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.
Indian Flood Basalt Volcanism • Freemasons' Hall • Faunas of Chengjiang and Herefordshire Lagerstätten • Athabaska Tar Sands • A3 Hindhead Tunnel • Antarctic Odyssey • Hobbit SOS • Northumberland • Sinking Slabs - Moving Plates • Hertfordshire Puddingstone • Walton on the Naze • Hammersmith Foreshore • Chilmark Stone Mine • Salisbury Cathedral • Darwin 200 • Mantle Xenoliths from Cape Verde Islands • Winter Weekend in Dorset • Members' Evening • LIPs, Meteorite Impacts and Mass Extinctions
Lecture: 'Assessing the rates and styles of major Indian flood basalt volcanism, and its environmental impact at the K-T boundary' by Mike Widdowson
1 February 2009
I think the best way into this is to copy the speaker's abstract:
Abstract Continental flood basalt provinces (CFBPs) are distinguished from all other volcanic activity by the repeated sub-aerial effusion of huge batches of basaltic magma over short periods of geological time. Their stratigraphies consist of many large-volume lava flow fields of 102 ->103 km3 generated during volcanic pulses that lasted of the order of 10 100 years.
High precision 40Ar/39Ar ages (67 64 Ma), and palaeomagnetic data (30N 29R 29N) demonstrate that Deccan CFBP volcanism straddles the KTB (65 Ma), and that much of its c.1.3 million km3 volume erupted in the latest Maastrictian and early Danian during a period less than 1 Ma. Atmospheric cooling associated with SO2 aerosols generated from just one Deccan lava field are likely to have been severe. Emissions of CO2, Cl, and NOx are also likely to have been prodigious, and together must have had an effect upon terrestrial, lacustrine, marine, and atmospheric conditions at a variety of scales. Accordingly, the record of repeated eruption of mighty Deccan flow fields across the Cretaceous Tertiary boundary (KTB) may provide a plausible explanation for global environmental deterioration at this time and, arguably, could have been a trigger for mass extinction. Recently, debate has been re-kindled as to whether the 'Deccan effect' may provide an alternative to the Chicxulub impact- extinction scenario. However, if Deccan volcanism did play a role in KTB extinction, then evidence should be apparent from fossils within contemporary regional sediments (i.e. infra- and inter-trappeans), such as those accumulated in water bodies located in depressions around the edges of the evolving Deccan lava fields.
This presentation seeks to explore these issues through an examination of the architectural complexity of the Deccan CFBP, its timing relative to the KTB, and assessment of its gaseous emissions.
Field Trip: Half Term Day Visit To Freemasons' Hall, WC2 led by Di Smith
19 February 2009
This trip was led by Di Smith, who met us outside the building at 1.00 on Thursday February 19th, and introduced us to the outside of this magnificent building, before we had an official tour of the interior. It was a cold and blustery day as we were first taken to see a plaque on the outside of the Connaught Rooms, commemorating the founding of the Geological Society of London, the first geological society in the world on that spot in 1707, when it was the Freemasons' Tavern.
The present Grade II listed building dates from 1927- 1933, and is the third meeting place of the United Grand Lodge of England on this site, built by subscription and known from 1919 onwards as the Masonic Peace Memorial. It is an art deco building, constructed throughout of fine limestones and marbles, richly decorated with Masonic symbols. The outer vestibule, completed 70 years ago, is surrounded by grey columns of Napoleon marble, partly metamorphosed carboniferous limestone from Northern France. The walls are of Italian limestone, and the star in the floor includes Italian Cipollino marble, a whitish marble with green chlorite, reddish serpentonite and yellow travertine, another whitish stone and a black border that looked like Carboniferous metamorphosed limestone with very few fossils.
