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.
Gold Giants of the Great Silk Road • Sumatra to Java • Royal Observatory Greenwich • Geology of the Moon • How Britain Became an Island • Quarries of Ardingly and Horsham • Magnetism in the Solar System • 20th Anniversary Pooh Geowalk • How to Make and Break an Asteroid • The Auvergne • Groundwater under London • Folkestone Warren • Wegener • Monitoring Volcan de Colima • Western Weald • Winter Weekend in the Mendips • Members' Evening • The Ancient Human Occupation of Britain
Lecture: 'Gold Giants of the Great Silk Road: Expedition across the Tien Shan in Central Asia' by Reimar Seltmann
17 January 2008
Dr Reimar Seltmann is coordinator for the Centre for Russian and Eurasian mineral Studies at the Natural History Museum. He brought with him amazing specimens of gold-bearing ores for us to handle, as well as a robe decorated with gold thread, a hat and slippers. The Gold Giants in question turned out to be huge mines in Uzbekistan and the Kirghyz Tien Shan in Central Asia. It is impossible in a brief account to do justice to the wealth of information we were given.
The deposits of gold, copper, silver, uranium, and base metals, some arc-related and some orogenic, stretch from the Urals in the west to Mongolia in the east, and result from magmatic episodes from the Ordovician to the Jurassic. Arc-related deposits yield higher grade ore, but lower tonnage and orogenic deposits, lower grade, but huge tonnage. We looked at two of the mines in particular, Muruntau in Uzbekistan and Kuntor in Kyrgizstan. These are both open pits: Muruntau is 6km across, its minerals the result of granitee magmatism, and Kuntor is more than 4000m above sea level in fault zone. The lecture ended with a brief account of a 2006 expedition to Mongolia and was followed by a lively question and answer session.
Lecture: 'From Sumatra to Java' by Nigel Harris
2 February 2008
The Tectonics of Risk along a Plate Boundary The AGM was well attended, so maybe this brief account is superfluous, but it was a fascinating lecture. Nigel is Professor of Tectonics at the Open University and the material, as well as being very relevant to those studying S279, S339 and SXR339, provided lots of interest to the rest. Discussing the continuing rapid northwards movement of the Indian-Australian plate and its subduction under the Burma plate, he moved backwards from the Sumatra earthquake of 2004, via Krakatoa (1883) and Tambora (1815) to Lake Toba c.70000ybp, all part of a collision system. Tibet a zone of active crustal thickening, Precambrian gneiss shunted over Holocene sediments in the Indus, high-grade metamorphic rocks being squeezed southward, partial melting of the crust about 20km down, and much more!
After referring to the devastating Kashmir earthquake of 2005, Nigel concluded by saying that climate-related disasters are much more dangerous than earthquakes and volcanoes. The Himalaya / Tibet disruption of normal Hadley circulation, the resultant monsoon with the floods on the Ganges and Brahmaputra, post-monsoon cyclones, and the East Asian monsoon with flooding of the Yangtse and Huang Ho kill a lot of people.
Field Trip: Royal Observatory Greenwich
20 February 2008
The outing at the February half-term this year was to the Peter Harrison Planetarium at the Royal Observatory Greenwich. Opened by the Queen in May last year, this is now the only planetarium in London dedicated to astronomical science. The group of 24 members met beside the distinctive bronze conical capping over the planetarium dome in time for the 2 O'clock show entitled 'Star Life'. The life-cycle of stars from the condensation from cosmic dust through to their ending, often in spectacular supernova explosions, was illustrated by images from modern space telescopes. There was not much that would be new to someone who had completed S282, but the images projected onto the planetarium dome were outstanding.
Following the planetarium show, the group divided into two parties. While one group explored the three new galleries with their interactive exhibits on modern astronomy, the other enjoyed a guided walk through the Meridian Building to learn how astronomical timekeeping solved the problems of navigation and how Greenwich Mean Time became the time standard for the whole world. Bradley's transit telescope of 1750 was the basis of time measurement for the Nautical Almanac, first published in 1776, and the meridian of this instrument became the de-facto standard for navigation. An international conference in 1884 accepted the meridian of the transit telescope at Greenwich as the Prime Meridian for timekeeping, although by that time the main instrument at the ROG was the Airy Transit Circle, some 6m to the east of the Bradley meridian. Accurate measurements in the 20th century showed that the Earth was not a sphere rotating at a uniform rate, but a rather irregular ellipsoid with mobile tectonic plates and irregular rotation. This required a re-definition of the basic geodetic coordinate system, and the resulting International Reference Meridian is currently 107m to the east of the Airy meridian. Using a GPS receiver set to WGS84, we found this meridian unmarked in Greenwich Park outside the Observatory.
Lecture: 'The Geology of the Moon' by Ian A Crawford
20 February 2008
Another really well-attended meeting after our Halfterm visit to the Greenwich Planetarium with Eddie. Ian is Senior Lecturer in Planetary Science in the School of Earth Sciences, Birkbeck College. With a long-standing research interest in astronomy and astrobiology, he has specialised in recent years in the study of the Moon. Interest in Lunar exploration has been reborn lately, with countries like China, Japan and India taking a hand, and in this context he gave us a fascinating overview of what is and what is not known about our nearest neighbour in space.
With samples of anorthosite and basalt he graphically illustrated the difference between the Lunar highlands and the maria, pointing out that we don't know why the two faces of the Moon are so different, nor why the huge impact basin near the south pole on the far side is not flooded with mare basalts. There is not even a consensus on the origin of the Moon though the giant impact theory appears the most likely. The very old surface of the highlands records the early solar system, with minerals never hydrated or altered in 3.8Gy, but the sparse samples we have of lunar rocks (from only nine localities on the near side of the Moon) leave a lot of gaps in our knowledge. Remote sensing with X-ray spectrometers can give information on the internal structure and stratigraphy, but the last visit to the surface of the Moon was in 1976. He ended with the question: is it not time to go back?
Lecture: 'How Britain Became an Island' by Sanjeev Gupta
13 March 2008
Originally billed as: 'The Great River of the English Channel', this lecture was an enthralling account of current theories of how and when Britain was separated from mainland Europe. Sanjeev works in the Department of Earth Sciences and Engineering at Imperial College, London, and his team, together with Graeme Potter of the UK Hydrographic Office, have been conducting detailed mapping of the bed of the Channel using multi-beam sonar and GPS, with some funding from English Heritage because of the archeological possibilities.
Speculation about the gap in the chalk cliffs at Dover has a long history; early suggestions show the Rhine and the Thames confluent and studies through the 20th Century consider a filled valley system, glacial erosion and tidal scouring. We were shown the bathymetry of the Channel, concentrating on the palaeo-Solent and the palaeo- Arun, giving us a 3D perspective view, so that we could see the streamlined islands and other landforms in a valley 10-15km across. The original valley could have been 45km wide. These islands, up to 10km long, are seen to be the result of a catastrophic flood that broke through the chalk ridge of the Weald-Artois anticline, from a huge pro-glacial lake dammed to the north by the coalesced Fennoscandian and British ice sheets and to the south by the rock ridge, perhaps 500,000years ago, resulting in the isolation of Britain, where for 100,000y there is no evidence of early humans.
He compared the resulting landforms to those resulting from the Missoula Floods in the United States, an extraordinary subaerial formation, and also to the same sort of islands in the Ares Valles on Mars. Questioned as to the mechanism of failure of the chalk barrier, there can only be speculation: perhaps an earthquake? We need to have a close look at the Straits of Dover.
Field Trip: The Quarries of the Ardingly Sandstone and Horsham Stone led by Roger Birch
5 April 2008
The day started in surprisingly good weather at 1030hrs, at Philpot's Quarry in West Hoathly (TQ357324). There were about 20 of us. After handing out a collection of notes and diagrams, including a copy of the famous Philpot's Quarry section by Allen (1989), Roger started by giving us an introduction to the geology of the Weald in general. We then made our way across to the large storage area, full of canvas bags filled with rough stone, where we were able to browse - searching for fossils, charcoal fragments and other fluviatile features.
From there, we went down into the quarry itself. The floor is, apparently at the surface of a ferruginous intra-formational conglomerate, containing debris from a lateritic soil although none of it was exposed to view. Ahead of us, was the lower lift at the quarry face, about 4m high, with the Ardingly Sandstone itself near to the top. This forms part of the Lower Upper Tunbridge Wells Sand Formation and is about 140 Ma old. Fluviatile in origin, it is a massive, clean, soft, fine to medium-grained and quartzose (>95% quartz) sandstone that has been used extensively as building stone in the Weald including nearby Wakehurst Place and Bateman's. Quoting from English Nature, it shows very low-angle, large-scale trough cross-bedding facing south-south-east. Water flow down the delta-fan slope was intermittent, varying from powerful (horizontal laminae with current lineation) to weak (ripple cross-lamination) to nil (wave ripples, often draped with trailed and burrowed or sometimes sun-cracked clay films). Molluscan body fossils Neomiodon, Viviparus and Unio record freshwater conditions. The bottom 1m contains abundant upstanding Lycopodites (club moss) in life positions, associated with small structures showing that the plants grew upwards as the sand accreted around them.
The eroded surface is overlain by the Top Lower Tunbridge Wells Pebble Bed, which forms the floor of the bench from which we were able to study the debris-strewn lower slope of the 8m thick Lower Grinstead Clays. There, we found minute ostracods (water fleas) and Neomiodon but no sign of any of the Equisetum lyellii (horsetail), Unio or Viviparus that have also been found there! Fine plant debris is apparently common and disarticulated dinosaur bones have been occasionally found near the base. Again, because of the debris, we were unable to study the Pebble Bed itself. At this particular face of the quarry, the Ardingly Sandstone is heavily jointed near-vertically (the result of valley bulging and cambering) and is overlain here by a series of thin sandstone layers. Calcitic flowstone was seen within some of the cracks.
Back up at ground level, we went for a tour around the mason's yard where we were able to see stone being worked ranging from fine cornices to rough walling stone. In the Mess Room, a fossilised Lepidotes (a carp-like freshwater fish) has been built very conveniently into the wall for all to see!
