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.
Icelandic Volcanism • Northmoor Hill • The Geology of London • Tectonic Evolution in Zagros • Impact Cratering and Ejecta Deposits • Riddlesdown Quarry and Croham Hurst • The Corsi Collection and Oxford Building Stones • Predicting Earthquakes • Samos, an Overview • Messel fossil-pit • Bargates of Guildford and Godalming • The Lost River Tyburn • Barton-on-Sea • Vale of Fernhurst • Evolution of the Thames Basin • Weald and Downland Open Air Museum • Fishbourne Roman Palace • Late Quaternary Climate Variability • Norfolk • Mantle Plumes and the Siberian Traps
Lecture: 'Icelandic Volcanism - What's the Deal with the Rhyolite?' by Stephanie Flude
6 February 2010
Steph is now Post-doctoral Research Fellow with the OU, having gained her doctorate at Manchester on 'Rhyolite Volcanism in Iceland: Timing and Timescales of Eruptions.' The lecture followed the AGM and a number of those present had been to Iceland and we were all delighted with the lively and interesting presentation of the rhyolite debate about Icelandic volcanism: fractional crystallisation versus melting. She introduced us to the tectonic framework of the Greenland / Iceland / Faroes structure with a bathymetric map and a structural map showing mid-ocean ridge spreading with older and newer rifting areas and fissure zones. She also discussed the difference between sub-aerial eruptions resulting in shield volcanoes and the very steep-sided cones from sub-glacial eruptions.
The rhyolites are interesting because they are a proxy for protocontinental crust, and are potentially dateable using Ar-Ar dating or K-Ar dating as rhyolites contain a lot of potassium. Trachytes are formed by magma mingling, the mixing of basaltic with rhyolitic magmas; we were shown micrographs of feldspars with two or three stages of growth. There was much more: discussion of ring structures, calderas, dating of eruptions at 50 300ky intervals, with many pictures of scree and snow, giving us some idea of the difficulties of doing research in a somewhat difficult environment!
Field Trip: Northmoor Hill led by Jill Eyers
18 February 2010
We gathered on a fairly damp morning, in a fairly damp car park for a fairly damp walk around Northmoor Hill Nature reserve, a site prepared and conserved with much hard work by the Bucks Earth Heritage Group. That hard work was immediately apparent on the raised walkway we followed over the boggy patch caused by a lens of Reading Beds clay. Immediately beyond and uphill slightly, the ground was much drier, showing that we'd passed from clay onto the sandy gravels more typical of the Reading Beds all this without seeing a single bit of outcrop.
A bit more wet walk through woodland, down some handy steps cut by the hard-working members of BEHG, and we were back on the clay again where the bed of a small stream showed gravel of rounded or sub-rounded flints. Jill told us that some of the flints (the more rounded ones) were from the Reading Beds, but that some had washed down from the Quaternary gravels that overlie the area. Jill explained how the various gravels of the Thames terraces each have their special characteristics that make them distinguishable to the practised eye. Those we were looking at were from the Winter Hill Gravels, which had a different source area to the slightly older Gerrards Cross Gravels, accounting for the differences. The change in source area probably resulted from the capture of the upper waters of the River Thames by a river flowing north into what is now the Wash.
Downstream, the little stream we had been looking at disappeared down a swallow hole it must have found the bottom of the clay, and drained into the Chalk immediately below. We were asked to imagine what would happen to the gravels at the swallow hole, and (with some prompting from Jill) came up with a picture of imbricated pebbles plastered onto the sides of the hole with clay. Jill then took us past a "Footpath closed" sign into an old chalk quarry where another flight of steps led up to the unconformity between the Reading Beds and the Chalk and lo and behold there was an abandoned swallow hole looking incredibly like the picture we'd come up with earlier: brecciated chalk each side of the hole, and the hole itself filled with sandy clay and imbricated pebbles.
By this time it was raining fairly steadily. We climbed back up the track and round the edge of the reserve to where we could look over to the level ground of Denham Aerodrome. Jill explained that the ground had not been artificially levelled for the aerodrome they had sited it on the level ground formed by the Winter Hill Gravels, one of the glacial outwash gravels of the early Thames.
From there, it was a short walk back to the car park, and the drive to a very welcome hot lunch at the Coy Carp in Harefield, overlooking the River Colne and the Grand Union Canal. Many thanks to Jill for a very interesting, albeit damp, morning.
Lecture: 'The Geology of London, and how it has influenced the Development of that Great City' by Diana Clements
18 February 2010
Di nobly stepped into the breech at very short notice after the Half-Term field trip to Northmoor Hill. As well as being a vital part of our committee, and working at the Natural History Museum, Di teaches Geology to adult students and gave us the benefit of that teaching. With a wealth of maps, charts and photos, she covered a vast extent both in space and time. The Anglian Glaciation, 400ky BP, altered the course of the Thames; we had in the morning seen the flat area, the location of Denham airfield, which was the site of a proglacial lake with lots of different gravels, difficult to correlate. The various Thames terraces are correlated using oxygen isotopes.
The Thanet Sand underlies much of London, lying on top of the Chalk, but separated by 20my. This was shown visually with pictures ranging from Herne Bay to Pegwell Bay, and details of various pits and mines from Pinner in North London to Chislehust in the south-east, references to SSSIs, Geomorphology (Hampstead Heath and Epping Forest), Thames tributaries and fossil-hunting. Finally a brief look at Geology and the development of London with Stone Age sites, Roman London, and the need for water, which brought us up to the present day with the construction of the London Water Ring Main. This gives just a few details of this fascinating talk. Thanks from all of us.
Lecture: 'Tectonic Evolution in Zagros' by John Cosgrove
18 March 2010
Prof Cosgrove started with a tutorial about fold structures. Going much deeper into the mechanics and mathematics of deformation and fracture than encountered on any of the OU courses I have taken, he showed how the development of sinusoidal and box folds, and kink bands, depended on the layered structure of sediments and on the isotropy of the assemblage. Kink bands concentrate the stress, reinforcing oblique deformation and eventually leading to fracture and thrust faults. The development of these fold structures depends on how the stress front propagates, and this depends on the presence of weak layers or lubrication fluids in interfaces. This frequently results in whale-back folds in echelon rather than continuous fold structures.
Tectonically, what is now the Zagros Mountains was in a passive continental margin for over 400 million years and accumulated a thickness of about 14 km of mainly carbonate sediments on top of the Precambrian basement. The collision of the Arabian Plate with the Asian Plate in the late Miocene produced the massive thrust belt on the eastern side of the Persian Gulf and the foreland basin now occupied by the Gulf and the Mesopotamian plains to the NW. There is a considerable thickness of late Precambrian salt below the sediment, and this has allowed the thrust to penetrate farther in the Fars region than in the Dezful Embayment to the North. Salt domes are common in the region and can be forced to the surface by the immense pressures and spread like glaciers.
Lecture: 'Impact Cratering and Ejecta Deposits' by Kieran Howard
15 April 2010
Dr Howard gave an explanation of how impact craters are formed and their modification by erosion over time. Smaller craters erode rapidly, and no craters below 1 km are found older than 1 Ma BP. Larger craters may have slumping of the steep crater walls and terraces on the crater rims, due to rebound of the crater floor, and a central peak. The meteorite itself is almost always vapourised on impact, and the ejecta surrounding the crater can be shocked or melted rock from the impact site.
Dr Howard has done extensive work on the 1 km diameter Darwin Crater, formed 800 ka BP in a remote part of his native Tasmania. First recognised in 1972, this much-eroded crater is now filled with 60 to 70 metres of lake sediment over breccia containing fractured quartz. Ejecta in the form of "Darwin glass" is found throughout 50 km2 surrounding the crater - a much larger area than normal for this much impact energy. This is undoubtedly from molten ejecta and consists of black glass with about 70% silica and white glass with over 90% silica content. The explanation for the extent of the field is that the meteorite impacted in a wet peat swamp which would have amplified the explosive effect, like the spitting when a drop of water falls into a pan of hot fat.
This conclusion has implications on the extensive tektite strewn field covering SE Asia and much of Australia. The origin and formation of tektites is still uncertain, and no crater has yet been found which correlates with this tektite field, but from the distribution of larger tektites, the crater is thought to be in NE Thailand, Laos or Cambodia. By analogy with the Darwin crater, the large extent of the strewn field suggests a role for volatiles at the impact site. Samples of tektites and meteorites were passed round for handling and examination.
Field Trip: Riddlesdown Quarry and Croham Hurst
17 April 2010
Riddlesdown led by Rory Mortimore
Part 1Riddlesdown quarry, formerly the Rose and Crown chalk pit, is located in the Caterham valley close to the little town of Whiteleafe and represents an outstanding record of the Chalk stratigraphy of the North Downs. It is a Site of Special Scientific Interest for the wildlife and is owned by the City of London Corporation. They conserve the quarry and have done an amazing job to Rory's specifications including installing a set of steps up the scree so that the most accessible rock face can be examined at close quarters and providing a resident flock of goats and sheep who help to keep down the vegetation. This quarry is the shining example of geoconservation in the London area and the City of London Corporation, particularly the local rangers responsible, are to be congratulated.
