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.
Gilbert's Pit • Thames Barrier • Land and Sea Level Changes • Student Day to Herne Bay • Denbies Vineyard • Sarsen Geowalk on the South Downs • Kensal Green Cemetery • Langdon Cliffs, Dover • The Building Stones of Canterbury • Southern Spain • Extreme Sedimentology
Field Trip: Gilbert's Pit, Charlton
22 February 2007, morning
The Thames Barrier is situated right beside the geological SSSI at Gilbert's Pit, Charlton and so here was an opportunity to look back into sea-level rises and falls in the Thames Valley from a very different era. Charlton was designated an SSSI as it is one of the very few places in the London Area that it is possible to view the Lower London Tertiaries. It is designated the Type Section for the Woolwich Beds and has a magnificent exposure of the Blackheath Beds at the top of the section.
This pit was operating from 1880 mainly for the Thanet Sand that underlies the Woolwich Beds. This is a very pure, fine-grained sand that was used mainly as a moulding sand for the Woolwich Arsenal and for glass making. The pit ceased production in 1938 and was partially backfilled with war-time rubble, burying the contact of the Thanet Sands with the underlying Chalk. Steep slopes surround a football pitch and for the most part are now wooded but there used to be an excellent exposure on the eastern face. The Thanet Sands are mostly covered with scree and access is difficult but nevertheless it is worth examining this famous section. Until about 1999 it was the favourite haunt of the local kids who enjoyed sliding down it on 'sledges' of corrugated iron but anxieties that the Blackheath Beds at the top would be completely eroded away by these activities led to a protective fence being built around the whole section. Sadly this has proved counter-productive as vegetation is now almost obscuring most of the exposure and access from the bottom is blocked by the build-up of the small, black, rounded pebbles fallen against the gate from the Blackheath Beds way above.
But this was February and the visit was not intended to be a 'Field Trip' in the normal sense. Wet gear, yes, but no boots. We examined the section and the pebbles from the bottom and then made our way up the path overlooking the Thames to see the exposure from the top. Although we did not go and examine the Woolwich Beds, the group was shown examples of the Shell Beds. This is a very restricted fauna, typical of brackish, estuarine conditions but 55 Ma ago the estuary in question was thought to be sub-tropical, more typical of mangrove swamps in Malaysia. Calcretes beneath indicate that the underlying beds may have been sub-aerial for a while and beneath them there is the marine Upnor Formation overlying the near-shore Thanet Sands. The Blackheath Beds at the top are still well exposed, although somewhat degraded, and it was possible to make out some graded bedding. These very rounded, very black pebbles are somewhat enigmatic. They originated from flint layers in the Chalk but have they then been re-cycled from similar exposures in the Upnor Formation near-by? With the Blackheath Beds the environment has returned to fully marine sediments at the base of the London Clay. How long will it be before this is the fate of the present Thames Estuary once more?
Field Trip: The Thames Barrier
22 February 2007, afternoon
Following the morning study of Gilbert's Pit and a substantial pub lunch, a short walk took us to the Visitor Centre at the Thames Barrier where the party of 25 Branch members assembled. Entry to the Barrier required some paperwork and security checks, but we were soon inside and welcomed by our guide for the afternoon, Roger Vine. Roger explained how the need for a flood defence scheme for London was recognised after the devastating floods of 1953. The original study by Prof. Herman Bondi in 1966 recommended raising the river banks and the construction of a flood barrier with movable gates. Construction started in 1974 and the Barrier was opened by the Queen in May 1984.
The flood risk for London depends on two probabilities: the probability of a tidal surge (which decreases with the peak height of the surge), and the probability that the peak of a surge will coincide with a high spring tide. The 1953 tidal surge reached a height of 2.59 m ODN, while the highest surge ever recorded was a massive 3.66 m in 1943. Neither of these coincided with the 2.71 m mean high spring tide, however. After considering the probabilities, the Barrier height was designed at 6.9 m ODN with adjacent riverbank defences at 7.2 m, to give protection against a once per 1000 year tidal surge. With global rising sea levels and the sinking of south-east England due to glacial rebound, this condition would be satisfied until until 2030. This seemed a long way in the future back in the 1960s but is beginning to get uncomfortably close now. It isn't as simple as predicting that the water will start lapping over the top 23 years from now, but it does affect the probabilities. The Barrier is still expected to give protection against a 500-year event until at least 2050, and a 100-year flood until 2070.
A ridge of hard chalk in which the massive concrete caissons could be founded, and a relatively straight stretch of the river determined the siting of the Barrier. Interestingly, the river banks had to be raised upstream as well as downstream, as exceptionally high river flow could pond up behind a closed Barrier and give a river, rather than tidal, flood risk to London. Although the talk by our evening speaker, Andrew Haggart, challenged the case for sinking land levels assumed in the Barrier design specification, since the Barrier was designed we have become more conscious of eustatic sea level rise due to global warming, now predicted at up to 0.31 m per century. It is comforting to know that studies are already being undertaken to find the best way of protecting London against flood into the more distant future.
After his talk, and a short film about the construction of the Barrier, Roger equipped us all with hard hats and smoke hoods and led us out across a bridge to Pier 9 - the closest one to the southern bank. Then we descended a long set of stairs to the tunnels which give access to the central piers between the navigable gates. On reaching Pier 8, the complexity of the machinery was revealed with the huge hydraulic actuators which raise and lower the gates. Roger explained how the system was designed with several levels of redundancy at all stages. If the power or any of the machinery should fail, there is always a backup system ready to cut in and close the Barrier. If any backup system also fails, there are still several ways of uncoupling the failed components and using other systems to close the barrier and provide the necessary flood protection. To me, this was the most impressive feature of the engineering.
A further safety procedure allowed us to see one of the gates in its closed position. If any maintenace operation on one of the gates means that the gate cannot be restored to fully operating condition at four hours notice, the gate must be closed before the maintenance work can begin. This explains why you sometimes see one of the gates closed even at a time when there is no risk of a flood, but it allowed us to look on one side of Pier 8 to an open gate, and on the other side to a closed gate.
The lengthy discussions which took place out on the deck of Pier 8, and our guide's enthusiasm in providing full and complete answers to any questions asked, put us rather behind schedule and it was getting dark as we returned through the tunnel and across the bridge. Roger was thanked for his excellent talk and guided tour, and we all left to get the bus to Greenwich for the evening lecture.
Lecture: 'Land and Sea Level Changes in South East England' by Andrew Haggart
22 February 2007, evening
With the location of the day's events, it seemed sensible to try and hold our evening talk in the magnificent Maritime Campus of the University of Greenwich, beside the Thames. We were delighted that Andrew Haggart from the Earth and Environment Department was able to talk to us about Land and Sea Level Changes in South East England. With global warming high on the agenda, it was an opportunity to look into the future of the Thames based on Andrew's research on Recent events, deduced from core samples.
Andrew was less pessimistic than Roger Vine at the Thames Barrier on the rate of sea-level rise in the Thames Estuary. The assumed sinking of south-east England, which had been assumed in the design specification of the Thames Barrier, was not supported by Andrew's research. He has examined river terrace deposits and concluded that at least some of the peat layers in his core samples may be due to compaction.
Diana Clements and Eddie Yeadon
Field Trip: S260 Student Day to Herne Bay
25 March 2007
Despite an early start under grey skies, those who remembered to adjust their watches for the start of British Summertime, assembled in a windy cliff top car park at Beltinge just east of Herne Bay, and were to be rewarded with an exciting day of Geological Discovery as skies cleared to reveal a pale blue spring sunshine lit horizon beyond the beach below.
