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.
How active is an active volcano? • Arabian Plate Tectonics and the Developments of Hydrocarbons • The Angela Marmont Centre • BGS activities in London • Geological Mapping Course in the Forest of Dean • BGS Keyworth, Nottingham • The Chalk of Steeple Morden and Ashwell, Hertfordshire • Sheridan Reservoir Dome, Yellowstone • Geoconservation Day at Gilbert's Pit, Charlton • Green Chain Geowalk • The Atlas for London Basin Geology • Pett Level • Isle of Man • Church geology in West Sussex; a study in historic building stones • Wiltshire Sarsens and Avebury • The Oceanography Experience • OUGS 41st Symposium - Dublin • Symposium Field Trip - Killiney • Symposium Field Trip - Dublin Institute for Advanced Studies, Geophysics Section • Symposium Field Trip - Tara Mines • Hog's Back Geowalk • Did the Sea Taste Salty to Dinosaurs? • Church Geology in West Sussex • The Geology of Pigments: Turning landscapes into colour • Mr Kipling's Exceedingly Good Rocks • Herne Bay • Suffolk • Members' Evening • Survivors: The Animals and Plants That Time Has Left Behind
AGM Lecture : How active is an active volcano? by Steve Blake
2 February 2013
Dr Steve Blake, Reader in Volcanology at the OU, came to give us our AGM talk. Steve has now been at the OU for more than 25 years. Steve’s motivation for this talk (and research) is from the perspective of understanding and predicting hazards.
The title question appears quite simple, as did his questions what is a volcano? and what constitutes an active volcano,? until you try to answer them.
Steve began by summarised volcano types: from monogenetic eruptions (e.g. the New Zealand volcanic field) through the sequences of eruptions at the Puu Oo vent in Hawaii to flood basalt eruptions such as the Deccan Traps.
Different definitions of “activity” include: currently erupting magma, having recently erupted magma, emitting hot gas and fluids, and seismically active. There is debate as to whether bubbling fumaroles should also constitute “active”. Concentrating solely on volcanoes erupting magma the Smithsonian/USGS Weekly Report (new activity) www.volcano.si.edu/reports/usgs lists 20 erupting now, 540 with historic records of eruption and 1500 where there is evidence of an eruption over the last 10 ka. This means only 1% of “active volcanoes” are actually erupting now.
Measures of activity are volume erupted, duration, intensity (volume/time), frequency of eruptions (number/time) and the proportion of time a volcano spends erupting magma (the latter measure being the one Steve is mostly working on).
From data plots of today’s (basaltic) eruptions such as those of Piton de la Fournaise, Hekla, Mt Cameroon, Nyamuragira, Kilauea and Kliuchevskoi Steve concluded they erupt between 1—100 km3 s-1 and large volume eruptions such as Laki erupted 600 km3 s-1. In addition to plotting volume against duration Steve also showed us a “cartoon” graph to include periods of repose as well as eruption (this plotted in a staircase configuration—the treads representing repose periods and the risers showing erupted volume) which may be useful to predict a volcano’s probable eruptive volume and time between eruptive episodes (assuming the volcano remains true to form!).
Steve’s work produced a similar result to that of George Walker in 2004 1.
Creating the question of why flood basalts, with such prolific eruptive outputs, are in the “wrong” place on the graph? Although there has been much investigation of, for example, the Deccan Traps it is still not known if there was a low eruption rate (1m km3 continuously for 1m years) or larger discrete events nor has a vent(s) ever been traced.
Possibilities explaining this “mismatch” could be the measurements made are wrong/the model is inapplicable or the mechanic behaviour of today’s volcanic systems are different (or both). Steve thinks mechanical differences are more likely — behaviour depends on both the magma supply and the mechanical properties of the whole system
A thought provoking lecture, I certainly hadn’t realised either the range or the magnitude of uncertainties, particularly when estimating erupted volume (probably 10% but possibly 50%). We started with some apparently simple questions that, for me, generated far more complicated questions but perhaps nothing is simple when talking volcanoes.
1.Canón-Tapia, E., Walker, G.P.L. (2004), Global aspects of volcanism: The perspectives of "plate tectonics" and "volcanic systems", Earth- Science Reviews, 66, 1-2, pp. 163-182.
Lecture : Arabian Plate Tectonics and the Developments of Hydrocarbons - by Saad Jassim
21 February 2013
We were delighted to welcome back Dr Saad Jassim, Senior Geologist at Heritage Oil Ltd and Research Fellow at Geotech, Leeds, less than a year after his last visit in May 2012, when he had offered to replace Paul Logan if necessary. He has studied at Baghdad and Leeds, compiled a tectonic map of Iraq, edited and contributed to ‘The Geology of Iraq’, 2006.
He situated the 40% of the world’s oil and gas reserves found in the Arabian peninsula within the overall tectonic framework of the area, in the context of the collision of the African Plate with the Eurasian Plate and the opening of the Red Sea, successive subduction zones and a continuously subsiding basin of the passive Gondwanan margin since the late Pre-Cambrian, which has resulted in an enormous thickness of sediments.
He started with photos and song of the Marsh Arabs who inhabit the area at the head of the Persian Gulf where the Tigris and Euphrates meet. He continued with the Geological map of the Arabian peninsula and the Bouguer Gravity Anomaly map, with lower values showing collision zones. The Geological map shows the Arabian Shield, the Basin area, and the fold belts of the Zagros orogeny. NE/SW tending structures result from the successive sutures caused by the sequential closure of the Tethys ocean and its predecessors over 820 Ma. Later structures cut across this lineation, creating domes and hydrocarbon traps.. He described the result as a horst and graben structure formed from extensional and collisional regimes.
The sediments increase in thickness from the west, reaching a depth of 25,000 ft by continual displacement, not faulting. 70% of Middle Eastern oil is generated by the Jurassic-Cretaceous sediments laid down when the area was situated between 30° north and south of the Equator. Oil and gas trapped below late-Permian anhydrites has mostly not been exploited.
In answer to questioning, he finished by saying that the gas, far from being exploited, is burned off as waste. GEO Expro 2009 http://www.geoexpro.com/article/Iraq_Resource_Base/6dffb777.aspx contains an article by Rasoul Sorkhabi, citing Saad’s work, with some of the maps and diagrams relevant to Iraq and the Arabian peninsula.
The Angela Marmont Centre - With Fiona Fearnhead
21 February 2013
The Natural History museum was packed with children and their families. The queues outside for both entrances overlapped. Fortunately Di Clements made special arrangements for us but even so we had to make our way from the Earth Science entrance to the, relatively newly opened, Darwin Centre for Biodiversity over the far side of the museum by way of the main entrance hall. There were zigzag queues for various activities and the building hummed with enthusiasm. Anyway we all arrived punctually - indeed some had time for coffee in the Darwin Centre café.
Fred Rumsey, a botanist, welcomed us with an introductory talk in a room, set up with stereomicroscopes, used for groups. The museum, recognising the importance of publicity, is going out of its way to welcome groups and individuals to the Angela Marmont Centre, part of the Darwin Centre. The centre is supported by Professor Sir John Marmont and his wife, Angela and named after the latter.
There has been wholesale transfer of specimens from other parts of the museum increasing available gallery space. Fred took us to see where plant specimens are stored stacked in drawers in banks of cabinets under sterile conditions. Hungry organisms must be excluded so we had to leave our bags outside.
The collection covers the range of plants found in the British Isles. Don’t imagine that it is complete. There is room for a ten percent expansion. There are two main reasons. Firstly not all varieties have necessarily been collected. For instance an expert in bramble varieties has been hard at work so the collection is thought to be nearly complete whereas the dandelion collection is probably only eighty percent. It is recognised that amateurs as well as professionals can contribute. Also we are all aware that foreign species, some not at all welcome, appear from time to time.
We briefly discussed when an alien becomes a native. It transpires that there is a Mediterranean orchid, which was first found in the wild twenty-four years ago, gradually spreading from Cornwall. In another year it will be considered a native. Rhododendrons are definitely considered to be native. Obviously climate change is likely to have quite an impact on this factor. Within the AMC there is space available for visitors to work on their samples. Indeed there was a gentleman earnestly working at a bench with what looked to my untutored eye like beech leaves mounted on paper. Each station has some reference books.
Fiona Fearnhead, the Earth Science Identification and Advisory Officer, ran a brief session using the microscopes and a few fossils. There were echinoids including one complete with spines. I assume it was modern but, if so, the spines seem to have changed very little with time. I really should have looked at the label! Indeed I picked up a sheet with labelling instructions together with a sheet of labels thoughtfully laid out on the benches for us. One or two people brought in samples for identification. This is also one of the functions of the Angela Marmont Centre. If necessary the staff are able to call on other experts working elsewhere in the museum.
Meanwhile Di Clements took the other half of the group to the Earth Lab over the other side of the museum. This gave us a chance to survey the stones on the floor and walls as well as those included in the circular table on the first floor landing. The main object was to look at the collection of fossils in the Earth Lab.
Di explained to us how she was involved with vetting the BGS fossil collection and getting it photographed as well as having some input into the design of the cabinets. You might think that banks of little boxes would be ideal but some larger fossils would not fit. The final design has a backdrop of a light coloured material divided into a grid with squares of a size that accommodates the majority. Larger fossils overlap. The specimens themselves sit on glass shelves, which are unobtrusive. Di has left space between some larger fossils, which sit on their own shelf. This gives an appearance of a bright airy display that is very easy on the eye. Needless to say each cabinet covers a single Period so start at the far left for the oldest specimens and carry on round for younger samples.
Di made it sound quite straightforward to select which fossils to display. The collection was the one originally displayed in the Earth Galleries and each one had to be checked for ability to withstand vertical display in the new cabinets. In some cases there was space within a particular cabinet so some extra exhibits were selected from the NHM collections to compliment the BGS specimens.
We were intrigued to see a display naming male and female ammonites. Apparently it is possible to sex them where many have been found together. The females are larger than the males.
Finally we made a whistle stop tour of the Cocoon. Florin Feneru was our guide. The Angela Marmont Centre is within the Cocoon at its base. To be honest the Cocoon does not lend itself well to a quick passage but I noticed that other visitors were looking with great interest so I took my husband the next week. There is a lift ride to the seventh floor and then you gradually wind down a spiral path surrounding the collections to the fifth floor. Museum staff are on hand to help and occasionally it is possible to see and talk to others working behind windows.
There are exhibits along the path explaining the role of science in the museum. First of all there is an introduction to NaturePlus. There are credit card sized cards that are used to select subjects that you would be interested to revisit at home. I must say that at this stage I wished I had taken a grandchild with me. It is worthwhile persevering. In fact it is noticeable that there has been a great effort to bring information dissemination up to date and there are various database stations available in the galleries.
When you go do make sure that you look at specimens in the drawers – someone has definitely had fun but I won’t spoil the surprise. There are also very old specimens displayed under glass and it is interesting to leaf through an old Herbarium with both specimens and drawings.
Recent news about misleading labelling of food products has lead to great interest in DNA studies. However rather than wondering about horsemeat turning up in beef products what about using DNA studies to help control insects such as the mosquitoes, which carry tropical diseases such as malaria. This would have a far greater impact on global health particularly as the changing climate is likely to result in insects spreading further afield. Find out more in the Cocoon but bear in mind that this is popular science and not in depth.
Our thanks are due to our three guides Fiona Fearnhead, Fred Rumsey and Florin Feneru.
(Photographs © Paul Hetherington)
Lecture : So what is the BGS doing in London? A not very systematic review - by Don Aldiss
21 March 2013
With this informal title Dr Don Aldiss, a Geologist with the British Geological Survey since 1978, after a PhD with the Open University, currently engaged in 3D modelling of the Thames Basin and the Future Thames project, embarked on a comprehensive survey of the many different aspects of the Survey’s work in general and in London in particular.
Starting with a traditional geological map and the distinction between bedrock and superficial deposits, he informed us that such maps were the past; now the development of 3D geological modelling gives a more realistic understanding of the ground beneath our feet. He illustrated his point with reference to the area round Holborn Viaduct, the site of the planned Farringdon Crossrail Station, where made ground is deeper than superficial deposits, while stressing that the information presented, obtained by the sinking of 135 boreholes, was not in fact reality, but a depiction of a possible reality. He regretted that he was unable to show us such models in 3D, in the way they are intended to be seen .
A great many aspects of BGS’s remit were covered: Engineering Geology and Geohazards, including the volume change potential of the London Clay; groundwater flooding; Structural Geology and the importance of faults; Hydrogeology and the effect of climate change; Geochemistry with reference to water quality etc. Lastly he spoke of the Research challenges of the Future Thames project.
Throughout Don stressed that this research is in the public domain with free downloads from the BGS website link to NORA (NERC Open Research Archive); in particular the paper on Farringdon can be accessed at: nora.nerc.ac.uk/20346.. A lively discussion followed, showing the interest stimulated by the material presented, only curtailed because of lack of time. For a more detailed description and illustration of 3D mapping with reference to Northern Ireland, see the article by Graham Leslie et al in Geoscientist, Volume 23 No 3, April 2013 and at: www.bgs.ac.uk/gsni/geology/3d/index.html..
Geological Mapping Course in the Forest of Dean - with Nick Chidlaw
22-24 March 2013
Nick is an OU geology tutor, among his other geological teaching roles, and runs the course to enable participants to map an area from scratch. The Forest of Dean is a good area for this, as the geology is sufficiently accessible (at least it is in March when the vegetation hasn’t grown). There were over a dozen of us, and we divided up into groups of two or three (our group was Anna Saich, Kris Palubicki, me and John Potter from SE Branch). The first two days were spent mapping in the field, then the third producing a fair draft geological map of the area.
We started at an outcrop with guidance on what to look for and how to record it – and what we were lo oking at. We also had Nick’s notes on the stratigraphic column and geological and industrial history of the area – as you would have from a desk study before commencing field mapping. We then worked our way around the area, using footpaths, an old railway line and quarries marking up the exposure locations on the plan topographic map, and recording the exposures in our note books.
Nick told us the locations he would be at particular times for guidance, tuition and to indicate key out- crops. After the first field day we drew up our results, and identified areas still to be mapped, missing data and errors to be corrected. This then determined our field work on the second field day. It should be mentioned that it rained heavily on the first field day, then snowed that evening – there was then 2-3” of snow covering the ground on the second day. Two people gave up and went home, the rest ploughed on!
We tied up our notes on the evening of the second day, and then went to a room in a pub for the Sunday to turn our drafts, maps and notes into a fair draft geological map. It should be said that in addition to the dips and written observations we also collected samples. All of these were used to produce maps, with co-operation between groups where some areas had been missed. As we finished a little early Nick suggested a visit to the Geomap at the New Fancy mine site. This is a map of the whole Forest, the size of a car park and made of the actual outcrop rocks with mines, roads, railways and towns marked on. Very impressive, after it took ten minutes to clear the snow from it!
We ended up with a presentation by Nick on the Forest and its geology, including his mapping of the area. It was good to see that all groups had similar maps at the end of the weekend! This is a good opportunity to map in groups, with as much or little help as needed – and to put into practice our OU studies and skills. Recommended, though I hope you have better weather!
BGS Keyworth, Nottingham
28 March 2013
Fifteen members of the OUGS met at the BGS in Keyworth (near Nottingham). We were informed that the British Geological Survey (BGS) is part of the National Environment Research Council (NERC) and that their main sponsors are the Department for Business, Innovation & Skills (BIS). The work of the BGS covers many areas including climate change, pollution, waste management, natural hazards, groundwater, oil and mineral sources.