Some 30 LOUGS members assembled at the main access point (just off the A22 near to a Jewson's store) in glorious sunshine. We were greeted by one of the rangers, Matt Johnson, from City of London Parks. However entry to the quarry proved to be not so simple a matter as could be expected; unlocking the gate gave us all the first real challenge of the day; the lock yielding to subtle dexterity only after 10 minutes had elapsed and plans made to purchase a bolt cutter. Rory explained that Riddlesdown Quarry was the last accessible quarry in the area and the cross section it provided was of critical importance to anyone wishing to advise on projects and geological conditions in the London area. Borehole evidence was insufficient and the information available over the full exposure has been used to assist in the interpretation of data from various London drilling projects, including the Croydon cable tunnel. Rory had prepared a comprehensive handout which contained a sketch of the quarry that Rory had done in the eighties when advising on how to proceed with conservation [this will be published in the forthcoming GA Field Guide to the Geology of London, due out in November 2010].
The flint bands within the Chalk are important stratigraphical markers (they also present a drilling hazard for tunnel boring machines!). The flints were deposited at the junction between an oxidising environment on the sea bed and a reducing environment caused by anaerobic microbes a few centimetres below the surface. Often the flints formed around a layered junction which could be seen within the formed flint itself. Some notable flint layers were the large flint layers above the Glynde marls, the very large flints between the Bridgewick marls and the Bopeep flints, the latter flints so named from the type section at Bopeep quarry in the South Downs near to Alfriston.
We first looked at the lowest exposure in the quarry. This corresponds with the Lewes Nodular Chalk Formation. At the base of the succession, where the New Pit Chalk Farm deposits are about 1.5m thick, we found the fossil bivalve Inoceramus cuvieri as well as scarce examples of the brachiopod Terebratulina lata (the former zone fossil in the Middle Turonian). Above the New Pit Farm Chalk were some 5 marl beds spaced at rather irregular intervals. The Glynde, Southerham, Caburn and Bridgewick marls seen here, were considered to have been derived from volcanic ash relating to the opening of the Atlantic. Interestingly the fossil record just before and after these beds always showed an evolutionary change. Analysis showed there was about 5% clay content, a composition which made these marls attractive as sources for talcum powders. The Lewes Chalk is very gritty and had been laid down during a period of relative shallowing. Conditions were such that the area could be described as a carbonate factory. Shallow enough to allow phytoplankton and zooplankton to proliferate and deep enough to allow thick deposits to build up. Being a long way from land the detritus that accumulated was largely composed of bryozoans, such as sponges, small brachiopods, bivalves and echinoids.
There were some conjugate pairs of fractures evident. Steeply inclined and polished, these fractures developed as a result of stress relief at the margins of the chalk. Importantly these fractures were important drainage routes for ground water occupying pore spaces within the chalk.
Part 2Moving up the succession we encountered the Bridgewick flint band of the Lower Lewes Nodular Chalk Formation. The Bridgewick flints are black flints used for flint knapping and together with the Bopeep flints above represent a zone of maximum flint formation the 'Turonian flint maxima'. We then made our way to the main quarry face where Rory gave an overview of the strata exposed at the quarry. These are the Lower and Upper Lewes Nodular Chalk Formation which are overlain by the Seaford Chalk Formation.
Sediment-filled dissolution pipes form the castellated skyline which could be clearly seen along the main quarry face. The dissolution pipes are paleokarst features formed during Quaternary as a result of the dissolution of the Chalk by relatively acid soil water particularly along fractures in the rock. The dissolution pipes have been filled with Clay-with-flints deposits. The Clay-with-flints is a head deposit (the term head being derived from head of the valleys deposits in Wales) which have been reworked ex-situ. This contrasts with the putty chalk seen at the first section which is an in-situ slope deposit. The putty chalk is reworked chalk formed during Quaternary by freeze thaw action. The presence of the marl seams reduced the downward percolation of water, and together with the high porosity of the Chalk (up to 40%), enhanced the freeze thaw process. Rory explained that it was possible to view these Quaternary deposits in the upper bench at the top quarry. Also visible at the top of the quarry were the flock of goats 'employed' by the City of London Parks to manage vegetation at the site.
The main quarry face also provides a good view of the conspicuous flint bands and marl seams that form marker horizons within the Chalk. The morphology of fossils, in particular Micraster, varies between the marl bands allowing the identification of the different horizons within the Chalk. Whilst different countries may have different nomenclature, key markers are consistent across large areas and in some cases the equivalent flint bands, marl seams and fossils can be traced throughout the Anglo-Paris basin. There is, however, a degree of lateral variation, with some lithologies being better developed in certain parts of the basin.
One of the most conspicuous and consistent marker beds in the English Chalk, the Lewes Marl and associated Lewes Tubular Flints, can be seen in the main quarry face. Some members of the group climbed higher up the face to inspect the Lewes Marl and underlying Lewes Tubular Flints . The fingerlike tubular flints were first described by Caleb Evans (1870) from the adjacent railway cuttings Whilst sheet flints are formed along a sub horizontal surface (slip planes) and conjugate fractures after lithification of the Chalk, tabular flints (beds of nodular flints) are formed due to stratigraphical controls. The group then went up a set of steps adjacent to the quarry face at the northern end of the quarry. This section is through the Upper Lewes Nodular Chalk and includes the Cuilfail Zoophycos Beds at the base, and the Beachy Head Zoophycos Beds towards the top. The zoophycos streaks and flints are trace fossils found throughout the Chalk but which are more abundant at certain horizons. These widespread horizons have been linked to transgressions and regressions during the deposition of the Chalk, as well as climatic controls related to Milankovich Cycles. The visit to the quarry finished at the view-point at the top of the steps where we thanked Rory and Matt for an enjoyable morning. The elevated position on top of the spoil heaps provided an overview of all the exposures in the quarry as well as a view across the Caterham Valley.
Croham Hurst led by Diana Clements and Jenny Parry
In the afternoon the group moved off to Croham Hurst for a mapping exercise devised by Di and Jenny. Members were divided into 6 small groups with instructions on which routes to take to the top of the aptly-named Breakneck Hill. Each group was given a contour map with the paths marked and coloured pencils to annotate the map with any exposures found. The objective was to meet at the top to compare notes.
Croham Hurst is an interesting location as the 1912 geological map and subsequent revisions show no Lambeth Group between the Thanet Sand Formation near the base and the Harwich Formation Blackheath pebbles beds at the top of the outlier, whilst the newest, 1994 revision has added a thin band of Lambeth Group between the two. The groups were not informed of this discrepancy before they set off. Don Aldiss from the British Geological Survey had been very helpful in supplying detailed notes about this problem which were discussed at the top.
All groups found white Chalk at the base of the hill and a clear change in slope seemed to herald the arrival of the Thanet Sand Formation. Exposures of yellow sand were also noted by all the groups either in the roots of fallen trees, badger holes, 'kids erosion' (a swing) or slumped sections and these were duly marked on the Master Map. Details of the vegetation were also noted. The problem began to emerge of material that arrived at the base as scree from above. This was particularly so in the case of the Blackheath pebbles which easily roll down from the top of the hill. Two groups found small patches of calcite-cemented pebbles. The pebbles are mostly loose and are very obvious in the open heathland at the top and only occasionally can they be found cemented.
So did we find any evidence of the Lambeth Group? One group found some reddish clay on the northeast face that seemed a possible candidate and were sensible enough to bring a sample (which seems less red now that it is dried out). Unfortunately there was not enough time to inspect the outcrop but its approximate position is marked on the map for another occasion. Another group noted the position of slumped sands near the top of the hill to the southwest which some of us did inspect at the end and collected a sample. This appears to be coarser-grained with more glauconite than the sand exposures nearer the base of the hill but proper analyses would need to be made to prove whether they came from the Thanet Sand Formation or the Lambeth Group. If the latter, the most likely candidate would be the Upnor Formation which immediately overlies the Thanet Sand.
The exercise was a really useful contribution in trying to nail down the stratigraphy of this problematic hill and I thank LOUGS members for their help. I think (hope) that they found it helpful and instructive too.
Field Trip: Oxford - the Corsi Collection and Building Stones
8 May 2010
This most successful Field Trip was in two halves: the morning visit to the Oxford University Museum of Natural History to look at the Corsi Rock Collection under the expert guidance of Dr Monica Price, Acting Director, and the afternoon Building Stones of Oxford walk led by our own Di Smith. The only downside was the weather cold and wet, as befits a May morning.
We met at the Museum, and were welcomed with coffee and an introductory talk by Monica, who curates the collection and is the author of a splendidly illustrated book: 'The Sourcebook of Decorative Stone: an illustrated identification guide' (2007). The Museum itself is a Grade 1 listed Neo-Gothic building designed by Irish architects Deane and Woodwarde, influenced by John Ruskin. Its major displays are in a large square courtyard with a glass roof supported by slender iron pillars. Stone columns surround both the court and the first floor gallery, each made from a different British stone, and all labelled: a study tool in themselves.
Monica's talk gave the history of the Collection. Faustino Corsi, a lawyer in Rome had made a collection of about 900 examples of mostly antique decorative stone, carefully organised and catalogued, which was in the early Nineteenth Century a magnet for those going on the Grand Tour. When in 1826 it became known that he wanted to sell, the British Museum was one of the organisations interested, but they haggled over the price, wanting only a part, giving the opportunity to a rich undergraduate at Magdalen, Stephen Jarret, to buy the whole lot with additions to make it up to 1,000 specimens, and including the catalogue. He then donated it to the University and it was installed in the Radcliffe Library in November 1827.
We split into two groups as the collection is housed in drawers, so one group looked at the other exhibits while waiting their turn. The first surprise was the substantial size of the samples, that of a smallish brick, and organised by colour according to Corsi's own catalogue number.