For many it was their first OUGS fieldtrip since starting their Geology courses in February and an array of bright hard hats appeared as we walked down the cliff path. At the bottom we paused as Brian explained how the soft clay layers of the cliff, slide over each other more readily when wet, than the sandy layers which have greater friction. Water penetrating the cliff causes instability which eventually leads to the failure of the cliff as it tumbles into the sea, where waves remove the rubble from the shore line. A concrete promenade has been built to protect the retreating cliff line but looking to the east and west it is possible to see two different cliff profiles behind the seawall. To the west the London clay beds of the cliff have been graded to form a slope of about 20 degrees and grassed over to prevent further falls, but to the east the cliffs dip approximately 55 degrees so further falls are likely in the future when the beds become unstable when saturated.
Once on the beach we were encouraged to become 'real amateur geologists' by getting our hands dirty. Looking at the composition of the beach was not enough. After a brief introduction to the material beneath our feet and the importance of accurate recording in our field notebooks, we had to observe at close quarters, on our knees if necessary, through our hand lenses, and collect as many different shaped, sized and coloured fragments of beach material as we could in a few minutes. We were then encouraged to use our hitherto clean and unused grain size charts to describe the appearance of our finds and extend our 'geological' vocabulary. Most people were amazed at the range of material found in quite a small and 'ordinary' piece of sand and pebble foreshore and more than willing to share their beautiful and diverse finds with their co-hunters who were all encouraged to guess at the provenance and geological history of the fragments by Diana Smith our leader, who was able to share her great enthusiasm for even the most humble of finds, with us all.
As the tide receded Di Clements who had been scouring the rock pools near the low water mark for fossils shared her finds, explaining some of the processes involved in fossil formation, the use of fossils in the dating of sediments and the importance of fossils in determining the 'way up' orientation of sediments. Eager to find our own fossils we splashed among the small pools turning over rocks and stones with many people easily finding their own specimens to share with colleagues and to add to their own expanding collections, Sharks teeth, Beds of Bivalves in a grey sandstone deposit, fossilised wood and many near perfectly fossilised pyritized bivalves, names of which escape me [mostly Arctica morrisi - Ed.] were verified and carefully stored in tissue and margarine tubs brought with us for such exciting discoveries.
Following this baptism into the association of avid seashore enthusiasts we split into smaller groups to explore other geological delights to be found close at hand under the guidance of our enthusiastic leaders. Along the coast at this point gigantic blocks have been tipped onto the edge of the beach close to the sea wall as added protection to wall and cliff, providing an opportunity to study non local rock specimens at close hand, their 'twinkly', crystal nature being obvious without recourse to hand lenses. However, once on hands and knees and viewed through a hand lens the wondrously intricate crystal structure of the granites and highly metamorphosed rock could be seen clearly and again we were given the opportunity to share our observations and raise our awareness of geological terminology in the field. [S260 students having just reached this Block in their studies].
Our next port of call was the nearby Bishopstone Glen where Iain met us and reiterated the need to make a quick visual survey of the exposed beds following the guidelines given earlier, paying particular attention to locality, grid reference, date, time of day and weather conditions before taking a closer look at the beds where safely possible. We were encouraged to draw a sketch section of the exposed face, some 20 m high, on which to note our observations following closer examination of the beds, again having the opportunity to relate previously gained concepts to field experience. We were able to see variations in size and colour of the sand and clay clasts between the sandy Harwich, powdery Upnor and Thanet Sand formations, identify green glauconite and black minerals in the sand samples, record grain size accurately using our charts and note iron staining and fossil evidence. Fortunately, the sun was shining at this point which illustrated the difference between observing grain colour and composition on the grain size chart through a hand lens in shade and sunshine.
A move to the north, seaward facing cliff face followed, to enable closer observation of the younger London Clay Formation which has been subjected to rotational slipping nearer the top of the exposed cliff beds. Brian met us there to encourage our observations and extend our geological vocabulary and powers of deduction as by this time we were clearly becoming real landscape detectives capable of seeing fine cracks, finding small selenite crystals, and spotting the irregular boundary of the unconformity between the fine plastic London Clay and the Harwich Formation. The latter provoked a discussion on the conditions during deposition, origin and transportation of the sediments. Above the London Clay the most recent deposits, the silty aeolian brick earths and the pebbles and irregular clasts of the solifluction layer, completed the geological picture of the area built up during the morning.
By now, we were all more than ready for a lunch break, so we arranged to meet after lunch to study the building materials used to build the towers at Reculver.
As expected, many locally found sandstones and flints were used in the walls but many had been brought by sea, from further afield, for example limestone to strengthen corners of the building, and some even possibly from far flung parts of the Roman Empire to augment Roman clay tile fragments. Much interest was generated in the features of flint; the white borders, chattermarks and their formation into such distinguishing shapes. The sandstones, showing bedding, cross bedding and ironstone layers were used to explain the principle of fining up, [finer grains on top in a sedimentary sequence] giving 'way-up' criteria. Fossiliferous shelly sandstone similar to that found on the beach had also been cemented into the west facing wall. It was a pleasure to study the wide variety of rock samples present in the walls out of the cold north-easterly wind. The building materials of west facing walls, subject to westerly winds, also show considerable differences in the rate of weathering.
There was time to take a further look at the enormous and varied non-local blocks of rock used to protect the point at the small Reculver headland. Our attention was caught by the coarse grains of the intrusive igneous rocks which glistened and twinkled in the sun. With the help of Diana Smith, closer inspection led us to identify feldspar, biotite, muscovite and pyroxene crystals, the latter with clear characteristic cleavage planes at 90 degrees. Large blocks of Limestone exhibiting different barely visible layers attributed to differing environmental depositional conditions, were also identified. Stromatolitic layers were seen in blocks as were brachiopod beds and rubble beds, sometimes delineated by bands where calcium had migrated to form wavy lines between the beds. The limestone was laid down in hot tropical conditions in shallow clear warm water, a fact that can be deduced as some contained corals, which can be found in similar conditions today.
This wound up a hugely enlightening Field Trip for fledgling OU Geology students and others with many opportunities for the hands-on experience of seeing and touching Real rocks as promised and I'm sure it will not be the last for many of us if the comments of departing participants were anything to go by.
Field Trip: Denbies Vineyard led by Richard Selley
28 April 2007
In weather right for growing vines, we gathered in Denbies car park where we met Professor Richard Selley. He was the "notorious local geologist" [his words, reference 1] who recommended that the owner of the Denbies Estate should set up a vineyard. I was curious to find out more, having walked in this area earlier this year.