Our tour started with a visit to the Henry De la Beche library passing a large William Smith geological map (made up of several sections stitched together). The librarian told us about the extensive collection of books, maps, reports and journals (the BGS publishes many of these documents). Recent library books and journals are now online. Recently, duplicate books were removed from the collections leaving two rooms holding the remaining titles on portable racking. The library was computerised in the 1970s and 1980s and provides an enquiry service and can be used by the public for reference.
The librarian explained some of the BGS history. The BGS was established in 1835 by Henry De la Beche (the first Director). The Chancellor of the Exchequer gave his approval for a museum displaying rocks and minerals of economic significance in 1837. The museum was initially housed in Craig's Court, Whitehall in 1841 as the 'Museum of Economic Geology'; Richard Phillips was the first curator. When Craig's Court became too small the museum relocated to 28 Jermyn Street, off Piccadilly, in May 1851. In the 1880s a photographic library was donated to the BGS library. However, movement of the foundations necessitated the eventual demolition of the building and the museum moved to Exhibition Road, South Kensington opening in July 1935. In 1966 the BGS merged with the overseas Geological Survey. The present offices in Keyworth were purchased in 1976, and by 1986 the transfer of all the collections from London and Leeds was complete.
After visiting the library, the OUGS moved to the main storage hall containing over 4,000 pallets containing cores and cuttings. The storage halls were initially commissioned in 1985. The core archives were set up to facilitate the use of core, which have been collected for specific projects, for a wide range of different investigations such as:
The main core collections include (the figures help to show the size of the collections):
We passed a fossil collection (the museum has been lost). Some fossils, including brachiopods and belemnites, were available for viewing.
Dr Don Aldiss, the Principal Mapping Geologist, gave a very interesting talk in the visualisation suite telling us about his work. A piece of software, GeoVisionary, created jointly between the BGS and Virtalis, is a powerful data streaming and visualisation engine with a geological toolkit. Geoscientific datasets are used for digital elevation models, topographic mapping and 3D models.
Most members spent some time (in the bitterly cold wind) looking at the stones in the BGS Geological Walk. This walk is available for any member of the public to view during opening hours. The Geological Walk was set up with the support of CED Ltd (who provided all the natural stone paving) and opened in May 2012. The walk extends 130 metres. The display covers about 3 billion years of Earth history: all rocks in order of geological period.
The BGS maintains its own informative website at: www.bgs.ac.uk and a website titled UK School Seismology Project 'real science with real data': www.bgs.ac.uk/schoolseismology. Information about the National Stone Centre can be found at www.nationalstonecentre.org.uk.
The Chalk of Steeple Morden and Ashwell, Hertfordshire - with Haydon Bailey
13 April 2013
The two chalk quarries on the itinerary for this trip are both near Ashwell, Hertfordshire and currently active. The day was led by Dr Haydon Bailey and his colleague Dr Liam Gallagher, both of Network Stratigraphic Consulting Ltd. We met at Steeple Morden station car park (TL29835 38665) and were given a handout by Dr Bailey which included detailed descriptions of the lithologies to be seen, sections, stratigraphic logs and colour illustrations.
Dr Bailey ran through the programme and objectives for the day and, having checked that we all had the appropriate safety wear, we walked for approximately five minutes, parallel to the railway, into Station Quarry. From this quarry (Figure 1) approximately 100,000 tonnes of the White Chalk is extracted annually by the operators, Omya. The Chalk is white with high calcimetry values, indicating that it has a high carbonate content. It is used as a whiting for paper, as a filler for paints, whitewash, putty, rubber and plastic manufacture. It is also used as a filler in pharmaceutical products, cable sheaths and UPVC window frames and is introduced as a calcium additive in flour, cereals including Weetabix and Kellogg's and malt drinks such as Horlicks.
The Chalk is transferred to Plantation Quarry, approximately 1 km away by a covered conveyor belt. Chalk extracted during the summer months is stored under cover to allow water to drain from it prior to processing. This also evens the work load by allowing for processing in the winter when extraction may be less practicable.
The exercise for our morning in Station Quarry was to consider the problems associated with correctly identifying the stratigraphy in the Chalk. This can be very difficult generally but in this location, with few flints as marker horizons, it is particularly difficult. We were told that we were looking at the lowermost part of the White Chalk Subgroup, previously known as the basal Middle Chalk.
From our initial position facing the chalk face, to the right (south) two fine flint bands could be seen but these were seen to taper out to the north.
Two marl seams define the 7 - 8 o north/south dip, deriving from our position on the northern boundary of the London Basin. The thicker (up to 1 cm) of these seams is the Odsey Marl, named after the local manor house, and the thinner lower one marks the top of the Holywell Nodular Chalk, a hard shell-detrital chalk. The shell fragments are of the inoceramus bivalve Mytiloides. This organism lived in the unconsolidated ooze at the bottom of the Cretaceous seas and evolved to form an increasing surface area so as to be able to settle in the ooze. Alternative strategies developed by other organisms for the same purpose include long spines.
The marls are mostly of volcanic origin, deriving from Mid Atlantic Ridge volcanicity and each seam has a specific geochemical signature which can be traced laterally on a global scale. The Holywell Nodular Chalk is overlain by 9 m of the New Pit Formation in this quarry. These chalks are relatively soft and smooth to the touch, contrasting with the Holywell Nodular Chalk. 'Finger flints', formed as burrow-fill replacement flints, are found in the New Pit Formation. Some isolated fossil sponge beds can be seen which are pale yellow/orange due to the presence of limonite and other iron minerals. The age of the Chalks seen here is from 93.5 – 92 Ma, lower to middle Turonian, and probably represents the highest sea levels of the period. The waters were at a latitude of 420 and very warm at 170C, with the nearest shoreline probably as far away as Scandinavia.
Approximately 2 m below the quarry floor lies the Plenus Marl (see below), alternations of soft marls and marly limestones. The unconformable base of the Plenus Marl marks the base of the White Chalk above and the underlying Grey Chalk. Historically this boundary has been the subject of much disagreement but following extensive talks is now accepted by the BGS although not necessarily by all interested parties.
Moving up-dip to the north end of the quarry face we were lower in the stratigraphy and tried again to identify the Odsey Marl. This proved very difficult, noting that the marl seams are only a few cm in thickness. Marl seams seen clearly at this position lower in the succession were named as the Aston Marls. There is work in hand to extend the lateral correlation of the marls on a wide geographical scale.
Towards lunchtime we moved to Ashwell village and took a brief stop to see the Ashwell Springs from which the river Rhee rises, a major tributary of the river Cam. The springs are an SSSI for biological reasons and are calculated to be fed from an aquifer in the underlying Totternhoe Stone.
After lunch in a room at the Bushel and Strike public house we took a brief look at Ashwell church, built in mediaeval times mainly of Totternhoe Stone. This is a poor quality building stone generally having a durability expectancy of about 150 years, hence the poor condition of the stonework of this building and the reason for the current conservation work in hand. The softness of the stone has facilitated the graffiti which was seen inside the church, including a carved sketch of Old St Pauls Cathedral as it was prior to the current Wren building.
Following the brief visit to Ashwell church we met outside Cliff House in the village at the junction of Kingsland Way and Ashwell Street (TL27046 39582). This is a residential address which has a rear garden parallel to the public footway. The other side of the garden is flanked by a shallow cliff some 6 m high. The Plenus Marl is fully exposed in this outcrop and comprises eight discrete seams. These marls represent a global oceanic anoxic event, sometimes known as the Cenomanian-Turonian boundary event, which may have been caused by sub- oceanic volcanism some 500 Ka earlier when crustal production was at its highest for 100 Ma. The marls have been correlated laterally as far apart as Texas and North Africa and can be a major oil source.
From Cliff House we moved to Plantation quarry (TL2953 40188), approximately 1.5 km away. This receives the chalk transported on the conveyor from Station Quarry and accommodates the covered storage sheds and processing plant. This is a considerably smaller quarry than Station Quarry and we again were given an exercise, this time to locate the Morden Flint and to consider why flints could be seen in the southern end of the quarry, in the Holywell Nodular Chalk but not to the north.
Within this quarry there is a weak fold pair comprising an anticline and syncline plunging south eastwards. In the highest part of the Holywell Nodular Chalk, within the syncline to the south, there are several conspicuous bands of nodular flint of which the most laterally persistent is the Morden Flint. This was readily identifiable from the abundant fragments of Mytiloides on the upper and lower surfaces of the nodules.
The lower limit of the chalk with flints is marked here by a thin marl seam tentatively correlated with the Aston Marl seen in Station Quarry. It was clear that these flints thin out and progressively disappear to the north- west and north-east extremities of the quarry as the succession thins over the anticline to the north.
The absence of flints in this part of the quarry is in part related to the mode of formation of flint as researched by Chris Clayton for his PhD thesis between 1979 and 1982 (unpublished). The work was based on Paramoudra Flints, vertical columnar or barrel shaped structures up to 1.5 m high with a cemented chalk core. Animals burrowed in the original chalk in a circular manner and this anaerobic activity beneath the sediment produced H2S which diffused upwards. The sea water above the burrow contained O2 so a redox boundary formed which during a period of low sedimentation remained stable. Oxidation to sulphate released hydrogen ions to produce an acidic environment which dissolved the calcium carbonate and precipitated the silica, Clayton (1986). The silica, believed to have derived from the skeletons of siliceous sponges, radiolarians and diatoms, initially formed a gel which whilst mobile was able to fill burrows and enclose material lying on the sediment (Figure 2). The gel in time lithified to become hardened flint.
Figure 3 shows the Chondrites burrows from Figure 2. These trace fossils are the remains of mm scale burrows originally inhabited by an organism whose identity remains unknown.
Figure 4 illustrates the principle described above and demonstrates the various forms that flint may take, as dictated by the nature of the organic material on which the flint originated,e.g. sponges, Thalassinoides, Chondrites burrows etc.
The absence of flints in the NE and NW parts of the quarry, which we were asked to consider, may be explained by the interruption of this formation process in the presence of a high proportion of clay minerals as found in marls, such as those in the New Pit and Holywell chalks. This inhibition of flint diagenesis in the presence of clay minerals has been demonstrated by experiment (Siever & Woodford, 1973).
A possible resolution to the matter having been posited time demanded that we leave the quarry having expressed our gratitude to our leaders for the day. Acknowledgements and grateful thanks are due to Dr Haydon Bailey and Dr Liam Gallagher for giving us their time and answering our questions, to Omya Ltd. for allowing us access to their quarries and to Di Clements (LOUGS) for organising the day.
Bryan J Carter
Lecture : Determining an eruption age using Ar-Ar geochronology: solving the mystery of Sheridan Reservoir Dome, Yellowstone, USA - by Millie Wilkinson
18 April 2013
Millie, fresh from being recently awarded a PhD at the OU, gave us an enthusiastic account of her research, starting with the basics of the dating technique, which is extraordinarily complicated involving for example distinguishing extraneous argon from inherited argon.
Factors affecting our ability to determine an eruption age include ash layers which provide marker horizons, and the evolution of the volcanic system. The Sheridan Reservoir dome is a crystal-rich, high silica rhyolite dome, which erupted through the Huckleberry Ridge tuff, dated to 2.1 Ma.
To analyse it, the dome rock was crushed to single crystals to see the age range and the grains melted by infrared laser. Mass spectrometry for 16 individual crystals gave an age of 2.25 ± 0.07 Ma: too old! Might this be due to inclusions, excess argon? In search of more precision U-Pb concentration in zircons was used, still gave a range of ages, but with less pronounced spread.
Argon loss was modelled, changing the temperature of the magma in the model, and using different crystals; the problem was defined as one of contamination, perhaps self-contamination.
Then volcanic glass was analysed, giving a very narrow Ar-Ar range of 2.04 ± 0.03 Ma (compared to feldspars’ 1.8-2.2 Ma), agreeing with the zircon range and with field observation.
The talk concluded with a discussion of magma processes and a few pictures of Yellowstone.
The very lively style of presentation could not disguise the fact that much research depends on minute, painstaking and repetitive labour, yet needs lateral imaginative thinking to see one’s way round obstacles. It gave rise to a stimulating question and answer session.
Geoconservation Day at Gilbert's Pit, Charlton
28 April 2013
Many thanks go to all of our members who took part in the geoconservation day at Gilbert’s Pit. We achieved our objective of clearing a swathe up the existing scree slope to enable us to see the best route to install a stepped pathway. A few of the smaller trees were felled as well, with thanks to Laurie’s colleagues from Shooters Hill Woodlands conservation group. Jackie Skipper took groups to the top of the exposure to explain the section more fully and we hope everyone enjoyed the day.
Gilbert’s Pit, Charlton holds SSSI status as it is the type section of the Woolwich Formation, part of the Early Eocene Lambeth Group. Although much of the original section is covered by scree, the Lambeth Group and Blackheath Beds at the top can still be seen and improved access to these beds is what we are trying to achieve.
The photographs show work in action through the day arranged in chronological order, taken by Laurie Baker.
Green Chain Geowalk
12 May 2013
A bright and blustery morning saw a small, select party from the London group of the Open University Geological Society gather beside the Thames at Barrier Park with the two leaders for the day, London Group members Di Clements and Laurie Baker, who are also members of the London Geodiversity Partnership who devised this trail (Figure 1).
The Thames Barrier, which became operational in October 1982 almost fifty years after the devastating floods along the East Coast and Lower Thames Estuary, stands on a relatively straight stretch of the river with steel reinforced banks visible on either side, and is estimated to offer protection against flooding until at least 2070. The concrete piers for the Barrier are built on 180 m thick chalk which faulted at this point, forms a firm foundation below the Thanet sands, river gravels and mud of the river bed. Walking away from the river we climbed about 22 steps before descending a similar number over a raised bank built to protect the low lying area to the south of the river. Industrial units and warehouses have been built here on the reclaimed land, but our path took us through a small park over what would have once been a muddy foreshore rising gently from the river to a low chalk cliff, the line of which was visible beyond the Woolwich Road. The railway line to Woolwich Dockyard and beyond was tunnelled through the chalk.
On climbing to the top of Cox’s Mount views towards East London, with Canary Wharf, the Millennium Dome, Queens House Greenwich, and Charlton Football Ground which was built in a chalk pit, were visible through the trees (Figure 2). The diversion of the River Thames to the south and subsequent redirection of tributaries as a result of the Anglian Ice sheet, the plateaux north and south of the river and possible local faulting were considered.
Gilbert’s Pit, a Site of Special Scientific Interest, provides information for Geologists and Engineers as a type section for the London area, is reached along the footpath south. Originally quarried for Thanet Sand used as moulding sand at Woolwich Arsenal and for glass production, the area has been recently cleared by members of the Geodiversity group and London Open University Geology Society to reveal a clearer view of the section.
Some of the marine Thanet formation can be seen overlain by the Upnor formation, a concretionary layer, which in turn is overlain by the lagoonal sediments of the Woolwich Shell bed and the Lewisham Leaf bed, parts of the Lambeth Group. This is topped by the Blackheath beds not visible from this point but are evidenced on the newly cut steps and at the base of the section by the fence where the smooth, well-rounded black flint pebbles form a thick scree. A further face can be accessed now the brambles and small trees have been cleared recently and shows the full range of beds including the Blackheath beds many metres thick at the top (Figures 3 and 4).
The path gently rising all the time follows a small stream through Maryon Wilson Park to an area just below Charlton Park Road where a depression has been formed by springs which feed the small stream. These appear at a point where water percolating through sands reach an impervious clay layer.
Local Blackheath pebbles set in concrete were seen in a wall bordering Charlton Park Road whilst across the road on the large level plateau at the top of the hill is the well used sports ground of Charlton Park.
