty fishing village of Audresselles. There were several cafes and
a small market selling various products.
Location 2 – Ambleteuse - Beach north of Ambleteuse
The wind dropped and the bright sunshine was very warm as we arrived at our second location of the day. The sandy beach north of Ambleteuse is easily accessed via a ramp. Here there is an excellent collection of Chatillon sandstone blocks, not all of them in situ, displaying many trace fossils. This is a world class location for ichnofossils and we observed many varieties of living burrows (such as Diplocriterian and Thalassinoides) and feeding burrows (Rhizocorallium) and other feeding or movement trails (Photo 9).
The high density of ichnofossils suggested that there was a high density of life at the time that the fossiliferous beds were laid down. However, we noticed that not all the beds seen contained trace fossils. It might have been that erosion had removed them, or that the preservation potential was different even in beds of very close temporal and physical proximity to each other.
Location 3 – La Pointe aux Oies Dunes de la Slack – Beach south of Ambleteuse
The sea-cliffs at Pointe aux Oies seen from the beach contain outcrops, from the base of the cliff upwards, of Wimereux Clay, Croi Beds and Oies Sandstone.
At the very top of the Oies Sandstone, we observed a 1 m thick bed of algal limestone, probably deposited in a freshwater environment such as a lake. This marks the top of the Jurassic succession at this point, above being deposits of Cretaceous Wealden Sandstones. The rock was layered, possibly representing seasonal growth cycles of the algae.
Beneath the algal limestone was a 10 m thick outcrop of the Oies Sandstone. The Oies Sandstone is a yellow/green calcareous fine to medium grain sandstone. The green colouration is explained by the presence of glauconite, formed by flocculation where freshwater meets seawater. This would imply an estuarine depositional environment.
A short walk along the coast brought us to an outcrop of the underlying Croi Beds. These are irregular beds of pale grey nodular limestone and dark grey calcareous sandstones and mudstones (Photo 10).
The nodular limestones are precipitated limestones which have been heavily bioturbated, leaving no bedding structures. They contain shell fragments. The dark grey material is argillaceous and fine grained mudstone. There is little sign of bioturbation, but fragments of small (5 mm) bivalve shells were found.
Tuesday 25 October (Day 4 and Departure)
Location 1 - Cap Gris-Nez
Our first location on Tuesday morning was touted as a ‘tourist stop’ although there was some geology to be seen. Cap Gris-Nez, which literally translates as the Grey Nose Cape, is the closest point to Britain and the site of an English fortress built by Henry VIII. Together with Cap Blanc-Nez, which we were to visit in the afternoon, it forms part of a “Grand Site de France”. These sites are recognised for their landscape, historic and cultural value and are managed according to the principles of sustainable development and to provide improved public access. At Cap Gris-Nez hard paths, information boards and viewpoints had been installed and ecological restoration had been carried out.
From one of the viewpoints we could see large cemented sandstone concretions or “boules” in the cliff face (Photo 11). These were from the same strata that we had seen at Audresseles (the Crèche Sandstone). Some of the boules had been eroded from the cliff and were visible at the base of the cliff, whilst other examples had also been conveniently placed around the paths which allowed us to inspect them closer. Graham explained that the boules formed when calcite precipitated from water was forced into the clean (high porosity and high permeability) sandstone from the overlying Crèche Clay and underlying Chatillon Clay. The examples of boules that we had seen in the Chatillon Sandstone at Audresseles were less well developed because the sandstone was thinly bedded with lower porosity and permeability.
Location 2 - Wissant
Slightly further north along the coast we stopped to look at the Holocene sedimentary deposits of Wissant. During the drive
from Cap Gris-Nez to Wissant we had observed several different modern day sedimentary environments; low lying marshy areas,
sand dunes and the shoreline (beach).
Behind us on the beach we could see poorly lithified windblown sands (see photo 13).
Graham asked us to consider the succession in relation to the rise in sea level which has occurred over the last 6,000 years.
We were also reminded of Walther’s Law (that sedimentary facies that were originally deposited laterally adjacent to each other
can be preserved as a vertical succession) and that “the present is the key to the past”.
After a cliff-top lunch at Cap Blanc Nez, making periodic sorties to the only bit of cover for miles around, Graham started our final afternoon, saying firmly: I HATE CHALK. This is because it is hard to get a chronostratigraphical sequence. In the mid-60s they used ammonites, and then foraminifera. Lately scanning electron microscopes have permitted the study of nanofossils, but it is still difficult.
We descended to the beach, (Photo 14) where, looking carefully at the cliff face, we could see the division between the middle and the lower chalk: above, the rock is more porous, with occasional flint; below, the plenus marl, thin beds of marl/limestone, overlies white chalk, grey chalk, and towards the base of the visible succession the blue chalk (Photos 15 and 16), its colour the result of pyrite and marcasite from the oxidation of pyrite.
This illustrates Walther’s Law, that a vertical succession of facies reflects lateral changes in environment, in this case transgressive and regressive phases of sea level. The rate of deposition of algae varied as sea levels varied. When a pause in sedimentation occurs owing to the distance from land, the sediments become deoxygenated, anaerobic bacteria thrive and pyrite results, dissolving the chalk while flat tabular flints develop.