White marbles came first, some true marble and some limestone (Palombino). The next set included variations in style and colour within the same stone according to the other minerals such as haematite visible in the veining. We went on through yellows, pinks and many different reds, such as the dark red rosso antico and the carboniferous 'Duke's red' from Derbyshire. (The Duke of Devonshire visited Corsi and sent him some samples.) In the following drawers were brecciated marbles with many different colours from Sicily, North Africa (cipollino, named for its resemblance to the layers of an onion), Turkey, and French black and white travertine from the Pyrenees and many other examples.
We moved on to different greens, with serpentinite from Greece, lapis from Afghanistan, smaragdite from Corsica etc. There were jaspers, mostly from Sicily, with amazing colours, and granites and microgranites from Egypt. Some of the provenances that Corsi could not discover have been subsequently elucidated, and some disproved, but the catalogue is still an astonishing document from nearly 200 years ago. After thanking Monica, we dispersed for lunch (not picnic weather, so it was either the Museum or a local hostelry)
We met again at 2.00 to be instructed by Di Smith on the Building Stones of Oxford. Di is on home territory here, and her enthusiasm was obvious from the word go. It was graduation day, so we would not be allowed into the Colleges, but she had (through choir connections!) fixed up for us to go into Keble in the late afternoon.
Before we set out we had a brief history: in Saxon and Mediaeval times they used the local ragstone. The coral rag from Headington flakes easily and cannot be cut to ashlar, so in the 13th and 14th centuries Burford Stone from the Cotswolds, previously used for 'fine work' became more prevalent. In the 16th century the price went up, so they rediscovered Headington freestone, an oolite, which at first was good quality, but the quality is variable as it comes from an epicontinental sea, which means that environmental conditions changed rapidly in both time and space..
Throughout the afternoon the problems of the effects of water on the stone was a recurrent theme, with 'before and after' photographs to illustrate the problem. With Headington hard, a medium-grained late- Jurassic bioclastic limestone, as a base, water cannot drain away from the stone above; it gets behind the case-hardening, thus encouraging more decay. If the stone is cleaned, the gypsum crust is removed, allowing yet more water penetration and exacerbating the decay process. We then set out on a circular (sort of) tour of the city, stopping first at a wall of Headington rag, seeing the variable state of the stone, and then to Wadham College, whose front quad was recently cleaned and resurfaced with mostly Clipsham, an oolite from Rutland. The tiles on this side of the roof are original and of Stonesfield slates, replacements come from a similar facies in Colleyweston over in Lincolnshire.
Pausing en route to look at igneous setts, perhaps from the Cleveland dyke, our next stop was the Sheldonian, built by Wren in 1668 on made ground parallel to the old city wall. Built of Headington stone, it has been refaced entirely in Clipsham, with an apron in front of the railings of decorative pebbles of Permo-Triassic sandstone. It would take too much space to detail everything. One of the highlights was the Radcliffe Camera whose bottom storey was open until it became a book repository in the 19th Century. The different stones used are rusticated Headington freestone, Headington hard, Taynton stone, (a Burford-type oolite, much valued by Wren), with the columns above of Portland limestone. The patching once again is in Clipsham stone.
In a modern accommodation block for Corpus Christi College concrete, with good local aggregate, is used sensitively, abutting natural stone older buildings; Oriel, originally only two storeys high, had a third added by the simple expedient of removing the gables and roof, putting in another floor, then replacing the gables when more rooms were needed; Bear Lane was used to illustrate 'way-up-ness' using graded bedding, erosive bases and burrowing; the Portland Roach was seen in one of the new Christ Church College blocks before we reached St Aldate's where the Post office is made of greenish Wardour stone from Wiltshire in the Vale of Wardour, the porch rests on a plinth of Shap granite (Hurrah!); Christchurch with the Tom tower (another of Wren's Oxford connections), named after the bell that is rung 101 times every evening at 9.05, one for each of the 'endowed scholarships' set up when the college was founded; Carfax and its tower witnessed 'town and gown' rioting under Edward III as the High Street 'ran with blood' on St Scholastica's Day; St Giles, the site of a fair on September 3rd with the Randolph Hotel, built of yellow brick from the Oxford clay; the Martyrs' Memorial of 1841 with statues of Bath stone, and the Ashmolean Museum, newly cleaned and showing off its Bath Stone and Portland, despite the weather; St John's College behind the Lamb and Flag has an accommodation block which contrasts Provencal limestone and its very varied structures with well-presented concrete.
Finally about half the original group, undaunted by the weather, arrived at Keble built of bricks made of Oxford clay. A triumph of Victorian brick architecture, it has been described as 'a crouching dinosaur in a Fairisle jumper'. Built in the 1870s to 80s, signed William Butterfield 1876, it is mostly of Oxford clay bricks, but with white courses from the Gault: an astonishing finish to the day. We went our separate ways: some to a café, some to cars, and at least one back to the station.
Lecture: 'Predicting Earthquakes' by Peter Sammonds
20 May 2010
Peter is Professor of Geophysics at University College, London and also Director of the 'Risk and Disaster Reduction Unit' there, so he is well placed to give us the latest research on a subject of obviously topical interest. Talking of natural hazards he pointed out that they have accounted for 1.5million deaths in the last six years, with about 50 volcanic eruptions, 100 sizeable earthquakes, 40-50 tropical cyclones and numerous floods, and of these earthquakes are the most deadly, with in 2009-10 alone L'Aquila, Taiwan, Sumatra, Haiti and Sichuan.
Referring specifically to L'Aquila, we were introduced to the tectonic setting: a regional map with dates of earthquakes. The Abruzzo mountains have many normal faults with extensive historical earthquake records, so that the slip-rate on a fault can be calculated, since if a fault is slipping faster than the rate of erosion there is a scarp, and a scarp of 18m, produced over 18,000y, gives a slip rate of c.1mmy-1. Using ground-penetrating radar at Piano di Pezza in the Central Appenines, students investigate fault-flow rates.
The magnitude 6.3 earthquake of 6th April 2009 at L'Aquila was responsible for a buildings loss of 30- 50%, 307 deaths and 80,000 homeless, many still living in tent cities with nothing to do, their livelihood gone. Modelling allows some prediction of the possibility of earthquakes, but there are two few data. Satellite interferometry, with measured slip used to calculate stress changes on nearby faults, can help, but we took away the impression that accurate prediction is still a long way off.
Field Trip: Samos - An Overview
28 May 4 June 2010
Like many islands Samos has had a complicated geological history. It is made up of a series of nappes that are connected to the Turkish mainland which is now only divided by 1.2 km of seaway (Fig. 1). Between the nappes are valleys filled with Miocene sediments, the youngest of which is famous for its terrestrial vertebrate fossils, well-displayed in the little museum at Mytilini.
The oldest of the nappes is a prominent massif of marble making up the Kerketas Massif in the west of the island (Fig. 2). Half way up are some famous caves, at one time inhabited by Pythagoras. The caves lie along the line of the Pythagoras Thrust dated at 30-35 million years old (Fig. 3). The Agios Nikolaos Nappe is the next in the sequence which is poorly exposed on the north of the island and was not encountered on the LOUGS trip.
Above that is the Ampelos Nappe which is prominent throughout the island, cut by valleys of Miocene sediments. This is composed predominantly of marbles and phyllite, the former exposed in the mountain tops, in particular the Ampelos Massif.
Outcrops were mostly examined in road cuttings where the relationships with the phyllites could be well seen. The landscape in late May was covered in swathes of bright yellow broom and this seemed to pick out areas of phyllites and scree, leaving the marble relatively barren as in the older Kerketas Nappe (Fig. 4).
On examination the marble ranged from very blue, to stripy, to white (rare) and was usually dolomitised. Where seen in the abandoned quarry south of Pyrgos it was stripy and cut through with faults (Fig. 5). The phyllites, too, were predictably variable (Fig. 6).
In most places crenulations were apparent but less often the metamorphism was of a higher grade and schists were encountered. Some were mica-rich with pyrite crystals and some were darker with graphite. Elsewhere crystals of chloratoid were identified. At one small location, close to the marble quarry south of Pyrgos, higher-grade blueschist facies rocks were found with blue glaucophane, green epidote and white lawsonite.
Within the Ampelos Nappe small exposures of the younger Selcuk Nappe are in faulted contact, dated as 42-32 Ma. This nappe consists of metabasic and ultrabsic rocks. Metagabbros were encountered in road cuttings, sometimes 'rodded' and making prominent 'knobs' in the landscape (Fig. 7). Following a gully down a small track they were found to be part of a melange alternating with serpentenite, chrysotile and phyllite (Fig. 8).
The youngest nappe is the Kallithea Nappe exposed in the far west of the island. It is the only nappe that is not metamorphosed and the contact with the Kerketas Nappe is only 10-8.5 Ma old. This was examined in a road cutting south of Kallithea where the predominant rock-type was a fairly uniform green serpentenite. At the southern edge pillow lavas were just discernable, rather disguised by their green alteration colour and in association with red, bedded radiolarian cherts and thin limestones with sylolites (Fig. 9). The ophiolite is interbedded with limestones of Triassic and Jurassic age.
The Miocene graben fill sediments
Basal Conglomerate was encountered at several locations. This was typically red, poorly sorted and containing clasts of rocks already examined in the nappes predominantly marbles and phyllites, probably deposited during flash floods. Occasionally small areas showed imbrication allowing current directions to be assessed. Everywhere they were subsequently tilted (Fig.10). At one location calcretes were found at the top of the section, indicating very arid conditions. In close association with the conglomerates in many cuttings there was a lava flow overlain by tuffs with a probable lehar above. It appears that the ash was predominantly laid down in water, in places alternating with thin-bedded limestones.