Dorking Caves and Museum
Sharing cars, we headed into Dorking town centre and the museum.. Here, we divided into two groups, Ammonites and Trilobites, I was among the Trilobites, visiting the caves first. We met Sarah, our cave guide, in South Street. I'd been totally unaware of the system of manmade caves below the busy streets of the town until now. Many of the shops and houses had cellars which were part of a network connecting passages. The entrance was a narrow doorway on the left of the War Memorial. At one time, there was even a covert route from pub to brothel! But, their main function was to storage of perishable goods before modern refrigeration, in this case wine; the temperature inside being a steady 14-16°C. Excavation began in the second half of C17th; the date derived from the style of lettering in the graffiti near the entrance. Originally, entrance was via a staircase in the grounds of Butter Hill House, but this is now blocked off. The cave system is on two levels, 4 tunnels of around 15 metres long on the upper level having, roughly forming a square. Adjusting from the bright sunlight outside to the inside dimly lit by candles and more recently installed electric lighting, we came to a tunnel on our right, a few metres from the entrance, a C19th wine vault, divided by brick walls into 42 bins, each could accommode 600 bottles. The caves were last used for wine storage in the 1960s. We continued through a well shaft into the southern passage, labelled Vintage Port. There were 2 more well shafts at the end, both now filled in to floor level. Some of us descended the 42 steep, narrow steps to the Lower Mystery Chamber, passing a bat roost in the well shaft. The roundly dug chamber is believed to be a folly. There was a dark, iron stained tide mark, where the water table had risen sometime after excavation. The cave geology is Folkestone Beds, a sandstone, in the upper part of the Cretaceous Lower Greensand Formation. Being soft, it was easily excavated but strong enough not to collapse, though similar caves collapsed below houses in Reigate. It was deposited in a marine environment. It contains iron staining derived from dissolution of glauconite, a ferromagnesian silicate, only formed in seawater and unstable when exposed to rainwater. In a number of places, current bedding was visible, formed by tidal currents flowing over sand.
Resurfacing into the sunlight, we retraced our steps to the Museum. Located in a former foundry building, there was a fascinating assortment of items on display; and more tucked away upstairs. Many of the geological specimens were from the collection of Lord Ashcoombe, the original owner of the Denbies Estate. In the display cabinets downstairs were some impressive looking mineral specimens, mainly from the Carboniferous Limestone in Derbyshire: fluorite, barite, galena, sphalerte, quartz. More locally, there were fossils from the Chalk, including ammonites and echinoids; along with an iguanadon tail from a well shaft in the Lower Greensand. Upstairs we saw woolly mammoth teeth and femurs from the Pleistocene river terrace gravels of the River Mole; and more Chalk fossils.
Ranmore Common and Denbies Vineyard
As we ate packed lunches and enjoyed view from the Chalk ridge at Ranmore Common, Richard gave a geological overview: we were on the northern edge of the Wealden Anticline, comprised of sedimentary rocks deposited during the Cretaceous Period. Moving towards the core of the anticline, to the south, the rocks became progressively older. Beyond the Chalk was the Lower Greensand ridge of the Surrey Hills, the highest being Leith Hill; its the tower visible through a clearing in the trees. Beyond this was the Weald Clay which forms the relatively flat land in the core of the anticline. However, it was too hazy to see the South Downs Chalk ridge, at the southern end of the anticline. Between Ranmore Common and Box Hill, the River Mole cuts through the Chalk, forming the Mole Gap. Denbies vineyard is situated near its southern end.
We walked down to the vineyard in good time for the 3 o'clock tour. Along the way, we admired the view over the Mole Gap across to Box Hill, the vineyard in the foreground. Denbies is the largest privately owned vineyard in Europe, covering an area of 104 hectares (260 acres). They grow 19 different grape varieties and typically produce 400,000 bottles of wine a year, sold exclusively within the UK; though last year was a bumper year, with 500,000 bottles produced. The site is so well suited to viticulture due to a combination of geology, topography and microclimate.
Most of the vineyard overlies the Chalk along with an extension up a dry valley comprised of Pleistocene river terrace gravels. We skirted round this on our way down the hill. Both provide mildly alkaline, well-drained, but not waterlogged soils. The Chalk is particularly well-suited because it porous and permeable. The permeability comes from fractures in the rock, along which water can flow easily. The vine roots often exploit these, thereby obtaining moisture.
The south and east facing slopes are sheltered from the prevailing wind. The angle of slope also maximises sunlight, particularly during the late summer and autumn. However, being in a valley, the vineyard is prone to frost, late spring frosts after a mild, early spring being particularly lethal. To tackle the problem they tried using diesel heaters, but they cost £3000 per night to run, so they now have a cheaper system involving a tractor driver on call.
Essentially, the geology is akin to that of the Champagne region of France, where vines are grown on the south facing Chalk slopes of the Paris Basin. This was why Richard recommended this site for a vineyard. It's likely to benefit from human induced climate change - at least initially - if this it brings hotter, drier summers and more sunshine. The top of the dry valley could be turned into olive groves.
Our Denbies tour began with a film in a 360° surround cinema. It featured Richard describing the geology and illustrated the working year in the vineyard. At the end, our guide was somewhat disappointed by our rather underwhelmed reaction only a few people got sprayed with wine. Nonetheless, there were inviting rich fruity smells as a train with open wooden carriages and onboard commentary took us round the working area of the building, where vine became wine. We past the fermentation tanks, some oak barrels, the bottling area, before disembarking in a dimly lit room for the highlight of the tour, the wine tasting.
We sampled three wines, all named after localities in the Mole valley area:
Before leaving the tour for the shop, we thanked Richard for a fulfilling trip. I'm certainly encouraged to come back and sample more of the produce.
Reference: In addition to what I heard on the trip and at Professor Selley's lecture, I have referred to:
Spring Geowalk - a Sarsen Hunt on the South Downs led by Brian Harvey
13 May 2007
On a rather soggy Sunday morning, 18 people gathered in the car park next to the Jack and Jill windmills on the South Downs south of Hassocks, West Sussex. From here, we set out on our 8 mile Geowalk.
Brian began by outlining the geology of the South Downs and history of land use: we were walking on Chalk, on the southern limb of the Cretaceous Wealden Anticline. Recently, geologists have reorganised the sub divisions of the Chalk. The former Upper, Middle and Lower Chalk divisions have been replaced with 2 main groups: Grey Chalk and the younger White Chalk. Both are themselves sub-divided and named after locations where they outcrop, e.g. Newhaven Chalk Formation in the White Chalk. The overall dip of the strata is south towards the English Channel, forming long rolling southern dip slopes and steep north facing scarp slopes. Smaller anticlines and synclines are superimposed on the overall structure, e.g. the Pycombe Anticline / Henfield Syncline. In places, there are isolated outcrops of younger, Palaeocene, sediments which were deposited after the Chalk but have been eroded in most places. These include Clay-With-Flints. In a number of places, south flowing rivers cut through the Chalk, including the Adur to the west. The topography has also been considerably modified by periglacial processes during the Pleistocene.
The South Downs Way, a long distance path running along the Chalk ridge from Eastbourne to Winchester, follows the route of an ancient trackway. Originally it ran through the forested natural landscape of the Downs. From the Neolithic onwards, the land has been modified by livestock and arable farming, including sheep grazing forming the open grasslands people usually associate with the South Downs. Today the Downs are under growing pressure from development and changes in farming practices from sheep farming - no longer economically viable unless subsidised to cereal monoculture. The latter makes for more monotonous landscape, reduced right to roam; and - crucially for geologists - the underlying rocks are obscured by growing crops. There have been moves to award the South Downs National Park status, meaning increased conservation and protection, but this is being held up by bureaucracy.