The wall surrounding Charlton Cemetery is made of yellow London stock bricks (Figure 5) produced using locally found brick earth, fine grained clay containing a little chalk and sand. London clay mixed with chalk and sand was also used and local Victorian maps show brickfields, clay pits, clay mills and potteries all evidence of the local industry. Looking closely at the bricks we saw they showed the clay had often been pushed quickly into the moulds and also contained black patches. These were the charred remains of household ash, incorporated to allow firing at low temperatures in a clamp on the local clay fields.
In many places in the area, we saw blackened misshapen bricks in blocks fused together during manufacture, which as they were unsuitable for regular building works, were incorporated into walls that from a distance resemble haphazardly assembled stone walls (Figure 6).
Standing on the corner of Ha Ha Road, with its fine avenue of trees, we had reached the northern part of Woolwich Common, separated from the level ground of the Royal Artillery Barrack Field to our north by a Ha Ha, dug to stop cattle roaming onto the parade ground. The Ha Ha ditch and the surface of the level path beneath the trees were well drained and dry, owing to the presence of the Black Heath Bed of the Harwich Formation, which underlies the fairly level plateau stretching towards Plumstead to the east (Figure 7).
The Common rises gently to the south, towards the 130m high land of Shooters Hill, the second highest point in London. The Claygate beds and Stanmore gravels, deposited over 1 mya, which form the crest of the Shooters Hill, and similar to those that cap other high areas in London, are thought to be either of shallow sea marine or fluvial origin. Much of the London Clay in this area has been eroded but the line of springs which emanate from the clay at the side of Shooters Hill were known for their medicinal properties, similar to those of Epsom Salts.
Our route took us to the east to Plumstead Common, once the site of a Quarry, also thought to be used for brick making as low walls retaining an area of higher ground and bushes were made of blackened burnt bricks, ‘seconds’ not transported far from point manufacture. In the shrubbery we found the ‘Dog Rocks’ of Plumstead Common. These are of naturally cemented Black Heath pebbles probably quarry discards, which have taken the shape of Dogs’ Heads (Figure 8).
Crossing several side roads we followed the open grassy areas of Plumstead Common, with occasional views to the north over Plumstead towards Thamesmead factories, warehouses and modern housing developments, until we reached a gorge-like valley known as ‘The Slade’ (Figure 9). Cut, possibly along a fault line, by torrents whilst the land was frozen and impermeable, the steep valley sides have Blackheath Pebbles several metres thick. These form the surface of the Plumstead Common plateau, and may have been exposed by slips of the clay below. At the base of the pebbles, springs mark the level of the impermeable clays of the Woolwich Beds and small ponds can be found in the valley bottoms at several places in this area.
Following the trail towards Bostall Woods and Heath from Winn Common we were shown photographs of the area in 1901 when it was a hive of industry. It was here the local clays, loess, and sand exposed in the valleys which dissect the plateau were exploited for brick making; the flint pebbles used in aggregates and mined or quarried chalk for lime production. All these were vital for the rapid expansion of housing in South East London and the industrial and warehousing developments on the flat Thames Plain to the north. Now with most of the signs of its industrial past obliterated by thick undergrowth and trees, the area is largely used for recreational purposes, although chalk pits, pond filled clay pits and hollows can be found in the vegetation.
One small and long exploited exposure of underlying chalk can be seen at aptly named Chalky Dell (Figure 10), an overgrown quarry in Lesnes Abbey Woods, where mine shafts known as Dene holes were dug as long ago as Roman times. I did not go into the quarry so please refer to the full description by Diana Clements which appeared in London Platform August 2012.
Walking downhill towards Lesnes Abbey we came across the Fossil Bed Enclosure, where over 100 years ago William Whitaker came across fossil marine shells and sharks teeth in the sand. Further investigations have found fossil remains of over 46 species of mammals over 55 million years old, making it one of the richest sites of that age and an SSSI. A detailed description by our leader Diana Clements, of the site and fossils found there appeared in London Platform August 2012 following a visit with South East Branch in May 2012. Though fossils are often found on the surface and ‘dry sieving’ is permitted, further digging of this very localised bed is restricted, although part of the site is opened annually for a day for further investigation. On the surface, we found some small marine fossil bivalves (Figure 11) and a couple sieving during the afternoon had unearthed a great many shark’s teeth.
The forecast rain began to fall as we left the shelter of Lesnes Abbey Woods and walked towards the ruins of Lesnes Abbey (Figure 12). It is thought that this foundation of 1178 by Richard de Lucy was accessible by boat and hence some of the original building stone may have been brought from Normandy. With the Dissolution of the Monasteries much of the stone was recycled but the extent of the buildings can be seen from the walls that remain. The Abbey, built about 15 m above the floodplain, commanding good views across the river, subsequently enclosed and drained much of the marshland although over the years repairs to the banks and walls to prevent repeated flooding proved costly and often ineffective. The area was badly affected by the 1953 surge and the foundations of more recent housing developments between the Abbey and the Thames at Thamesmead have been built on concrete platforms to raise them above flood level.
Having thanked our leaders for sharing their extensive geological and historical knowledge of this often overlooked area with us, some members of the group had a brief look at the stones of the Abbey before heading homeward from Abbey Wood station, a short walk from the Abbey grounds.
The Green Chain Walk was devised by the London Geodiversity Partnership: www.londongeopartnership.org.uk
A pdf of the Geotrail giving more details is available on the Green Chain Walk website: www.greenchain.com/greenchainsite/downloads/file/1/green_chain_geo_trail_guide
Those who helped out at the Gilbert's Pit Geoconservation day, or those who went on this walk, might be interested to see photos of the Woolwich Beds on the South Wall of the pit up close: www.lougs.org.uk/tmp/gp.htm
Lecture : The Atlas for London Basin Geology - by Michael de Freitas
14 May 2013
Dr Michael de Freitas, Emeritus Reader in Engineering Geology, Imperial College, and author of ‘Engineering Geology, Principles and Practice’, introduced the topic of his talk by showing us through historical maps going back to the 1930s and 50s, how the structure of the London Basin has been poorly understood. The Regional Guide 1947 refers to buckle folding on the outer edge of the Alpine orogeny, while in 1972 the Water Resources Board plotted a map of the top of the chalk, still talking in terms of synclines and anticlines. And as recently as 1996 the Regional Guide shows only two faults.
Barton’s ‘Lost Rivers of London’ (1992), however, contains a map of the tributaries of the Thames, upon which a grid has been superimposed, using the course of the rivers and significant points where they divide. When the rivers are taken away and the grid left, we see a compartmentalisation, which when plotted along with groundwater data showing the level of groundwater beneath the chalk, reveals that the lines agree with numerous faults, and that geotechnical problems in construction coincide with the river courses.
This compartmentalisation is perhaps the result of the opposing stresses on the Variscan Front caused by the push from the Alpine orogeny from the south-east and the opening of the Atlantic from the west. The sinking of boreholes reveals the extent of the faulting, but is necessarily patchy. An example is the Plaistow graben in North London where there is 20 m displacement of the chalk along a fault. The subsequent effect of the ice ages was to cause adjustment along the faults.
In view of all the problems these findings have created, a group of geologists formed the London Basin Forum following the 9th Glossop Lecture given by Michael in November 2008, with the aim of creating an electronic atlas under the umbrella of the BGS in order to put site geology into perspective: the London Basin Atlas. It will be used for risk management and the drawing up of contracts, with the research paid for by the client.
Details can be found at : www.bgs.ac.uk/londonBasinForum and relevant maps and charts at www.bgs.ac.uk/londonBasinForum/docs/281009/Mortimore.pdf.
Photographs: Anna Saich, Pam Pettman and Gavin Mair
Pett Level - with Peter Austen
2 June 2013
The fieldtrip was led by Peter Austen, unfortunately Ken Brooks couldn’t be there, but Peter had borrowed his footprint casts. There were fourteen OUGS members, with Peter and Joyce Austen. It was a good sunny day - almost the first of the year. We met up at Pett Level and Peter showed us the casts Ken had previously made of dinosaur footprints (Figure 1). It was a good low tide, which is essential for getting round to Fairlight Cove (and back!).
Starting on the beach, we passed the sand covered remains of the drowned “fossil forest”, thought to have been used by Mesolithic hunters (about 5.2 ka), as their axes have been found in adjacent caves.
We headed west and first looked at the Cliff End fault, a classic normal fault (Figure 2), from the Alpine Orogeny. This brought the Wadhurst Clays down level with the Ashdown Sands. These beds were laid down 140 Ma, when Sussex was at 40o N, in a landscape of meandering rivers, flood plains and lakes. Passing along the sedimentary structures in the cliff, we saw mammillated Tilgate Stone, and the Ashdown fossil beds (plenty of remains of plants and fish – and occasional crocodile, turtle and iguanodons, etc.).
Passing round the headland we examined Haddock’s reverse fault (Figures 3 and 4), where the Ashdown Beds have been pushed up 60m to the level of the Cliff End Sandstone. The slip surface was plainly visible in the cliff. The fault originated in the Tertiary, from the collision of Africa into Europe. The line of the fault could be seen across the beach and into the sea, as could many Equisites in the rocks.
Then there is an area of regrarded and stabilised slope, with new drainage and limestone protecting the toe. By Fairlight village were large banks of Larvikite, placed in 1990/91 after the major erosion of soft clays leading to cliff falls in the 1970s. This stopped the erosion, and vegetation grew on the cliffs, thus stabilising them. It also led to the formation of a shingle beach on the foreshore, hiding both the exposure and dinosaur footprints. Today we were lucky enough to find both therapod (Figure 5) and sauropod tracks, apparently an Ankylosaur brain case has also been found, along with dinosaur bones.
We went as far as the Fairlight Cove Fault, which raised the Ashdown Sands, including the Fairlight Clays, sixty metres. These clays include early Wealden plants, and, further west, 140 Ma carbonised wood from forest fires.
Then we retraced our steps, finding more dinosaur footprints, and dinomush (beds thoroughly disturbed by trampling herds). We thanked Peter in the usual way – it was an excellent day and a location well worth another visit.
Isle of Man - with Dave Burnett and Dave Quirk
14-17 June 2013
Day 1—Friday 14 June
We went to The Sound Cafe at the southern-most tip of the Isle of Man for a welcome cup of coffee and introduction to the geology and history Isle of Man. We were here to see the geology but there is so much more to the Isle of Man than the rocks that it is made up from. Just a quick run through of the things that you might know – it is the home of the TT races, Manx cats (we later got very excited to see a tailless cat only to find that it had lost it in a car accident) and the Tynwald (the oldest parliament in the world). It was also the first to give the vote to women some 125 years ago.
The understanding of the Geology that we were to study in this trip is based on the Victorian survey of the Island carried out by GW Lamplugh from 1892 – 1897 and the most up to date surveys making use of increased sophistication of survey methods on the ground and in the laboratory. Dave Burnett and Dave Quirk have published very recent work on the geology of the Isle of Man and we were very fortunate to have them as our leaders to share their love of the Island and their extensive knowledge with us.
The main geological spine of the island is a NE – SW range of Caledonian mountains formed by the folding and deformation of Ordovician sediments forming the Manx group of rocks which make up about two thirds of the Island. These rocks are marine turbidites derived from the northern margin of the continent of Gondwana/Avalonia deposited in the Iapetus Ocean around 490-470 million years ago. But that is not the whole story. The Isle of Man has moved over time from 600 South to its current position of 550 North thus changing the climate from close to the South pole towards the equator during the Carbonifersous and on up towards, but not quite reaching the North pole as it is today. There are Silurian rocks of the Dalby group, similar to but different from the Manx group on the west coast, the Peel sandstone sequences exposed in cliffs just north of Peel and in the Southwest outliers of Carboniferous limestone and across the island some granites of different ages too. So much to look at and explore; it was a very exciting prospect and made all the more interesting for having such knowledgeable and enthusiastic leaders.
Our first look at the rocks was a visit to the Chasms near the village of Cregneas (Figure 1).
We parked at the top of the hill and had a misty walk out towards the sea. We had to be very careful where we put our feet as the whole cliff top was cut by deep chasms where large areas of the cliffs were falling into the sea. The Chasms are in the Mull Hill formation made up of thick bedded quartz arenite sandstones grading up from medium-grained sandstone to a very fine sandstone or siltstone hence their lack of strength and propensity to fall into the sea (Figure 2).
From here we moved on to Port Erin and went to the end of the beach looking towards Ireland. Here we were able to see the Lonan Formation, highly folded thin bedded quartz wackes draped with laminated to very thin bedded mudstone (Figures 3 and 4). These rocks represent distal turbidites, either further away from the shore or moved sideways across the sequence. Here too the folding was caused by the Caledonian Orogeny. It is hard to remember that we are looking at not an instant in 100 years apart or 10’s of hundreds or years apart.
A brilliant introduction to the Geology of the Isle of Man with lots to discuss and think about and a well earned lunch stop.
Cass-ny-Hawin, meaning ‘foot of the river’, is the site of an Iron Age hill fort which was used to defend an important landing place in this part of the island. Geologically it is at the fault contact between the Manx Group and the Carboniferous limestones (which we would see more of later in the trip). The fault itself is near vertical and marked by iron staining and fault breccia.
Here the Manx Group consists of interbedded mudstones and sandstones; the white sandstone and dark grey mudstone giving the rocks a ‘pin-striped’ appearance. There is evidence of bioturbation and early basic dykes intrude the Manx Group sediments (Figure 5).
Ronaldsway Airport is close to Cass-ny-Hawin. As part of the runway extension a promontory was constructed out into the Irish Sea and rock-armour was imported by barge from Norway. The larvikite blocks shimmered in the sunshine as we made our way back to the minibuses.
In Victorian times a tramway ran along Marine Drive taking tourists along the coast from Douglas to Keristal and on to Port Soderick. With the sun shining it was easy to appreciate the coastal views that the Victorian tourists admired. The Manx Group rocks exposed in the cliffs are sandstone dominated and the beds are dramatically folded (Figure 6). Folding occurred during the Caledonian mountain-building event following the closure of the Iapetus Ocean.
In general the Manx Group is sandstone dominated in the east and mudstone dominated in the west. Although dating of the Manx Group is somewhat limited, the sandstone dominated beds at Marine Drive and the mudstone dominated beds (Cass- ny-Hawin and Port Erin) are thought to be contemporaneous and represent lateral facies variation within the turbidite deposits. The Keristal Member is thick quartz-rich bed thought to be a channel within the turbidite.
Although Marine Drive is now a roadway, the route is partially closed after major rockfalls and we headed back inland to return to Douglas.
Day 2—Saturday 15 June
We set out for Niarbyl on the west coast on a far more promising day than previously; blue sky and sun but still a strong wind. We parked up and walked down to a picturesque cove complete with newly renovated thatched fisherman’s cottage (Figure 7). The added bonus was protection from a bitterly cold wind!
This bay is the setting for a major shear zone at the boundary between the early Ordovician Manx Group (c485 Ma) and the Silurian Dalby sandstone (c425 Ma) which is thrust over the intensely deformed Manx Group (Figure 8). The boundary is also believed to represent an exposed part of the suture where the Iapetus Ocean was subducted beneath Laurentia but this subject seems to be open to discussion. Our leaders thought it possible.
Previously the Dalby sandstones had been correlated by GW Lamplugh as part of the Manx Group. However, graptolite dating now indicates that the Dalby sandstones are Silurian (Wenlock epoch) and some 60 million years younger than the Manx Group. Evidence from palaeocurrents also indicate that the flow of Dalby sediments was roughly SE whereas sediments of the Manx Group flowed in the opposite direction. The following may help explain this.