The Pythagorion is the oldest of the Miocene fresh water limestones dated at approximatedly 11 Ma. Ribbed gastropods were found just north of Mavratzei, across the valley from a disused marble quarry in the Ampelos Nappe. Further north along the same route the Hora Formation was brilliantly displayed along a length of road cut where the sediments appear to be rhythmically bedded. There were magnificent slump structures probably generated by earthquakes (Fig. 11). Small gastropods were found here too, along with trace fossils on the platy slabs and probable sinuresis cracks caused by sub-surface de-watering.
The overlying Mytilini Formation is found close to the village of Mytilini. An exposure identified as a river channel deposit was seen by the group at the back of the town. The town is justifiably famous for the large collection of fossil vertebrates from the Mytilini Formation which are displayed there in the Natural History Museum of the Aegean. This Late Miocene fauna (8.5-7 Ma) has yielded, amongst the 60 different species, bones of giraffes, horses, rhinoceros and bovids. The display includes models and dioramas as well as the individual bones and teeth (Fig. 12).
The top of the graben-fill sediments was seen in the north of the island. In a road cutting close to the delightful sea-side resort of Kokkari, tufa was examined from the Pliocene Kokkarion Formation. In some patches the tell-tale holes left by the decay of vegetation that the tufa surrounded were very obvious, but initially it was difficult to recognise that almost the entire face was composed of tufa (Fig. 13). There were some thin marl seams and the tongue test confirmed that these were bentonite, probably from ash falls (the marl sticks to the tongue).
Raised Beach deposits
It appears that over the past 4,000 years the northwest end of the island has been rising and gently tilting the island to the southeast. At Potami Beach, beneath the striking new church of Ag. Nikolous, there are 3 obvious notches in the cliffs thought to relate to earthquakes 1) between 3,900 and 3,600, 2) 1,500 and 3) 500 years ago. The notches also have associated cemented beach rock. At this location serpulid worms were found associated with the highest notch. Further along the coast vermitid worms can be seen but these were not reached by the group. Approaching the northwest coast from the south, on a separate occasion, the raised beach deposits were seen again at Aliki Beach, close to Ag. Isidoros. Here fragments of gastropods and bivalves were seen in the cemented beach rock. The most remarkable feature at this location was the borings into the rock by rock-boring bivalves (probably pholads), picking out the different heights of the raised beaches (Fig. 14). Similar borings were recognised by Lyell on a visit to the Roman temple of Serapis at Puzzuoili as proof of variation in the sea-level of that area and featured as a frontispiece to his Principles of Geology. Aliki Beach is also worth visiting as a lesson in traditional boat building techniques with boats in all stages of construction.
A visit to the Temple of Hera at Ireon neatly summarised aspects of the geology of the island. The only column of the basilica left standing seemed impossibly precarious and was a timely reminder of the devastation the earthquakes can bring to this part of the world. Much of the site was destroyed in the earthquake of 262 AD and it seems impossible that this column could survive even a small earthquake in the future: the large circular blocks of which it is made are already considerably offset. At least 20m in height, it is though to be only half its original height of 40 m (Fig. 15).
Habitation on the site dates from the Bronze Age right through to 5 AD and the building stones reflect the development. We were fortunate to meet the architect responsible for the current excavations, Nils Hellner. He told us that much of the basilica was constructed from tufa from a quarry almost within site of the excavation. Large pillar and stall mines off the road between Hora and Mytilini have been identified as the source of the material and it has been calculated that 25,000m3 of material have been removed of which 18,000m3 have been located on the site at Hera. Blue-coloured marble has been correlated with a quarry on the other side of the Ampelos Massif at Manolates, about 800 m up the mountain from the north coast. Nils is still trying to work out how they achieved getting the stone down the mountain for shipment round to Ireon. The white marble of the columns and decorative work seems to have come from quarries on Phourni, a small island to the west of Samos, more famous for its lobsters. The green alterstone capping where sacrifices were made is reportedly fire-proof. It is said to be an ophite, possibly form the island of Patmos but our leader, Paul Grant, thought it was more like a very green sandstone. We had a lot of fun trying to make the connections to the rock outcrops that we had seen during the week.
The LOUGS trip to Samos in May 2010 was a select group (Fig. 16) and we are extremely grateful to Paul Grant for his time and enthusiasm in introducing us to this fantastic island which has been intently mapped by students from Imperial College for the past decade, supervised by Paul. Some of us plan to be back!
Lecture: 'The Story of Messel fossil-pit: how Ida and company became unearthed' by Susanne Feist-Burkhardt
17 June 2010
Susanne is a Researcher in the Palaeontology department of the Natural History Museum. Her research is in the field of palynology, but the subject of her lecture, Ida, the 'missing link' in primate evolution, one of many fossils dating from 47my during the Eocene, preserved in oil-shales from a volcanic lake or maar in the 'Messel fossil pit' near Danstadt in Germany, has a family connection and she showed us a photo of herself as a teenager with her father, Otto Feist, helping to excavate in the sediments in the early 1970s. The exploitation of the mine had finished, but scientists had not yet taken over.
Ida, Darwinius masillae, made headline news in May 2009, with press articles, a television programme, but Susanne gave us interesting information with photographs of the splitting of the shales, showing how techniques were evolved to preserve the fossils in the fragile sediments. She brought casts and one actual specimen for us to examine. In 1974 the site was closed to the public, and after the defeat of a proposal to turn it into a landfill site, in 1995 it became a World Heritage Site.
The particular interest is the perfect state of preservation, with hair, guts, details of the teeth, including milk teeth, the feathers of a bird, a boa with a crocodile in its gut, a female horse with a foetus, and many more. This was not a catastrophic burial: the animals probably died as a result of volcanic gases, emanating from the lake, the anoxic conditions accounting for their preservation. If you are interested, there is lots of information at: www.messel-fossils.eu
Field Trip: The Bargates of Guildford and Godalming led by Graham Williams
20 June 2010
On Sunday 20/06/2010 a group of OUGS members took part in a very interesting field trip when Graham Williams introduced the group to the Bargate Formation. The Bargates are a particular form of sandstone local to the Guildford / Godalming area. Several sites were viewed during the day.
The day started at a viewpoint near The Chantries (St Martha's Hill area) where it was explained how geology relates to the landscape. For example, Folkstone Sands are found on the tops of hills where there are trees as little else grew there. The trees were often well spaced. The Bargate beds formed either a plateau or steep slopes as they did not erode easily. The agricultural land is on the valley floors.
The Folkstone Sands were laid down at the end of the Albian period and the Sandgate Beds (Bargate Beds), Hythe Beds and the Atherfield clay Formation were laid down in the Aptian period roughly 112 million years ago. Both the Albian and the Aptian periods form part of the Lower Cretaceous.
The sites visited included a visit to a sunken lane meant where both the Bargate and Hythe beds were studied. A large outcrop of Bargate in Godalming (with several sand-waves) was also viewed before visiting two quarries located in Littleton. Graham Williams also had some fossils including ammonites and bivalves to view through a microscope.
Bargates, like other sandstone, are pieces of grit which have been collected by the moving sea currents and become cemented or compacted together. The strength of the water movement can be worked from the thickness of the rock layers and rock pattern. The sand-waves viewed at each outcrop were all running in the same direction (southerly) and provides us with further information.
Field Trip: 'Looking for the lost River Tyburn' led by Diana Clements
15 July 2010
In Hampstead, where rainwater seeping through the permeable Claygate meets the impermeable London Clay, there is a springline. One such spring, the Shepherds Well, is known as the source of the Tyburn, but is now bricked up and diverted directly into the Upper Level Sewer. There is no river to see, except on rare occasions when rain overruns the storm drains.
Starting in Regent's Park by the boating lake once fed by the river, we heard how the city once reached only as far North as the Euston Road. Here the London Clay gives way to gravels and land much better suited for buildings.
We made our way through Marylebone gathering clues. Street names such as Aybrook and Welbeck, and drinking fountains in parks such as Paddington Gardens hinted at a watercourse nearby. We paused to consider if Aybrook may have been a local alias of the Tyburn. The name Tyburn may mean 'boundary stream' or maybe 'two streams'. Aybrook may have originated as 'A brook' or 'Ea brook'.
By now we were becoming aware of subtle variations in street level, suggesting topography mirrors the shape of former landscapes, and watercourses would have flowed in the dips. Marylebone Lane curves its way through the otherwise angular street pattern, we concluded we were standing on the Tyburn's former banks. At the lane's Southern end, Wigmore Street has a pronounced dip - we crossed it to find a plaque announcing the location of an ancient conduit owned by the City of London which took water from the Tyburn to East Cheap.
We passed by another fountain once fed by the river in St Christopher's Place, and crossed Oxford Street at a dip to come to Grays department store in South Molton Lane. Those of us with good vision might have glimpsed in the basement a koi pond said to be fed by the Tyburn. We were not particularly convinced by this as it seemed to run sideways to the route we were following, but it is a nice story.
The topography was becoming steeper as we passed Brooks Mews but found our route blocked by a row of garages at the bottom of Bourdon Street. And it was the bottom - we had a significant slope to climb out of the valley to continue on our way.
We stopped on the winding Bruton Lane to read a plaque which stated "THIS WALL FACE IS THE BOUNDARY BETWEEN THE CITY'S LAND AND THE BERKELY ESTATE 1894". We know from old maps that boundaries often followed watercourses, but it is tempting to think this boundary was important enough to lend the Tyburn its' name.
We finished our walk in Green Park, where the river had carved a deep hollow in the edge of the River Terrace that runs along Piccadilly. Here the land flattens out and evidence for the river becomes ambiguous. It is generally accepted that it flowed under where Buckingham Palace now stands. From there, it may have flowed West, bifurcating around the Thorney Island on which the Palace of Westminster now stands; but there is also reason to believe it flowed South, along Tachbrook Street and into a tidal inlet near Vauxhall.