There is little flint in the lower chalk. We spread around looking for fossils, including brachiopods echinoids and corals.
Our final stop on the way back to Eurotunnel was Sangatte, where along the length of the cliff our time-line went from the Cenomanian, through the Turonian to the Quaternary, the Sangatte marine sequences. Our first finds were a sponge (Photo 17) from the Lower Chalk, and old worked flints.
The Pleistocene epoch is characterised by two and a half million years of fluctuating temperature, with seven warm and seven very cold periods, with resulting sea level rise and fall. In this sequence there is a well-bedded chalk outcrop, but then the bedding goes and the face is ‘rubbly’, with pebble beds, fossil cliffs and wave-cut platforms, under which are wavy fractures. We stopped at many places to look at solifluction as a result of repeated freezing and thawing during the Ice Ages, chalk pebbles in sandstone, loess deposits from cold periods, and shattered flints, again the result of alternate hot and cold periods.
Our final puzzle led to a debate on the origin of a strange deposit of cemented sand containing a lot of iron and numerous pebbles (Photo 18).
Here we said Goodbye to Graham, many thanks for a splendid trip, got back into the coach and arrived safely back in London.
Photograph credits and copyright:
Student day to Herne Bay
13 November 2011
The group met at Beltinge car park at 10.30 am on Sunday 13 November 2011. The weather was fine and sunny, with a blue sky and a light breeze. It was cool until the mid-afternoon, when it started to get very chilly. We were warned that not all field trips enjoy such good weather!
Brian Harvey started the trip off by giving a brief summary of the geology of southeast England. He explained that the rock in this region is the youngest in Britain. We walked down on to the promenade, where Brian talked about coastal erosion and said that money is the main limitation on what can be done to save the land from the sea. The vulnerability of land to erosion is largely determined by its slope. The land to the west of where we stood had an angle of about 20°, and it was much less prone to landslip than the cliffs to the east, which had an angle of about 55°.
It was Remembrance Sunday and so, at 11 am, everyone observed the two minutes’ silence.
Diana Smith took us down on to the beach and briefly explained how to differentiate between the three rock types: sedimentary, metamorphic and igneous. She said that sediment — such as the sand and shingle we were standing on — is the precursor to rock. To describe sediment, we must look at the grains and note down their size, shape, colour and work out what material they are made from.
We then went off individually to collect samples of sediment. We brought back sand, pebbles, shells, a tin can and a plastic bottle top. Diana explained that they were all samples of sediment, but that a futuregeologist might have a hard time working out what Fanta was and why a can of it was in the bedrock!
She pointed out that some of the sediment is actually igneous or metamorphic rock. The nearest igneous rock to Herne Bay is in Brittany in France. Non-sedimentary rock could be brought to Herne Bay naturally by the sea, or used as ballast on ships and dumped when it is no longer needed.
We then split up into small groups and went to look at rock exposures in greater detail. Iain Fletcher told us that, when looking at an exposure, we should first stand back and study it from a distance. We should then sketch it, and include in our diagram the scale and words to describe the rock features, such as the colour and any peculiarities. We looked at glauconite in the Thanet Sand Formation, and learned that this mineral forms only in the sea. Although the grains in this sand were fine, the grains in the London Clay Formation are even finer. The Thanet Sand Formation must have formed in a shallow sea, because the heavy grains of sand would have sunk quickly after being brought to the sea by a river. The London Clay Formation formed in deep sea, because the light grains would have stayed afloat for longer and the river’s momentum would carry them far out to sea.
Diana talked to us about the protective armour on the beach at Herne Bay, which is large chunks of igneous and metamorphic
rock imported from Norway.
After lunch, Geoff Downer talked to us about the history of the Roman wall at Reculver. Geoff has written two books about the geology and history of this area. He described how there was once a Roman fort at Reculver, but that only the wall remains now. In Roman times, the sea level was much lower. Much of the fort has now been lost to marine erosion, leaving only part of the wall.
We looked at the rocks used to build St Mary’s Church, which stands on the headland at Reculver. Geoff pointed out that the headland would not be there at all if it were not for the protective armour on the cliff. Where there was no armour, the land had been eroded back to form Herne Bay.
Science does not exist in books, videos or websites. Those media are a great way to learn about science, but they do not in themselves constitute science. To understand science and fully enjoy it, you have to get out there and see it in action. That is what the London branch of the Open University Geological Society did today: we studied geology at first hand. Everyone had a great time, made some new friends and learned more than we could from home study.
Lecture : Members' Evening
17 November 2011
Members evening comprised four short presentations by Gina Barnes, John Lonergan, Richard Trounson and Eddie Yeadon and a display of some hand axes by John Jarvis.