About 490 Ma ago Laurentia and Gondwana were separated by some 3000 km of Iapetus Ocean. The Isle of Man at this time formed part of the seabed to the north of the microcontinent Avalonia which was attached to the northern edge of Gondwana. The deep water Manx Group sediments which comprise two thirds of the Isle of Man originated from upland areas of eastern Avalonia or Gondwana which were transported northwards to the Iapetus Ocean where they were eventually deposited in the Manannan basin. About 470 Ma ago Avalonia rifted from Gondwana and drifted northwards as the Iapetus Ocean closed in front and the Rheic Ocean opened behind. Some 30 Ma later western Avalonia docked with Laurentia in the area of today’s Newfoundland and by c430 Ma eastern Avalonia was colliding with the European part of Laurentia. Closure of the Iapetus was complete by c423 Ma. The Dalby Group sediments derived from Laurentia and were deposited into the narrowing Iapetus Ocean 427-423 Ma ago as eastern Avalonia approached Laurentia. Thus the different flow directions of the Manx Group and Dalby mudstones are explained.
Our next stop was Peel where we walked along the cliff top to the disused Peel Hill quarry (Figures 9 and 10) to examine the Peel mudstone, part of the Dalby Group . The calculated upper age of these and the overlying red coloured manganese-rich rocks had been very varied until the discovery of Silurian graptolites in the mudstones and derived Lower Silurian age corals, brachiopods and crinoids etc. in the sandstones.
More recently palynological dating has suggested that the Peel Sandstones are early Devonian. We were to examine these on Monday but were told here that the sequence of fine and coarse grained sediments together with conglomerate horizons probably represents a braided river setting. Interestingly, until recently it was believed that the fossil material in the conglomerate pebbles had been derived from an upland area to the north but palaeomagnetic data now suggests that the Peel sandstone block was rotated anti-clockwise by 45° from it’s original position so the source of sediment for these sandstones now seems to be somewhere in the direction of the Lake District.
After lunch, we proceeded from Ramsey to Maughold Church. In the churchyard we were shown round the collection of cross stones contained in a purpose- built shelter. Andrew Foxon, formerly of Manx National Heritage, but now a free- lance archaeological consultant, showed us round the crosses and gave us an explanatory talk.
The crosses had all been collected from within the parish, which was one of 17 parishes on the Island. Some had been recovered from within only a few metres of the shelter. The parish was called after a saint who had been sent across the Irish sea, and had founded a monastic settlement there in the Irish tradition. The current church was probably on the same site as the saint's original chapel, and the site contained 4 keeills, cells for a monastic priest with an oratory attached.
The crosses were wide-ranging in date, from the Celtic Kingdom and the Norse period. Some were simple grave markers, a plain incised cross. However, many were both carved and inscribed, with inscriptions in a range of different languages and scripts, reflecting the different dominant cultures of the island.
The oldest cross slab, probably dating from the 7th century, marked the grave of a monastic bishop. It bore a Latin inscription in Hiberno-Saxon characters. We were also shown a cross which was in similar style to crosses found in SW Scotland, the nearest part of the UK mainland. This bore the name of a Welsh prince called Guriat, who was thought to have taken refuge in the Island in the 9th century.
Other stones included:
Ogham (often pronounced "Oh-am") script, is a form of early medieval alphabetic writing, consisting of straight and slanted strokes cut onto the edge of a piece of stone or wood. It was mainly used for writing monumental inscriptions in early forms of Irish, but was occasionally used for writing in other contemporary languages.
About 95% of the stones were made from local rock. A few were of stone which had been imported from Northern England, and one was made from a piece of Norwegian gneiss. It seemed likely that this last piece had been brought over as ballast in a Norse ship.
We re-embarked on the minibus for a brief visit to nearby Port Mooar. Here we looked at the Lonan Formation of the Manx Group. The grains were coarser than we had seen earlier, about 50-50 mud and sand, with planar lamination and lots of ripples. We were told that the beds had previously been thought to have been laid down in a deltaic environment. However the present consensus was that they had been formed by turbidites (Figure 11).
Our visit was cut short by the onset of rain, and we abandoned the search for a tertiary dyke, which is the other main feature of this locality.
On the way back to Douglas, we made a brief stop to take a look at the Laxey Wheel (Figure 12). We viewed this from outside its enclosure, as the attraction was just closing as we arrived.
The wheel, also known as the Lady Isabella Wheel, was built in 1852-1854. It was part of a system to provide power, transmitted by linked flat rods, to pumps in the nearby mine workings. The workings were as much as 540 m below the top of the main haulage adit. The pumped water was raised to a separate drainage adit through which it flowed out into the valley. The mine had been worked for lead, zinc and copper, in a north-south trending area of mineralisation in the Manx Group and the underlying Foxdale Granites. The mineralisation cuts across the granites, and is therefore post-Caledonian. It could be Devonian onwards, but current research suggests that it mainly occurred in the late Carboniferous.
It was pointed out that the Wheel had been intended from the start as a tourist attraction, as well as industrial plant. This was evidenced by the prominent and elaborate viewing gallery on top of the Wheel. Profits from the then burgeoning tourist industry had thus helped to finance mining technology. Following this brief stop, we returned to Douglas.
Day 3—Sunday 16 June
The visit on this day was to the bay of Castletown in the south of the island, following the Lower Carboniferous sequence westwards from Langness to Scarlett Point, then round the corner to Poyllvaaish. The sequence began at the Langness Arch (Figure 14) with a spectacular unconformity (Figure 13 ) in which, at an interval of 150 million years, slanting red shales of the Manx group were overlain by massive conglomerates of pebbles to boulders, angular as well as rounded, set widely or closely in a fine red matrix, with some bedding of larger and smaller stones, and some faulting. Iron was clearly present, with, we were told, copper mined on the ness.
The conglomerate (Figure 15) was interpreted as a consequence of the splitting apart of the Caledonian mountains with the breakup of Pangaea at the end of the Silurian, causing extension faulting in the rocks of the Manx group, followed by rifting and the creation of a series of half-grabens, the first of which was filled by energetic flows of rocky material from the erosion of the Manx group mountains: the Langness site is at the furthest tip of this infill.
From here we headed round through Castletown to Scarlett Point at the far, western side of the bay, to see the Early Carboniferous, past slightly sloping beds of fine grained fossiliferous limestone lying conformably on the conglomerate as a result of the sinking of the deltaic-coastal environment to a sea floor. At the Point itself, however, the limestones were overlaid by a slope of volcaniclastic basaltic rock, greenish and layered, out of which, at Scarlett, protruded a large dyke of ropey lava (Figure 16).
The volcaniclastic material was the product of explosive underwater eruptions producing thick layers of basaltic ash; the dyke was the result of later Late Carboniferous subaerial eruption, rising up and cascading down on either side like a fountain. Columnar jointing could be seen on the island plug just off-shore.
Lunch was at Castletown, the old capital of the Island, eaten in Rushen castle after an introduction to the fortress by the museum curator Allison Fox. Originally a monastic settlement, Rushen Abbey, the castle was constructed with a single-storey square keep in 1190; its centrepiece now is an impressive keep of the 14th-15th century, which housed the governor of the island down to the 17th century, and which with its surrounding buildings remained in use as a prison up to the last century. It had thus survived (at least so we were told), as the finest example of a mediaeval fortress in Europe, despite being built of a limestone which was declared to be a cross between a sponge and a sieve. It is now a well-equipped museum of the history of Castletown, but one that required a climb of ninety or more steps to get up and down, leaving little time to take it all in.
After lunch we drove still further round the coast to the west of Scarlett Point, where at Poyllvaaish Bay, younger, fossiliferous limestones underlay reefs of pale limestone. In between these were shallow bays covered in thin flat beds of dark muddy limestone, rising up to vertical against the scarps of the reefs on either side. The reefs themselves are Carboniferous, and the reef-builders probably algae, plus bryozoans and crinoids, in a quiet lagoonal environment. On the seaward side were thick carbonate prograding beds. Blocks of reef material on top of the muddy limestone on the floor of the bays were evidence of reef collapse, perhaps as the scarp face became too heavy, perhaps because of earthquakes. The series of reefs suggested a large and complicated reef structure of uncertain length and width. In its final phase, when a drop in sea level may have exposed the original reefs, new reefs had grown underwater on top of the muddy limestones on the floor of the bays. Subsequently, as a result of rapid sea level rise, reef-building ceased; instead, there was deposition of thin, finely-grained layers of dark to black carboniferous limestone containing pyrite, which had hardened into a building stone in the quarry we passed as we walked back towards Scarlett Point.
Here, we came up against a slope of pillow lava (Figure 17) at the western end of the slopes of volcanic material we had examined in the morning; like the dyke, the pillow lavas were evidently created by subaerial eruption, in this case flowing into the sea. The sequence now became clear. Occurring at the end of the Lower Carboniferous, the reefs and the hard black limestone mark the end of the sedimentation process from conglomerate to limestone. This was terminated by the volcanism exposed between Scarlett Point and Poyllvaaish Bay, whose heat probably hardened the black limestone at the top of the limestone sequence.
All can be interpreted as the end result of the rifting which created the half-grabens in evidence at Langness. Thereafter the half-grabens were superseded by a single basin caused by thermal sag, that is, sinking after volcanism. This produced a second major unconformity in which the limestone is overlain by Bowland shales, themselves overlain by Millstone Grit.
This very varied and interesting day ended archaeologically with a walk up to a pre-Viking hill fort, introduced by Andrew Foxon, before our return to Douglas and refreshment.
Yvonne and Michael Brett
Day 4—Monday 17 June
Again we took the now familiar road back to the west coast and on to our first location, south of Kirk Michael, at Glen Mooar. To our left, as we walked south along the beach towards Ooig Mooar, were unconsolidated sandy cliffs, a wire fence hanging suspended above a ravine indicating the rapidity of their erosion. These thick Pleistocene glacial sediments probably marked channel fills from an extensive river system draining glaciers to the north.
Then we were back with the Manx Group, specifically with the youngest unit, the Lady Port, c472 mya, characterised by laminated mudstones and sandstones but especially by pebbly mudstones and beyond! The first rocks we saw were massive finely planar laminated grey mudstones criss-crossed by veining in 3 clear directions, an indicator of later tectonic events. In places the veins bordered on boudinage. As we walked south thicker pale sandstone layers appeared in the rocks. At one location there were planar layers of dark mudstone, pale sandstone, and a pale mudstone with burrows indicating a more oxygenated environment. We also walked past an fault, well marked because here the erosion of the sands above the Lady Port rocks was meeting erosion of the fine fault-gouge material and the cliff was eating backwards.
Then we came to mudstones which now contained sandstone lenses up to 4-5 cm long amidst swirling sandstone layers, then mudstones, still laminated but now acting as a matrix to pebbles and cobbles: largely of sandstone but some mudstone too. At the final stop we were walking on lithosomes, narrow blocks of sandstone several feet long standing proud of the more eroded laminated mudstones and sandstones around them.
Most of the clasts in these rocks are characteristic of the Manx Group as a whole and they are interpreted as debrites: debris flow deposits into the Iapetus Ocean, initiated by sinking of the ocean basin with associated extensional faulting upslope and compressional thrusting downslope. It was explained that these processes can proceed over hundreds of kilometres along a continental slope and move tens of meters of sediment thickness in huge coherent blocks. Triggers could include tectonic events, sea level changes or slope effects as the receiving basin sinks. Wherever an escarpment formed, then sand could be deposited. The different levels of disintegration of the sandstone layers into pebbles, cobbles and large blocks, as we saw at this location, mark the stages of development of a debrite.
A little further south on the Peel Promenade, with a splendid view of Peel Castle (Figure 18) where many of us had lunch, we were with a very different rock, the Peel Sandstone Group (Figure 19). Here the beds dipped about 50°, much of the cross-bedding was about 0.5 m long with changes in direction evident, there was lensing and behind the rubbish bins a fine ripple bed. Trough shapes showed that the rocks were ‘right way up’. One narrow bed was filled with small clasts and moulds of broken shells and there were several narrow conglomerate beds. The sandstone grains were of moderate size, many angular, some frosted and all iron-stained.
The Daves interpreted this formation as laid down by a braided river with a seasonal flow in a semi-arid environment, but, from possible dune features elsewhere, an Aeolian deposition has also been suggested. Dating has proved problematic in the past as only one trace fossil has ever been found, Beaconites, possibly the burrow of a crab. More recently, however, the rocks have been dated palynologically as early Devonian, identified as a period when the Isle of Man lay at c29o S. The parallels with the Old Red Sandstone on the mainland are obvious. The dating of rocks found in the conglomerate layers fits with this analysis, many of them being Silurian: volcanics, limestones and sandstones. The direction of flow seems to have been southwards.
Sandy Colville Stewart
On Monday afternoon we travelled from Peel Castle to our last location of the trip, Foxdale, which is located in the central spine of the island. We went to the aptly named Stoney Mountain, a former quarry, to look at the surface exposure of the Foxdale Granite (Figure 20).
Gravity and magnetic data show that the surface exposure of the Foxdale Granite is only the top of a huge batholith of tens to hundreds of cubic kilometres. The granite is S-type, rich in muscovite. The granite contains late-stage pegmatites (very coarse-grained crystalline veins) formed by hydrothermal fluids flowing through fractures, and composed mainly of quartz with some feldspar and small quantities of beryl (Figures 21 and 22).
The granite formed as the last stage of the Caledonian mountain building event, by which time collapse had actually started. The granite is cut across by a younger east-west trending vein which was mined for lead sulphide (galena). It is postulated that such a large volume of granite was able to get through the crust as a result of faulting. A combination of normal faults and an extensional setting created space for the granite to reach the surface.
Photographs © Yvonne and Michael Brett, Di Clements, Gill Hetherington, Anna Saich, Ursula Scott and Sue Vernon.
Church geology in West Sussex; a study in historic building stones - by David Bone
20 June 2013
David’s fields of interest can be found at the website at the bottom of this account. He is well-known to some of us, having led a field trip to Boxgrove and Bracklesham last year, and this year in September will lead another based on Chichester and Pulborough.
He started with the rationale for the study of church geology by pointing out that historic building stones are the key to local distinctiveness and with safe and easy access offer ideal research opportunities, inviting community engagement and useful information for repairs and restoration.
Factors affecting the use of a particular stone include geological factors, suitability for the purpose and availability. Human factors include patronage, transport, the status of the building, aesthetics and cost, all factors that have changed throughout history.
Generally speaking West Sussex is made up of relatively soft, poor quality sedimentary rocks, so imports have been used to create variety. In chronological order the list includes Devonian sandstone from Torquay and Plymouth, Jurassic limestone from Caen, Chilmark etc, Lower Cretaceous from Purbeck, Ardingly, etc., Sussex ‘marbles’, plus Greensand and Kentish ragstone, followed by Upper Cretaceous chalk and flint, Palaeogene septaria, Quaternary erratics, and finally re- used building materials such as Roman bricks and tiles.
Given this list, it would fill too much space to list all the buildings considered. There are 250 church centres in West Sussex (some have more than one!) from East Grinstead in the north to Brighton and Chichester in the south, and David has visited them all – and is revisiting many. His study concerns the exterior fabric of the buildings; to include the interiors would be another lifetime’s work.
The whole talk was illustrated with maps showing the distribution of the dominant building stone with reference to the geological substrate – not surprisingly there is a connection - and also with transport links through time. The inclusion of images of many of the churches added to our enjoyment. Learn more at: www.westsussexgeology.co.uk and go on the Field Trip.