We went through the Processional Corridor to three vestibules forming the approach to the Grand Temple. The first, immediately above the Ceremonial Entrance, holds the bronze Shrine, with the Memorial Window, representing 'Peace through Sacrifice', behind it. The second is an ante-room, and the third, the richest, has a coloured ceiling with a star at the centre, echoed by the mosaic floor with a central star in lapis lazuli, a semi-precious stone, mined for 6,000 years in Afghanistan. It is a metamorphic rock, produced by contact metamorphism, and is usually blue with gold flecks. The main component of lapis lazuli is lazurite (25% to 40%), a mineral composed of sodium, aluminium, silicon, oxygen, sulphur, and chloride; (Na,Ca)8(AlSiO4)6(S,SO4,Cl)1-2. Most lapis lazuli also contains calcite (white), sodalite (blue) and pyrite, (metallic yellow). On close inspection one can see that the points of the star are made of lots of small pieces fitted together in a mosaic.The great bronze doors separating the Vestibule from the Grand Temple itself were each cast in one piece in Chelmsford, using brickearth as a mould, and weigh one and a quarter tons each. They depict on the outside the building of Solomon's Temple , whereas inside they are adorned with symbolic designs. The Temple can hold 1700, and the ceiling, as is traditional in a Freemasons' Lodge, depicts the 'Celestial Canopy', surrounded by the Great Cove, a frieze entirely in mosaic showing allegorical designs incorporating the Classical Orders of Architecture. The walls are faced in Botticino marble, with a base of Belgian black marble, inlaid with panels of Ashburton; the door surround is also Ashburton, a Devonian limestone, containing stromatoporids and corals. Notable also is the alabaster light strip, a form of gypsum, probably from Northampton.
This more or less concluded the official tour and we were left free to wander round the Museum, full of fascinating historical objects of pottery, glass, furniture, regalia and much more. However I will just mention a few things of interest. There were 'training tablets', made of inlaid marble, depicting the tools of the trade, with the ever-present set-square and compass, hammers, rules etc. There is also a Master's Chair of 1780, with decoration in the form of Ionic columns (for wisdom). The Senior Warden has Doric (for strength) and the Junior Warden Corinthian (for beauty). Behind the chair are two pillars surmounted by celestial and terrestrial globes, and the pillars are engraved with maps representing the discoveries of Captain James Cook FRS. This concluded a fascinating afternoon. Many thanks to Di, for leading it, and my personal thanks to Di Clements for help with some of the detail of this report.
Interior and Exterior pictures courtesy of Diane Clements, Director, The Library and Museum of Freemasonry, Freemasons' Hall.
Lecture: 'Soft-bodied sensations: the faunas of Chengjiang and Herefordshire Lagerstätten' by Derek Siveter
19 February 2009
Our Half Term outing to the Freemasons' Hall was followed by a lecture by Derek Siveter: 'Soft-bodied sensations: the faunas of Chengjiang and Herefordshire Lagerstätten'. Derek, Professor of Earth Sciences at Oxford, is a paleobiologist who works on fossil invertebrates and the preservation of soft tissue. There are just a handful of sites known where exceptional conditions of burial are helping to increase our knowledge of early animals. In marine conditions 60-70% of faunas are invertebrate, but preservation is rare.
In the Chengjian area of Yunnan Province in South China, 525 million year old mudstones have preserved the detail of Cambrian worms etc including the digestive system, which must indicate anoxic conditions. There are inarticulate brachiopods with pedicles and sponges, and the earliest vertebrates. In Hereford the site of deposition is relatively restricted, about 5m by 30m, and contains Silurian fossils from 425million years, a period for which knowledge of soft-bodied faunas is very limited. Particular conditions such as a dust cloud must have led to their preservation in nodules in amazing detail and in 3 dimensions.
The lecture was illustrated with photographs of the finds, including a trilobite with all its appendages, giving an insight into how fragmentary the fossil record is.
Lecture: 'Athabaska Tar Sands Origin and Extraction' by Joe Jennings
19 March 2009
For the purpose of comparison, Joe started by introducing us to a conventional oil field, the Elgin field 200km east of Franklin in the North Sea. The reservoir is the Franklin sands, a restricted area 5œkm below the sea-bed, with the oil at 190º and a pressure of 100bars, so there is no problem getting the oil to flow.
By contrast the Athabaska Oil Sands, 5 hours north of Edmonton in Canada, cover an area of 54,000 square miles, frost-free for only two months of the year, are in a reservoir of lower- Cretaceous sands and clays, sitting unconformably on Upper Devonian deposits. Joe passed round a jar of the oil, looking exactly like black treacle, to show us how viscous it is. The overburden varies from 30m to 500m and the deposits from 20m to 70m. At present deposits with more than 75m of overburden cannot be worked economically. The source is thought to be Devonian shales under the present Rocky Mountains.
He finished by explaining some extraction techniques, using water at 80ºC or steam at 400ºC and answered questions, particularly on the problem of how this very viscous material reached its present location.
Field Trip: A3 Hindhead Tunnel
In March, soon after the breakthrough of the first tunnel, a party from LOUGS visited Balfour Beatty's A3 Hindhead Tunnel construction project. The trip was led by Sarah Chilton, the Lead Geotechnical Engineer for the Project, assisted by her Tunnel and Earthworks Geotechnical Engineers. All three work for Mott MacDonald, but as part of the integrated site team. Their enthusiasm and knowledge showed in the way they led our part round and answered our questions.