The first part of the walk was eastwards along the Chalk ridge to Ditchling Beacon, following the South Downs Way. We didn't see any sarsens here, but Brian did point out:
On Ditchling Beacon, we turned south to walk down the dip slope of the Chalk to Stanmer. We upped the pace as time was getting on and the sky was darkening. However, Brian did highlight the view westwards to a dry valley, North Bottom. This was more periglacial topography, formed by rivers flowing eroding a dry valley. They were able to flow over the Chalk without draining away because it was rendered impermeable by deep permafrost. The slope on the far side was tree covered, being too steep to plough. There were cultivated fields on the shallower slopes. Initially, we didn't see any sarsens, but we did see some iron stones, ferrocretes (laterites). These were formed at the surface from the leaching of iron from the underlying rock and precipitation as a surface cap during tropical weathering. They possibly came from the now eroded Palaeocene Reading Formation.
We crossed Ditchling Lane and entered Highpark Wood, possibly ancient woodland which has never been cultivated. However, trees were coppiced to provide wood for sheep folds and fuel as well as charcoal fuel for limekilns. Here we saw our first sarsens on and near the path. They became more frequent and larger as we descended through the woods to Stanmer.
Stanmer lived up to its name which means Stoney Bog. When we got there, the rain was so torrential that we had to eat our sandwiches in a barn. Here, Brian told us about sarsens: they are hard, non-porous sandstones cemented with silica. They occur throughout southern England, on various rocks, including Chalk, Tertiary and Quarternary deposits; but are not found in the sediments in which they originally formed. They may be the remnants of a Tertiary erosion surface, like the Clay with Flints. How they formed is a "geological embarrassment". They are not glacial erratics: many of them, including those on the South Downs, are well beyond the southern limit of the Pleistocene ice sheets and none have been found associated with till. Presently, they are believed to be silcretes, surface deposits formed in a tropical climate with periodic heavy rainfall, in a similar way to flints, involving a change in pH precipitating silica as water permeates strata. However, they also occur in long lines on valley sides where water seeped out. This could occur in a cooler climate. Brighton University has identified clusters of 30 or more sarsens on the Downs in this area, but many have been removed for use as building or decorative stones. Dodging large puddles in a road which had become a temporary stream, we walked past the church to the village pond which was bordered with sarsens on the nearside. Some people tried visiting the donkey wheel on the far side, only to find it closed.
Fortunately, the rain eased as we left Stanmer and walked back up the hill, this time taking a track eastwards past Limekiln Wood. We were now walking on Clay-with-Flints. Turning north into Upper Lodge Wood, we passed a tree sculpture and contemplated the view towards Chattri, the Indian War Memorial. Beyond this were examples of the modern pressures on the Downs: housing development spreading north from Brighton, as well as the busy A23 dual carriageway. We crossed back over Ditchling Lane and continued past New Barn towards Lower Standean. In the garden of the house here was a sarsen stone rockery. After this came the steepest part of the climb, past Rock Pond, but the rain was now clearing. This path brought us back to the Chalk ridge, near the ploughed flinty field we'd seen earlier. Before walking the last mile back to the car park, we thanked Brian for leading the walk and indeed finding us some sarsens.
Before heading home, we looked at the windmills, now preserved by a trust of volunteers. Jack, the more easterly windmills is a private house, but Jill, the westerly is open to visitors on some Sunday afternoons. We sampled some of the homemade cakes in the small café on the ground floor. Outside were some millstones. Brian had told us to look out for millstones made from three different rocks: French chert (Burr Stone), Millstone Grit and German volcanic rock. The sails of the windmill were being refitted, with two of them on the ground, but smaller sails were turning a gear system slowly moving some wheels on a track. We climbed the steep staircase to look round inside and then the two steeper ladders to the upper two levels. It was quite cramped inside. Other than the millstones, the workings were predominantly made of wood. There were large hoppers for feeding grain and flour and a central shaft leading from the sails. Owing to the repair work we unfortunately we had to head for home without being able to purchase any flour.
Field Trip: The Geology of Kensal Green Cemetery led by Eric Robinson
19 July 2007
This walk had to be rescheduled for 2.00 as the cemetery closes at 4.30, so only about a dozen of us managed to get to the rendez-vous, for a visit in the context of the 200th anniversary of the Geological Society, and the forthcoming 150th anniversary of the Geological Association, both with the theme of Local Heroes.
The title above does not do justice to the wealth of information that we were given. We met under lowering skies at the Main Gate, a neo-classical arch with semi-circular gardens and paths leading in various directions through the cemetery. Here Eric gave us a brief introduction, before going off to the left, to the Dissenters' cemetery.
Kensal Green was the first commercial cemetery in London, opened in 1833 by the General Cemetery Company, which still runs it, to cope with the health risk and overcrowding in churches and churchyards. The style, with a mixture of formal avenues and informal planting was influenced by Pere Lachaise in Paris (1804). At first slow to take off, it became popular following the burial there of HRH Augustus Frederick, Duke of Sussex in 1843, and his sister Princess Sophia in 1848.
There were two sections: 39 acres of consecrated ground for Anglicans, and 15 acres, unconsecrated, for Dissenters. Each has a chapel, with catacombs underneath. The Dissenters' Chapel has recently been restored with the aid of the Friends of Kensal Green. Both chapels are listed, as are many of the memorials, and the perimeter wall, that is in a sorry state at present.
Our walk focused on two different aspects: firstly changing fashions in gravestones and the stone used; secondly the people commemorated here. The earliest stones we looked at were simple limestone slabs, the engraving all but corroded away, with the engraving barely visible, contrasting with Scottish granite nearby, looking pristine. On our walk round the Dissenters' ground we also saw Shap granite, gabbro from South Africa, slate, Bath stone, sandstones, a garnet-bearing metamorphic rock from India, weathered marble, Aberdeen, Cornish and Irish granites and sculptures in artificial stone.
In fact with all these different stones, and interesting memorials, e.g. Sir William Beatty, the surgeon who attended Nelson, Robert Owen (1771-1850) and the neighbouring 'Reformers' Memorial' (1885), we didn't have much time for the rest! The cemetery lies alongside the Grand Union Canal, and there used to be a gate, so that coffins could come by water. We could not in the time available see the whole of the cemetery, but among personalities whose graves we saw were Charles Babbage, George Cruickshank, George Birkbeck, founder of the Mechanics' Institute, and finally Isambard Kingdom Brunel. There were pretty temples, angels, grand vaults for a whole family; some had been restored, some were impossible to restore (even by Eric!), and some were overgrown.
So I end with a few pieces of advice, gleaned from listening to Eric, for your own memorial. Do not have it made in limestone: your name will disappear. Do not use different stones in its construction: they will weather at different rates. Always make sure your sandstone blocks have the same orientation. But the best bet is a nice bit of granite. And one final suggestion: go and visit. It was a fascinating afternoon.
Field Trip: A Chalk and Talk Walk on Langdon Cliffs at Dover led by Rory Mortimore
8 September 2007
All the good things about an OUGS field trip were evident on this late summer day when 35 people gathered to follow Rory Mortimore on a fascinating walk exploring and analysing the chalk of the Langdon Cliffs at Dover: good weather, good company and an interesting and knowledgeable leader. Having spent many years regularly walking the paths above the famous white chalk cliffs with their repeated seams of flint I was looking forward to finding out more...