Eddie Yeadon described the dramatic limestone scenery seen on a holiday in France: The Aveyron region is on the southern slope of the Massif Central, where a thick cover of Jurassic limestone lies over the older basement. Although now uplifted to around 1000 m altitude the strata remain largely horizontal, forming a plateau known as the Grands Causses. This plateau has been cut by three rivers, the Tarn, Jonte and Doubrie with deep and spectacular gorges. In places, the plateau has formed dramatic karst landscape with the beds of limestone and sandstone eroding to produce odd-shaped pinnacles. Caverns with underground rivers can also be visited. The main purpose of his visit was to see the new motorway viaduct at Millau, and Eddie concluded his talk with a video taken while driving the 2.5 km length of this remarkable bridge.
John Lonergan spoke about his SXG390 project on landslip hazards in the Isle of Wight. Details of this topic are given in the next article.
Landslip hazards in the Isle of Wight - By John Lonergan
A 2011 BSc Geosciences SXG390 Project
The Isle of Wight contains most of the geological strata found in Southern England, and is structurally a monocline with a steeply dipping northward limb forming the spine of the island. The more gently dipping southern limb ends in a coastline with cliffs that are eroding rapid- ly, owing to the chalk and sandstone cliffs overlying Gault Clay, which is both easily eroded and forms an underlying failure surface. Unfortunately, this area also contains the holiday towns of Ventnor, Shanklin and Sandown. This project investigated the landslips along this coat, based on five case studies dating from 1995 to 2010.
There are various types of landslide, as members will remember from Brian Harvey’s exposition at Herne Bay. Broadly, these are rotational, translational slides, falls, toppling and seepage erosion – and the island has examples of all of these. The underlying causes and final triggers are very often water - rainfall, sea erosion, reduced material properties, increased loadings, leaking drainage and perched water tables.
There are many definitions of hazard and risk, but In OU Geosciences, a hazard is a potential source of danger, and a risk is the chance of that danger occurring. Hazards can be categorised in terms of their potential effect on: people, property, infrastructure, utilities, habitat or amenity. And these are quantified in terms of the effect on numbers of people, price of property, replacement cost of infrastructure, etc. Risk is very difficult to quantify, as the probability and timing of geological failures are impossible to predict accurately.
Thus the mitigation strategies chosen and implemented are based on the estimated severity of the consequences of the hazard, compared to the cost of mitigating the hazard, essentially a cost-benefit analysis. Mitigation strategies divide into: holding, managing or retreating the coast line – or taking no action. There are two types of coastal protection engineering: hard or soft, with the aim of reducing erosion and increasing stability of the coastline. Hard measures include reinforced concrete retaining walls and steel sheet piles; soft measures in- clude regrading and draining slopes, and offshore reefs.
A third strategy includes planning policy and controls and educating the local population about the risks and what they should (or should not) do. These include restricting uncontrolled excavations, regrading of slopes, unloading of toes and loading tops of slopes. These local discussions are essential to the local understanding of the costs and consequences, and thus political support, as they directly affect local land use, planning and taxation.
Engineering solutions are expensive, and are generally beyond local council resources. Thus the major coastal protection schemes have been funded by national government. On the Island the Isle of Wight Council produced an Isle of Wight Shoreline Management Plan, and has since updated this. This aims to align local priorities with the Defra criteria and thus gain national funding for local schemes. This process successfully led to the implementation of significant engineering works from 1998 to 2010.
The chosen solution will always be a balance of cost and risk; there is little specific quantitative data on these. As landslides are geographically widespread, there is the opportunity to learn from others’ experience (both good and bad) and the EEC Response programme was set up to encourage this across Europe – and the Island played its part in this.
In conclusion it is essential to understand the local hazards, and this can only be done by understanding the local geology; and to analyse as both a system and a set of individual components.
Lecture : The History of the London Branch
8 December 2011
To mark (almost) 40 years of LOUGS, Brian gave a talk that was nostalgic for some and informative for most of the assembled group. It was a social, ‘Christmassy’ event with wine and nibbles, followed by the talk.
He started with the story of the origin of the Open University itself, a ‘University of the Air’ in the mind of Harold Wilson, given substance through the persistence of Jennie Lee, with Foundation Courses starting in 1969. Thus our Branch was formed early on in its history.
There was a pioneering spirit evident throughout the talk. There were field trips organised without the benefit of up-to-date maps, field trips camping in tents, Winter Weekends that really happened in winter in atrocious weather, visits to working coal mines(!) and slate mines, the whole accompanied by photos, some of younger (but still recognisable) manifestations of those present. The field trips to the United States in the 1980s were particularly impressive.
From the beginning conservation of sites of geological interest was a theme, along with help and encouragement to students. The first Revision Day was in 1985 and that tradition has continued through the changing syllabus: this year’s was more successful than ever. From 1990 to 1995 there were field trips in period costume, even down to the food and cutlery, again with photos as proof that women really could do geology in long skirts.
In 1992 to mark the 20th anniversary of the OUGS there was a geological cake with the strata extending through the whole cake, not just the surface.
There have been 9 Branch Organisers in those years, the AGM was started in a flat, pubs were also used, then the Royal School of Mines (when this writer came on the scene!) and more recently the Natural History Museum.
Thanks to Brian for a really interesting evening.