We then drove off into the nearby village of West Hoathly (TQ363326), for a picnic or pub lunch. This beautiful little village is full of fine old houses and cottages, many roofed with Horsham Stone as is the lych-gate to St Margaret's Church. After lunch, we had a look at the Sussex Marble font in the Church and took note of the local sandstones used in the walls. This marble, otherwise known as Large 'Paludina' limestone or winkle stone, is made up largely by Viviparus fluviorum and comes from just above the Horsham Stone in the Weald Clay succession.
From there, we drove westwards to the Lower Broadbridge Farm (TQ142314). Here, Horsham Stone in all shapes and sizes was on display from a very large ripple-marked slab to small slabs and heaps of rough walling stone. The quarry or delve is inactive at present and, somewhat disappointingly, over half a kilometre away to the NW. Horsham Stone is a hard, fine-grained and calcareous sandstone deposited at the front of an alluvial fan, the sediment being originally derived from Cornubian granite, as is evident from the presence of tourmaline. This lower, arenaceous phase of the western Weald is near the base of the Lower Weald Clay Formation and about 120 Ma old. It caps the escarpment that forms an arc around Broadbridge Heath, Horsham - on the west side, from north to south-east.
I personally had read and found a paper by Jonathan Radley (Geology Today, May 2006) and an Information Sheet produced by English Nature (referred to above) to be excellent preparation for this visit, not least for writing this report, and I am sure that Roger's book on "Sussex Stone" will be just as interesting! Nicole expressed our thanks to Roger at the end of what had been a fascinating day's geology especially to those like me who have grown up with the Weald on their doorstep!
Lecture: 'Magnetism in the Solar System: the Journey from the Sun to the Centre of the Earth' by David Price
17 April 2008
David, a Professor in the Department of Earth Sciences at University College, London, gave a packed room a fascinating and intellectually demanding lecture on current knowledge about the nature and interaction of the magnetic fields of the Sun and the Earth, and the puzzles presented by other planets in the Solar System.
The magnetosphere serves to deflect charged particles from the Sun, whose magnetic storms peak every 11 years. The importance of the Earth's magnetic field has been recognised for a very long time - a compass was developed in China about 200y BC and as far back as the 16th century in Europe the distinction between the magnetic and rotational North Pole was discussed. This angle of declination varies with time, from 10°E in 1600 to 25°W in 1800 and back to 6°W in 2000. At present the magnetic North is migrating about 0.5km per week.
After a discussion of mechanism for generating the magnetic field, David posed the problem of its periodic reversals, as recorded in ocean floor basalts and other iron-bearing rocks, and whether we are overdue for another reversal (the last one was 780,000y ago), and its intensity has dropped by about 10% over the last 400y. What effect this will have is hard to estimate: many birds and other animals use the Earth's magnetic field to navigate. (For some pretty diagrams see 'Metro', Friday May 9th: 'Compass Consternation' by Ben Gilliland)
London Branch 20th Anniversary Pooh Geowalk
11 May 2008
On 11th May 2008 an eager group of the young and young at heart set off from The Gills' Lap car park in Ashdown Forest for the London Branch 20th Anniversary Pooh Geowalk, ably led by Brian Harvey and Dr Sue Hay, with Brian being in charge of describing the geology and Sue being in charge of all things relating to Pooh. The whole of the walk was on the Wealden Group, formally known as the Hasting Beds. These are the lowest group in the Cretaceous sediments of the Weald and most of the walk was confined to one Formation of that group, the Ashdown Beds. The Ashdown beds form the arenaceous part of one of the megacyclothems described by Prof. Percy Allen. Allen originally postulated that these cycles were the result of eustatic sea level changes producing regressive and transgressive bodies but in 1975 he produced a new model of the Weald in which sedimentation in the Early Cretaceous was controlled by tectonic changes in topography in a land locked basin where fresh and brackish water were never more that a few metres deep.1
Usually being regarded as anticlinal, the Wealden Structure is actually a pericline with the folds closing both to the west and east, the eastern termination being in Boulonnaise in France. In Early Cretaceous times the Weald Basin would have been at a latitude of 30o north, with the elevated topography of the London Platform to the north and the Portsdown High to the south being bounded by Hercynian thrust faults, reactivated as thrust faults during the Permian. The Wealden area would have consisted of an alluvial mud trap of freshwater pools, banks and brackish lagoons traversed by muddy and sandy channels. Occasional earthquakes signalling movement on the bounding faults would have uplifted the highlands and increased river flows.
Our first real stop was in a quarry so that we could look at a bedding surface with ripple marks, although we had stopped briefly twice before; the first time so that Brian could point out a North-South valley, and the second time so that Sue could practise jumping in sand pits. The bedding surface was a very hard quartz-cemented rock, which had probably been used for road building. In addition to the ripple marks there were also possible trace fossils of a branching burrow. It was difficult to decide at this location whether the ripples were symmetrical or asymmetrical. The significance being that asymmetrical ripples would have been formed in flowing water while symmetrical would have formed in still water. The elevation on the bedding surface at the quarry was measured at 193m above sea level.
As we walked down the valley we walked through clay until coming across another hard sandstone at 181m above sea level. Then came more clay and another sandstone at 172m above sea level. These were the cyclothems, or coarsening upward cycles described by Allen and which were the result of the changing elevation along a major fault line. At the bottom of the valley we came to a very ironrich stream where we could see how quickly the streambed had been eroded in the last 20 years because a concrete apron had been undercut by more than half a metre in that time. We walked along the bank of the stream to look at an outcrop that Sue and Brian had spotted on their reccy of the area. The outcrop showed convincing symmetrical ripple marks, indicating that the sands had been deposited in a shallow lake with the raised ripples being formed at the bottom of the lake.
Walking back along the stream bank we came back to the main path that we were at the North Pole. We decided that it was quite warm for the North Pole but that this was probably due to global warming. We also discussed its location and decided that for the North Pole to be there must mean that we were in the middle of a polar reversal.
There were no outcrops on the way up the other side of the valley and we pondered on the N-S valley with a small stream at the bottom. It may be that there was a N-S structure underlying the Weald but there was no evidence for that. It was also difficult to say whether there was a strike slip fault because there were no marker beds and so it would be difficult to see the fault without geophysics. The final theory was that the whole valley may have been excavated by the little stream at the bottom by mass wasting. Given time it could have eroded the valley, bearing in mind that we had evidence that it had eroded 2/3 of a metre in 20 years. Originally there had been another 1500 metres of strata above the rocks we were sitting on and Charles Darwin had used this to estimate the time taken to erode the Wealden Dome at 300 million years. This was at a time when the age of the Earth was estimated at 6000 years. He was ridiculed when he put it in the 1st edition of Origin of Species.
For a short while we followed the Weald Way, the long distance footpath from Gravesend to Beachy Head. This was part of a Roman Road and we found some of the iron furnace slag that they had used to construct the road. At this point we were at 188m above sea level and we had found no sign of any exposure of the sandstone that we had found on the other side of the valley, which suggested a fault going along and down the valley.
Next we entered the Hundred Acre wood and our next stop was to admire a heffalump trap that Sue had found. She said that there were no sign of heffalumps now because they had all been trapped. Brian's more prosaic explanation was that the heffalump traps were actually sawpits where sawyers had cut trees into planks. Moving down into the wood we looked for a sign of Owl's old house, which had been blown down, and of the Wolery, the house he moved into but our search was inconclusive. Although there was a good contender for the honey tree that Pooh climbed. It was as we walked a little further on that we came upon some heffalump tracks in the soft mud and a strong container that could only have been a honey pot. We also found a place were the Woozle wasn't.
After lunch, which was extended because "We can't go yet because Richard hasn't finished his TMA." we set off again and found a second quarry at 136m. This was the same stream that we had found earlier but we were now 3 m lower and the stream was flowing on the cemented bed that we had seen earlier. We were now in Posingford Wood, described as a "gay friendly wood with willows, hazel and sweet chestnuts". There would have been charcoal burners here in Christopher Robin's time.
At this point we made a side trip to visit Poohsticks Bridge. We had been collecting racing sticks for some time because Sue had warned us earlier that the ground around the bridge would have been stripped of sticks by tourists. The sight of more than twenty adults and one child rushing from one side of the bridge to the other with cries of "My one is winning!" or "Mine's in the lead!" was definitely worth seeing.
On the long walk back to the car park we looked out over a valley covered with trees. Brian pointed out that 20 years ago there had been no trees growing in the valley because until 1985 the commoners had the rights to graze their sheep. In 1693 commoners had been awarded the rights to pannage (grazing) breaks and litter (the right to collect bracken and heather for bedding) and estovers (the right to cut birch, alder and willow). In 1985 the sheep were removed and the trees had been allowed to regenerate. " and by and by they came to an enchanted place on the very top of the forest called Galleon's Lap"
Back at Gill's Lap we were able to look out over the beautiful landscape of Ashdown Forest. Here there is a plaque that reads "Here at Gill's Lap are commemorated AA Milne 1882-1956 and E. H. Shepard 1879- 1976 who collaborated in the creation of Winnie the Pooh and so captured the magic of Ashdown Forest and gave it to the world."
Prizes were given out to both the youngest person attending the walk and the youngest person to have attended both the original Pooh Geowalk and the final Pooh Geowalk and a good day was had by one and all. Brian and Sue were thanked for this excellent culmination of 20 years of Geowalks; Brian having led 28 walks in all.
1. The geological description of The Weald has been extensively cribbed from Brian's excellent field notes.
Lecture: 'How to make and break an Asteroid' by Hilary Downes
15 May 2008
Hilary, professor in the School of Earth Sciences, Birkbeck, has been involved with the Lunar and Planetary Institute, Houston, in the study of ureilites, ultramafic, achondrite meteorites. Starting with the question: What is an asteroid?, we were given an insight into current research on their significance as early building blocks of our planetary system through the study of ureilites, very abundant achondrites which date from a very early phase in the development of the solar system.
The ureilites, containing pigeonite, olivine and pyroxene, show features suggesting extreme impact shock. All ureilite breccias have a similar range of olivine composition, indicating that they come from one parent body. One possibility is that they come from the mantle of the asteroid that suffered collision and reassembly at a time when the process of differentiation, melting due to decay of 26Al, causing metamorphism and the transformation of carbonaceous material into graphite, was incomplete. Therefore they may provide important information about that part of the solar nebula from which the asteroid accreted. Billions of years in a hand specimen or thin section!