Wiltshire Sarsens and Avebury - with Geoff Downer and Diana Clements
7 July 2013
Our first stop was at Lockeridge Dene, near Avebury, to view Sarsens both on the ground and used as building stone. Sarsens consist of silica grains cemented with silica, and are in consequence very hard and dense.
The first area consisted of a field taken into the care of the National Trust in 1908, following concerns that the natural sarsens were being destroyed by quarrying. The boulders here are up to 1 m across and 0.5 m high and mainly tabular in shape. They are scattered irregularly over the field, mainly a few metres apart. In one place a fresh surface was visible, and was much paler than the smooth grey weathered surfaces of most boulders. The fresh stone consisted of poorly sorted rounded grains, grain-supported with a sugary texture in some places. A few boulders show deep holes on the upper surface up to 1 cm in diameter, which are interpreted as root holes (in which case most of the boulders appear to be right way up).
The sarsens in this area have been exploited as a building material for millennia (including in Windsor Castle in the fourteenth century) but from 1850 they were quarried on an industrial scale, both for road setts and for building. The method used was heating followed by quenching to induce splitting through expansion/ contraction, and wedges were driven in to control the direction of splitting. Many wedges could still be seen in situ: perhaps they broke, or the boulder proved unsuitable. Many – perhaps the majority – of boulders had one or more planar vertical faces where part had been removed in this way.
In the adjacent field, the boulders are larger and more numerous: being further from the road and the village, they are likely to have suffered less from quarrying, though here too some wedge pits and cut faces were seen. The largest boulders here were 2 or 2.5 metres across, also mainly tabular, and in the main no thicker than those at the first locality. This locality is in the bottom of a shallow valley, with one steep and one gentle side, and the sarsens are distributed in a ‘train’ or 'stream' along the valley bottom. Later in the day we discussed the possible reasons for this, concluding that solifluction, or earth creep down the slope during the frequent freeze/thaw episodes in ice ages, may have carried the stones down the slope.
Finally at this locality we looked at sarsens in the walls of houses and especially in a garden wall. This demonstrated the wide range of colours and textures, from pale to dark grey with some pinkish patches. One stone had a particularly high density of root holes.
Just before lunch, we visited the stone circle at Avebury. Geoff Downer took us through the history of the site.
Avebury Henge is now surrounded by a ditch that was dug around 3000 BC. However, the ditch predates the henge: the stones were positioned 4-600 years later. The sarsens probably come from Fyfield Down, which is about 5 miles away. It is not known how they were moved. The stones were buried around 1310 (the date is so precise because some silver coins from that year were found buried with the sarsens). They might have been buried for superstitious reasons. In medieval times, there was a village in the middle of the henge. With the surrounding ditch, this was in effect a fortified village. For hundreds of years, the locals used the sarsens as building stones. This practice was satirised by William Stukeley (1687-1765). It was not until Alexander Keiller began excavating the site in the 1930s that the villagers were stopped from taking stones for their own uses.
During the lunch break, a number of us explored the village of Avebury, and one or two were able to take a quick look at the Alexander Keiller museum, which is well worth the visit. Housed in the Stables and Threshing Barn of the Manor, it is named after, and partly concerned with, a member of a family of successful marmalade manufacturers in Dundee, and sole heir to its vast fortune, who devoted a great deal of money and energy to conserving the antiquities of Avebury between the two world wars. Many of these antiquities are on display.
We then reassembled, and drove in convoy to Clatfield Bottom, a valley below Fyfield Down.
We walked along a country path strewn with sarsens and then through a patch of trampled-down vegetation to an open field. There stood a dolmen, a former passage tomb, or place of temporary burial, made of large sarsens, which probably was originally covered by a mound. This is known locally as the "Devil's Den", and was constructed between 4000 and 3000 BC.
another enclosure with high vegetation and a field beyond. This was the site of former warrens near the well-known Manton racing stables. Continuing up the valley, we came across a train of sarsens, one of which bore a wedge -pit, testifying to the activity of the Free family, who had apparently worked here. They had been carrying on a long tradition of human exploitation of sarsens. Geoff profited from the stop to show us a map which displayed the surmised routes taken by the sarsens used for Stonehenge around 2200 BC.
Geoff then suggested we move up to a shady area under trees on high ground for a general discussion. Geoff said this was an area of high concentration of sarsens, at least 8-10,000 having been found.
Having reviewed some of the speculations of early antiquaries ("Currants in a spinning bun" (Stukeley); sarsens "growing out of the ground" (Pepys)), Geoff turned to the thoughts of modern geologists.
First he invited a discussion of what we had seen during the day. There was consensus that we had been looking at mostly grey, but sometimes red (from iron), very hard sandstones cemented to form silcretes. Fresh rock was notably paler, but with a coating of iron in places. The majority were grain-supported. Some stones included flints, some rounded, some angular. Geoff said that fossils were unusual: none had been found in French sarsens, but a few had been found in the Harwich Formation. There was no graded bedding in the sand, but occasional burrows and root holes. The sand was probably water-lain, there being no trace of dune structures.
The ultimate source of most of the silica cement that had formed the silcretes was probably flints, but some plants were also very siliceous. Groundwater was very likely involved in the transmission of dissolved silica to the sediments. Different types of cement were found in sarsens, in optical continuity with the grains, having been accreted grain by grain. Opaline and chalcedonic cements were often found in one block, the nature of the cement changing from one part to another.
It would seem likely that post-depositional processes were involved. Silica-rich groundwater may have been formed at a pedogenic surface where silica and silicate minerals were weathered in soils under a warm humid climate. With a changing water table, this silica-rich water could then move to another horizon.
In Fontainebleau in France, substantial layers of sarsenstone had been found in situ. In the UK, by contrast, evidence for in situ silcrete formed from sand, as opposed to conglomerate silcretes such as Hertfordshire Pudding Stone, was equivocal, and none had been found in this locality.
There were two main models for the formation of sarsenstone:
In terms of age, the abundance of silica used to form the rock suggested that formation was subsequent upon the erosion of flint from the chalk. most of the in situ silcretes seemed to be associated with the base of the Reading Beds/ top of the Upnor Formation. That would put formation around the Palaeocene- Eocene Thermal Maximum, which would be appropriate in climatic terms.
The silcrete (Hertfordshire Pudding Stone) found in the Upnor Formation at the Pinner Chalk Mine was of this age. However there was evidence of silcretes forming at later times in the Palaeogene. For example there were sarsens at Windsor and Virginia Water in the Bagshot sands which were clearly later.
Geoff passed round a sarsen stone found in Essex and made reference to a video on the formation of sarsenstone in the Fontainebleau area available on the internet at www.sarsen.org/2012/08/how-sarsens-are-formed-video.html.
Geoff concluded by pointing out that the sarsens we had seen here had clearly been moved, initially by uplift and erosion of the cap rock, and then by gravity and solifluction. Sarsen stones were therefore now found mainly in the valleys, though also on high ground. The work of geomorphic, had been completed by anthropogenic, processes, particularly farming and quarrying. These activities had often been carried out in co-operation, as had been suggested earlier.
Following this, we retraced our steps to the cars and returned home.
The Oceanography Experience
25-26 July 2013
Based at the National Oceanography Centre (NOC) in Southampton the Oceanography Experience is a fabulous motivation for current S330 Oceanography students and really brings the textbooks to life. Although the majority of those on the two day course were current or past students there were also those who were there ‘just for fun’ and it definitely was a fun filled couple of days.
The first day was spent out on Southampton Water and day two was back on dry land in the NOC analysing the data that we collected. Southampton Water is an estuary which receives freshwater inputs from three rivers – the Test, the Itchen and the Hamble. We investigated changes along the length of the estuary from relatively fresh river water of the Itchen down to the more saline waters of the Solent.
RV Callista is a purpose built research vessel operated by the University of Southampton and based at NOC. Split into watches we spent a full day on the boat making various measurements and taking lots of samples at positions along the estuary. Each watch rotated around the different tasks so that everyone had the opportunity to assist in the various tasks required including:
Command and Control
The somewhat grand title of ‘command and control’ reflects the importance of this task which involved liaising with the crew of the RV Callista and communicating with those carrying out the different scientific operations. Results from the activities were recorded in the ship's science log together with details such as the position, tidal flow and water depth. Hand held probes were also used to make temperature – salinity profiles at each location and Secchi disks were used to test for light penetration.
Conductivity – temperature – depth (CTD) profiling
At each position a conductivity – temperature – depth (CTD) profile was recorded. This utilised a CTD – rosette water sampling system which was lowered over the stern of the boat. Sensors on the device recorded the conductivity (salinity) and temperature as the CTD was lowered through the water column to a metre or so above the sea bed. Data from the CTD was displayed in real time in the dry lab of the boat. As the CTD was raised back up through the water column the group controlled the depth at which water samples were taken by triggering the closure of sample bottles on the rosette.
Collection of water samples for nutrient analysis
Water samples were collected from the CTD – rosette lab. Samples were filtered to remove any plankton – the filters turned slightly green from the chlorophyll! Bottles were then prepared for the silicon (Si) and phosphate (PO4) analysis which was carried out for us in the NOC labs.
Plankton samples (plankton trawl) and benthic samples (oyster dredge and grab sampler) were also taken. The plankton sample was prepared and taken back to NOC for microscope identification.
Back in the lab we spent time plotting up our sampling positions on charts of the Solent and reviewing the CTD data. Depth profiles along the length of the estuary allowed us to examine mixing between fresh river water and saline water in the Solent. Data from the silicon and phosphate analysis allowed us to consider the behaviour of dissolved nutrients in the estuary. We then had an introductory talk on the biology of Southampton Water, including the exotic inhabitants, before breaking for lunch.
Lunch was taken in the NOC canteen with a great view of the RRS James Cook, a large research vessel which was being prepared for another research expedition. In the afternoon we carried out plankton identification under the microscope and had the opportunity to tour the NOC aquarium.
NOC staff and OUGS volunteers helped to make this a great practical experience of oceanography.
OUGS 41st Symposium —Dublin
9-11 August 2013
This was held in Dublin City University, between the city centre and the airport. Our last Irish symposium was also held there – twelve years ago. The title was “recent developments in Irish geology – all kinds of everything” and it ranged widely, with excellent speakers and lots of new geological developments and knowledge. The symposium programme included illustrations from George Victor du Noyer, who worked for the GSI in its early years and produced superb geological watercolours - Ireland was the first country in the world to complete the national geological survey and produce a geological map!
On Friday Professor Jenny McElwain discussed how studying past environmental and biological catastrophes can provide lessons for the future. She investigated the extinction and recovery rates of plants in the fossil record, during four mass extinction events and rapid changes in climate. Starting from her research on the warming at the Triassic – Jurassic boundary, she showed how the recovery of species when environmental conditions revert to their pre-excursion norms leads to drastically altered ecological compositions and unstable ecosystems for sometimes millions of years.
Saturday started with a talk from Dr Koen Verbruggen, the Director of the Geological Survey of Ireland (GSI). The GSI is a smaller and more nimble version of the BGS, involved in many of collaborative exploration and research projects. Including the search for oil and gas far into the Atlantic, he also highlighted some interesting resources. All GSI data is available free over the web, even more from the ngdc website
Professor Ken Higgs, of University College, Cork discussed New Developments in the investigation of the Valentia Island Tetrapod Trackway. These track ways are very impressive, and exposed by progressive coastal erosion of the shore. They are mid Devonian, as are the recently found Polish footprints – contradicting the fossil evidence for the evolution of the first tetrapods in the upper Devonian. Around them is evidence of the fossil environment – they seem to have been grazing on the shoreline.
Tom Blake, Irish Institute for Advanced Studies discussed the Irish National Seismic Network, part of the Nuclear Test Ban Treaty organisation and what it has learnt about earthquakes in Ireland, and also the seismology for schools project which gives students the chance to monitor and discuss earthquakes monitored in their own schools.
Prof John Walsh, Fault Analysis Group in University College Dublin discussed new analysis into the post Caledonian faulting of onshore Ireland. He showed that much of the post Caledonian faulting is characterised by multiple reactivation of older faults (and not the development of new faults), with significant effects on the older rocks and the flow of fluids within them. This had plenty of good OU structural geology sections.
Dr John Ashton, Chief Mine/Exploration Geologist at Boliden Tara Mines Limited, explained the discovery, geology and operation of the Tara Mine, near Navan. This is the largest lead and zinc mine in Europe, extracting 2.5 Mt pa from sulphide lenses in Lower Carboniferous shallow water carbonates, thus the mine is a series of parallel levels, gradually getting deeper (and further from Navan).
Dr John O’Sullivan, Technical Director, Providence Resources discussed very recent developments off the Irish coast in offshore hydrocarbons, and the challenges (and costs) of today’s exploration industry, and it’s new technology.
Dr Maria McNamara, University College, Cork explained her research into the original colours of fossils. This involved taking recent coloured beetle bodies and decomposing, heating, squeezing, or storing them in water for long periods. The iridescence is structural, and not due to colouration, but this survives, colours generally didn’t, though patterning could be seen from areas of lighter and darker colours. The wavelength, and thus colour change, with temperature. Thus patterning can sometimes be seen, but rarely original colours.
Sunday morning was for field trips;
Then on Sunday afternoon there were two more talks, the first was on the recent Irish-led expedition to the Mid-Atlantic Ridge, by Professor Andy Wheeler of University College, Cork. On this they discovered a previously unknown field of hydrothermal vents – the first to be found between the Azores and Iceland. And, as the discover can propose the name, and the professor’s mother in law gave him a book on Irish myths to read on the boat, it is named Moytirra, after a mythical battlefield “the plain of the pillars”
The last talk was on the Tellus Project, which is mapping the geology and environment of the border counties, by Dr Marie Cowen, GSNI (and separate from the BGS and GSI). The area is mapped from the air, with geophysical and geochemical surveys – this has found regional scale structures and located previously unmapped buried features.
Dr Nick Rogers of the OU gave the closing address, and highlighted that the OUGS is the largest amateur geological society in the British Isles.
On Monday there were two field trips; one to Tara Mines: the largest lead mine in Europe , and the other to Newgrange in the Boyne Valley with magnificent stone tombs and landscapes built in the Neolithic.
This was a very friendly and well run Symposium, with excellent talks and a credit to the Irish Branch of the OUGS.
OUGS Dublin Symposium Field Trip — Killiney
11 August 2013
Dr Patrick Roycroft led our coach party to White Rock, Killiney (near Dalkey Island and just south of Dublin). Once out of the coach it was clear why this area is called Dublin’s Bay of Naples, with Sugar Loaf Mountain taking the part of Vesuvius – and sun shining on the palm trees. We looked at the contact between the 450 Ma Devonian Leinster granite and the Lower Palaeozoic deep sea sediments.
The Leinster Granite was one of the earliest granites to be studied (and led to Richard Kirwin naming igneous rocks in the late nineteenth century) and is the largest batholith in the British Isles. The area is known as White Rock as the light coloured granite contrasts with the darker limestone (at Black Rock, near Dun Laoghaire, for instance). The sediments are Ordovician turbidites, metamorphosed during the closure of the Iapetus Ocean in the Silurian. The granite was intruded, and then uplifted and unroofed in the Devonian and Carboniferous.
We examined the sediments first, and the metamorphosed mudstone and siltstone layers could be identified. The folding and cleavage being caused by the closure of the Iapetus. The granite is interfingered with the schistose sediments, with boudinage and larger quartz veins sweated out. The actual date of granite intrusion is controversial, due to cross cutting aplite veins, as is the speed of crystallisation – it could have occurred relatively quickly. We then went along to an old mine entrance, where lead and barytes were extracted, and then removed by sea.