Rory began by outlining the origins of the cliffs: chalk deposition followed a time 100-65 million years ago during the Late Cretaceous Era when the supercontinents of Laurasia and Gondwanaland were breaking up. Tectonic plate movements resulted in a superplume event under the Pacific which caused the oceanic crust to bulge thus displacing sea water onto the continents (see research by Peter Skelton). Sea levels were exceptionally high. In addition, vol volcanic activity along the mid-oceanic ridges pumped greenhouse gases into the atmosphere, drastically increasing global temperatures and preventing the formation of polar ice. For 30 million years, in the warm shallow seas which covered the flooded Cretaceous continents, calcareous nannoplankton such as coccilithophores and formaminifera thrived. After death their tiny, calcareous and spherical skeletons, coccospheres, fell onto the ocean floor to become the pelagic limestones; stretching from Devon and Yorkshire at their most northerly, under the Eocene clay and sand deposits of London across France to the Paris basin and then on to the chalk hills of Crimea, Kazakhstan and the Judean Desert.
In the South of England, uplift and folding of the horizontal chalk beds over the last 30 million years have given us the Downlands we are familiar with: the North and South Downs originally a huge chalk dome whose top has since been eroded to expose the older rocks of the Weald. The cliffs at Dover are the result of erosion of the soft sediments when the shallow valleys of Southern England were flooded and the Dover Straits were broken through during the Ice Ages. (Hopefully more information in 2008 talks! Ed.).
Recently revised, the terminology used by the BGS in the mapping of the Chalk Group, divides it into two sub-groups, the Grey Chalk and the White Chalk. These sub-groups are then further divided into 9 formations and 4 members. In the Grey Chalk subgroup much more clay is present; the White Chalk subgroup, however, is on average, 98% pure CaCO3. Geo-physical natural gamma and resistivity borehole wire-line logs, together with density and observational data have been used to map the stratigraphy of the chalk beds throughout the region identifying marker beds and consequently any lateral variations. The Langdon Cliffs fall into the White Chalk sub-group and particularly the Seaford and Lewes Nodular Chalk Formations.
Rory gave us an overview of the scene before us as we stood at the top of Langdon Cliffs, highlighting the importance of the numerous detailed geological borehole surveys carried out prior to the construction of the Channel Tunnel. The final route from Shakespeare Cliff to Sangatte being chosen to avoid various aspects of the geology such as deep weathering along valley axes and areas of paramoudra flints.
As we walked along the cliff top towards Langdon Hole, Rory pointed out the surface Seaford Chalk bed which was homogenous but destructured for at least one metre due to deep periglacial freeze/thaw action. Below this formation the chalk was more uniform and solid with flints visible. Some flints were very large these are the paramoudra flints. As we carried on along the path it was possible to see the Bedwell columnar flints and sections of the wavy shell of the bivalve Cladoceramus undulatapictatus which is a characteristic fossil for this depth of rock and is therefore an International Index shell.
A discussion about the formation of flints was inevitable. Rory explained that the general structure of a flint nodule is often a pyrite core (surrounded by glauconite if formed in a shallow water environment) encased in a thin, very hard chalk layer further surrounded by crystalline silica with an outer cortex of more porous silica. In the cliff, each flint horizon has nodules of specific characteristics reflecting the unique conditions under which it was formed. The silica originated from sponges and silicaceous plankton. Normally soluble on the seabed, the silica precipitated within the carbonate sediment when under pressure. Some flints show evidence that the precipitation has occurred around or within the shell of an animal such as an echinoid. The shapes of other flints indicate that they were formed inside the burrows of animals, similar to those of marine crustaceans of the present time, but have overgrown the original confines of that burrow. Trace fossil such as Bathichnus paramoudra are sometimes preserved but more often evidence of that fossil can be found as the pyrite in the middle of some flint nodules.
The bacterial breakdown of organic matter within the burrow produces a mixing zone of methane rich gas rising up and oxygen gas diffusing down. Under pressure these conditions result in the instantaneous dissolution of carbonate and the precipitation of silicate. Sheet flints are formed as silicate precipitates in cracks to produce conjugate flint sets during periods of lateral movement in the chalk.
Horizons of flint with unique and specific characteristics can be used to date rock. They can be traced right across Europe for hundreds of miles indicating their formation in conditions where the climate and chemistry of the oceans was the same. This could be related to precession of the cosmically controlled Milankovitch cycles. Favourable climate conditions over a period of time could increase rainfall and consequently the organic productivity leading subsequently to a high probability of flint formation. Rory commented that where there is a high marl content to the chalk, there would be few flints. He explained that marls are lime-rich muds containing various proportions of clays and calcite. They are present as conspicuous and characteristic horizons and grooves in the chalk cliff. Rare Earth Elemental analysis used to characterise the minerals in the clays confirm their volcanic origin. The environmental conditions at that time would therefore have sustained less of the organic life essential for the formation of flint. The characteristic composition of each type of marl reliably identifies each specific marker bed.
We continued our walk...
From Langdon Hole, Rory guided us on a journey through time - down the cliff by a zig-zag path and descending a very steep metal staircase onto the beach. He particularly wanted us to investigate and try to identify the marker beds of flints and marls that are a feature of this wonderful section through the cliff. Past the uppermost exposed chalk of the Cuckmere Beds of the Seaford Chalk, then past the Shoreham Tubular Flints and Marls, the Lewes Marl and Tubular Flints, the Bridgewick Marls and Flints and finally finding the Southerham Marls and Flints at beach level. Rory emphasised that 95% of Geology concerns stratigraphy and that these layers and their contexts would be recognised and understood by geologists worldwide.
We had our lunch sitting on the beach watching the ferries sailing past and some very brave souls swimming in the sea. The rest of the afternoon was spent looking at the rocks on the foreshore, which proved to be a rich source of fossils, taking great care to keep our safety helmets on as we explored the areas under the cliff. Echinoderms, bivalves, sponges and molluscs were found in just a short space of time. Rory was unfailingly enthusiastic in sharing his knowledge with us: indeed he hardly seemed to stop for lunch as we all had so many questions to ask him. Thank you, Rory, we all thoroughly enjoyed our trip to Dover and will never be able to gaze upon those evocative White Cliffs in quite the same way ever again.
If you want to know more, read Rory's article: www.jncc.gov.uk/pdf/V23Chap1.pdf
Field Trip: The Building Stones of Canterbury led by Geoff Downer
23 September 2007
Since this was a joint trip with South Eastern Branch, there were about thirty of us as we met in the car park of the Canterbury Law Courts, but the trip had proved so popular that many were disappointed. We hope that it will be possible to organise another at a future date, since even for those of us who were lucky enough to participate, it was obvious that there was lots more to see in this World Heritage Site, with three buildings of major religious importance: St Martin's Church, St Augustine's Abbey and the Cathedral.
Geoff had prepared really informative handouts, so that even the more knowledgeable amongst us learned something new. Before setting off we were given a brief history of Canterbury, from an original Iron Age settlement, through the Roman period from the Battle of Bigbury until the end of the fourth century AD, the return of Christianity under Ethelbert and Bertha and the coming of Saint Augustine, the development of the Abbey and Cathedral, the Normans in 1066, the dissolution of the monasteries and finally the coming of the railway in the nineteenth century.
Thus armed, we followed our leader out of the car park and up the hill to St Augustine's Conduit House, dating from the late 12th/ early 13th century. Built mostly of flint with ragstone arches, it acted as a holding reservoir for the Abbey, collecting water from the spring line and sending it through a lead pipe to a water tower within the Abbey precinct, now vanished. The conduit House itself was in use until the 19th century.