The project will complete the dual carriageway link between London and Portsmouth and remove the notorious traffic jams at Hindhead. The new road will be 6.5km (4miles) long and includes 1.8km (1.1miles) twin bored tunnels under the Devil's Punch Bowl Site of Special Scientific Interest (SSSI). Work started in January 2007, and the main tunnelling works started in February 2008 and both tunnel bores broke through on 26 February 2009, shortly before the LOUGS visit. The Project is planned to minimise disruption to the SSSI, by using dump trucks and a conveyor in one tunnel to move a million cubic metres of fill from the cuttings at the north to the embankment at the south, and this started in April. The LOUGS party were taken around the site by bus to see the tunnels and earthworks, and shown the panoramic views over the weald, as well as introduced to the geology and environmental protection measures in place. The Project is planned to be open for traffic in mid 2011.
The tunnel route generally runs within the Hythe beds, which are mainly formed of layers of sands and sandstones which can be highly variable in their nature. Underlying the Hythe beds is the Atherfield clay. The top of the Atherfield clay is marked by a spring-line which can be seen in the Devil's Punch Bowl and within the Boundless Copse areas. Boreholes at the top of Hindhead Common will encounter the full thickness of the Hythe Beds while those within Boundless Copse will encounter the Atherfield Clay near the surface.
The tunnel is 1.83km long comprising about 1.77km of bored tunnel and approximately 30 metres of cut and cover at either end. The maximum depth of tunnel below ground is about 65m at Gibbet Hill, (measured to the top of the tunnel). The tunnel will have two separate bores. Each bore will include a 7.3m wide 2 lane carriageway with 1.2m wide verges on each side. The verges are wide enough for emergency use by a disabled person in a wheel chair. Each bore will have maintained headroom of 5.03m, together with a further clearance of 250mm to the underside of any plant suspended above the vehicle gauge as a safeguard against flapping tarpaulins and ropes.
The tunnel bores will be approximately parallel and linked by pedestrian cross-passages at approximately every 100m throughout the tunnel. The tunnelling method used by Balfour Beatty is the Sprayed Concrete Lining method (SCL). This allows excavation to proceed in 1.2m sections, then a concrete is sprayed on to a designed thickness. Note that the site geologists inspect the face at every advance, mapping the features and checking for joints, faults, water and other potential hazards. The tunnel alignment is in the middle of the Upper Hythe beds at the southern portal. These beds consist of silt sand with rock bands. Further north, it passes through the Upper Hythe beds where the percentage of rock starts to increase and then in the Lower Hythe beds there is rock with minor sand bands. There is a visitor centre onsite, just off the southbound A3, and public viewing points are located over the tunnel portals.
Diagrams and photos from the Highways Agency and Balfour Beatty websites.
Lecture: 'Antarctic Odyssey' by Linda McArdle
16 April 2009
(with help from Jenny Forrest)
Linda, the Information Officer of OUGS and therefore well-known to many at the meeting, treated us to a fascinating and informative slide-show of a journey in Antarctica with glimpses of lots of rocks, ice and snow . and penguins! She started with statistics: 5th largest continent, the World's largest desert, 97% permanent snow cover, 3% bare rock. Starting from Ushuaia in Tierra del Fuego with its subduction-related volcanoes, down the Beagle Channel, passing Cape Horn and across the Drake Passage we arrived at the South Shetland Islands, island arc volcanism that ceased 4my ago.
To recite everything would take up too much space, but even from the photos we could see different geological phenomena from pale grey crystalline rocks on Half Moon Island, to a beach of black volcanic ash on Deception Island, a collapsed strato-volcano with a maximum age of 780ky, where the most recent volcanic activity dates from 1969. We saw icebergs, a magnificent sunset .and we learned that penguin poo is pink.
Lecture: 'Hobbit SOS' by Lorraine Cornish
21 May 2009
An illustrated talk on the conservation of the controversial holotype skeleton Homo floresiensis (aka Hobbit). Lorraine, in the Conservation department of the National History Museum went to the National Research Centre for Archaeology in Indonesia to help conserve the bones of Homo floresiensis, a hominid discovered on the island of Flores in 2003. The bones date from 18ky, but are very different from those of modern humans. So far the remains of 15 individuals have been discovered in the Liang Bua Cave. The creature was only a metre tall with a very small brain and a jawbone unlike that of Homo sapiens. The climate in Jakarta makes conservation difficult and facilities were limited to one fridge.