Field Trip: The Auvergne led by Dee Edwards and Dave Williams
24-31 May 2008
Most of our group travelling from London had an eventful journey on Saturday, first on Eurostar from London St Pancras International to Paris, then a very interesting traverse of Paris with the able assistance and fluent French of our organiser, Yvonne, via the underground train to the station for our internal train journey to Clermont Ferrand. We were met at our hotel by our leaders, Dee Edwards and Dave Williams, and other members of our group travelling independently by various means and our first day came to a happy conclusion over a group dinner with plenty of local wine.
Sunday Sunday started with our first visit to the local epicerie to stock up for lunch before we set off in bright sunshine for our first location, a disused quarry at Gare de Volvic, north of Clermont Ferrand. First, Dee explained the overall geology of the area. The volcanism in the Auvergne is considered to be intracontinental, ?hot spot type, or possibly failed rift due to extension. At Gare de Volvic the source of the lava flows was Puy Nugère to the west about 12,000 years ago. The lavas became more evolved from vesicular basalt to more acidic trachyandesite, all within a short space of time, deposited at such speed that it is possible, Dee told us, that bodies are buried beneath, as it is known that humans occupied the area at the time. Banded rocks indicating mixing of the two types of magma have been found here. We searched for evidence of these, but unfortunately none was found. The scoriaceous rocks here were quarried as they made good building blocks. They are very loose but when mixed with water they solidify without the need for cement. The project was however very dangerous as the material was removed by driving at the face. (Of interest, Dee told us that she is considering a retirement project exporting this material as the ultimate slug repellent due to its richness in manganese, calcium& iron)
Our next location was at Source de Volvic to the west where Volvic is at one end of the Limagne Fault and an 'Enval' NW-SE trending line of spatter cones. Here at Maison de la Pierre we followed the 'son et lumière' exhibition detailing the hard and hazardous daily lives of the quarrymen at the beginning of the 20th Century. A basaltic lava flow originated as the overflow of a lava lake at Puy Nugère. Quarrying began at the flow front and continued underground and had taken place here for 700 years but it was from 1900 that Jean Legay-Chevalier invested in and eventually worked 32 quarries in the region. Orders for the stone came from all over Europe. The processes involved in working the stone required large quantities of water and it was due to Legay's initiative and vast investment that water was eventually discovered after digging down 80 metres. Unfortunately, it was only 30 years later, after Legay's death, that water was piped to the community who had been so involved in the work.
On Sunday afternoon, after a packed lunch at damp picnic tables under some pine trees, we found that the information centre for the famous bottled mineral water at Volvic was closed until 14:30. We therefore drove down from the Chaîne des Puys and visited Grand Gandaillat Quarry in the Limagne plain instead. Here we would examine some exposures of sedimentary deposits.
The major N-S trending Limagne fault lies just to the west of Clermont Ferrand and forms the western boundary of the long Limagne basin, some 40 km wide and over 100 km in length. The Massif Central was an emergent horst structure during the Oligocene and sediments were deposited all around. South West of where we were, in the Cantal, these sediments formed the basement for subsequent volcanism. However, the earliest volcanism of the Auvergne had occurred near Grand Gandaillat quarry, here in the Limagne, around 25 Ma.
The nearest puy to the quarry is the Puy Crouel and my handbook of walks ('Le Volcanisme en Auvergne', Chamina, 2003) says there are also dykes of peperitic basalt within the quarry. However, all I saw on this wet afternoon were sediments. These had been formed during the Oligocene and into the Miocene, when extension and rifting connected with the opening of the Western Mediterranean led to thick deposits of waterborne calcareous muds being laid down in the subsiding basin. It is uncertain whether these deposits were truly marine. My handbook (ibid.) calls them lacustrine. We found beautiful stromatolites and the u-shaped tubes of burrowing organisms, but as it was raining we didn't linger.
The next stop was in Royat, near Clermont, where there was a lava flow that was only 44,000 years old. The flow had a cave in it called the grotte des laveuses (washerwomen's grotto). I'd already seen this volcanic feature on a previous trip, in 2003, and the consensus at that time was that it wasn't a lava tube. There were none of the little stalactites coming down from the ceiling that you get in lava tubes. I think we were told that it was thought to be the result of the lava flow having met some water, like the river that currently flows down the same valley. The water would have evaporated to produce a gas bubble that lifted the lava. Not so, we were now told. It is now thought to be the result of the flow going over a pocket of scoria that has subsequently been washed out.
The grotte des laveuses is situated in a car park below the Saint Leger de Royat church. We took a lift up the side of the cliff to the church. Near here some samples of local rocks had been mounted along a wall to show the following succession, the oldest being at the bottom:
By now the weather had brightened up and I could hear a cuckoo's call somewhere in the valley. It was time to move on again, back to the information centre at Volvic, which opened at 14:30. The friendly receptionists there invited us into a cinema to view three videos and they even arranged for us to see the English versions. The first video had beautiful landscape photography but rather purple prose. The next was an amusing tale of a couple of teenagers who encountered some extra terrestrials needing pure water for their tree of life. Naturally, they found the water here in Volvic! The final video was all about the geology of the volcanoes of the Chaîne des Puys.
There were other displays in the centre, some geological and others less so, including flavoured Volvic water to sample and a radiator made of a slab of basalt that I found intriguing. Before we left, we were each given a free goody bag containing a copy of 'Volcanology of the Chaîne des Puys', published by the Parc Naturel Regional des Volcans in 1991, in English. This was the last visit of the day and we now made our way back to our hotel at Orcines, beside the Puy de Dôme.
Monday The weather forecast for Monday was not good - heavy rain all day and as we approached the base of Puy de Dôme we could see that the top was shrouded in mist. One of the principle reasons for climbing Puy de Dôme is for the view from the top. This is the highest volcano in the Chaîne des Puys and on a good day it is possible to see a large number of the 80 cones that make up the Chaîne. The Chaîne rises from crystalline basement adjacent to the Limagne Plain, in linear fashion 2-3 km west of the Limagne Fault. Why it doesn't exactly follow the line of the fault remains a mystery especially as earlier, smaller cones follow the line of the smaller, parallel, Volvic- Enval Fault.
The low cloud not only prevented us from seeing the Chaîne des Puys but it also prevented us from seeing the earlier strato-volcanoes to the south, Mont Dore in the middle distance (3 0.23 Ma) and in the far distance the older Cantal (c. 11 2.9 Ma). On a good day it is even possible to see Mont Blanc far away to the east. Instead we had to make do with the magnificent enamelled Volvic stone dioramas picturing what we should have seen. But at least the rain held off and we could examine the rocks at the top as there is an outcrop right beside the Visitor Centre. The rock here is competent and dense without structural features and inspection bore out our earlier theories, based on the steepness of the slope, that this is a volcanic plug that had solidified in situ. It is a palish rock (even when wet) with occasional well-shaped hexagonal bronze crystals (up to about 2mm) identified as biotite mica and transparent twinned crystals of feldspar of about the same size visible with a hand lens. Some people also spotted quartz. The matrix is fine-grained. The composition suggested that the rock lay towards the more acidic end of lavas, perhaps a rhyolite. The map confirmed that we were in the right area but that the rock was not quite so evolved. It is shown as a trachite. In fact there are two plugs of trachite at Puy de Dôme and so the cone is known as a cummulodome. Further down, where the slope is less steep the rock is a pozzoulane (scoria) but we were not able to examine that on this volcano although there were plenty of chances to see similar rocks elsewhere on our trip. It is amazing to think that Puy de Dôme only erupted 11,000 years ago and that it is by no means the youngest the maar crater eruption of Lac Pavin occurred only 7,000 years ago. The debate continues about when the next eruption may be ...
We were frustrated again at our second scheduled stop. The small maar spatter-cone at the base of Puy de Dôme has been turned into a car park! Dave likened it to the Vogan's Inter-Galactic Bypass in The Hitchikers' Guide to the Galaxy. Tant pire! But the afternoon at Lemptégy more than made up for it and at least we had no need for our 'Towel' as the promised rain held off until the evening.
Since we couldn't spend time with the spatter cone, we went straight to Lemptegy, the 'volcan a ciel ouvert'. This relatively small volcano is in fact two, Lemptegy 1 and Lemptegy 2. It was used as a quarry for the re construction of Rouen and other cities following the destruction of World War II, since the pouzzolane of which it's composed makes excellent building material, light but strong. Quarrying has ceased, the site has been protected since 1970, and, since 1977, as part of the Parc Naturel des Volcans. It is now used as a teaching resource to show visitors the internal plumbing of a volcano, since the basalt and trachyandesite 'chimneys', dykes and lava flows have been exposed, free of the overlying scoria cone..
After an introductory film, we were guided round the site by a geology student from the University of Clermont Ferrand. Lemptegy 1, basalt, erupted about 30ky and is called Petit Lemptegy. It was followed almost immediately by Lemptegy 2 (trachyandesite): dating by thermoluminescent dating gives the same age. In the wall of scoria, red deposits from the first lie against black from the second, the whole covered by fine deposits from the nearby volcano of the Le Pariou. Further along we distinguished seven layers: Lemptegy 1, then black basaltic scoria from the Puy des Gouttes, a narrow band of periglacial deposits, Lemptegy 2, followed by a period of calm. Above there is 'trachyandesibasalt' ash from the Puy de Come at 16ky, a layer of soil and finally a white trachite deposit from a pyroclastic flow from the Puy Chopine at 9.5ky. This is the last volcanic event to affect this area.
From this position, looking into the centre of the volcano we saw the necks of Lemptegy 1 and 2, with dykes radiating from the necks, and a trachyandesite lava flow from the base. Perched on top of Lemptegy 1 are two huge 'bombes', the larger weighing 60 tonnes, ejected at the time of the eruption and still lying where they fell. Others in the shape of spindles, cowpats, cauliflowers result from different styles of eruption from multiple vents in the region. Fortunately the rain kept away and Lemptegy ended with a visit to 'La mine explosive', where, locked in our seats and wearing 3-D specs we were jolted about, deafened, and had rocks and fire thrown at us in a mine disaster complete with rats!
To round off the afternoon, on our return to Orcines we stopped at the 'Champ des martyrs' a memorial to twenty-four patriots, shot by the Germans in 1944. The memorial is in trachyandesite Volvic stone and behind it is a volcanic lava flow, thought until recently to be from Puy Pariou, but in fact myocene and 4my old.