Dr Roycroft finished with an explanation of current igneous research, which suggests much quicker crystallisation times and investigates the convection cells in the granites, the crystal shape (most spherical in a pure melt), and big crystals not necessarily inferring slow growth or a large igneous body.
Symposium Field Trip — Dublin Institute for Advanced Studies, Geophysics Section
11 August 2013
On the Sunday morning of the symposium weekend, a group of us bussed into central Dublin to visit the Geophysics Section of the School of Cosmic Physics of the Dublin Institute for Advanced Studies (DIAS) at 5 Merrion Square.
DIAS is a postgraduate institution, modelled after an institute at Princeton, and established by Eamon de Valera, while Taoiseach, to further research in two areas of study in which he was keenly interested, Theoretical Physics and Celtic Studies. It is thought that the institute was also intended to provide a post for Erwin Schrödinger, to encourage him to come to Dublin, and to provide an academic bridge for contacts between Trinity College Dublin and University College Dublin, which at that time were deeply divided by denominational and political allegiances.
The Geophysics Section is housed in one of the houses which front on Merrion Square, one of the finest surviving Georgian squares in Dublin, which was first laid out in the 1760s, and largely completed by the early 19th century. We were welcomed at No 5 by Dr Tom Blake, who had lectured us the day before on the Irish Seismic Network, which is managed from there.
Before showing us around the house, he took us across to the gardens of the square to see the statue of Oscar Wilde, which is of significant geological interest. The gardens were leased in the 1930s by the freeholders, the Pembroke Estate, to the RC Archdiocese of Dublin, to provide a site for a new Cathedral to replace the Pro-Cathedral in Marlborough Street. (There are already two Cathedrals in Dublin, but both are Anglican.) However the then RC Archbishop of Dublin, Dr McQuaid, decided that the funds collected for the new cathedral were better spent on new churches and schools in the then burgeoning working class districts in the Dublin suburbs. The gardens were subsequently transferred to Dublin Corporation for use as a public park. They were initially named after Archbishop Ryan, who transferred the land to the city, but were renamed Merrion Square Park, following public criticism of the former Archbishop for failure to take adequate action against child abuse by some members of the clergy of the archdiocese.
The statue to Oscar Wilde was erected in the gardens because he used to live at No 1 Merrion Square, which belonged to his father, an eminent surgeon. It is flanked by two pillars with quotations which look like, but are not, graffiti, illustrating his thoughts on art and life. The statue, which was sculpted by Danny Osborne, is apparently known locally as “The Quare on the Square”. It is set on a 35- tonne boulder of quartzite weathered out of the Leinster Granite in the Wicklow Hills. It is carved out of exotic rocks to display Oscar Wilde in suitably flamboyant costume.
He wears a smoking jacket of green nephrite from northern British Columbia, with quilted collar and cuffs of pink thulite from western Norway. His trousers, of larvikite, are also from Norway. His shoes (equipped with bronze laces), and socks, are carved in charnockite from India. Head and hands are in white jade from Guatemala (apparently replacing porcelain). He even wears a TCD tie, made in glazed porcelain!
Having seen the statue we left the gardens, crossed the road back to No 5, and were shown round the house. In the hallway we were shown the basic seismological equipment used for the School Seismology Project, run with the University of Middlesex, which had been one of the topics of Tom Blake’s lecture the previous day. We saw a poster on the experimental work carried out on Killiney Beach by Robert Mallet, the Irish scientist and businessman credited with being the founder of seismology. There was a brief discussion of the subsequent work at Potsdam of Ernst von Rebeur-Paschwitz, the German astronomer who drew the connection between the mysterious disturbance of a pendulum set up to measure inter-planetary gravitational attraction, and an earthquake in Japan, and the work of the English mining engineer John Milne, who developed the global seismological monitoring network which Rebeur-Paschwitz had envisaged.
We also saw a poster on the work of the Geophysics section on seismic hazards and risks. We were taken up to the particularly fine Georgian mezzanine room, with a skylight ceiling, which is used by the Geophysics Section for coffee breaks, and then taken round a number of the rooms used for the work of the Section.
The garden of the house is largely occupied by the Irish National Data Centre for the Comprehensive Nuclear Test Ban Treaty Organisation (CNTBTO). There is a plaque commemorating the former Jesuit observatory at Rathfarnham Castle, Co. Dublin, which was equipped with a seismograph and for much of the early twentieth century was the principal source of earthquake information for the national media in Ireland.
In the Data Centre we were shown a computer display of international geoseimic recordings of earthquakes powered by Seiscomp3, a package of software devised after the Banda Aceh earthquake. We were given a demonstration of the different seismic signatures of a natural earthquake and a nuclear explosion, and shown a video illustrating the work of the CNTBTO inspection teams. Those who were interested were also provided with a copy of the treaty and further information on the work of the Organisation.
We then adjourned for coffee near Trinity College before being picked up by the coach and taken back to the Symposium.
Symposium Field Trip — Tara Mines
12 August 2013
Tara Mines are located on the edge of the town of Navan, north of Dublin and currently owned by New Boliden (a Swedish company). The ore body was initially identified from lead and zinc anomalies in stream sediments, then Tara Mines were granted prospecting licences in 1969. They located a mineralised outcrop, and drilled boreholes from 1970. Production commenced in 1977, and the mine extracts up to 2.77 Mt per year - this is the largest base metal ore field yet found in Ireland. Two other deposits have recently been mined. The mine is on the outskirts of the town of Navan, but the environmental precautions mean even some locals don’t realise it is there.
The ore is in Lower Carboniferous Limestone reefs, which were in a shallow water carbonate succession, and then eroded and then buried by the Upper Dark Limestone. The stack of seven ore lenses in the reefs dip at 15-20° to the south west (see photograph of model) as a series of “steps”.
The ore is blasted on the working faces at the end of each shift, after the mine is cleared of personnel, and then the faces checked at the start of the next shift, then loading shovels put the ore into dumpers and are driven underground to the base of the conveyor for the journey to daylight.
Our contact was John Leahy, former Branch Organiser for the Irish Branch of the OUGS, who worked in the mine several years ago. Once at the mine, and equipped with full protective equipment (see photograph) and having received their safety induction, we were given an introduction to the mine and its geology, and then taken on a tour of the below ground mine and above ground mineral processing works. We were led underground by Jim Gerahty, the mine’s chief geologist (and an OU Geosciences graduate), who also explained the geology.
We were taken underground in their minibus, down the spiral ramp, and allowed out to see the faces where it was safe. There are many large machines underground, including 50t dumpers, and safety on the roadways is critical. It was noticeably much hotter at depth (and wet in places), and as the rock is shotcreted to provide structural support after excavation, and then the excavation adits filled with waste (only ore is taken to the surface), we had to follow the new excavations, which are dispersed over the mine. This is because the ore body is drilled, and analysed, so that the specific areas with the highest mineral content can be targeted. Mineral mining is all about the selling price on the metal exchanges, compared to the cost of production.
After lunch in their excellent site canteen, with the mine manager, we were taken around the above ground processing works, and to the end result, the sidings where ore is loaded onto trains to be taken to ships in Dublin and on to Sweden for processing – and then onward sale to China.
It is a very slick and impressive operation – and everyone was both very friendly and professional.
Hog's Back Geowalk - with Iain Fletcher
8 September 2013
Brian Harvey devised and led a number of Geowalks for London Branch over many years. These were always popular, attracting families as well as the regular LOUGS members. Iain Fletcher has begun to write up these walks with a view to Web publication.
The first to come to fruition, the Hog’s Back route west of Guildford, was trialled by a small number of LOUGS members on 7 July and, apart from the rain, worked beautifully. Iain has managed to retain the flavour of Brian’s 2002 Hog’s Back geowalk whilst adding a few geological gems of his own. As he has written up all the details and illustrated them with his own photographs there is little point in duplicating them here. Instead, please look at the LOUGS website where we will soon have a new page dedicated to Geowalks and Building Stones in the London area: www.lougs.org.uk/localgeo/geowalks.htm. It will have a link to the pdf of this Hog’s Back Geowalk, and to other walks in the south-east. It is intended that these write-ups will be used for self-led walks.
To whet your appetite, the photographs on this page were taken during the LOUGS walk on 8 September, which was really excellent despite the few heavy showers of rain that began in the early afternoon. It is a circular country walk of seven miles from central Guildford westwards to Compton and back. As there are few accessible geological exposures, Iain explored the relationships between geology, topography, land use and building stones. The route was mostly on footpaths and pavements, including a section along the river, and mostly of gentle gradient except for the Chalk slopes up and down the Hog's Back ridge, which tested us right at the end of the walk. Many thanks to Brian for devising this walk and to Iain for resurrecting it for a new generation of LOUGS members.
Lecture : Did the Sea Taste Salty to Dinosaurs? - By Chris Pearce
19 September 2013
Dr Christopher Pearce, Research Fellow at the University of Southampton and the National Oceanography Centre, proposed to discuss what controls the composition of sea- water, and how that knowledge can be used to reconstruct past environments. For those of us who had studied S330 Oceanography, (in particular Ocean Chemistry and Deep-Sea Sediments) along the way the lecture included much that was familiar (though in my case more than ten years old!), including inputs and outputs, residence time, thermohaline circulation, mixing time and so on.
Within the great mixing pot of the oceans a kilo of water contains 34.4 g salt, made up of many different salts of which sodium and chloride are the most abundant. Balancing the rate of supply through rivers, groundwater, hydro-thermal pore waters from the mid-ocean ridge system, precipitation etc. against the rate of exit can help to determine past climate, since some of these ratios are measures of oxygenation or carbonate deposition, for example.
Chris’s research is concerned with isotopes such as carbon, strontium as a tracer of marine inputs, neodymium and REE. Carbonates record climatic variation through time, as for instance increased weathering as a result of the Himalayan orogeny.
There is however an imbalance in strontium (Sr): the riverine flux is more radiogenic than the hydrothermal flux and is currently higher: the question is why? Could it be glacial melting? The clue is to look at marine particulates since particulate transport dominates the flux of most metals in the Ocean. Meanwhile, experiments on basalt dissolution are important for showing the influence of Sr and Nd on fertility and productivity. Organic carbon burial itself is associated with black shales; thus at the Palaeocene – Eocene Thermal Maximum, where there are no black shales, there is an abrupt loss of deep-sea carbonate. The chemical composition of marine sediments in general is affected by climatic events, and is correspondingly reflected in changes in levels of ocean oxygenation.
This was a talk based on painstaking ongoing research; thus many questions remain. Asked for the answer to the question posed in the title, Chris’s response: ‘Yes, but we don’t know how salty!’
Church Geology in West Sussex - with David Bone
29 September 2013
David has spent many years investigating and researching the types and sources of the building stones used to build the many churches in West Sussex and for this trip he had chosen 4 of these churches which would illustrate the great variety of building stones and also demonstrate the reuse of stone. We would mainly be looking at the building stones from the outside of the churches.
We met on the morning of Sunday 29 September in Priory Park, Chichester, in the north-east corner of the city and within the Roman walls. This part of the city has not been developed and historic remnants have been found within the park, including a medieval tower, WWII underground storage tanks, a Victorian folly, and a mound which is the remains of a Norman motte & bailey.
The first of the churches we had come to view was Greyfriars (the Guildhall) which stands within Priory Park. This is the chancel of a 13th century Franciscan church, which was completed around 1282. The building was used by the City Corporation after the dissolution and is now part of the district museum service and has also served as a magistrates court. The building illustrates well the variety of stones used in its construction. There is no good building stone available in Sussex and therefore a variety has been brought from far and wide, possibly more so for building in this area than anywhere else in the country. David gave us a list of all the stones used in each of the 4 buildings we would be viewing today, and he has identified 22 types used to construct Greyfriars.
In early medieval times and particularly following the Norman conquest, there was a lot of church building. The Normans knew Caen stone well and therefore this was used extensively. It is an easily-identifiable good quality yellow stone which was used around doorways and supports in 3 of the churches visited today. Lavant stone from nearby in the South Downs and Bath stone were also used extensively. David gave us much detailed information about the origins and content of many of the cobbles and concretions making up the rubble infill of the walls, and it soon became obvious that the sources varied from re-used stones, erratics and ballast to local flints. There was much discussion about the geological features and provenance of the stones used in the construction of Greyfriars, and to conclude this first building, he set us the task of finding certain stone varieties in the walls.
Following a lunch stop in Chichester, we continued to St Peter’s at Westhampnett. The origins of this church are pre-Norman but it has been mostly reconstructed, with a tower added in the late 12th century, further extension in the 13th century and later restoration. Here 20 stones have been identified in construction and rubble infill – including Caen and Bath, but also noticeable use of Lavant and Quarr. The most remarkable features here are flint galetting (using flaked flints pressed into mortar) and re-used Roman bricks enclosed in the chancel walls, where there were 2 very characteristic Saxon windows. Put-log holes (where scaffolds were originally placed) were also noticed high in the tower walls.
Our next stop was the church of St Mary Magdalene, Lyminster. The church dates from about 1040 AD, with many additions and much rebuilding through the succeeding years. Its steeply pitched roof indicates that it was originally most likely to have been thatched. David had arranged for the church to be opened for us to see some of the remarkable interior features – including the gravestone of the dragon slayer made of Hythe stone, the 12th century font of Sussex marble, the high narrow arches characteristic of Saxon architecture, and medieval stained glass windows. Again, David has identified 20 different stones used in the construction of the church, including again Bath and Caen, but drew out attention also to Pulborough stone used on the corners of the building and Wealden sandstone around the porch and windows. An interesting geological feature can be found a short distance from the church (although we did not have the time to visit this and it is on private land) – an ancient ‘knucker’ hole (the home of a legendary water monster), said to be a bottomless pond, but in fact a sheer-sided doline or solution hole.
The fourth and final visit of the day was to St Botolph’s, Hardham. This is a small 2-cell 11th century church with 13th & 14th century windows. The church is especially important for its very remarkable early 12th century wall paintings and we were able to go inside to view these to their best advantage as there was no bright sunlight to obscure them! Unusually, the church is whitewashed externally, as was almost universal in medieval times. David has identified 6 stone types used in the construction of the church. Re-used Roman bricks and tiles in the rubble of the walls of the church remind us that Hardham is close to the line of Stane Street, and several Roman sites have been found in the vicinity. Again, Bath stone is prominent as dressed stone, as is Pulborough and Portland.
All four churches chosen for this transect of West Sussex were remarkable for their very varied building stones and also for their historical and architectural interest.
Photographs by John Lonergan
Lecture : The Geology of Pigments: Turning landscapes into colour - by Ruth Sidall
17 October 2013
Dr Ruth Siddall of the Department of Earth Sciences, UCL, and Associate Fellow of the Wiener Laboratory, American School, Athens, is a geoarchaeologist with a special interest in archaeological pigments and painting techniques, who together with colleagues from the Slade School of Fine Arts is a member of the Pigmentum Project for research into the chemistry of colour.
Throughout the talk she showed us slides of pigments under the microscope and stressed the need for a good polarising microscope and ideally UV fluorescence. She started with classifying pigments into minerals, plant and animal dyes, and synthetic compounds. After a brief discussion of organic dyes such as madder, she concentrated on those derived directly from minerals, from the very expensive such as Orpiment and Lazurite to the cheapest, the Ochres.
By way of demonstration, she took five British pigments which the Project has investigated: Salt Green and Haematite from Geevor Mine in Cornwall; Bideford Black from Greencliff, Bideford, Devon; Chalk White from Oxted, Surrey; and Old Meadows Ochre from Bacup, Lancashire.