So we looked for the Tyburn, but we didn't see it. What we did see was a variety of indications of the Tyburn's former course, something to bear in mind when it rains and puddles form in the road, or when we hear an unusual street name or see a winding lane.
Field Trip: Fossil-hunting at Barton-on-Sea led by Iain Fletcher
25 July 2010
We had a fine breezy day for our fossil-hunting, as we met Iain in the car park at Highcliffe on the Hampshire coast beyond Christchurch for an introduction to the day. He gave us a simplified stratigraphic column of the Barton Beds and three pages of illustrations of some of the fossils. He also had some examples of the fossils to whet the appetite.
The field trip was advertised as being suitable for children and in the end there were two (see illustration) plus thirteen adults. We were warned against climbing up the unstable muddy clay cliffs, since the fossils are washed out of the clay on to the foreshore, but the temptation was too great for some!
We had hand trowels and plastic bags for our finds, and walked along the beach eastwards to the most productive areas. The boys were very enthusiastic and sharp-eyed; it is an excellent way of introducing children to one of the pleasures of Geology. To most of them fossils mean huge things like dinosaurs; so to find on a beach shells that resemble today's, but are 40million years old, together with sharks' teeth, and dig out from the clay different shells and fragments, similar to those found nowadays in warmer climates introduces them to the concepts of climate change and sea level variation over time.
In the end we had quite a good haul, with several shark's teeth of varying sizes; bivalves: crassatella, microcallista; gastropods: volutospina, clavithiles, haustator(?) plus worm tubes and scaphopods.
Naturally the children had the cream of the finds, but the photograph shows that even the remnants were not negligible! Many thanks to Iain for a very enjoyable day.
Vale of Fernhurst Geowalk led by Brian Harvey
12 September 2010
It was on a sunny, if slightly windy, Sunday morning that we assembled at the National Trust Woolbeding Car Park. Once we had gathered, Brian explained that we would be taking a shorter walk than on normal Autumn Geowalks, in order to participate in the Fernhurst Open Day. This was a fund-raising festival on the site of the Wealden ironworks at North Park Furnace. It was to raise funds to preserve the site, which was very important in terms of the industrial archaeology of iron working in the Weald. This now wholly rural area had once been the location of an industry of key importance for the country's defence. The organised activities at the Open Day would include the participation of the Sealed Knot, the society which re-enacts battles from the English Civil War.
We would be walking across the vale of Fernhurst and the Fernhurst Anticline, part of the broader Wealden anticline formed during the Alpine Orogeny, as a result of Africa colliding with Europe, following erosion of the chalk after tilting. The Fernhurst anticline showed signs of ongoing erosion, which would become apparent. We would be looking at the evidence of the erosion and landslips, and the general geology as it related to the iron industry.
En route to the site to the furnace we would be walking down the succession of the Lower Greensand Group and onto the Weald Clay formation in the Wealden Group.
We started off walking on the Hythe Beds, sandy sediments laid down in a marine environment relatively late in the history of the Weald, but before the Chalk. Much of the ground was covered by bracken, which likes well-drained and acid soils such as those which form on the Hythe Beds. We stopped by a trig point, still equipped with a mounting for a theodolite, but now no longer used by the Ordnance Survey, which relies on GPS.
Nearby, Brian stopped at a point on high ground overlooking lower ground and said we had reached a place where in a previous visit , some years earlier, he had predicted that the ground would slump. This had indeed proved to be the case. The Hythe Beds lay on Atherfield Clay, and the build-up of water on the clay surface had provided lubrication, triggering the collapse. Notable collapses had occurred in the area since the last glaciation, and the process was ongoing, part of the general erosion of the Weald. Breaching of an anticline, through erosion of strata weakened in tension at its core, may often lead to a reversal of its original topography. In the Weald it is common to see inward facing escarpments of higher ground formed of harder material, separated by valleys of softer clay in the eroded core of the anticline. There was a brief but inconclusive discussion as to whether the Weald was still subject to uplift.
Further on, we walked down a sunken lane or hollow way. Brian referred to the possibility of dating hollow ways by counting the number of different species of tree. The rule of thumb gave 100 years of age for each distinct species. Species were counted as we walked down the lane: lime, beech, holly, rowan, sweet chestnut, hazel, silver birch, oak, hawthorn. Some identified elderberry, and there was some uncertainty as to whether this counted. On the basis that there were nine distinct species, the hollow way could be dated to 1200.
Discussion turned to the use of the sunken lane. It had probably been used to transport to the coast cannon cast at foundries such as the one we were to visit. Cannon could be transported on large carts, drawn by oxen, with up to 30 oxen to the cart. Oxen remained an important means of transport up to the settlement of the American West in the 19th Century, contrary to the impression often given by Hollywood films which showed horses! In that context oxen could also be relied on, if necessary, as a source of food. The problem with this means of transport in the Weald, was that the sand hills alternated with valleys of clay, which bogged down transport, particularly in the winter. It took about three years to transport oak cut down in the Weald to the coast for shipbuilding. There was then discussion of a possible alternative means of transport for cannon, relying on the river Avon for access to the coast.
The problem became evident as we passed through a very clayey stretch in a wood at the junction with the Atherfield Clay. This was a very silty clay, the beds of which were laid down in a marine transgression. There was evidence of big landslides on the scarp face. Brian pointed out that the colour of the ground was not so different from the Hythe Beds. This was because the clay contains iron and rapidly oxidises from grey to yellow on exposure. A little later there was the sound of a gun going off. This was a welcome sound, as it indicated activity by the Sealed Knot at North Park Furnace, which we were gradually approaching.
We were passing through a wooded area, and there was discussion regarding the sustainability of the iron making industry in the Weald. This had been a matter of contemporary controversy when the industry was still flourishing. Fears had apparently been expressed in certain quarters in the Admiralty that the cutting down of wood to make charcoal for use in the blast furnaces was prejudicing the use of the woodland as a source of timber for shipbuilding. The consensus view of modern researchers, however, is that the industry was well-managed from the point of view of forestry, and that customary law had imposed tight controls on the use of the woodland resource, which ensured that it was sustainable.
We passed by depressions in the ground, which probably represented the site of old workings for iron ore. We were now on the Weald Clay, a formation which also contained sandy horizons and harder layers of iron pan, which was worked for the iron-making industry here. These took the form of pits or shafts some 15 ft (4.6m) deep, and were dispersed, reflecting the fact that the workable iron ore was found in lenses and was not continuous. Approaching the North Park furnace, the path passed through a cleared area and began to be covered with slag, which was thought to be original slag from the historic iron-working activities.
The site of the furnace itself was in a wooded area. We passed over the site of a dam with sluice gates behind which the river had been ponded up to provide the water power which operated the blast furnace. Wheels in the sluice provided the motive power for the bellows, the heavy machinery and the hammer, as illustrated by a working model which was on display.
The use of waterpower was a revolutionary innovation which had originated in the relatively industrialised Burgundian lands (now in eastern France and Belgium) in the later Middle Ages, and had been introduced into England in the late 15th century. This created a more effective process for ironmaking, which replaced the "blooming" process for ironfounding. That process had relied on hand bellows to raise charcoal fires to the necessary temperature to reduce the iron from the haematite in the iron pan, and had been carried on with little change since the Iron Age. The introduction of the water-powered iron furnace led to the production of guns on an industrial scale in the Weald, which continued there until it succumbed to competition from coke-fired furnaces in the North in the early part of the Industrial Revolution.
We looked at the archaeological map of the furnace and examined the wall of the dam, which clearly threatened collapse. English Heritage recognised the importance of the site, but further funding was dependent on fund-raising by local initiative, hence the Open Day. The proceeds, including those from a collection at the main entrance to the site, would go to the preservation fund.
Beyond the dam in the woods and in an open field beyond, which had a car park behind it, there were various attractions, in addition to the Sealed Knot. They included a display of the process of making charcoal, for use as fuel and in barbecues etc. Refreshments were available, including local beer, and there were a number of stalls selling books and other items of local interest. Some of us were entertained by a member of the Sealed Knot, who kindly explained 17th century social customs.
After an hour or so the main part of our party turned back, leaving a rear guard to watch the Sealed Knot and then return by car. Those of us in the main party retraced our steps slowly, the walk being uphill most of the way, and punctuated by the sounds of occasional cannon-fire from the Sealed Knot. We passed briefly going up the hollow way we had passed down earlier, to examine an exposure of the Hythe Beds. Brian said that these had formed in a marine environment in a subsiding basin fed by sediments eroded from high ground on the Cornubian and Armorican massifs, and from the London Platform. They showed flat bedding with very little current bedding, contained glauconite, and were bioturbated, demonstrating an oxygenated shallow marine environment.
Walking along higher ground towards the car park, some of us stopped to look at the fine specimens of spotted red fungi sporadically present to the side of the path.
Lecture: 'The Geological Evolution of the Thames Basin' by Andrew Newell
16 September 2010
Dr Andrew Newell, based in the Hydrology Group of the BGS at Wallingford, gave us an exhaustive account of the complexities of the Thames Basin, taking in Geological formations (125), and the effect of underlying Geology on Geomorphology in the global context of tectonic and climate change. Much ground was covered, both literally (an area of streams and rivers covering 20,000km2 including the Medway system) and figuratively in time (from the Silurian through the Carboniferous and the Jurassic to the Ice Ages and the Holocene.).