Tuesday Rochefort-Montagne was our first stop on our way to Thiezac, on a rainy Tuesday morning. Dave first told us about the Mont Dore volcano which is sandwiched between the Chaine des Puys to the North and Le Cezallier to the South. A caldera was formed about 3.3Ma (K/Ar dating). The magma chamber is believed to have been situated about 1km down and contained about 10% water and other gases, thus creating an explosive eruption. Subsequent eruptions have erased most traces of the caldera morphology. Consequently, the vent location is unknown, but believed to be somewhere between La Bourboule and Mont Dore. The Mont-Dore eruptions produced 8km3 of material and fine rhyolitic ash material covers an area about 50km in diameter.
The locality visited is on the edge of the Mont-Dore deposits. Ash air fall deposit forms a cliff some 15m high. The ash is very fine with some linear stratification and contains small clasts of pumice which were elongated during the explosion. There are also clasts from the basement material (granite and gneiss). Pumice from the area has been used as an abrasive and in toothpaste.
Our next location was a comfort break in Bort-les- Orgues. We were now in the Cantal region. We stood on the bridge over the river to have a quick look at the basaltic columns at the top of the hill above the town. On our way to Salers, we stopped at the Cascade de Salins on the L'Auze river, where we could see basaltic columns (about 8m high) sitting on top of Oligocene sediments (30Ma). The waterfall had cut through the lava flow. Our visit was cut very short due to very heavy rain.
Driving along, from the coach, we could just about see the central peak of the Cantal volcano in the mist. Its last volcanic episode was a lava flow deposited on top of eroded deposits from previous explosions. We reached Salers, a medieval fortified town (against the English), which was restored in the 19th century just in time for a delicious lunch in a pleasant restaurant and an interesting walk around the town.
After lunch in Salers the bus took us on to the town of Aurillac. During the journey the rain started again heavily this time accompanied by forked lightening. (We were glad we would be indoors for our next visit). On route we were told we were skirting the edge of the Cantal stratovolcano. The rocks comprised huge debris avalanches, some from Nuee Ardente or pyroclastic flows. A jumbled cyclic chaotic mass of bombs down to fine dust with some lavas on top, which are the exception, not the rule. Volcanism in the area occurred between 14 1.5 Ma with no actual consistency or trend. We arrived at the Maison des Volcans at Aurillac to look at their displays of The Geology of The Cantal and to see 2 films, one on the geology and formation and one on the natural history of the area. The museum was informative and some of the displays could be manually manipulated.
After leaving the museum we continued on our way to Thiezac approaching from the SW through a "U" shaped glacial valley of an eroding deep river valley of the River Cere. In Thiezac the valley is less "U" shaped. On our way we passed road cuttings exhibiting the rubbly reddish basal beds of the volcano including the pre-volcanic basement rocks of the Limagne Basin. Later we stopped on route to Thiezac at a place called Cascade de la Roucelle in the valley of the Cere to look at the avalanche deposits. These brown deposits were finer grained within a mixed matrix deposited first.
Above these were the mega blocks comprising huge chunks of rocks, which have flowed within the massive slumping action and have come from several miles away. These breccias can contain the older basalt blocks, which the river has cut through and in turn the mountain becomes too steep and collapses in sections. Fluidisation of the flow takes the uncemented breccias likened to floating on a carpet and is later deposited some miles dstant. (Likened to Mt. St. Helens). After viewing this outcrop and waterfall we continued on our way to Thiezac and the next hotel.
Wednesday Today's plan was to visit Puy Mary and the Diatomite Quarry and hopefully see some amazing views of the residual peaks of the Cantal and find some fossils at the quarry. We left Thiézac heading north passing evidence of volcanic activity in the roadside cuttings and took the pass to Super Lioran in the hope of seeing Puy Griou, but unfortunately the clouds were low and so we had to wait until the return journey. We travelled North West along D18 with the unseen Puy Mary ahead of us. The roads are narrow and winding and follow a meandering misfit stream up a Ushaped valley winding round a number of hairpin bends before arriving at Pas de Peyro. Dee assured us there were three reasons for the visit: firstly the view, secondly the café and thirdly the path to the top of Puy Mary. Currently one and three were hidden behind the low cloud which left the second option - the welcoming café. We all piled in for tea and cakes. The cakes on closer inspection were a form of thermal metamorphism, so we were already thinking geology before exploring the road cutting along road D17. The rocks were very weathered, but from the mineral content it was decided they were probably a trachyandesite in a massive flow. The joints were not opened inferring the blocks were still in situ and were therefore not mega blocks. As you walk down the road away from the café the rocks become more evolved and the material changes from massive lava flow to more of a breccia/conglomerate.
The mist was starting to lift and so we headed back towards the café and our first clear view of an intrusive plug on the other side of the valley. The first noticeable feature is it's light colour and the vertical joints running through it. It was part of the late intrusion phase. It is formed of very viscous sticky lava that did not flow and is called a phonolite. Next to this is a planeze, one of the last lava flows that was originally responsible for the first misinterpretation of the volcanics of the area. Its coverage made the volcano look like a shield volcano rather than a stratovolcano. Most of this lava has now been eroded away and the present landscape has been heavily influenced by glaciation. The cloud had continued to lift and the group decided to make a bid for the top of Puy Mary, an ancient lava dome altitude 1783m. This is reached by a steep path with views of the radial valleys and a bird's eye view of the central area. Puy Mary itself is a mass of porphyritic latitic andesite with phenocrysts of hornblende clearly visible. Surprisingly on the top of the Puy there was a large lump of schist which had apparently been lifted in by helicopter.
Leaving behind the Puy Mary our next stop was the Diatomite Quarry at Foufouilloux (Virargues). Here the rock is a light coloured bedded rock dated at least at 5.8 Ma by a dyke running through it. We have switched to a porous sedimentary rock composed entirely of the frustules of eukaryotic algae diatoms. These unicellular algae were encased within a cell wall made of silica (hydrated silicon dioxide) called a frustule. When processed into "diatomaceous earth" it is used as a filtration agent (beer, wine) as an additive (paint) and more intriguingly as an activator in blood clotting studies and as a component of dynamite. The typical chemical composition of diatomaceous earth is 86% silica, 5% sodium, 3% magnesium and 2% iron.
It is theorised that the lake in which the algae lived was created by a phreatic explosion. The depth of the deposits is around 30m but highly variable with deposition occurring over a period of 50,000 years. Varves have been recorded at approximately 0.3.mm per year. The deposition occurred in a warm period in the Tertiary and was later protected by a covering of glacial moraines (25m deep currently). The diatomite layer is sitting on top of Brèche Andesitic. After recovering from spelling and pronouncing Foufouilloux, the group was immediately galvanised into intensive highly motivated searches for the fossils and found many delicate but well preserved leaves and stems. Some of the full imprints of leaf fossils were carefully collected and packed under Di's supervision. At Chastel sur Murat the Chapele Saint Antoine was built in 1878 on top of a basalt outcrop consisting of columnar basalt around 10cm wide with phenocrysts of olivine and augite. A process that was speculated to have dictated the width of the columns was the rate of cooling of the intrusion; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns. This is one of three outcrops that are aligned along an eruptive fissure. They are the result of the most recent feeder pipes in the Cantal at 4.3 Ma.
On our return journey over the pass luck was with us and this time the beautiful view at Font de Cere could be seen. We could see the distinct chaotic slumping covered with trees below the cliff line. There was also the opportunity to compare the Puy Griou, a trianglar plug with vertical jointing and the Plomb de Central, with its flat top.
Alison Matthews and Mark Biswell
Thursday Today was to be spent locally around Thiezac, itself built on an ancient landslip in the valley of the Cere. Our driver, Pascale, had to put up with a lot of condescending advice from other (male) drivers about the difficulties she would encounter! From the hotel, l'Elanceze, named after one of the major nearby, our coach took us through the town, following the valley upwards, and heroically branching off up a minor, twisting road, through the village of Lasmolineries, to get us as near as possible to our morning destination: the waterfall of Faillitoux.
We made our way for almost a kilometre across the saturated hillside, fording a stream, the waterfall visible in the distance, narrow and white, pouring over a notch in the wall of basalt.
Arriving at the stream at the bottom, we could see the black columnar basalts of a lava flow, approximately 9.5ma old. Some of us climbed up to the base of the columns, diameter 50cm. The rock is dark and porphyritic with phenocrysts of augite and olivine.
According to the authors of 'Le volcanisme du Cantal', this rock is ankaramite. Dave described it as cumulates from the base of the magma chamber. Beneath the waterfall the basement of the Elanceze massif is visible, and among the broken rocks bordering the stream, the contact with the country rock can clearly be seen.
Back to the coach and down the valley, between the cascade and the village we stopped by the roadside to examine a series of successive mud flows and debris avalanche, separated by erosion surfaces.
Back to Thiezac and up the hillside to picnic at chapelle Notre-Dame-de-la-Consolation, whose roof of schist tiles was being repaired.
After a very pleasant picnic lunch at Notre Dame d'Esperance in the hills above Thiézac we set off to view 3 nearby sites on the N122 and practise our mapping skills.
Abandoned road cutting: we divided into teams, each being assigned a sector of cutting to examine. The writer's sector appeared to contain 3 different breccias:
Carrière de Payot: much of the same in this disused and overgrown quarry a "breche" andesite, much altered by super heated sulphurous steam. This apparently is an area of a really large debris avalanche and sector collapse. Active road cutting: An interesting exposure showing
Friday Best weather so far, not the day to be leaving this beautiful country. Off to the boulangerie to buy lunch and onto the coach, all ready to go before the scheduled departure time. An hours drive along ridges and across classical U-shaped valleys to a quarry at Neussarges which had just started work when I was here in 1999. Then there was broken columnar basalt strewn all over the floor and it was at the risk of a broken ankle to examine the newly fallen rock. This time all had been neatly cleared away and we could see a clean rock face some 100 metres high and some 200 m wide.
The site manager, who had forgotten we were visiting, told us that they would be blasting in one hour and that men were setting the charges at the top of the face at that moment so unfortunately we could not get close. The lowest part of the face appeared to be a scoria, rusty coloured and fragmented. Above it was the basalt, tall, straight columns but with some fantastically curved. Much discussion as the the cause of this curving, but no final conclusion.
Back onto the coach and exit the quarry, we stopped at the road side to look at the layout of the working when WHOOOOMP, and the blast brought down the next supply of basalt just 5 minutes after we had left.