Thence we walked to St Martin's Church, the oldest church in the country in use for worship continuously from the 6th century until now, although the building itself was originally Roman, 'a geological encyclopaedia' (to quote 'England's Thousand Best Churches'). We had a fascinating time examining the south wall, identifying (with Geoff's help) the different components, from Roman bricks and tiles to Kentish ragstone, Caen limestone, Thanet sandstone and the ubiquitous flint, to less common ones: Marquise limestone from France, Quarr stone from The Isle of Wight, and Tufa. In the graveyard is the tomb of Mary Tourtel (1874 1948), creator of Rupert Bear, and we also looked at a grave with a sarsen stone and a block of gabbro(?) of a certain Walter Cozens (1858-1928). He must have been a geologist.
On to the County Gaol (1808 - seven different categories of prisoner!) of Portland stone and the Old Sessions House with a modern extension made of reconstituted Portland stone to match.
St Augustine's Abbey is a beautiful, vast and complicated ruin, covering a large area, of which we examined part of the outside wall, in which, having got our eye in, we identified the same stones we had found at St Martin's, including Tufa, Quarr and Marquise, plus several others.
Before lunch we looked at new building stone, notably Baltic Brown, a very decorative, dark granite from Finland with two different feldspars, orthoclase and plagioclase, and a query about its formation, since the orthoclase is surrounded by plagioclase. Bertha's Walk led us back to the city wall, where we saw the oldest architecture Roman tiles forming the lower part of arched doorway, since blocked, at the Quenin Gate.
The afternoon was devoted to the Cathedral. We started by looking at St. Peter's Gate, the first example of another theme of the day: restoration, which sometimes works well, but sometimes not, as here, where the artificial stone used to replace ragstone has weathered badly.
Once inside the precinct, gazing at the Cathedral itself, and looking at our splendid series of drawings, we learned of its development from Lanfranc's original replacement of its Saxon predecessor, its extension under Anselm, its huge development as a pilgrimage centre after the murder of Thomas Becket, the earthquake of 1372 and subsequent rebuilding of the nave, and the construction of the 'Bell Harry' Tower (out of bricks, duly faced with stone!) around 1500. The final 19th century touch was the replacement of the 11th century North-West Tower with a copy of the early 15th century South-West Tower in the interests of symmetry!
All this was information was conveyed despite the accompaniment of pealing bells, perhaps more beautiful, but not as informative, as Geoff's voice.
A close-up examination of the exterior followed, as we made our way slowly round from the west end, round the east end, through the cloister, unusually on the north side of the church, and back to our starting point. On the way round we passed the water tower, which gave us an idea of of the appearance of the destination of the St Augustine's conduit with which we had started our journey, though the Cathedral was on a different system.
Much of the stone arrived via by from France, the Isle of Wight, Devon and Yorkshire, finishing its journey in flat-bottomed boats up the Stour, getting as far as Fordwich. Although the dominant stone is Caen limestone, the mediaeval church used the same variety that we had already met, plus various marbles and some, perhaps Roman, pillars. Successive re-buildings reused old stones as well as importing new ones. Present-day restoration uses Lepine stone from France and Purbeck marble and is very sympathetically done, but is hugely expensive.
I haven't done justice to the interest of the day, accompanied by our enthusiastic leader, and helped by lovely sunny weather. Many thanks to Geoff and, of course, to Di for organising it.
Field Trip: Southern Spain
22-26 October 2007
Day 1 (Monday morning)
The first stop to get the field trip under way was on the Mesa Roldan, a prominent 214m high hill just south of the seaside town of Carboneras where we were staying. From this high vantage point overlooking the calm Mediterranean, the leader, Paul Grant, explained the Betic region we were to study during the week. The land on which we were standing had been formed as a volcanic island arc and forced into collision with Iberia in the late Miocene. Paul pointed out a line of hills which marked the Carboneras Fault, a sinistral strike-slip fault running north-east from the city of Almeria. This fault marks the boundary of the Cabo de Gata volcanic block to the south-east from the metamorphic basement rocks to the north-west.
The Mesa Roldan had volcanic rocks lower down and carbonates at the top, characteristic of an atoll which had been exhumed by later uplift. The capping limestones had centimetre-size round holes from Porites, a long tubular coral which gets colonised by algae and later dissolved out to leave holes which serve as conduits for fluids. Much of the top of the hill had been quarried for the cement factory at the bottom, and there had been extensive quarrying in the area. Lower down the hill we saw interfingering deposits of volcanic and carbonate rocks with fossils, which indicated that volcanism was still active while the carbonates were developing. A peculiarity in the rocks here was the presence in layers of rounded brown nodules, red in the centre with concentric brown material. Paul explained these as lavas which had been altered from the outside inwards by warm fluids circulating through cracks in the rock. There was extensive stockwork of calcite veins visible. A little farther down we saw a conglomerate deposit with rounded hornblend andesite cobbles infilled with calcite. This would have been a volcanic debris flow - a lahar.
Fossils detected in the limestone on the mesa included pectins and other bivalves, clams and oysters, mostly as internal moulds.
On the way to the second location of the morning, on the road out from Carboneras, Paul stopped at a roadside cutting which showed an interesting succession. At the bottom was a loose red flood deposit of what looked like laterite, volcanic in origin. At the top of this deposit there were clearly fossil plant roots, showing that the volcano had emerged above sea level. This was sharply cut off, however, by a layer of carbonate rocks which must have been deposited after a subsequent sea level rise, submerging the whole thing - plants and all.
The final stop of the morning was on the beach at Agua Amarga. The feature here which Paul wanted us to see was a big sand wave in the cliff to the west. This had been deposited as a large under-sea dune of carbonate sand which had become stationary and covered with later infill layering. Above this, the hill was capped with an unlayered mass of carbonate which had resulted from a submarine debris flow - an olistostrome. The cliffs were not climbable, so we just sat on the beach in very warm sunshine to eat our lunches, while watching three Dutch art students carefully wrapping up a large boulder in aluminium foil. They couldn't understand what we were doing there, either.
Day 1 (afternoon)
After lunch we left Agua Amarga, and after a short drive stopped at a bentonite quarry at Cerro la Mata Lobera. This was a vast pit with layered clay sides, the upper parts of which were an iron-rich brown, the lower white montmorillonite. The sides contained hints and ghosts of the original igneous rocks of which they had been composed. Paul explained that the original rocks had been pyroclastic flow deposits, including hornblende andesite and reworked lahars. These had been transformed by hydrothermal alterations to leave clay. In terms of the economic output of the quarry, the different layers on the quarry provided cat litter (the brown stuff), absorbents for chemical spills, and clay for the paper industry and fulling.
We then moved on, and stopped again at a bend in the road half-way down a hillside near the small town of Hortechuelas. Leaving the people carriers at a lay-by on the side of the road, which enjoyed a view over the neighbouring hills, we walked down into a gully and scrambled up the next hillside. This formed the flank of a former strato-volcano. We sought to identify the minerals in the rocks on the hillside and thus the rocks themselves, which Paul said represented archetypal pyroclastic flow deposits, such as would be found near Snowdon. The rocks were identified as dacite, which would place them in the middle phase of a typical volcanic sequence.
We then walked back to the cars, as there were a number of further stops in the day's planned itinerary, but a nasty surprise awaited us when we got back, we discovered that one of out three people carriers had been broken into. This was the car which had been parked on the road side of the other two, so that it had to a degree been hidden from view. One of the windows had been smashed, and the rucksacks and personal items inside belonging to its six occupants had been taken. Paul said that he thought that some strangers who had stopped after we parked, and had taken pictures of the hillside, but were evidently not geologists and had spoken in a Eastern European language might be the culprits.