The whole talk was vividly illustrated by an enviable range of graphics to clarify the sequences of deposition, uplift and erosion, folding and faulting that have led to the moment we experience now, but which is just part of the continuing drama. The London Platform itself, laid down in Silurian seas, was an island in the Jurassic with marine, tidal oolitic limestones offshore. The effects of the Late Jurassic opening of the North Atlantic and the rifting that accompanied the opening of the South Atlantic in Early Cretaceous times, the deposition of the Hastings sands of the High Weald and the Wealden clays, and the Chalk which in the Late Cretaceous covered even the high point of the London Platform when relative sea level was 100-300m higher than today are significant Geological events.
At the start of the Tertiary the Icelandic mantle plume , located under North Britain, and causing 2.5km of uplift led to a karstic erosion surface on the chalk, and with the Alpine collision tectonic forces folded and faulted the region. Not quite finally, the Ice Ages changed the course of the Thames, as witnessed by the stratigraphy of the river gravels prior to the Anglian Glaciation. The Holocene has been dominated by sea level rise. Today GPS technology confirms that in the Thames Basin the combined effect of sea-level rise and land subsidence adds up to 2-3mmy-1. A chilling end to a stimulating lecture.
Field Trip: The Weald and Downland Open Air Museum
17 October 2010, Morning
What a cold day - I had to scrape ice from the windscreen, even though the sky was cloudless. We arrived at Singleton in time for me to change from fleece to cagoule so I committed the geologist's cardinal sin of leaving my hand lens in the wrong pocket. Unfortunately Di Smith had to cry off so Yvette introduced David Bone, a local geologist, who kindly took over. David started by introducing us to the geology of the area. We gathered in the shade (therefore chilly), on a grassy bank overlooking the site. His ploy, he said, to prevent us wandering off to have an advance look at the exhibits.
Geology of West Sussex
Singleton is situated on the South Downs just north of Chichester. The Hampshire Basin is to the south and the Weald to the north. It is on the southern limb of the Wealden anticline where the beds dip south. The geology is complicated here by a number of east west oriented smaller synclines and anticlines. The boundary between the Hampshire-Dieppe High and the Weald basin lies along the Portsdown-Middleton faults (just to the south) that underlie the Portsdown and Littlehampton anticlines. The River Lavant rises at East Dean and flows west to Singleton before skirting West Dean to turn south. It flows along the Singleton anticline. The river is a winterbourne and in summer, when the water table is lower the bed is dry. The riverbed itself is alluvial. Such small rivers cannot be taken for granted though and in a really wet winter Singleton can be flooded.
A glance at the topographical map shows quite a complicated system of dry valleys associated with the river valley. Just north of Chichester the alluvium passes downstream into Alluvial Fan Deposits. The Wealden Group crops out in the north east of the Chichester district. Both Horsham Stone and Sussex Marble occur as beds in the Wealden Group. Going south the succession continues with the, younger, Lower Greensand Formation and then the Gault and Upper Greensand Formation. At Cocking, about 4 km north of Singleton, the Chalk Formation outcrops and continues south of the village. There is an unconformity still further south and the younger, Palaeogene beds, the Reading Formation of the Lambeth Group and London Clay Formation (Thames Group) are preserved largely concealed beneath quaternary deposits. The Bracklesham Group, the youngest, is found only in the most southerly area, Selsey Bill, beneath Quaternary deposits. Its offshore extent is poorly known. What about below? Organic rich shales forming petroleum source rocks were laid down in the Lower Lias, the Oxford Clay Formation and the Kimmeridge Clay Formation. During Cainozoic uplift the hydrocarbons migrated towards the margins of the Weald Basin. On the southern margin they accumulated mainly in the Great Oolite carbonate rocks (Middle Jurassic). Under Singleton there are east-west faults resulting in two horsts on either side of a narrow graben in which the hydrocarbon is trapped at 5000 feet. The 'Goodwood Gusher' is in Singleton Forest about 2km to the north so is not visible. The oilfield is under Singleton itself so has been drilled at an angle. Oil is trucked out to the Holybourne rail terminal. It is planned that gas, at present flared, will be used for power generation. Since production started in 1986 recovery is about 3.5% (2009). A recovery factor up to 10% should be achievable.
The Weald and Downland Museum
The Weald and Downland Museum was set up to establish a centre that could rescue representative examples of vernacular buildings from the south east of England. This is important to geologists because the building materials will be locally available and inform about the local geology.
A house from Walderton, Sussex Walderton, SU 790107, is south west of Singleton but still on the South Downs. This flint and brick house has been through various designs in its history even before it was rebuilt at the museum. We ignored the inside and looked carefully at the flint on the left side wall.
Fig. 1 shows a representative sample. First some flints have a white coating but others are dark outside. Immediately we have to sharpen our thinking. No! It isn't just flint. Weathered flints turn white so they are field flints. The dark coated flints have not been weathered so must have been quarried. Also shown is an example of sheet flint (in a minority) and below it a banded flint. These parallel lines may be a sign that the silica has deposited rhythmically in bands, which differed in their water content. There are other theories though. The front door steps, fig.2, seem to be a later addition as the picture of the original building, in the guidebook, does not appear to show steps. The impressive symmetrical ripples with flat (so eroded) tops occur when ebb and flow currents wash gently backwards and forwards over sand grains on a beach or when river currents gently meander back and forth in the shallows. The sandstone is well cemented with a calcareous cement so is hard and strong. Those who had been to Horsham with Roger Birch recognized Horsham stone. As Horsham is quite a long way north of Walderton and medieval roads were poor it seems even more unlikely the steps are original.
Medieval shop from Horsham, Sussex (TQ170306). The shop was part of Butchers Row (Middle Street) and was last occupied by Robert Dyas, Ironmonger. The roofing slates are Horsham Stone, fig. 3. The stone is split along its natural cleavage to make the slates.
This roofing material is very heavy and the wooden beam supports often bend under the strain. The slates at the top have a smaller height than those at the bottom.
School from West Wittering (SU7898), a coastal village. An open-ended cart shed was converted to the village school in the early eighteenth century. The stable, in the yard, was possibly for the schoolmistress's horse. First the stable, which has flint walls but these flints are rounded with chatter marks, fig.4. The school is next door with more decorative walls. Each row of flint is followed by a course of stone, fig. 5.
Our flint lesson continued. The flints are cobbles from a beach. There was some discussion as to their origin. Some could be ship's ballast but these are Tertiary. David suspects that they were transported up the narrowing Channel by ice (from an ice sheet not from a glacier). This process could have been quite lengthy as sea levels rose and fell; or it could have been a single event, say driven by a tsunami. Anyway if large blocks had been transported they are likely to have made their way down through the sediment. There are gouges in the channel to support this idea. Then we examined the stone. David's acid produced a fizz so they are calcareous. In fact they are sandstone with calcareous cement. Some had holes bored in them. Fig. 6, shows one hole with its late owner still in residence. This is a modern Piddock shell and suggests the rock was collected by boat from an offshore source. It is from the Bracklesham Group.
Granary from Littlehampton (fig. 7). Littlehampton (TQ0202) now is a coastal resort lying east of Chichester. The date above the granary door is 1731. We were interested in the sixteen stone staddles (or steddles). They are calcareous and close inspection showed shell debris and oolites. They are Portland Stone. The museum guide comments they are identical to those under a granary from Goodwood so were probably mass produced and available over a wide area.
Cattle shed from Goodwood. Goodwood is just along the road from Singleton. Here we examined flint galleting (also called garneting or garreting), fig. 8. With so few flakes this is not a particularly good example. Flint flakes have been pushed into the lime mortar. It is likely that these flakes are new and specially bought in imagine cleaning the mortar from so many flakes! We discussed their purpose. It could just be for decoration but, as lime mortar is slow to set, they could be useful as spacers so the weight of the wall does not squeeze out the mortar.
Market Hall from Titchfield Back at the market place is a more prestigious building. The Market Hall was built at Titchfield, Hants (SU541047) almost certainly by Henry, third Earl of Southampton. Perhaps he thought it would contain the riotous behaviour by locals who had already had their market days reduced to five by the second Earl. Here the paving of the open arcade, used for trading, interested us, fig. 9. The black stone was quite shiny. There were numerous shells showing e.g. Viviparus. This limestone is Purbeck Marble. Note that Purbeck Marble is not a true marble but polishes nicely. The base of the low wall just outside is unpolished Sussex Marble. It is a rough and lumpy stone as it consists of Viviparus shells. Again this stone is not a true marble but would also polish nicely. Purbeck Marble slabs lie on top. The Viviparus species e.g Viviparus fluviensis are freshwater gastropods. The Viviparus shells in Sussex Marble are larger (Viviparus sussexiensis) than those in Purbeck Marble. The flints in the path along side are brownish and subangular so there is yet another source. They have been washed off the Downs to the coastal plain and suffered frost shattering. They could have been picked up as field brash. They are not high enough quality for building. They tend to be too brittle as well as not looking good.
House extension from Reigate (TQ254503) The original house stood at 43 High Street. The extension dates from the early seventeenth century. It illustrates the problems associated with restoration. The basement, walls and magnificent chimney were built of Reigate Stone (from the Upper Greensand Formation), which has very external weathering properties. The museum would have liked to rebuild it using Reigate Stone but, unfortunately, it is no longer available so similar stone from Chilmark takes its place (Chilmark Stone is an approved replacement). Reigate Stone was used in many prestigious buildings. The Earl of Surrey, William de Warenne, the earliest recorded Quarry owner was an influential man in England which may explain why Reigate Stone was used in many royal buildings during the Middle Ages. Reigate Stone was mined from hills around Reigate. It is a freestone which can be cut and carved easily.