A rock fall had blocked to road we had planned to use to reach the Autoroute for Clermont so we diverted to the town of Saint Flour. After our driver had parked to coach with true Gallic panache we took our lunch and walked along the side of a very busy road boarded by a wall of more columnar basalt for a distance of some 600m then turned through a small gate into a grassy area again facing this wall of columnar basalt. This was topped by a rubbly scoria very similar to Staffa of the coast of Mull.
Again, much discussion about cooling rates and the formation of the jointing. Dave gave a number of possible theories but I got the impression he was not convinced by any of them.
As to the falcon we watched as we ate, the only species I can offer is Kestrel, perhaps male French Kestrels are not so markedly different from the female as our British ones.
Friday During the lunch break, a couple of us walked back into the upper town of St Flour, in advance of the rest of the party who drifted up for shopping after lunch. We were just able to peek into the late Gothic Cathedral, which was built of local stone, because it was about to be closed for lunch, but the custodian agreed to let us have a very quick look. The town, which is a significant administrative centre and the ecclesiastical capital of the area, has a number of interesting museums and would repay a further visit. After lunch and shopping, we rejoined the bus for the journey back to Clermont-Ferrand. Geologically, this was uneventful for most of us, but Di had with her the Geological Map of France from the BRGM (in two parts, like the BGS ten-mile maps of Great Britain) and followed it assiduously.
Di reports that we picked up the ancient crystalline basement (which looked very schisty from the bus) as soon as we hit the motorway from Saint Flour. Our driver Pascale pointed out the twin chapels on outliers of volcanic rocks at Massiac (she had told us an elaborate story about not crossing the river at the bottom with the animals, hence the need for summer chapels on each peak). Then we came over the Limagne fault and onto the Plain of Limagne (mainly Oligocene) near Lempdes - very obviously flatter, but a disappointingly undramatic descent. The road passed through an outlier of Hercynian granitic basement around Issoire (well-exposed in cuttings). We passed a dramatic hill-top village on top of a lava flow, probably associated with the Chaine des Puys, around Montpeyroux. The other peaks close to the road from there on related to the older, Miocene, volcanics of the Plain, including the elevated ground on which stood the Cathedral at Clermont. Off to the left we could see through the mists the Chaine des Puys, including glimpses of the Puy de Dome, as we approached Clermont.
After arrival in Clermont-Ferrand and after checking into our hotel, which was not far from the centre of the city, and unpacking, we took a tram into the city centre for a guided tour. We were met at the Tourist Office, which was close to the Cathedral, by our guide, Adeline, an art history graduate. As it had started to rain, we were given an introductory talk on the history of the city on a room in the upper floor of the Tourist Office, which contained some ancient mosaics, and our tour was subsequently planned to minimise exposure to the rain.
We were told how the city of Clermont had evolved from the Gallic settlement which had been used as a base by the Gallic leader Vercingetorix when fighting against Julius Caesar. The Gallo-Roman regional capital had been built on a maar to the west of the centre of the modern city. The settlement had then moved to its present location, on a volcanic plug, in medieval times, in response to the need for a more readily defensible site.
The rain having abated a little, we left the shelter of the Tourist Office and walked over towards the Cathedral, viewing on the way the 19th century statue of Pope Urban II, who preached the First Crusade at the Council of Clermont in 1095. The statue was an example of the 19th century eclectic style, i.e. a mixture of gothic, renaissance and romanesque. Some adverse comment was made, in the light of the events which followed, on the Pope's call for pilgrims to defend themselves. However the preaching of the Crusade at the Council was prompted by attacks on Christian pilgrims to the Holy Land. The suggestion that Christian arms might be better used in defending the rights of Christians to worship at Jerusalem, can be seen as consistent with a general move at the time against armed violence between warring factions at home. The Pope would doubtless not have envisaged many of the brutal consequences of his sermon.
We were then taken around the Cathedral. We learned that the Cathedral had originally been built in the romanesque style in an arkose sandstone from the locality of Clermont. However in 1248 work commenced in constructing a gothic choir, ambulatory and radial chapel, under Jean Deschamps, a builder who had also worked at Narbonne. This work used Volvic stone for the first time, this stone being located at depth, and therefore requiring innovative working techniques to be exploited. Further work in replacing the arkose (and romanesque) fabric continued during the High Middle Ages, but building stopped in the 15th century and until the 19th century the cathedral remained a mixture of Volvic gothic, and arkose romanesque. By the 19th century the arkose fabric of the narthex was in a fairly ruinous condition. Nineteenth century attitudes to building conservation were different to those which prevail today, and the French Gothic Revivalist Eugene- Emmanuel Viollet-Le-Duc was instructed to replace it. He rebuilt the narthex and the few remaining arkose parts of the nave, and constructed the spires, using Volvic stone and a Neo-Gothic style following 13th century models.
In the interior of the cathedral, we admired in particular the very detailed and high relief carving in Volvic stone in the sacristy door which had been damaged in the French Revolution. There had been a great deal of damage to the fabric in that period, including the destruction of the rood screen. We also saw wax wall paintings in a side chapel which had been mercifully protected from those depredations as a result of having been concealed behind wooden screens when the paintings ceased to be fashionable. Passing out of one of the Cathedral doors, our attention was drawn to an inscription declaring that "The French People recognises the Supreme Being and the Immortality of the Soul" (the implication being, nothing more). This was a relic of a phase of the French Revolution in which prominent churches were de-Christianised, and dedicated to the deist cult of the Revolution. The lettering had recently been rediscovered in the course of restoration work, and restored as a memento of the period.
Our guide showed us the exterior of the Neo-classical Town Hall, typical of so many public buildings of the period in France, but unusually without an accompa nying square. This was a result of constraints of space, as the legal functionaries who conducted their business in the adjoining law courts would not countenance the demolition of their quarters to make way for a square.
We were then shown the Fontaine d'Amboise, a Renaissance fountain which had been a gift to the city by Jacques d' Amboise, one of the Bishops of Clermont. This had been moved from its original site, slightly damaged in the course of re-location, and furnished with fenders of more recent date. Although the gift of a churchman, it was entirely secular in its inspiration. Indeed the décor included images of wild men etc, some of which were shown performing indelicate acts. At the conclusion of the tour, we were taken down a little street to see a 17th century house with its original staircase. The house had belonged to a Conseiller (judge) at the Cour des Aides (a regional court, which dealt with tax and associated criminal matters, and was an important instrument in the assertion of royal authority in the area during the seventeenth century). The judge had clearly either benefitted significantly from his office, or else was fairly well-off to start with. In an apartment upstairs, part of the original interior had been preserved, including allegorical paintings by François Lombard, an artist from the Auvergne.
This concluded the tour, and we dispersed, some to carry out last minute shopping, others to enjoy a drink in town, before taking the tram back to the hotel for dinner.
Richard Trounson and Diana Clements
Lecture: 'Groundwater under London' by David Hartwell
19 June 2008
Having been involved in many of the major projects in London from the Thames Barrier, 1971, to the Channel Tunnel Rail Link, 2000-2004, David was able to illustrate the problems caused by the solid geology of the London Basin to a range of engineering developments.
With the aid of diagrams, sketch maps, tables and photographs, he graphically showed us the problem caused by the fact that under the hard London clay lie the saturated sands and gravels of the Thanet sands, exacerbated by the difficulty of getting rid of excess water in an urban environment. A lot of tunnels under London were built when the water table was at its lowest, and though it appears to have stabilised this century, Tideway and Crossrail, scheduled for 2009, still have a question mark.
Note: As I was composing these inadequate summaries of two very different lectures, from the early universe (Hilary Downes' Lecture, above) to groundwater under London, full of material new to many of us and illustrating the huge range of interests among members of the OUGS, I thought how privileged we are, particularly in London perhaps, that specialists such as Hilary and David will give their time for our benefit. Thank you.
Field Trip: Folkestone Warren led by Peter Golding
7 September 2008
My preparation included reference to my copies of the British Regional Geology: The Wealden District (1965) and Gibbons: The Weald (1981) and this trip seems to be a repeat of his Excursion 1! I downloaded papers by Hutchinson et al (1980), Popescu (2002) and Birch & Warren (2006) from the Internet and had a look at Google Earth, which includes a few very good photographs in particular, those by Tim Little.
As the Flyer said, a chance to see the classic section of the Folkestone Beds and Gault Clay, a rare exposure of the Glauconitic Marl (base of Lower Chalk), the Lower Chalk, the Middle Chalk and (last but not least) the magnificent landslips and stabilisation works along Folkestone Warren with the opportunity for a brief look at some Gault fossils.
From the East Pavilion Car Park and armed with the six-page Hand-Out provided by Peter Golding, about fifteen of us made our way west to Baker's Gap steps and down onto the East Promenade. There we were given a general introduction to the geology of the area and to Folkestone Warren itself with particular emphasis on the slip plane being at the bottom of the Gault. The general dip in this locality is about 1° NE. Then he produced some Lower Greensand rock samples from the uppermost Folkestone Beds, including some Sponge Rock complete with small white spicules believed to have been used along with fish scales by the burrower as lining and also some gritstone containing phosphatic black grains believed to be Carboniferous chert. Alternating layers of Folkestone Beds can be seen in the near-vertical cliffs to the west of the steps at Baker's Gap and to the east from the end of the Promenade.
Note the thicker yellowish Sulphur Band marking the top of the Folkestone Beds on this photograph. Gibbons describes the Folkestone Beds as " . soft coarse pebbly and yellowish glauconitic greensands with bands of calcareous and glauconitic sandstone"; and the fossiliferous sandstone Sponge Rock as being " whitish weathering and cherty" and "... sands that are rich in sponge spicules and usually have a calcareous cement". The Lower Greensand/Gault Clay cliff to the east of here is sloping at about 45°, with sandstone layers showing through with the boulder-strewn beach stretching away to the east towards Copt Point, the type-site for the Gault Clay.
Anyway, we made our way back up the steps and along to our second stop, above the East Pavilion. Here, the grass cliff top stops suddenly at a fenced off area which is thought to be at risk of slipping the cause being either slipping within the Gault Clay or the collapse of the Folkestone Beds cliffs below or a mixture of both. From the end of the old cliff-top road, up past the hotel and behind this area, relatively recent landslips have cut the road. From the severed end, we could see the Glauconitic Marl near the top of the slip surface - marl that forms the base of the Lower Chalk.