That was the end of geology for the day. The stolen effects included the debit and credit cards of one of the party, Nicole, and the passport of another, Laurie. Phone calls were speedily made by mobile to cancel the debit and credit cards, inform the authorities of the loss of the passports and find out how to secure replacement documentation. We learned that the thieves had already tried to use the stolen debit card at a cash machine in Hortechuelas, but they could only try to guess at the PIN, and the machine had retained the card on the third unsuccessful attempt.
We also had to contact the car-hire company regarding the damaged people carrier. We were told to go to the company's office at the airport at Almeria, so we drove there in convoy. Fortunately a replacement people carrier was available there, but having lost the better part of half of the day, we then drove back to Carboneras.
Day 2 (Tuesday morning)
Despite the dramatic events of the previous day, we set off for another day of geology and took the road out of Carboneras north towards Mojacar. After 1 to 1 1/2 km, we stopped by the side of the road to look at an exposure of grey/green angular rocks surrounded by a pinkish red infill material. The blocks seemed to fit with one another. Large crystals of hornblende (up to 2 cm long) were present as well as xenoliths. Paul confirmed that it was a porphyritic hornblende andesite. The infill was extremely fine lime mud with some laminations. Fossils including forams (base Messinian) are apparently present, but I did not see any.
A lively discussion ensued as to how it was formed. Eventually, Paul explained the latest thinking that it was either a sill or a lava flow that had intruded into unconsolidated wet sediment. The heat had produced steam which had brecciated the lava/sill (autobrecciation) and fluidised the sediment allowing it to infill between the blocks. This is called the Brèche Rouge (red breccia).
We then drove on to a scenic spot at the top of the road in the Cabo de Gata Natural Park. We could recognise the now familiar table top of the Mesa Roldan visited the previous morning.
Next, a stop on the right bank of the Rio Granatillo allowed us to look at some crushed rock created by the Carboneras Fault. The Carboneras Fault consists of a series of slices within a 0.5 to 1km thick fault zone. It separates the Betic Terrane to the North West from the Cabo de Gata Terrane to the South East. It has moved 22 km since the Tortonian/ Messinian about 7.1 Ma ago. To our left we could see crushed basement rock (Palaeozoic schist) whilst in front of us, the crushed rocks were Permo-Triassic or Tertiary. The vivid colours of the rocks prompted Paul to call it the Alum Bay of the area.
Further along the river bed, below the road bridge, we continued studying the crush zone where we saw reidel fractures and gypsum veins within fault brecciated red, purple and yellow coloured sediments. Also some evidence of boudinage and fibrous quartz veins.
Walking along the river bed, we then looked at the deformation of the basement rock (schist) from the Sierra Cabrera (which is a bit of the continental block of Iberia). Many deformation features could be recognised: sheared cleavage, raft, chevron folding, crenulations, boudins, en-echelon movement, and some folded cleavage. The rock contained por phyroblasts (minerals that have grown metamorphically), possibly andalusite. Down slope we came across an angular breccia of andesite with evidence of hydrothermal alteration and extensive faulting. Then a heterogeneous mass of rounded blocks or blocks with rounded edges suggested a post eruption mass flow, possibly a lahar deposit.
Finally we arrived on the beach where we looked at the geomorphology and recognised raised beach deposits about 3 metres above sea level, rising higher on the horizon. This suggested that the land was rising and tilting tectonically. Time to have lunch and look at a memorial to a young woman who had been drowned by flash floods whilst camping on the beach. Sombre thoughts
Day 2 (afternoon)
A leisurely lunch on the beach was followed by a view along the coast of a wave-cut platform stretching off and rising towards the distant headland. At one point there was a suggestion of a raised beach deposit.
Refreshed, we retraced our route up the dry river valley and soon found that the heavy rain of the previous week made the surface less than secure for our vehicles as one of our vehicles was soon bogged down and needed digging out. A little further forward and the same minibus found itself balancing on a large rock. The ingenuity to recover from this situation was eventually mustered and in view of the lost time it was decided to go straight to two sites we had missed the previous day.
At Cerro del Joyazo, one of two volcanoes in the north of the area, we parked by an area of recent industrial activity and at once saw why it is known as the Garnet Volcano. The ground was littered with small garnets. We were on an allu vial fan extending from the volcano and the ground had been worked to recover garnets for commercial abrasives.
We followed the pink glistening path up hill and were aware of clusters of garnets of up to 4mm between the stones. It was possible to collect by the handful! The path led us to a volcanic pipe. The magma had risen up through microschists which had provided the minerals for the garnets. This is the type locality for cordierite and we soon found the bluish crystals. A ridge forms a semi circle around the pipe but there is doubt about its identification as a crater. The ridge is topped with reef carbonates which at one place drape down the side. There was discussion about the submergence history of the reef which must have had the right water cover for a relatively short time because of the lack of slope debris.
We ended the day at Playa de Monsul. This area has been much used by film makers and scenes from Indiana Jones and the Lost Crusade were filmed here on the beach. But first to a dome of basalt columns. "How did we know they hadn't been distorted into this shape later?" asked Paul. The orientation and shape of the vesicles should tell us their position when they cooled. Unfortunately there was no consensus on what the vesicles were doing!
Down on the beach a wildly overhanging cliff showed the clear rising track of a fumerole through volcanic fragments.
The east side of the bay was marked by a wall of hexagonal blocks which could only have been built with great precision by the Incas. In fact it was one face of a vertical dyke with horizontal columns. Paul told us that small amethysts could be found here. It didn't take long to find the tiny blue crystals. The end of an interesting afternoon.
The morning of Day 3 began near the junction of the motorway with the road to Carboneras, with an examination of the contact between the Paleozoic basement of graphic schist and onlapping marine sediments dated at 7.1 Ma overlain by Quaternary fluvial breccia. The schist was similar to that seen in the Rio Granatilla the previous day but here was not quite on the fault line. Further up the road we could see the tell-tale 'Alum Bay stripes of the Permo-Trias sediments on the fault-line itself.
Next we went round the corner to the western side of the motorway near Penas Negras to view the southernmost boundary of the Sorbas Basin where it was faulted against the northern side of the Sierra Cabrera. Paul found us some Permo-Trias gypsum which acted as lubrication for the fault. From here was a good vantage point across the Sorbas Basin with Late Miocene (Tortonian) turbidites in the foreground and overlying (top Miocene) Messinian sediments in the distance including a prominant notch in the mountain that looked from here like a road cutting but is infact the the gypsum layer (dating from Messinian salinity crisis) which is still actively quarried in this area, particularly where the outcrop is at the surface as on the right of the peak (see photo).
In the afternoon it rained. Paul took us down to the 'dry' river bed of the Rio Agua to look at the Los Molinos section of Messinian Beach limestone with abundant coarse shelled oysters much as we had seen in the morning onlapping the schist. He proposed that this may have been a storm beach but by now he was losing his audience to the heavy rain. Paul's intention had been to take us on a traverse up the sequence, thorough the deep-water sediments to the gypsum above but rain stopped play and we headed back to the vans. The members of the large vehicle had all had their rain gear stolen on Day 1 so they got particularly wet.
We stopped again briefly on top of the plateau where we could pick up crystals of gypsum. We were very close to the large quarry that had set off the puffs of smoke seen from our vantage point on the other side of the valley. We then headed off for a view of a spectacular coral reef near Los Alias. From our vantage point we could pick out the geometry of the reef prograding over the talus slope.