I wondered if Peter Burgess, the author of Surrey's Ancient Stone Mines, could throw any light on the quarry. There was a quarry on the west side of Nutley Lane (which is a north turning off the High street. He replied: 'It would be almost impossible to tell. The most likely source would be a quarry close to Reigate, but in the early 17th century there may have been a number of possible sources. The quarries at Chaldon were definitely still being worked, and it is likely there were others near Merstham and Gatton. One important potential source at that time may have been the castle which was being robbed of its building stone, although I wouldn't like to say when this really started in earnest. Sorry not to be more definite, but as you might imagine, documentary sources are extremely rare or nonexistent, so all we really have to go on is conjecture'.
Pugmill House from Redford A wide variety of local building stones are used in the museum exhibits but extensive restoration has been necessary, as many buildings were not moved there until they were in pretty poor condition. After all most buildings are not designed for museums. A few have fallen foul of redevelopment and have been donated. However this is not a good area for building stones. There are no large quarries as good stone often comes from thin seams. This means other building materials such as wood, wattle and daub, and thatch have an important role as do the man made materials bricks and tiles. Fortunately clay is widely available. So it is appropriate that our next stop is at Pugmill House from Redford (SU865263). People did not live in this, open, hexagonal building. It is home to a pugmill, which used horsepower to prepare clay in a small rural brickyard (fig 10). The brickyard that it served is shown on the Tithe map of 1838 but all trace has now disappeared. Redford is just northwest of Midhurst on the Hythe Formation of the Lower Greensand Group.
We saw Woolbeding Common, overlooking Redford, on Brian Harvey's geowalk this year. The only Lower Greensand Group building stone is from the Hythe Formation. It is glauconitic fine- to coarse-grained sandstone and is of mixed quality. It is greenish grey where fresh and greenish buff to pale yellowish brown where weathered. Iron is oxidised rather than glauconitic in these stones so the stone is brownish. Some sands have been hardened to beds of compact non-calcareous sandstone. The sandstone has been used locally for building stone. The compacted stone is favoured as the building stone as it is easy to dress. The hills around Redford show a stepped topography indicating that they were quarried so possibly supplied the stone.
To me Pugmill House is a bit of an enigma. What on earth is stone doing in a brickyard? What is more the stones are rectangular and fit in quite well with the bricks; although it seems that the sandstone beds around Redford are quite thin. Other walls look as though they may have been patched with stone but this wall seems more than just patched. Surely it must have been more convenient to use bricks, which would have been a much better advertisement for the product? I asked David if he could clarify this point: 'Redford is located to the west of Midhurst / Haslemere and the Sussex Industrial History Society in 1972 record 'Redford, former brickworks SU 862262 all that remains of a small brickyard is the horse-gin house which may have contained a pug-mill. It is hexagonal, built of brick and local stone, with a tiled roof and brick floor. It is in fair condition and now houses agricultural machinery'.
Regarding the use of stone in the construction of the Pugmill, I suggest that (a) the stone panels perhaps make more of a feature of the surrounding brickwork and/or (b) the Hythe Formation sandstone is so cheap and abundant that it makes the first choice building material. The Hythe Formation deposits are extensive but inconsistent in producing good quality building stone and a high proportion often has to be discarded'.
Today there are two quarries that supply Hythe stone. Local Stone Co. at Little Bognor near Fittleworth supplies stone for anything but building. The Cowdray Estate at Easebourne near Midhurst sells stone for building walls as well as rockeries.
Lurgashall Mill Now to the museum's pièce de resistance - a working water mill. Lurgashall is Northwest of Midhurst on the Weald Clay Formation. There are several ponds around there but the one at SU940258, marked as Mill Pond, is its original home. It is no mean feat to transfer a water mill to a site where the river is ephemeral; even more so when it has an overshot water wheel. The cast iron waterwheel is 12 feet in diameter. Millponds are usually big as the water demand is high. When grinding this water wheel turns at 6 r.p.m., each turn using about 320 gallons of water. Two millponds, at about 12 feet height difference, had to be excavated and lined, then filled with water. As natural replenishment is inadequate a powerful pump has been installed and the water is reused. Pumping from a borehole makes up losses. However there is a neat way of reducing pumping costs. The bottom pond is in two sections. One section is lined and that section is used in summer during low rainfall. In winter water from the chalk is able to run into the other section and both sections are connected.
The Leconfield Estate donated the mill in1973. The building itself was dismantled and re-erected on site stone by stone. The stones, from the Hythe Formation are square cut. Some stones are quite substantial (fig. 11) so the sandstone beds in the quarry must have been quite thick. The mortar includes galleting, which could be made of Horsham stone. Lurgashall is on the river Lod, a tributary of the River Rother so water transport may have been used to move them upstream. It is not obvious from the guidebook whether the steps up the side are original. They are Horsham stone slabs resting on Sussex marble risers. At the top of the steps is an oolitic steddle stone - but it is not obvious why.
We shopped inside the mill for stoneground flour and Lurgashall Mill biscuits. There is a small set of grindstones to try grinding flour by hand. All the time the mill was working too. It sounds so contented. It should be obvious by now that no local stone has the right characteristics to be a millstone. Although stones quarried near the mill have been used to grind animal feed where quality is less important. Derby Peak, a millstone grit, is the best of English stones but millstones are no longer produced there. The stones were cut in one piece, roughly shaped with the central eye cut, and then sent out ready for dressing nearer the site. The museum uses French Buhrstone (Burrstone) millstones from La Ferte-sous-Jouarre (highly prized by millers). This freshwater quartz occurs in smaller lumps, which have to be lightly cemented using plaster of Paris and then bound with iron hoops.
Finally: We owe many thanks to David for sharing so many of his ideas with us. How lucky we were that he could step in at such short notice. It is not often that visitors test museum exhibits with hydrochloric acid but David's bottle seems to be quite at home here. Anyway nobody rushed up and told him to put it away. He is obviously a well-respected member of the community. Best wishes to Di Smith. We hope she gets better soon.
Field Trip: Fishbourne Roman Palace
17 October 2010, Afternoon
About twenty-five members of London OU Geological Society spent the afternoon visiting Fishbourne Roman Palace near Chichester in Sussex. Numerous finds from the excavations are on display; the information boards give background reading followed by a large collection of Roman mosaics at ground level where parts of the heating system can also be seen. In the outside area are the reconstructed Roman gardens. David Sharp (a tour guide from the Fishboune Roman Palace) gave an introductory talk about the palace telling us about the many excavations (and difficulties) which have occurred while building up the knowledge we have.
A model of the palace helped give a visual impression of what the palace may have looked like in its heyday. The palace was a chance find in 1960 while the previous landowner, a local farmer, was preparing the land to sell. About half the site of the palace (6.5 acres) was bought from a generous donation by Ivan Margary and left in trust to the Sussex Archaeological Society, and the palace officially opened to the public some years later in May1968. It is thought that Fishbourne palace was the largest Roman building to be found northwest of the Alps and was a display of great wealth and status. The site had to be levelled before the palace could be built; and a large drainage channel excavated to take water to the sea where a shipping basin existed nearby. Stone was brought in from other parts of the Empire to use in the construction. The completed palace building would have surrounded the gardens on all four sides.
David Bone, a local geologist, then gave an informative tour of the palace talking about the building stones of the palace and grounds. We were shown several types of stones mostly in the palace gardens. As well as the stone brought in from elsewhere in the Empire a variety of local stones were used. Chalk and possibly some greensand (of irregular shape) were used to build some of the walls. Malmstone (Upper Greensand with a cream or white colour) is also in evidence.
The stones used in the columns and gutters have come from the Hythe Beds, and there are also examples of Lavant Stone (a phosphatic chalk, coarse grained and partially cemented) from the north west of Singleton. Also present are Bembridge Limestone from the Isle of Wight. Very few stones used by the Romans remain and were probably reused elsewhere.
The afternoon's visit to the palace at Fishbourne followed a visit to the the Weald and Downland Open Air Museum which I felt was an interesting contrast. Both museums displayed building materials from different periods of time and in different settings. Overall the field trip was informative and very interesting.
Lecture: 'Reconstructing late Quaternary climate variability in the Russian Arctic' by Angela Self
21 October 2010
Angela, a Post-Doctoral Research Assistant on Climate Change in the Arctic at the UCL Environmental Research Centre, gave us fascinating insights into the nature of her work in the Russian Arctic.
Records are very sparse, and most proxies for climate change, such as tree rings, lake varves, corals or icecores cannot be used where the vegetation is tundra. Her chosen proxy is the chironomid, a non-biting midge, the early stages of whose life-cycle take place in water. Only the head capsule of the larva is preserved in lake sediments and the distribution is studied in cores taken from the deepest part of the lake. Angela passed round two specimens of the tiny creature. Chironomid distribution is strongly related to mean July air temperature and continentality, and thus palaeoclimates can be reconstructed.
Certain areas are out of bounds, but the lecture dealt with two widely separated regions: the Putoran Plateau in western Siberia and Lake Kharinei in European Russia. One of the pleasures of working there in the short summer is 24h daylight, all the flowers out at once, birds nesting in tiny trees. However quite apart from problems of getting equipment to remote locations in mostly bleak weather, current conditions for the population sounded grim with chemical and nuclear pollution, high unemployment, a high suicide rate and a life-expectancy of 45-50y. Climate change is predicted to have a particularly strong effect in the Arctic, with the northward migration of tundra and the loss of habitat, and the extinction of maybe 60% of species, so research into past changes is vital.