The nearby South Martello Tower is founded on this outlier of the Lower Chalk. The Geological Map shows it to be about 150m E/W and 250m N/S. It presumably evolved in much the same way as the inland spur near Etchinghill (ie. Tolsford Hill). Back up to the cliff top and along to the South Martello Tower (P16). Between here and the Chalk Cliffs, the wide and flat top of the Gault Clay col can be clearly seen gently - sweeping down below us, with suburban Folkestone to the left and East Wear Bay to the right. The panorama of the High Cliffs as they sweep eastwards towards Abbot's Cliff is magnificent, with the chaotic and heavily-vegetated Warren to be seen below. Imagine what the view would look like if all the slipped material were to be removed! Down, then, to East Wear bay and onto the beach!
Debris from the Roman Villa has been found here in the past. Now, it's just strewn with boulders, stones and black pebbles including building refuse and ironstone - but no chalk or flint. The low cliff along back of beach is just a cascade of highly disturbed and saturated clay - some white and slimy, some pale grey. Movement here looks more like slumping of saturated clay than the result of rotational slips! This is where the top of the Folkestone Beds reaches sea-level and where the Gault Clay is first subjected to erosion by the sea. We concentrated on finding fossils in amongst the fallen and highly disturbed Gault Clay to be found on and at the back of the beach. Plenty were found, including small ammonites and bivalves so typical of the Gault.
Most were casts of phosphatic carbonate (CaPO4). Although the ammonites are apparently generally crushed, we found some whole ones such as those collected by Anna Saich and Sue Vernon. Anna's ammonite collection includes what looks like a Hamites gibbosus; and Sue's is apparently Hoplites dentatus. Di Clements tells me that plenty of the small bivalve Pectinucula pectinata were also found, and she tells me that some fossils were so fresh that their aragonite shells had not had time to decompose!
Someone had found a long ironstone nodule and had broken it to reveal radial acicular crystals of iron pyrites, FeS2. Di gave me a piece for my collection! Otherwise, hard friable blocks of displaced but intact dark grey Gault Clay were to be seen exposed on the beach. Out to sea, we could see the wave-cut platform formed in the Folkestone Beds and we were told about the phosphatic nodules to be found there. This platform is extensive, about 200m E/W and 400m N/S. We, unfortunately, did not have time to investigate or to have a look at the inappropriatelynamed Sulphur Band (should be Pyrites Band) or any of the in-situ Gault Clay above it! Gibbons, by the way, describes this band as "a distinctive yellow-coloured layer which consists of hard phosphatic nodules veined and encrusted with iron sulphide and embedded in a mixture of clayey greensand".
The presence of Gault Clay at sea level is, of course, why East Wear Bay exists. At this location, the Gault Clay is about 45m thick and dips at about 1° NE so the length exposed to the sea works out at approximately 2,300 metres. At the east end, the underside of the Chalk reaches sea level and, soon after, the Gault is no longer subjected to erosion by the sea. It is interesting to note that the chord to the arc is in line with the cliffs beyond Abbot's Cliff, and that the Folkestone end is set back behind Copt Point by about 500m. The versine of the arc is about 400m and that is, of course, where erosion is at its maximum. No surprises then that it is also where the landslips are greatest!
After lunch on the beach, we spent the rest of the afternoon investigating the stabilisation works and the cliff faces behind. Various stabilisation methods have been used from buttressed seawall to cellular reinforced concrete apron slabs filled with chalk all constructed between 1938 and 1950, and instigated by the Great Slip of 1915. It must be remembered that this is highly disturbed ground, the most recent slips being due to reactivation of faults initiated 8,000yrs ago notoriously difficult if not impossible to define. But engineering solutions had to be found nevertheless!
Since the construction of the works, various studies and investigations have been carried out, using modern techniques and methods as soon as they became available and monitoring systems have been installed. These stabilisation works are best shown in plan by Trenter & Warren (1996), as reproduced below. You will need a magnifying glass!
So, off we went to the start of these works. This first wall is about 500 metres long and is a buttressed concrete sea wall, with timber groynes extending down the beach. The chalk cliff at this first stop is in two layers (see photograph below), with brecciated chalk overlying displaced but relatively intact Lower Chalk the top of which is dipping landward at about 60O. At this point, the High Cliff is about 300m away the whole slip is shown by Trenter & Warren (1996) to be a series of rotational slips (their Section 2-2). Our next stop was along the next length of buttressed seawall, beyond the large concrete apron and at the entrance to one of the drainage adits opposite Warren Halt. The outfall pipe was, at the time of our visit, discharging water at about a quarter full. There were several along this section of the wall that were discharging lesser amounts.
This photograph shows the group inspecting one of them. Here the chalk is all rubble, as shown by Trenter & Warren (1996) on their Section 2-2. It again shows that the whole slip is a series of rotational slips and that the High Cliff is about 300m away. However, at this location, the Sulphur Band is about 10m deeper than before and most of the Gault Clay has been pushed out to sea by the collapsing chalk. Note that, for a dip of 1°, the Gault Clay slopes about 1m in every 60m.
Our next and last stop was along the toe protection in front of Horsehead Point. This photograph gives a very good idea of the scale! The toe protection consists of an extensive reinforced concrete cellular slab infilled with chalk designed to not only to add weight to the toe but also to reduce the weight of fill behind the wall. Here, the slip on which the slab sits is a simple rotational slip in which intact chalk has rotated and is now sitting directly onto the Folkestone Beds below. This is shown in section by Warren & Trenter (1996) as being Melbourne Rock, the lowest member of the Middle Chalk. It is labelled as Slip 2 on their plan. After the recent movements of 2000/2008, the concrete apron had to be repaired.
Hutchison (1969) suggested that "sets" behind the top of the chalk cliff were the causal effect of the slips along the chalk face. It is now known that they form as a result of gradual extrusion of the softened Gault Clay from below the chalk as it becomes softened by the water flowing out of the chalk (Warren & Palmer, 2000). Many are scornful about the wisdom in constructing the railway through the Warren in 1844 but it has to be said that it served its immediate purpose for over 30 years before the major slip of 1877. No-one mentions how long it was supposed to have lasted or what its design life was!
Gibbons describes the Melbourne Rock as "a hard nodular yellow-white limestone". From there, we made our way up the path towards the railway cutting near what used to be Warren Halt. This was as close as we got to the High Cliff, still some 130 metres away! The railway cutting is surprisingly steepsided and deep, with water seen to be ponding in the cess! At about 4 o'clock, we were on our way back to the Car Park after a long and most interesting day, ably led by Peter Golding.
Lecture: 'Wegener a man well before his time' by Paul Grant
18 September 2008
Paul, who was for many years at Imperial College, London, began by introducing us to Paul Wegener (1880-1930) and his major work 'The Origin of Continents and Oceans' (first published 1915 and continually revised till 1929), which for the first time set out a comprehensive theory of 'continental drift'. He then recommended the most recent book on the subject: 'The New Theory of the Earth' by Don L Anderson.
The observations underlying Wegener's theory of Continental Drift were not new, and Paul cited amongst others a 'Carta Cosmographica' of 1554, and Ortelius, 1596, who said: "The Americas were torn away from Europe and Africa by Earthquakes and Floods". Among more recent authorities we have Lyell, Suess, Milankovitch, Holmes etc. Up until the time of Wegener, those who had noted similarities between flora and fauna or geological formations on different continents thought of pre-existing 'land bridges' that had subsided. In proposing his alternative solution, what Wegener lacked was a mechanism. The mid-Atlantic Ridge had been discovered by the Challenger expedition in 1872, while Holmes proposed mantle convection, but it needed the discovery in the early 1960s of symmetrical reversed magnetic stripes from spreading centres at the mid-ocean ridges for plate tectonics to be generally accepted.
In conclusion, Paul briefly talked of the history of the discovery of the structure of the Earth, the 'hot spot' hypothesis showing the mantle is not homogeneous and finally came to Don Anderson's proposal that the Crust drives plate tectonics and mantle convection is not needed as a mechanism. It is in fact the reverse of mantle convection.
Lecture: 'Monitoring the most active volcano in Mexico: Volcan de Colima' by John Stevenson
16 October 2008
There was an extremely good turn-out for Dr John Stevenson, a Researcher in the Structural and Petrological Geoscience Department of the University of Manchester, who spent two years monitoring the Colima volcano, 2,400m, near the west coast of Mexico, and gave a fascinating account of the equipment used, the difficulties of the monitoring process, the results and the hazards represented by the volcano, in an area where about half a million people live in close proximity.
Although the volcanic complex is 5 million years old, the active peak is much more recent, only 4,000 years, and there have been Plinian events at irregular intervals in the past few hundred years, the latest in 1913. It has been continuously active since 1998 with daily explosions since 2003. One of the problems facing the scientists is that the volcanic complex straddles two states, so that monitoring the volcano, civil protection etc. are the work of many organisations.
The observatory on Nevado de Colima, the extinct volcano 6,000m from the active crater, is equipped inter alia with a thermal camera to measure the temperature of the fumeroles, to see whether there was any change in advance of explosions, but one of the problems is cancelling out the effect of the weather. The main regular hazards are gas (SO2) flux, ash and pumice.
However more serious events occur, in particular the activity in 2005, with lava flows and bombes, pyroclastic flows and lahars. We saw a video of a terrifying lahar or debris flow, in which the camera appeared to be just a few metres away. We also listened to recordings of a tectonic earthquake, explosions and the 'harmonic tremor' of the volcano. John finished his lecture by listing the hazards posed, including the possibility of a debris avalanche resembling that of Mount St Helens, and the practical difficulties of palliating these hazards.
Field Trip: Western Weald led by Graham Williams
19 October 2008
About 20 of us gathered in the car park at Waverley Abbey, near Farnham, Surrey. Here, we met our leader, Graham who introduced us to the two themes for the day:
Waverley Abbey We arrived to find a film crew in the Abbey grounds, filming footage for the TV series Midsomer Murders. Faux policeman patrolled. There seemed to be a cycle race; but as far as we were aware, geology wasn't part of the plot. Therefore we had to keep out of their line of sight as we eyed the building stones. Waverley was a twelfth century Cistercian abbey and the first abbey to be built in Britain. Its then remote countryside location by the River Wey was favoured by the work hard - pray hard monks. Indeed, the building stones were predominantly from the western Weald.