As soon as we had finished with the geology for the day the rain stopped and we headed for a mockoutback hut built for the film set of a VW Ad. This was within the Field Centre at Urra and now served as a very welcome bar. The walls were decked with the memorabilia of visiting university groups including the Imperial College students that Paul had brought so often. No wonder this was one of the most favoured places for field work!
Day 4 (morning)
We took the road back to Sorbas this morning but with a short stretch of motorway to speed up the journey. We were in the middle of the Sorbas Basin and we were to look at a cross section in the deep valley that runs through Sorbas that is formed by the Rambla de Sorbas.
Sorbas is in the centre of one of the best examples of a karst landscape composed of gypsum (yeso) in the world. Nearby is Los Yesares, Europe's largest gypsum quarry. The valley is a cross section through alluvium to turbidites with a turbidite fan a great slurry that terminates in slump banks.
There was evidence of strong currents along the sea floor with each pebble re-worked. At the bottom was a small cliff. Walking back out of the valley we saw a bimodal cross-bedded unit below a laminate bearing plant debris, which is overlain by a limestone that has attributes of still sand.
Climbing out the valley we came back to the town where, as it was Thursday, there was a market. Some of us used the opportunity to buy local goods. We discovered the visitors' centre, Centro de Interpretación Los Yesares where, fortunately for us, much of the information was available in English. The centre was most informative about the area's three main habitats: desert, caves (including a model of a cavern) and river and the unique character of the landscape brought about by the gypsum.
We saw that the tumuli, unlike in Britain, refer to gypsum layers that are generated by the increasing volume of crystals on absorbing water - unique to Sorbas. Whilst Gypsum is highly toxic for most plants, gypsophyte species thrive in this type of soil and are abundant in this area. The area contains 50% of Spain's gypsophyte plant species, six of which are in danger of extinction.
Day 4 (afternoon)
This afternoon we walked the Barranco de los Sierras, a gorge cut through the Tabernas Monocline. The traverse commenced with grey beds separated by a fault zone from terrestrial red beds, a conglomerate containing massive clasts. Possible flash flood deposits? Further down the gorge a river channel in filled with Quaternary sediment was seen cutting across the gorge. Clasts of schists and gneisses indicated huge debris flows from terrestrial mountain areas, but the discovery of a block containing fossil barnacles indicated a change to foreshore deposits. Further changes to well defined turbidite deposits indicated a progression to fully marine conditions.
Now it rained, and a retreat to the nearest bar was called for. Suitably refreshed and the rain abated, Paul took us on a walk which he assured us was about 1 km. The general consensus was that it was considerably further than that, but it was well worth the effort when confronted by a spectacular Tortonian (Late Miocene) turbidite deposit. A huge overturned deposit of soft sediment here has been over ridden by a conglomerate producing the most remarkable slump structures.
This was our last day in the field and Paul chose an itinerary for us to examine on our way north to Alicante for our final night. First, some unfinished business on the northern margin of the Sorbas Basin. On the rainy afternoon of Day 3 we had looked over a valley to an incised coral reef further east along this northern margin.
Now, near Los Rimirez Paul pointed out a reef remnant on the top of the hill in front of us (we were too far away to verify for ourselves). Beneath it, the rest of the hill was strewn with boulders of gneiss and Paul tried to persuade us we were looking at an exhumed scree. In the end he succeeded. In situ gneiss on the hill to our right, and, in particular, tor-like structures within the boulders persuaded us that the blocks could have resulted from in situ weathering. The rest of the coral reef had been eroded to expose the boulders and the marl underfoot and in the hill behind could be compared with the remaining deeper-water facies as we had seen on Day 3. As we were about to clamber back into the vehicles, twinkly plates caught our eyes and the search was on for large slices of muscovite mica. The vehicles had conveniently parked right beside veins of mica and feldspar associated with the gneiss.
The next stop was Shear Delight! High in the Sierras de Los Filabres with spectacular views to the coast from above Bedar, we examined a road section with metamorphosed Bedar 'granite'. Paul told us emplacement of the granite took place in the Cretaceous, about 80 Ma ago, and the metamorphism to gneiss has been dated at around 48 Ma. As we walked down the section we looked for evidence of contemporaneous shear. We looked for lineation in the augen and of the laths of tourmaline and the asymmetric S-shaped distortion of the gneissic layering caused by shearing as one layer was pushed over another in opposite directions. We could see that all the features were aligned, and more conscientious students than us have calculated that transport was probably to the south.
Just round the bend there was a dramatic change in lithologies with lower-grade metamorphic garnetmica schists. By the time we reached the next bend the rock was different again. Fizz test confirmed that the brown, uniform, sugary-textured rock was a dolomitic marble. Several bends further we could see a green outcrop in the road-cutting with a similar- coloured green quarry higher up the hillside. It was now about 2pm and our jinx once again came into force prohibiting us from examining it in detail. There was no parking on the newly-fenced road and in the village of San Machael the exposure had been sealed with concrete since Paul's visit in the summer. We were content to believe him that this was serpentenite, or more specifically, chrysotile asbestos.
For obvious reasons the quarry is now closed. Paul explained that the different facies were a series of different thrust slices. We saw a final slice a little further along the road beyond the village of Lubrin. Here was a massive marble quarry. This seemed to be Spain's answer to Carrarra. The blocks at the entrance seemed of high quality but we were warned off from entering by the new chains, red 'flags' and the sound of a large vehicle reversing inside. The quarry has clearly re-started operations after years of abandonment and visits by IC students. Later, as we drove up the motorway we overtook a large flatbed lorry carrying 2 enormous cubes of marble.
Our final destination was a small volcano on the Betic side of the fault dividing Iberia from the island-arc plastered onto its SE corner. After describing the texture and mineralogy as far as we could Paul had to tell us it was pitchblende, whereupon several members of the group kicked themselves hard. Small residual patches of sediment adhered to the side. While most of us went with Paul to try to find the contact with the underlying basement of micaceous schist, Ina and Mary raced up to the top of Cabezo de Maria, clearly a popular shrine on the tourist route. We finished with a group photo under the backdrop of a dramatically castellated dyke associated with the volcano: the geologists' substitute for a Moorish Castle. Thank you Paul for a superb trip you made us all think hard! And thanks to Wilf for organizing it.
Lecture: 'Extreme Sedimentology - Field work in Chad and the Antarctic 2005' by Charlie Bristow
6 December 2007
Dr Charlie Bristow is Reader in sedimentology in the School of Earth Sciences at Birkbeck . His talk was based on the use of ground-penetrating radar (GPR) in sedimentology with reference to two very different environments: Antarctica and Lake Chad. In Antarctica, 'the coldest, driest and windiest place on Earth', his team studied sand dunes in the ice-free dry valleys, using GPR to image the internal structure of the dunes, to discover their age and structure, how they might have formed and migrated.
From there he took us to one of the hottest places on Earth, and discussed the palaeolake Megachad, which as recently as 6000 years ago was the size of the Caspian, 1000km from north to south. Now the dry and dessicated lake bed is the single biggest surce of dust on Earth (as we saw from his photos!). They found it to be very low-density diatomite lake sediments from silicious algae. They found paleoshorelines and beaches with pottery shards on the surface. It probably dried out and filled up several times in the last 10000 years, with links to the North Atlantic cycle.
This was a fascinating lecture on ongoing research, waiting for the results of carbon-14 analysis to carry the story further.