We were shown in turn the landscapes from which the colours were extracted. Salt Green is from the copper ores leached out by water; Haematite or iron oxide has accumulated on the spoil tip. Greencliff, Bideford is part of the coal measure. Chalk White, very distinctive under the microscope, was much used in the Low Countries during the Renaissance. Finally the Ochres are the product of acid mine drainage, where the ferrihydrite settles out, staining the water bright orange.
Once these pigments have been obtained they have to be ground, using a muller, a sort of upside-down glass mushroom, a long, laborious job. Finally the medium must be added to make a paint: water, egg-yolk, beeswax, oil.
The results can be seen at the website: turninglandscape.com where the exhibition earlier this year at UCL is documented.
This was a fascinating talk, followed by a lively question and answer session.
Mr Kipling's Exceedingly Good Rocks - with Graham Williams
19 October 2013
On a rather overcast Saturday morning a group of us assembled at the car park at Harrison’s Rocks, a little south of the village of Groombridge, near Crowborough, East Sussex. We were there to view, under the guidance of Graham Williams, outcrops of the Ardingly Sandstone at Harrison’s Rocks and at the nearby Eridge Rocks.
The Ardingly Sandstone constitutes an horizon of the Lower Tunbridge Wells Sand Member of the Tunbridge Wells Sand Formation and is of Valanginian age i.e. about 135 million years old 1. The connection with the 1960s advertising slogan for a well-known brand of factory-made cakes was that Graham’s original plan was to follow the inspection of the rocks in situ by a visit to Bateman’s. This is a Jacobean house near Burwash, partly built of Ardingly Sandstone, which was bought by Rudyard Kipling in 1902, and was his home for many years, but is now owned by the National Trust.
The plan was to examine the use of the rock as building stone there, and then sample rather more up-market cakes than the factory variety, over tea at the National Trust café. Having spent a fair amount of time at the second location, however, we decided that Bateman’s was a little too far, and substituted a visit to Bowles Rocks, a third outcrop in the Crowborough area.
The outcrops take the form of high crags, the faces having a crust which has resisted weathering, and are popular with rock climbers. Indeed Harrison’s Rocks are now owned and managed by the British Mountaineering Council, and having left the car park, we passed a large group of climbers come to practice their art.
We moved away from them to the southern end of the exposure. There Graham divided us into groups and dispatched us to examine the outcrop separately, with the aid of hand lens and grain-size chart. After a while we reassembled to discuss our findings. We were agreed that we had been looking at a fine to medium grained quartz sandstone, with possible very faint traces of plant remains, but no evidence of other fossils, and no traces of mudrocks in the crags. The bedding appeared to change from laminated units lower down, through trough cross- bedding, to more massive and somewhat coarser units further up. In a few places some of the cross-bedding seemed to exhibit a herringbone pattern.
The consensus was that we were looking at a non-marine water-borne environment, a braided river system, immature, but carrying mature sediments, and prograding through time. (There was evidence of clays on top of the outcrop). While the occasional signs of “herringbone cross-stratification” might indicate tidal conditions, the consensus was that it was more likely to represent the switching of braided river channels. Graham pointed out that there was a modern analogue in the braided channels of the Brahmaputra. The climate of deposition appeared to be Mediterranean, with hot dry summers and warm wet winters.
There was rather less clarity of view regarding the origins of the present topography, and recent geomorphology. Some of us speculated that the crag cliffs might have been created by faulting, and subsequent erosion in a periglacial environment. At the two following localities where the faces were less coated in algae, it became clearer that the sandstone was in fact very poorly cemented and only protected from weathering and erosion by an outer crust. This was consistent with notices on the site which had been put up by the rock-climbing interests, on the need to take care against damaging the faces by ropes etc.
A curious feature of the faces was honeycomb weathering, generally associated with calcite cementing. Possibly the crag faces had at some stage been locally cemented by calcite infiltrating down in solution from previously overlying chalk, and this may have contributed to their preservation, whereas other parts of the deposits had been eroded.
We then moved on by car to Eridge Rocks. There the cleaner rock faces gave a clearer view of variation in the grain sizes and the transition from a mainly laminated lower division, through sandwave structures, to a more massive upper division. At some levels, layers of coarser grains stood proud of finer-grained layers. In one place, the erosion of rock faces to form an amphitheatre also gave a better three-dimensional picture of the sandwave structures.
The third locality we visited, Bowles Rocks, is owned by an educational trust, which manages it as an outdoor learning centre. The rocks here also clearly showed a succession from laminated through cross bedded to more massive units, which may have been cyclical. In many places, however, the sands seemed coarser. In places where the rock was more friable, no doubt in consequence of climbing activities, we were able to remove grains and examine them under the lens. As well as the quartz grains, there were some smaller and darker grains of iron-containing minerals. Graham pointed out that these grains, though much smaller than the quartz grains, were also denser, and in consequence would have a similar settling-out behaviour in a given energy environment.
We noticed that some layers of the face displayed iron-stained laminae: apparently these were similar to those found in building stones used at Bateman’s.
We also saw at Bowles Rocks:
Graham concluded our visit with general remarks on the importance of making observations and measurements first, then attempting to put the evidence in a three-dimensional context, and only then attempting to apply models. He said he had followed that procedure himself at these very localities some years previously, before reading an article in the PGA by a distinguished sedimentologist by the name of Allen. He had then been gratified to discover that they had both independently come to rather similar conclusions.
After thanking Graham for a most interesting and instructive day, we then made our separate ways home.
Photographs all © Diana Clements
20 October 2013
The forecast was not good. Heavy rain showers, not what you would hope for when spending time outside hoping to find inspiration from the landscape and the wonderful array of sedimentary bedding, rocks and even fossils on display around the Herne Bay and Reculver County Park coastline.
So upon arrival in the little car park above the beach we met with members of the London Branch of the OUGS and set about signing in for what we hoped would be a fascinating day. Suffice to say it started to rain just as everyone arrived and signing your name on an increasingly wet piece of paper became the first challenge of the day!
Miraculously, once we made our way down to the beach below armed with some handy notes and diagrams we were given, the sky began to clear and so our introduction to the geology of the region was a dry one. In our first task, everyone was encouraged to look around them and find as many different items from the beach as possible. Within a few moments a long line of differently coloured and textured rock fragments, sea shells, seaweed and the ubiquitous plastics were lined up on the sea wall (Figure 1).
This was a clear demonstration that the area has a varied history geologically, dating back many millions of years since all the layers of sediment that had been eroded from the sediments nearby left their mark.
At this point we split into small groups to look at some local features more closely. The coastline is subject to fairly rapid erosion due to the soft sandstones and mudstones that make up a large part of the cliffs here (Figure 2).
To help slow this process, large pieces of rock from elsewhere have been piled up below the cliffs by the local authorities. Many rocks were of a crystalline nature (probably originally granites) exhibiting areas of metamorphism. There was some very nice alignment of the crystals in the rock demonstrating this metamorphism occurred in a high grade deep environment (Figure 3).
There was another rock used, a local sandstone, with numerous fossil sea shells throughout its layered texture (Figure 4).
We then moved to a stream cut valley leading to the beach. The stream had cut through sedimentary layers to reveal a very clear history of the site. It demonstrated that the area has been subjected to a range of climates and environmental conditions (Figure 5).
We were able to get close to the cliff to examine each layer in turn providing practice at recognising the possible environments in which they formed.
Lunchtime arrived and we all drove a couple of miles along the coast to a site known as Reculver to eat our sandwiches. This is a place where the Romans chose to build a fort. When constructed, it was some distance inland and was beside a wide channel that split the Isle of Thanet from the mainland. Since then the channel has completely silted up and the Isle is now unrecognisable as such.
We took a walk around what remains of the fort today. Much of the fort stonework has been robbed out to construct some local buildings which include the large church on the site. Sadly this fine building was subsequently almost entirely demolished and rebuilt by the Victorians further inland. Luckily for us two towers and some walls remain. Removal was considered essential at the time as the rate of cliff erosion meant that it would soon have succumbed to the sea. However today substantial sea defences have been built below the cliffs and erosion has slowed. This has left the site as a prominent point with rapid erosion continuing elsewhere.
We were able to examine the stone used to build the church in some detail. This revealed an array of rock types including oolitic limestones, flints, banded sandstones and even some finer cut stone brought from Marquise near Boulogne. (Figures 6 and 7)
Lastly we headed back to the beach looking at some more rock deposited for sea defence, showing an ancient sea bed revealed in a section of limestone (Figure 8) with another showing substantial crystalline deposits (Figure 9).
One deposited rock has a very large dinosaur footprint which was a surprise to see on the beach (Figure 10).
We continued to the beach to view the cliffs from below (Figure 11). Large parts of the cliff had collapsed recently showing just how easily the coastline is eroding (Figure 12).
Numerous shells were visible, often in bands in amongst the sandy sediment, which were as soft as paper in some cases so almost impossible to free from the sediment without their breaking up but one or two came out virtually complete (Figure 13).
So the day came to an end and miraculously the rain had stayed away, well almost! The sky opened as we headed back to our cars. So we were lucky to have just escaped a good soaking having had an enjoyable day led by some very experienced geologists.
Suffolk - with Roger Dixon
25-27 October 2013
Based in Woodbridge and led by Roger Dixon of GeoSuffolk, the aim of the weekend was to study the Red Crag and Coralline Crag, both in situ and as used in local buildings. The weekend officially started with a tour of Woodbridge, led by Roger.
After dinner Roger, accompanied by his wife Rosemary introduced the business of the weekend with a presentation of the geological development of the area, together with some of the buildings we would see.
For the first day we were located in the southern part of the exposures from Ufford Church to Orford Castle with many interesting sites in between; the second was spent in and around Dunwich. On both days Bob Markham was a mine of information.
With a storm threatening, some of the group left a little early, but without exception this was considered to be an informative and excellently led investigation of the Crag formations, taking us to sites we would not have known existed. Many thanks to Roger and Bob.
Day 1—Friday 25 October
Having settled into the Ufford Park Hotel, we drove down into Woodbridge to park by the station at the foot of the hill, just across the railway line from the quay, before climbing up to the Bull on Market Hill, the triangular square (!) in the middle of this very old town on the old A12 (as the Thoroughfare, this is now the main shopping street). The hill was fortified by the pagan Saxons of the Wuffing dynasty (sacred animal clearly the dog), whose kings were buried at Sutton Hoo across the river. Come Christianity, St Mary’s church on Market Hill dates from the tenth century, the present building from the fifteenth, by which time the town was an important river port, trading in animals, cloth, salt and flax, building ships and making their tackle.
Our first stop, though, was across the road from the Bull, for a peek through the gates at The Abbey, a brick mansion built in the grounds of the old Priory by Thomas Seckford, Queen Elizabeth’s Keeper of the Great Seal, a vastly wealthy lawyer whose money endowed the present Woodbridge School, and built all sorts of things we passed on the way. The first was the Shire Hall up from the Bull, originally housing a court house on the first floor and a market below. St Mary’s church just beyond, containing Seckford’s tomb, was remarkable for the flushwork of the porch, a sort of marquetry in stone picking out the initials of Christ and the Virgin. The church likewise has the grave of James Pulham, the inventor of Pulhamite, an artificial stone for modelling, rock gardens etc., made by his company down to WWII.
We passed odd little specimens along the way – a wreath, the head of a water pump. Opposite the church is a plaque to Edward Fitzgerald, he of the Rubaiyat, who with Tennyson, Thackeray et al. used to meet at the Bull, a fine old coaching inn, as the Woodbridge Wits. Leaving Market Hill for Seckford Street, we passed the great man’s dispensary, and further down turned short of his hospital and almshouses to go over the crest of the hill to the Angel, a pub once apparently of ill-repute, whose habitués may well have ended up with sundry other malefactors in the House of Correction along the street. This building of the Napoleonic era is somewhat incongruously matched with a theatre for the 5000 or so troops garrisoned at Woodbridge during the Napoleonic wars.
A plunge down the cobbles of Angel Street took us then to Chapel Street, on the line of the Saxon ditch and bank halfway down the hillside to a stream. The name comes from the Old Chapel, a handsome eighteenth-century Gothic building which has just been converted into a rather splendid house. Back up to Market Hill and down New Street (new in a manner of speaking, since it was built in the fifteenth century for access to the quays from the market place), we were into the timbered buildings of the fifteenth-sixteenth century, from the old grammar school up the hill to the Bell and Steelyard, an inn from the time of Henry VIII complete with a projecting steel beam, a steelyard, for the weighing of cartloads up and down from the market place.
A tortuous route took us round and up through Doric Place, a Regency housing development named from the Freemasons’ Doric Lodge, before we went back down to the river, its quays cut off by the railway which put an end to the business of the port after 1859. No sign now in the evening sunlight of the barges which had taken grain and flour to London in exchange for coal; instead, from the footbridge across the railway, we looked out over the wide estuary of the Deben to the woods which concealed Sutton Hoo on the far bank, and upstream towards the eighteenth-century Tide Mill, now restored and occasionally put to work.
Lime Kiln Quay survives only as a reminder of the Lockwoods’ Portland cement industry in the early nineteenth century, out of which came Pulham and his Pulhamite. Alas, the imposing cement Castle which was William Lockwood’s mansion had to be pulled down when cracks caused the timber frame to rot. But given that the Abbey has replaced the mediaeval Priory, that seems the only notable loss to this historic town whose past is most certainly still present.
Day 2—Saturday 26 October
Saturday started with a visit to the church at Ufford (Figure A), only a short distance from the hotel.
The weather was mild but beginning to cloud over as we gathered at the village stocks just outside the 12th Century church. Roger took us to a section of the church where red-brown cobbles were arranged in a herringbone pattern characteristic of Saxon style (Figure B). These were of Red Crag which is not a common building material, but has survived here among the flint, sandstone and septaria from later reconstructions. We looked inside and admired some of the other features of this old church.
Moving on to our second locality, a pit in Rendlesham Forest (Figure C), Roger introduced Bob Markham who would be accompanying us and was particularly strong on the fauna of the Coralline and Red Crag.
The pit here revealed a spectacular sand wave with obvious cross bedding (Figure D).
The sand is actually mainly of pieces of broken up shells much less than 1 mm size, with occasional larger pieces of gravel and pebbles. The grains are in contact with very little filling matrix or cementing and would be porous to water flow. This is the Red Crag, a marine deposit of the early Pleistocene (1.8 Ma to 1.5 Ma). This pit had two clearly worked faces more or less at right angles, which allowed the structures to be visualised in three dimensions. There was fine horizontal bedding above and below the sand wave, and a lot of the material had crumbled to sand to form a heap at the bottom. U-shaped burrows could be seen at the base of some units, and the bottom of each wave was marked by a distinct thin bed of coarser material.
Roger explained how the waves formed as the result of currents at the bottom of the sea, and how these currents could be inferred from a close examination and measurement of the variation in dip. From many years of study at this site, Roger had deduced a water depth of about 20 m and flow speed of about 0.5 ms-1 towards the South West, but this was variable with small counter-currents.
Returning to the cars, Bob pointed out the location where a famous UFO incident had taken place in 1980, when American servicemen from the nearby Woodbridge air base claimed to have seen an alien craft which landed in the forest here. Natural explanations (some of them geological) exist for most of the observations reported and no hard evidence remains.
Our third locality, Neutral Farm at Butley (Figure E), was difficult to get to, after forcing a way through tall nettles and past strange fungus growths. This is the type section of Harmer’s Butleyan stage of the Red Crag, but, although it had been cleaned up in 2005 by Natural England, was found to be so overgrown that it was difficult to see the face and only the most hardy managed to get close up to it. Roger did well to explain the two distinct units at this site. The lower unit is a sand wave characteristic of high intensity but intermittent, possibly tidal, currents in a water depth of 15 - 20 m. Grain sizes are typical for coarse to medium sands with sorting by shell avalanche and winnowing.