Moving up the sedimentary succession these were:
We also saw a few recycled Roman tiles. Someone spotted some Paludina limestone, fossiliferous with freshwater snail shells. This would have been Imported - either from Sussex Weald or the Jurassic Purbeck Beds.
Mellow Farm Quarry near Bordon - Folkestone Beds, Lower Greensand Formation Sharing cars, we headed to our first rock outcrop, down a narrow lane north of Bordon. We came to a rock face several metres high generally a golden orange colour. The bedding planes were clearer higher up the cliff, being masked by loose washout at the base. This was a quartz-rich, medium-grained, medium - thinly bedded sandstone, with darker orange-brown staining. Being poorly cemented and crumbly, this wouldn't have been used as a building stone. The colour and staining were due to the oxidation of iron from Fe(II) to Fe(III). The source of this iron, and the black specks, was the mineral glauconite which only occurs in marine sediments. The crossstratification was characteristic of large ripples forming as waves moved across sand in relatively shallow sea water. Possibly a tidal race in a channel which developed as the sea level rose.
Here, too was carstone in situ. Much harder than the sandstone, it criss-crossed the bedding and even formed tubes. There was discussion about how this might have happened; perhaps leaching of sandstone by percolating water.
Honey Lane Brickworks (Tower Brick & Tile) near Selborne - Gault Clay We stopped here on the way to our lunch stop at Selborne. Recently reopened after closure of the business, they make modern bricks and designer bricks for restoration / building maintenance. At the gate, Graham showed us a sample of the material dug from the brick pit, about a mile away. This was a dark grey siltstone containing 80% silt, also quartz and gypsum (selenite). He had a sample of selenite consisting of clear, colourless crystals. This is a troublesome impurity in brickmaking as it expands dramatically on heating reducing the quality of the bricks. Walking past the kiln and stacks of bricks / tiles, we looked at the material dug from the pit. Again dark grey. Most of it was dry but the wetter areas were plasticy and malleable. There was a considerable amount of sand, too (from the underlying Lower Greensand, perhaps?).
Selborne - Gilbert White's House and grounds After lunch in Selborne, we walked along the main village street to Gilbert White's House. Here, we were given an introduction by one of the staff . Graham added a few words about the geology. Gilbert White was Vicar of Selborne in the eighteenth century and also a naturalist. His book, The Natural History and Antiquities of Selborne was published in 1789 and has remained in print for more than 200 years. The original manuscript was housed in the downstairs room where we had our introduction.
Since Gilbert White's death, the house has been added to and has had a succession of owners, until its creation of a museum, partly financed by the Oates family, hence the Oates Museum upstairs. Among them was Lawrence Oates who accompanied Scott on the ill-fated 1911/12 Antarctic expedition and uttered the famous last words, "I'm just going outside. I may be gone sometime." His uncle, Frank Oates explored Africa and undertook biological studies at Victoria Falls. The house is now owned by the Gilbert White and Oates Memorial Trust and has been open to the public since 1955, though sadly they had a burglary earlier this year. Valuable furniture owned by Gilbert White was taken from a now empty looking room downstairs.
Geologically, White studied local fossils and, nearly 200 years ahead of his time, he deduced that they formed in a tropical ocean. The 25 acre garden and cattle field behind overlay Upper Greensand. The Hanger hill beyond was Chalk. We could have done with more time to look round the house, though it was pleasant walking round the garden in the autumn sunshine (White's sundial, kitchen garden, plants for sale). From here there was a good view back to the house. One section was faced with blocks of Upper Greensand limestone, as were some of the cottages in the village.
Road cutting near Hartley Mauditt / West Worldham - Upper Greensand Leaving Selborne we headed along busy, narrow lanes to a steep, sunken lane. If Graham hadn't said otherwise, I'd have thought the outcrop here was Chalk, being a creamy pale grey, soft and fine grained. It was limestone, but richer in quartz and contained more clay (marls). Graham's samples from here contained a few bivalve fossils. They were largely intact but had thin, fragile shells meaning that the sediments must have been deposited in calm, relatively deep water below the fair weather and storm wave bases, consistent with rising sea level.
Lower Froyle Chalk Pit - Lower and Middle Chalk Before we entered the quarry, Graham showed us a couple of markazite (iron sulphide) nodules gathered from the Chalk here, along with a trace from a nearby borehole. This illustrated some of the techniques used to identify rocks below the surface, eg during oil exploration. The gamma ray trace measured the amount of radioactivity at different depths. Variation in the speed of sound was an indication of rock density, hence porosity.
We spent about an hour interpreting what we could see in the quarry. At the end of it, Graham admitted this was a tricky quarry. Clearly, from the thicknesses he gave us for the Cretaceous rock formations in this area and the fact that the quarry sides were less than 100m high there couldn't have been more than two formations exposed here. The harder part was distinguishing the rock types in the bedding within these formations. There were no distinctive boundaries or obvious changes in colour or texture. All the beds were fine grained, creamy / grey in colour, though the lower layers looked orangy (Lower Chalk) and some of the upper layers looked whiter and harder (Middle Chalk). As we were at the western part of the Wealden anticlinal dome, the beds dipped west, but this was hard to see. There was flint on the quarry floor but we couldn't see any in the face. Perhaps it had tumbled down from Upper Chalk at higher level. A couple of people found some fossils in the upper part of the quarry: a bivalve and an echinoid (Ananchytes).
After gathering our evidence and hearing Graham's summing up, we thanked him for leading the trip and giving us practice in interpreting rocks in the field. Thanks, too, to Di Clements for organising the day.
Reference: Handout to accompany the field trip produced by Graham Williams.
Field Trip: Winter Weekend in the Mendips led by Paul Olver
7-9 November 2008
I had decided to set out early on Friday so that I could make the most of the extra time to see a little of the Somerset countryside on the bus journey from Bath to Wells, and then have time to explore Wells and the beautiful cathedral, and I was not disappointed. The sun was shining and the hillsides were a blaze of colour from the autumn leaves, and a visit to the Cathedral and the information given on the building stones used was very useful for the forthcoming weekend.
On Friday evening, with our group congregated, Paul explained the geology of the Mendips and what we would be seeing at localities during Saturday and Sunday. The Mendips are composed of rocks ranging in age from mid Silurian to mid Jurassic and we would be seeing outcrops of a variety of these which illustrate the complexity of the geology. The Mendips were formed during Variscan mountain building and therefore this is a region of complex tectonics, with thrusting from the south causing folding and overthrusting of strata.
On a bright, dry Saturday morning, we set out to our first locality at Vallis Vale. This involved a long trek along muddy paths to a small disused quarry. The walk was well worth it when we entered the quarry and saw this site of the classic unconformity, described in detail by de la Beche in 1839 in his first memoir, and now an SSSI. The beds in the lower half are grey Carboniferous fine-grained, unfossiliferous limestones, up to a height of approx 4 m, with beds dipping at approx 40°. These are topped by paler planar horizontal deposits of oolitic Jurassic limestones. The boundary between these beds is an angular unconformity, where rocks of the intervening periods of the Permian and Triassic are missing. Paul explained that uplift occurred during the Variscan and the Carboniferous limestones stood as an inselberg in Triassic deserts before being flooded and eroded by seas during the mid-Jurassic. Quarrying in recent times had been for the harder bands of the Jurassic rocks known as Doulting stone, some of which had been used in Wells Cathedral, and Carboniferous limestones had been used for ballast in road building.
Our next location was a short distance away and a climb up to Tetbury Hill quarry, where we were able to stand on the unconformity on top of the Carboniferous limestone. The Jurassic sediments above had been removed, ready for blasting the underlying limestone, and then work had stopped. Various interesting features were noted on the quarry floor, including faulting in the Carboniferous limestone with drag folding along the fault, fissuring with infilling, segments of chert bands, and fossils such as corals and brachiopods, with worm borings through the sediments at places along the edges of the pavement. After much investigation and discussion on our finds, a short drive then brought us to our lunch stop at the Talbot Inn at Mells.
Saturday afternoon was spent looking the results of the Hercynian Orogeny. The compressive forces acted from the south / southeast, and the Hercynian Front coincides with the Mendip Axis, with a stable foreland to the north. This Orogeny led to the northern translation of the Culm coal measures onto the northern foreland, and periclines (of which the Beacon Hill Axis is one) with the northern limbs dipping more steeply than the southern ones. The culmination of the Hercynian was the major over thrusting from the south, this resulted in Klippen of Carboniferous Limestone and Namurian Sandstone overlying highly contorted and overturned Coal Measures north of the Mendip Axis, as seen at Vobster.
We first visited Upper Vobster, where the flooded former quarry is now used for diving and diver training; as there is little sediment the visibility is very good at depth. The quarry was initially dug for limestone, but is part of an inverted sequence of carboniferous limestone on coal measures. Thus, while the aim was to dig down to coal, it had actually been eroded away from above the quarry location as this is the bottom limb of a large inverted Nappe of which the klippe in a remnant inlier (see Paul's sketch). The inversion is demonstrated by the fossils found in the limestone; worm burrows, current bedding and corals. Productus shells are also found, though these are disarticulated and deposited after death. There is red Triassic haematite in the limestone joints, as brecciaed shatter zones are common in these deformed rocks.
We then went on to Downhead Quarry on the Beacon Hill periclines. This was originally thought to be Llandovery from fossil correlation, but is now dated as Wenlockian. This is the most southerly Silurian surface exposure in Britain. Trace element analysis on the Andesite lavas, and occasional tuffs, shows they erupted in the Welsh Borders or Midlands. The quarry was excavated along the (east-west) axis of the pericline. The St Mary Hill Quarry is also on the Silurian outcrop. The Silurian Wenlock Volcanics were erupted as the northern Iapetus Ocean closed and subduction ended (before the southern part closed, as the Earth is spherical thus the last Silurian volcanics are found in Dingle). From paleomagnetics and radiometric dating this area was 30°S, and the volcanics were deposited in a warm limestone sea.
As we left the quarry it was definitely getting dark and after a good dinner Paul described what we had seen, going through the geological column for the Mendips, followed by limestone stratigraphy and fossils, and then paleoclimate change. Then most of us went to the bar.