Field Trip: Norfolk Winter Weekend led by Jill Eyers
29 - 31 October 2010
This residential Field Trip was advertised as being led by Jill Eyers, but when she was forced to pull out, she was replaced by Paul Logan, who agreed to take over at very short notice, and proved very knowledgeable and friendly. We are extremely grateful to him for giving up his weekend for us. We gathered at the Royal Hotel, Mundesley, on Friday evening in time for a drink in the bar to meet Paul and get to know him over dinner. The hotel helpfully allowed us to use our part of the dining room afterwards, so that Paul could introduce the geology of the region with its Cretaceous and Pleistocene sediments deformed by the effects of the Anglian and other glaciations. We were warned that the tides were not in our favour this weekend, and early starts were imperative. Russell Yeomans, an enthusiastic local geologist, filled in some of the gaps with photographs and his collection of fossils.
Saturday 30th October - morning
Happisburgh Our first location on Saturday morning was Happisburgh. In keeping with the OUGS custom, Paul encouraged us to make our own observations and consider what they suggested about the environment in which the rocks were laid down.
The cliff was approximately 8-10m high. At the base of the cliff we observed a grey clay containing shell fragments and some gravel. The clay had no structure and there were small chalk rafts incorporated in the clay (we were to see more impressive examples of these later in weekend). Overlying this structureless clay was a sequence of laminated clays and silts which generally coarsened upwards with sands towards the top. These were distorted at the base and there was evidence of ripple marks towards the top of the unit. Immediately above this sequence was a pebble bed followed by sands. The interpretation presented was that the structureless clay at the base of the cliff was a subglacial lodgement till (the Happisburgh Diamicton) formed at the base of the Anglian ice sheet. This is succeeded by deltaic sediments (the Happisburgh Clays) deposited in a glacial lake derived from meltwater. The presence of ripples indicates a lake environment (asymmetric ripples) and turbidite events (symmetric ripples). The upward coarsening of the sediments implies that in general the delta system was prograding. The overlying sandstone unit (the Mundesley Sandstone) represents the deltaic fan building out into glacial lake with the pebble bed representing the base of the fan.
Di Smith explained that as part of the Ancient Human Occupation of Britain (AHOB) Project excavations have been carried out on the beach at Happisburgh. These excavations revealed finely laminated estuarine silts and gravels deposited by an ancient river channel (the proto-Thames). Excavations have revealed flint tools and butchered bones, making Happisburgh the oldest recorded site for human occupation in Britain (800,000 years).
Blakeney Esker Our next location was Blakeney Esker. We parked up at Wiverton Down and started by walking around the site noting the variations in vegetation and what this indicated about the soil and geology. The dominant vegetation on the esker was gorse indicating acidic conditions (sands and gravel). Looking out from the esker the surrounding area was generally cultivated with crops signifying rich soils (chalky till which represents subglacial lodgement till similar to that exposed at Happisburgh).
The structure of the esker is a sinuous, linear ridge with steep slopes to the sides. The ridge runs northwest to south-east from Blakeney towards Glanford. Wiverton Down is thought to represent the southeastern extent of the esker although isolated hills to the south-east may represent eroded remnants.
Sand and gravel was extracted from Blakeney Esker from the 1940s to 1980s leaving an overgrown quarry pit adjacent to the car park. Inspection of a quarry exposure revealed a poorly sorted deposit, predominantly flint gravel within a fine (sandy) matrix. The flint pebbles show imbrications (i.e. they were aligned with the direction of current flow) Flow was generally to the east however variation in the alignment of the pebbles suggests that the flow direction varied possibly as part of a meandering system. The large size of some of the pebbles implies high current flow. However the poorly sorted nature of the deposit and layers of fine sediment indicate that the flow was variable.
Interpretation of the esker indicates that it was formed by the deposition of sediments from a subglacial river carrying meltwater; the flint pebbles being derived from the underlying chalky till. The surrounding area was generally flat and represents the glacial outwash plain with mounds of glacial deposits (e.g. the Cromer Ridge and Beeston Bump).
Saturday 30th October - afternoon
On this glorious autumn day, we stopped for lunch at Cromer where there were many places to refresh ourselves and to see. Cromer museum with its Victorian fisherman's cottage is well worth a visit as it has a geology gallery that includes some of the bones of the West Runton elephant are on display.
After lunch, we all gathered in the car park to drive the 2œ miles westwards to West Runton, to go onto the beach to investigate what was billed as "one of the most important Quaternary localities in the British Isles", where the skeleton of a woolly mammoth (Mammuthus trogontherii) the West Runton Elephant was found in 1990. This SSSI shows deposits from several glaciations and an interglacial. We would see the deposits but were not to be so lucky with elephants!
The cliffs represent the extreme fluctuations that occurred during the Ice Age with temperate and cold stages. Allied to the changing temperature, were major advances and retreats of the sea. We first examined the cliffs to see the different types of deposit. At the base of the cliff, which is where fossil mammal remains had been found, we found sand and silt with some peat. This was the West Runton Freshwater Bed, which is part of the Cromer Forest Bed formation.
As the tide ebbed, the Wroxham Crag became evident. It is a relatively thin layer of tidal flat and coastal sediments (about 200 mm), which has shell debris, is iron rich, well cemented and with both angular and rounded flints. This is the base of the Pleistocene at 2m years and lies immediately on top of the Beeston Chalk (Campanian) that contains some paramoudra flints a gap of 65-70m years! It was not certain whether there had been deposition between, which had been eroded, or there had been a large gap in deposition. As there is no evidence of intervening deposition the latter is thought to be most likely. In fact at the end of the Cretaceous there was uplift with no sediments being deposited until the early Pleistocene when the marine Crags and freshwater Cromer Forest Bed Formation were deposited.
At the regression from the marine Wroxham Crag we observed 3m of tidal silts, the West Runton Freshwater Bed at the base of the Cromer Forest Bed Formation where the elephant was found. This was deposited in the Cromerian an interglacial when sea level was falling, prior to the onset of Anglian glaciation that started about 480,000 years ago.
Paul Logan gave a description of sequence stratigraphy that associates deposition with falling or rising sea level, which was born in the oil industry because of the importance in finding sediments containing oil. Looking at the cliff to the east of West Runton we observed the thick layer of glacial deposits that overly the Cromer Forest Bed. Fortunately, West Runton had a station as I had to return to London on the Saturday evening, therefore missing further wonders that day and on Sunday.
Sunday 31st October - morning
After an extra hour in bed thanks to the clocks changing, we arrived at Overstrand in good spirits despite a grey sky and watery sun. Our objective was to view some spectactular glaciotectonic structures resulting from actions of the Anglian ice sheet. We made our way to near-beach level down a very steep and slippery slope. It was then decided that the rocks were far too slippery for us to proceed safely and anyway the tide was advancing rapidly. I think I heard Paul, our leader, mutter something about it being better if we'd set out at 0600h!
Undaunted we moved along the beach in the opposite direction where we were able to view some chalk rafts in the cliff above us. There was evidence of thrust faulting and someone likened the procedure to pushing a fridge along a carpet. What we had missed seeing on the other side were double thrust faults and overturning but thanks to Di Clements who later sent us a photograph from her previous visit to Overstrand, we were able to get the idea. We spent some time discussing various sub glacial features visible in the cliff face and puzzled over a "V" shaped structure filled with overlying deposits which Paul thought might be an ice wedge. We weren't entirely convinced.
We climbed back up to our cars and made our way to Kelling Heath. A pretty walk through oak woodland and a short climb took us to the top of this fluvial deposit outwash which marked the limit of the Wolstonian glacier. From the top we looked out over the outwash plain towards the sea and admired the nearby kame, another glacial structure. A whole series of braided river systems would have carried the sands and gravels from beneath the melting glacier with the volume of sand and size of gravels depending on river strength, hence on season or climate change. The river system could have been tens of kilometers wide and there might have been up to 100m of till on the plain. Awe inspiring.
We made our way back to our cars, stopping on the way to watch one of the North Norfolk Railway steam trains progress across the plain. We then headed for Salthouse where we enjoyed a good pub lunch before heading homewards.
Lecture: 'The Role of a Mantle Plume in the Formation of the Siberian Traps' by Andy Saunders
9 December 2010
Our last lecture of 2010 was given to a big audience after our usual little Christmas celebration by professor Andy Saunders of the Department of Geology, University of Leicester, who started by acknowledging his main collaborator, Dr Marc Reichow. The lecture would deal with Large Igneous Provinces (LIPs), mantle plumes and environmental catastrophe, concentrating on research into the Siberian Traps, erupted about 251 Ma. With a diagram of extinctions and LIPs, he showed that the eruption of the Siberian traps coincided with the end-Permian mass extinction. The cause could be the huge volume of dissolved gases brought to the surface, followed by an ocean anoxic event, and global warming.
After discussing the evidence for mantle plumes, referring to the plume trail that can be followed from the Deccan Traps to Réunion, and also to the North Atlantic/ Iceland province, he looked in detail at Siberia, with a map showing the estimated extent of the main outcrop of basaltic rocks on the Siberian craton and beneath the West Siberian Basin, one of the World's biggest swamps, extending under the Kara Sea to the north.
There followed the fascinating possibility that the plume under Iceland today could be the same one that produced the Siberian Traps, having tracked via the high Arctic. Both have the same high temperature lavas, with large volume. In that case, could there have been a precursor to the Siberian Traps? The origin of such plumes may lie in the latent heat of crystallisation of the Earth's core, with an excess temperature of 1000º C at the core-mantle boundary. Many thanks to Andy for an exciting lecture.