There is an angular unconformity between the upper and lower units, suggesting erosion before the upper sand wave was deposited over the lower one. Here, trough bedding and small ripples suggest shallower water with wave action and reverse flow in the vortex downstream of the sand wave. The shallow water fauna assemblage and sequence at this site characterises the Red Crag.
Behind the church at Chillesford is a large pit which was most recently worked and deepened for use at the US Air Base at RAF Bentwaters in 1968. The section exposed has the silty, laminated Chillesford Clay at the top overlying a silty brown sand with localised shell beds (Chillesford Crag) beneath which is a thick unit of cross-bedded sand and broken-up shell pieces. This bed contains the bivalve fossil Scrobicularia, so is called the Scrobicularia Crag. These units are all in the Norwich Crag Group and lie unconformably on top of the Red Crag. After the two leaders had explained the sequence, Roger intended to dig down to the Red Crag at the base of the pit which had been excavated in 1968 but there would have been quite a lot of digging involved and it would have put us late on the afternoon’s schedule.
The church at Chillesford is a rare example of Red Crag being used as a building stone, together with septarian nodules from the underlying London Clay. The Red Crag is not particularly durable and weathers to a warm creamy brown and repairs have been made using Coralline Crag, recognisable from the Bryozoans occasionally present.
Our final stop of the morning was a clay pit only a couple of hundred metres from the church, but we used the cars to get there. Here, the 1.7 m of Chillesford Clay is overlaid unconformably by a similar thickness of Boulder Clay from the Anglian glaciation. This clay was deposited in a lagoonal environment in the proto-Thames estuary and has been exploited for local brickmaking. The pit itself is long and shallow unlike those we had seen earlier. Once again Roger had brought his spade in order to dig down to the Chillesford Crag beneath the clay. Jim took over the digging (Figure F) and had soon revealed several examples of the large bivalve Mya truncata, in the red loose crag.
These were very fragile and one just fell to pieces while resting on Michael’s hand - fortunately after it had been photographed (Figure G).
It was then time for lunch, and while some drove into Orford to find a cafe or pub, the rest of us moved on to the church at Sudbourne, which was close to the first visit of the afternoon, to have our sandwiches.
After lunch we went to Crag Farm in Sudbourne, and there we were welcomed by an amazing sight: walls made of in situ Coralline Crag. The farm’s cattle pens, barns and stackyards were made by removing a vast amount of bedrock, the remaining rock functioning as walls (Figure H).
There is no evidence that the cows had any interest in geology but, if they did, they could not have been better placed. Roger explained how the Coralline Crag was formed during the Pliocene (c. 4.2—3.6 Ma). Crescentic sand waves, similar to Barchan dunes formed by wind, formed on a ridge of carbonate sands in the North Sea Basin. The sand waves migrated SSW under the influence of the current. The shifting substrate made it a difficult area for fauna. However, some species, particularly bryozoans, were able to form communities during calm periods. These were then swept away when the current strengthened, and their remains were deposited locally.
We then walked down to Crag Farm pit, which is about 150 m and 4 m deep. This is probably the largest remaining Coralline Crag pit. The farmer dug it out himself, and the rock faces have vertical grooves made by the digger. The rock excavated has been used for restoration work at Chillesford Church and Greyfriars Monastery, Dunwich. When in situ, the rock crumbles and seems to be too weak for building. However, after being cut out, the blocks are left to dry for about three days. This significantly increases their strength, making them suitable for use in building and renovation. We spent some time looking around the pit at the bedding planes, sedimentary structures, joints and fossil bryozoans (Figure I).
We then moved on to Richmond Farm, where there is another exposure of Coralline Crag, rich in sedimentary structures. The four walls of the pit allowed us to see the large-scale ripples in three dimensions. Fossils here include bryozoans and echinoids.
At Orford we walked down to the River Ore. In the distance, on Orford Ness, we could see some pagodas. These were used to test nuclear detonators during the Cold War. Orford has a long history of breakthroughs in military technology: radar was developed here around 1935.
Orford Ness is a shingle spit, one of only three in Britain (the others are Dungeness, Kent and Chesil Beach, Dorset). The spit has longitudinal ridges, which are the result of storm deposition over a long period of time. This is an ideal habitat for a wide range of plants, and more than 40 species can be found on the 11 mile spit. Astonishingly, nearly 15% of Earth’s vegetated shingle is found here.
We walked through the village to St Bartholomew’s Church (Figure J), where septarian nodules taken from London Clay have been used in the walls.
From there we went on to Orford Castle (Figure K), which was commissioned by Henry II in the 12th Century.
The building stones used on the outer walls include septarian nodules, Caen Stone and Barnack Stone (Figure L). Inside, Coralline Crag has been used to make the arches.
Day 3—Sunday 27 October
We started at Dunwich, “Suffolk’s Lost City”, built upon the Norwich Crag, deposited 1.6 Ma, just before the Ice Age (Figure M).
The city was founded by the Saxons as a port, and flourished as one of England’s top ten towns in Norman times. Unfortunately coastal erosion from strong north easterly gales and storms washed away the soft sediments the city was built upon, and this, combined with southward long shore drift forming a gravel spit across the harbour after 1350, led to it fading away.
We started at the church, built in 1850 (and “gothicked” in 1937), to replace the last city church which was abandoned in 1870. The last service in the old church was in 1855, but a buttress was brought from there and set up in the new churchyard. St Felix came to Dunwich in 625 AD, and it had a strong religious community from then, leading to the founding of several religious communities. Adjacent to the new church is the remains of the leper chapel, which contains Sarsen stone, not normally found north of Woodbridge. The south wall of the church has more than twenty types of stone (including basalt, granite, limestone, quartz, flint, septaria, brick and gneiss) (Figure N). Some from ships ballast from trading ships from Scandinavia, Northern Europe and beyond, and some from recycled local materials.
We moved on to the Greyfriars Priory, again this was moved inland away from the erosion. There is on large building left, with a long boundary wall (Figure O) and the remains of a gatehouse. This includes Roman brick, and chalk on the internal areas. There is also Melbourne Rock from the chalk, mainly from west Suffolk. In 2013 English Heritage funded the rebuilding of part of the boundary wall, using original wall and newly dug Crag rock from Crag Farm, seen yesterday in situ.
From here we walked to the site of Middlegate Street, a leafy and muddy sunken lane, but a reminder that in 1200 AD there were 800 houses and 5,000 people in the city. It now ends in a steep cliff – the erosion averaging a metre a year over the last millennium. In the storms of the thirteenth century hundreds of yards were eroded in fifty years, large quantities of shingle moving south and extending Orford Ness. Over the last sixty years the erosion has reduced, protected from storm waves by a broad shingle beach and the offshore Dunwich Bar. This is a sea bed outcrop of crag and limestone, which now blocks long shore drift and erosion – also making Sizewell suitable for a nuclear power station.
Walking south of the car park, current bedding can be seen in the crag in the cliff face. Also mud drapes, but no fossils – this was an estuarine deposit of the Norwich Crag Series. The Westleton Beds are seen 250 yards south of the car park, this was one of the clay and gravel layers in the Crag series, and is now thought to be separate from the other East Anglian gravels. Chatter marks on the Westleton Bed pebbles show a high energy environment, and increase in both depth and dip to the south, with rip current channels eroded into the sediments. The fauna was similar to the modern North Sea, and there were cold periods. Very rarely, Norwich Crag (1.6 Ma) is exposed at the base of the cliffs – the maximum extent of the ices was 440 ka, the Anglian.
After a good fish and chip lunch we went on to Westleton Common Pit. This is a county geodiversity site, a former gravel pit last used for WWII airfield construction. Freeze thaw structures can be seen in exposed slopes (Figure P).
The currents were mainly SW, with local estuaries and local NE/SW cross bedding. The gravels are poorly sorted, some are marine rounded, others are brown reworked flints and not directly from the chalk. The chalk is 1000’ below the surface, laterally the nearest exposures are Ipswich, Bury St Edmunds and Norwich. The quartz pebbles were from earlier rivers and deposited into the Westleton Beds – this area formed part of the proto Thames. And the dry valleys on the heath were formed in glacial / periglacial conditions.
After a welcome stop at the Hetherington’s new house for tea and cake, we went home in a storm. It was an excellent weekend. We covered a small area intensively and, hopefully, gained a good insight into its geology - Roger and Bob were generous with their knowledge and formed a memorable double act.
Photographs © Jim Camp, John Lonergan and Eddie Yeadon.
21 November 2013
Dr Tony Grindrod: Balcombe (Sussex): the Truth, The Whole (Hole) Truth and Nothing but the Truth
The recent drilling at Balcombe by Cuadrilla Resources was accompanied by myths, misunderstanding and misrepresentation in the press. Despite many references to shale gas and fracking in the media the Balcombe 2 and 2Z boreholes were purely conventional exploration boreholes for oil. The rig used the same site as Conoco had done in 1986 for the Balcombe 1 hole. Although a number of oil and gas fields are already in production in Kent, Surrey, Sussex, Hampshire and Dorset, they all produce from sandstone and carbonate reservoirs, not shales, and very few have needed hydraulic fracturing.
Nevertheless, companies face a very different surface environment to that encountered by Conoco in the 80s. Recent antipathy by Viscount Cowdray highlights problems explorers will face in obtaining a social licence to operate. Even though Cuadrilla have now left the Balcombe site, the road is littered with abandoned (bar one) tents and hastily discarded tarpaulins. The protesters caused more disruption than the drilling, and whilst Cuadrilla kept within the specified noise limits, any drilling noise was drowned out by the protest. This last comment does not imply any support for drilling onshore in the UK by the author, who has repeatedly highlighted the commercial challenges faced by the industry.
Returning to the coverage of shale gas and shale oil, progress remains very slow. Talk of drilling 20,000 boreholes is premature – not a single shale gas borehole has been drilled in 2013. According to the DECC website only 6 shale gas boreholes have been drilled in the UK, all by Cuadrilla, all in the Blackpool area. Where are the rigs, the personnel and the finance going to come from to drill this large number of boreholes?
Heidi Barnes: The Fairy Chimneys and the Villages of Death
In the Miocene and Pliocene, Cappadocia in Central Turkey was a subduction zone with explosive stratovolcanoes. These erupted the Ürgüp Formation of 9 high-silica tuffs, lavas and ash layers. It is up to 450 m thick and covers 20,000 km2. The Ürgüp Formation has eroded to form the phallic Fairy Chimneys of Cappadocia, but where the volcanics erupted into saline lakes, the mineral erionite was formed .
Erionite is a rare zeolite, a secondary mineral which precipitates from saline groundwater. George Walker identified zeolite zones in Iceland in 1960; as lavas are buried between 200 and 2000 m depth, different zeolites grow in turn as the temperature and pressure increases to 230°C.
Erionite resembles asbestos, forming woolly submicroscopic fibres which cause zeolitiosis – lung fibrosis – when inhaled. In three Cappadocian villages, dubbed the Villages of Death, up to 50% of locals die of mesothelioma, the cancer usually associated only with inhaling asbestos. People live in homes made of rocks containing erionite, it is in the dust and soil. But erionite doesn’t cause mesothelioma elsewhere. A researcher named Carbone found that certain Villages of Death families carried a mutant gene. This made them prone to mesothelioma.
Jim Camp: Geology in Film
My talk was about films with geological themes. Most of the films were science fiction, but some were documentary dramas. I ranked the films in decreasing order of personal preference, and said that only the top four are really worth watching:
John Lonergan: The Geology of the Pennine Way
The Pennine Way National Trail runs 268 miles the backbone of England, from Edale in the Peak District through the Yorkshire Dale, over Hadrian's Wall, through the Cheviots, to finish just over the border at Kirk Yetholm in Scotland. The Pennine Way was the first National Trail, opened in April 1965.
Geologically the strata get younger to the east, not always as the youngest is the Permian to the west, and this outline is by stratigraphy, oldest first, rather than geographically. There are three main bands of rock that make up the Pennines: Carboniferous Limestone, The Yoredale Series and Millstone Grit, with volcanic rocks to the north and some igneous intrusions. From 360 million years ago this area was covered by a shallow coral clear sea, forming a layer of limestone up to 150 m thick. There are classic exposures near Malham and Settle.
The calm clear conditions of the Carboniferous seas allowing the formation of limestone changed with the spread of the large river delta from the north. The river forming the delta carried mud and sand into an irregularly sinking basin, forming the Yoredale Series.
Overlying the Yoredale Rocks and capping the hilltops of the Pennines is the Millstone Grit. This formed as rivers flowing from a northern continent deposited sand and mud over an extensive delta. From Kinder Scout to Cross Fell these grits form prominent edges and sometimes flat topped hills. The impervious rocks and flat areas led to poor drainage, bogs and deep erosion. In contrast to the steady deposition in the south, further north a large volcano poured out ash and lava into which the granite of the Cheviot itself was later intruded. The youngest rocks on the Pennine Way are in the Eden Valley. The New Red Sandstone accumulated in desert conditions, and is seen in the buildings.
The Pennines were formed from these deposits, first by rivers carving out their valleys then by glacial erosion, and finally by plastering the lower grounds with till and boulder clay. Comparing a topographic map with a map of the Pennine Way shows why it is often walked in rain, as the Pennines form a watershed and lead to rain from both the northerly and south westerly weather systems. I’ve only walked part of it, but is highly recommended, with plenty of good, varied scenery – and geology.
There is a good leaflet on this on www.nationaltrail.co.uk/pennine-way/leaflets
Lecture : Survivors: The Animals and Plants That Time Has Left Behind - by Richard Fortey
5 December 2013
It was a pleasure once more to welcome Professor Fortey and hear his reflections on why some creatures survive essentially unchanged for hundreds of millions of years, despite extinction events that have killed off the majority of species.
He started with a picture of the trilobite, going back to the Car- boniferous or even the Ordovician, and continued with a photo of Delaware Bay, swarming with horseshoe crabs, its nearest living relative. So what are their properties? They’re very tough, have a very special blood, responsive to bacteria, grow slowly, taking 20 years to get to breeding age, and lay a very large number of eggs.
We then moved to New Zealand and the velvet worm, living inside a tree. It goes back to the Burgess Shale, dating from the Cambrian, when it was fully marine. Thence on to the intertidal zone in Hong Kong, where the inarticulate brachiopod, Lingula, of similar age, burrows vertically into the beach. In this zone a creature must adapt to varying temperature, degrees of salinity and anoxia. But since there have always been intertidal zones, it is here we first gathered that it is perhaps the survival of the habitat that is crucial for the survival of a species.
This message was reinforced by Richard’s next example: stromatolites, their photosynthesising properties so important for the development of life on Earth, and still after billions of years in the same high temperature, highly saline habitat with no predators. Then the greatest survivors of all: bacteria.
Other examples followed, beginning with the modern nautilus, very common off the coast of New Caledonia, and which like all nautiloids, including the ammonite, grows very slowly. On to plants, particularly the gingko, and then to the coelacanth, the jawless fish or lamprey, the New Zealand tuatara lizard that lives to at least 100, and the spiny echidna from Kangaroo Island, also long-lived. Ice Age survivors include the bison, despite having been hunted by us almost to extinction, the ibex, and the magnolia.
Last, but by no means least - the cockroach! (When working in Ghana, after a 3-month absence we came back to find cockroaches in our 5-star freezer – alive.)
Habitat, adaptability, toughness and longevity seem to be the key. Many thanks to Richard for an informative and lively evening.