[On Saturday evening after dinner, Paul gave us an excellent lecture, reviewing what we had seen, previewing what we would see the next day at Milton Lane and Burrington Combe, and introducing us to the Vaughan Scheme for the Zonation of the Carboniferous Limestone.]
On the Sunday morning, the bells of Wells Cathedral were half-muffled to mark Remembrance Sunday, as we left, in a small convey of vehicles, because of limited parking at the first locality. A pad had been placed over half of the clapper of each of the bells, so as to give a subdued tone each alternate time the bell was struck. We drove a little way out of Wells to the driveway of Wilton Hall, where we parked the vehicles and were met by Mr Simon Tudway Quilter, the landowner. He joined us briefly for the visit to the nearby Milton Lane locality, to learn more about the geology of the succession exposed there.
Milton Lane is a sunken wooded lane, running up a moderately steep hill, which cuts through a continuous succession from the Tea-green marls in the Triassic to the Blue Lias in the Jurassic, a sequence of approximately 21m vertically. The period of deposition of those beds reflected a significant a change of environment, the Tea-Green Marks at the top of the Mercia Mudstone Group representing a coastal area, the Westbury Formation reflecting rising sea level as the sea flooded over the Triassic Landscape. At this stage sea levels were mainly shallow and fluctuated, but with an underlying rise in levels which continued during the Jurassic, so that an archipelago in the Jurassic sea was formed by the higher ground in the Mendip area. There was also a shift of palaeolatitude during this period, from the 100 N earlier in the Triassic (representing the latitude of the Sahara) to latitudes of 20-25° N, i.e. the tropics and just beyond, in the Jurassic.
At the start of our walk up the lane the evidence for the Triassic strata of the Tea Green marls and the Westbury beds was somewhat confused, there being re-worked deposits of pebbles and red soils, but we were assured that the Triassic strata were clearer at depth.
A few yards further up the position did become more evident, with clear exposures of strata. A very white limestone layer in shales showed that we had reached the Cotham beds in the upper part of the Rhaetic. This was a period of rising sea levels, but the Cotham beds were still probably deposited in a lagoonal environment. Paul said that the limestone beds in the Cotham were marbled with a decorative "landscape marbling", floral dendritic pathways formed by escaping gas from anoxic conditions. Beyond the Cotham beds we reached the Triassic/Jurassic junction. Above the lane, in the blocky White Lias, there was a fine example of Dog-tooth Calcite Spar. The White Lias, layers of calcite-mudstones, marls, and thin limestones were laid down in shallow water, probably around islands which then stood in the Mendips area, surrounded by the Jurassic sea.
A few yards further up we came upon the Blue Lias. Paul explained that this was better described as the Blue-hearted Lias. This is because the limestones which alternate with the mudstones are as white as those of the White Lias on their exterior, but on hammering, the interior shows the difference. Reflecting the more abundantly fossiliferous nature of this section, we came across pieces of ammonite, and brachiopods. Having reached the top of the hill, some of us climbed up out of the lane and into an adjoining field, where there was a fine view over to Wells and towards Glastonbury Tor. On the way down, we came up on a loose piece of rock with desiccation cracks formed by the sun in an hexagonal pattern, a sun-bed. There was an inconclusive discussion as to whether the team "sun-bed" properly referred to the surface in which the desiccation cracks formed, or the layer of infill on top.
We returned to the cars and drove back to Wells. There was then a change of cars, as we were to take all the cars with us for the journey back home. Nevertheless, we would leave some of the cars at the car park at the Castle of Comfort Inn, where we were to have lunch, because parking near the next location would again be limited. Some of us went back into the hotel to collect our luggage, and as we came out a very light rain shower began to fall. We stood in silence by the cars as 11 o'clock struck, and we then listened to the strains of the Last Post and Reveille coming over from the Cathedral.
Having driven to the Castle of Comfort car park and changed cars as necessary, we proceeded in a north easterly direction and left the cars at another car park close to the next locality, Wurt Pit. This was a swallow hole in the middle of a field. The entry into the swallow hole enclosure was not entirely easy, and required the negotiation of mud and barbed wire fences. Once inside, there was a sort of ledge from which it was possible to view the exposures and peer down into the pit. At the entrance to the swallow hole were examples of a fungus called Jew's Ear, the name evidently being a relic of a less politically correct age. Paul explained that what we had before us in the swallow hole represented an interesting piece of geological history. What originally had been a plateau of Carboniferous Limestone gradually turned into a Karst Landscape with swallow holes. In Triassic times this became a desert landscape with occasional flash floods, and underground rivers. Subse quently deposited rocks laid down on a poor substrate then gradually collapsed into the swallow holes like this one at Wurt Pit.
There was then a process of silicification as a result of hot silica-rich solutions coming up. This was widespread in the Central Mendip area, and was related to lead and zinc mineralisation, giving rise to a sequence known as the Harptree Beds. What we had here was a condensed sequence of the Harptree Beds, containing virtually everything from the Middle Jurassic down to what we had seen earlier, which had all fallen down the swallow hole. At the bottom of the hole were Rhaetic deposits of iron ochre, which in the past had been worked to provide pigments for paints. There were a number of fossils, moulds preserved in the silica.
Paul summed it up by stating that the swallow-hole was a largely Jurassic veneer in the Carboniferous Limestone. Ten feet in from the pit there would be solid Carboniferous Limestone. Paul repeated his explanation briefly outside the swallow hole enclosure, for the benefit of those who had preferred not to venture within. He also produced a representative piece of stone from the pit, originally limestone, but which had been gradually silicified and stained with red ochre. We then returned to the cars and repaired to the pub for lunch.
Burrington Combe We set off in good cheer after a warming meal at the aptly named Castle of Comfort. Naturally, the rain which had held off for most of our trip started to fall heavily when the writer's turn came to take notes so technical information is a little scarce.
Burrington Combe lies within the steeply dipping northern limb of the Blackdown Pericline and exposes an almost complete sequence through the Lower Carboniferous with strata ageing southwards. It is believed that the gorge or combe would have formed along a fault line and was probably a wadi in Triassic time coping with flash flooding. The first location we looked at was a disused quarry made rather spectacular by rain slicked dark limestone covered in bright red leafed cotoneaster. Here thinly bedded Clifton Down limestone lies up against the younger, more massive Hotwells limestone forming a junction between two fossil zones. Some rippling was visible as well as slickensides along a fault line. Some examples of calcite mudstone with concoidal surfaces were found. These are an extremely fine limestone suitable for printing purposes.
In the second location, also a disused quarry, the upper part of the Burrington Oolite formation could be seen. This is thickly bedded and light to medium grey in colour. Of historical interest was a cleft in the rock face where in 1762 Rev Toplady sheltered from a storm and was inspired to write the hymn Rock of Ages.
The cleft would hardly have offered much shelter and we felt he would have been rather upset to know that just around the corner was Aveline's Hole, a sizeable cave which in early Mesolithic times had served as a burial chamber for some 70 to 100 people. It is accorded the honour of being England's oldest cemetery.
The third location further up the combe showed us the lower beds of the Burrington Oolite formation as well as very thin friable layers of yellowish dolomitic limestone. For a while we were studied intently from across the road by a small flock of white goats.
As daylight was fading fast we decided not to visit the fourth location. Some of us settled in at the local tea room whilst others started their long journeys home.
20 November 2008
First to go was John Jarvis with an introduction to a display of prehistoric tools from sites at Rainham, Kent. There were examples of Paleolithic, Mesolithic, and Neolithic stone tools, starting from early 'pebble' handaxes, just sharpened at one end, through Mesolithic re-sharpenable axes to later microliths. John can tell the difference between polished flint fragments, used as tools, and debris resulting from tool production. One piece of advice was to make a replica if you find something good, before it goes. Before and after the talks we were all invited to handle these ancient tools.
From the detailed to the broad sweep and Georgina Barnes: A quick trip through Japanese geohistory. Geologically young, we were looking at an accretionary complex from eastern Pangaea dating from the Permian-Triassic period at 270-230 Ma. We looked in particular at the Hida Mountains in central Japan, going on to the late Jurassic and Cretaceous periods with a band of granite as a result of the subduction of the ridge. Miocene rifting at 19-16 Ma preceded late Miocene volcanics caused by seduction of the Philippine plate and at 6Ma with an arc-related new volcanic chain, plus a Pleistocene caldera and the current volcanism which goes back to 700,000years BP.
Staying in the East we were treated to: Southern Thailand: a field trip in November 2007, led by Michael Ridd of the GA, a talk by Jenny Parry. We were treated to wonderful pictures from an adventurous trip, starting from Phuket, with limestone formations intruded by granites, with three prominent granite headlands, and at Phuket Patong Beach, granite intruded by migmatite. Amazing limestone towers in Phang Nga National Park, and a shell cemetery at Laem Pho, underlain by a bed of lignite were other interesting sights.
From the Far East to Africa, and a talk by Yvonne Brett: A visit to Mali. After a brief introduction on the importance of the River Niger in the prosperity of the region, we talked about the mud architecture of Djenne, the world's largest mud-brick building, and Timbuktu, the importance of the transport of salt from the Sahara, and the Bandiagara Escarpment, a strange sandstone outcrop with cliffs extending 200km, the home of the Dogon people.
Lecture: 'Homo Britannicus: The Ancient Human Occupation of Britain' by Chris Stringer
11 December 2008
Chris is a Researcher at the Natural History Museum and gave us the latest findings on the history of man in Britain. The evidence comes from the tools used, paleontology and archaeology, helped by new techniques such as CT scanning, direct dating, isotopes, and DNA.
Chris started with the 3 phases of human evolution, the first two in Africa, and then the spread of homo erectus who reached Southern Europe about one million years ago. The evidence in Britain dates from c.700,000 BP with sites such as Boxgrove. But it appears that the Ice Ages were too inhospitable for man to hang on here and at 125,000 years there is not a single artefact to be found. We find Neanderthals in Norfolk at 60,000y and by about 45,000y there are modern humans in Europe shown by a big change in technology. One of the most interesting problems is why there is such a big gap in the human record in Britain, even in interglacials when there were tropical animals, but the answer may lie in the changed configuration of watercourses, the Channel and river systems.
It was a fascinating lecture and, if my experience is anything to go by, led to a dearth of copies of the book in the NHM itself and nearby book shops! The lecture was preceded by our small Christmas party and made a very good evening.