The bones are very fragile, and one of Lorraine's problems was that some of the bones had been broken and glued together. In particular one jawbone had been stuck back together, wrongly, with superglue! She hardened the surface with resin, and made replica casts of hands and feet. When first discovered, the bones were thought to be of a child, or a diseased individual, but it is now recognised as a new species. Stone tools were also found in the cave. To watch the 'Looking after hobbits' video, go to the NHS website.
Field Trip: Northumberland
24-28 May 2009
Sunday 24 May, Morning
The site visited on the morning of first day was, for me, the most interesting of all - Siccar Point. I don't need to explain the historical significance of this place, which was made clear by explanatory notice boards at the entrance to the path and again at the top of the cliff. The weather was fine and warm on the Bank Holiday Sunday, which was good because the path down to the unconformity would have been dangerously slippery in wet weather. Although the main features of the exposure could be seen from the top of the cliff, the real treat awaited those who made the tricky descent to sea level. The fine grain grey Silurian beds were laid down in distal turbidites (Bouma units D or E) 425 million years ago. They were uplifted and rotated to be almost vertical in the accretionary prism formed at the Iapetus closure. These strata were very variable in thickness, and veining and tension gashes resulting from the tectonic forces could be seen. After a gap of 55 million years while they were eroded to a new topographic surface, red Devonian sandstones were laid on top. These red layers were deposited on a broad coastal plain with periodical flooding by the sea. They tended to be conglomerates at the bottom, trending to finer micaceous stuff at the top. The erosion surface of the unconformity was not flat - there would have been hills and valleys on to which the Devonian sediments were laid - and complex cross stratification could be seen in the Devonian strata where they would have lapped against a Silurian shore. Was this the result of wave action?
To the west, only the red Devonian rocks were above sea level and to the east, the unconformity rose to the top of the cliffs and only the Silurian was exposed. The unconformity was thus confined to a very small section of the coast. James Hutton was wise to explore this by boat from the seaward side, rather than down and back up the cliff! On returning to the cars, we walked on to a quarry where the hard Silurian greywacke had been exploited. The beds here were dipping at 70° and we decided they were coarser at the top so must be overturned. There had been low-grade metamorphism to develop a slaty cleavage in the finer micaceous mudstone, and veins of crystals including barite (barium sulphite) from a time in the Devonian when there was volcanism in the area. Then it was on to Eyemouth for lunch on the promenade.
Sunday 24 May, Afternoon
Refreshed by the break spent sitting in sunshine, watching the tide turn, we moved left along the beach to continue our story of the development of the rocks. There are many different rock formations to be seen. Looking northward along the western shore of the bay, big red sandstone blocks lie above and behind the Silurian greywackes, which also outcrop in small off-shore islands. We walked along this shingle beach, littered with bigger blocks, to examine a variety of rocks, from more Silurian material to massive blocks of red sandstone and volcanic material related to the Iapetus suture. The sandstones are from the Upper Old Red Sandstone and are late Devonian in age, waterlain but under arid conditions. The paler rocks are more friable and contain feldspar and biotite, suggesting granite, but also pyroxene, associated with basalt, indicating an intermediate igneous rock, which we were told was not andesite, being too acidic, and hence is dacite, an explosive material, self-brecciated in the course of solidification, from a sub-surface flow. There is secondary calcite. This is interpreted as a debris flow and is Silurian material showing igneous activity related to the closure of the Iapetus Ocean.
Cutting down into the brecciated dacite is a Quaternary river channel with several horizons visible. Making our way back, we studied a number of the boulders on the beach. These are of basal conglomerate from just below the big sandstone blocks. There is some imbrication in the Quaternary till, suggesting currents from NNE (whereas the Silurian material suggests currents from NNW). Also visible are pyroxene andesites from Mid- Devonian volcanic activity. These rocks are reddish with noticeable phenocrysts of plagioclase feldspar. We continued to Burnmouth and parked just by a narrow bridge over the railway, which here runs through a very steep-sided valley. We walked steeply downhill towards the sea, looking at the bedding and cleavage refraction in the red rocks. In the thicker units were unfilled fractures. These rocks are Devonian sandstones formed in an arid environment though water-lain. Further down were sole marks in steeply tilted red rocks. The angle of tilting and the sole-mark structures are consistent with turbidites, evidence that these rocks are part of the Silurian. If the rock is split it can be seen that the red colour is superficial. We continued to another large structure, igneous with vesicles and slightly porphyritic. This was a broad mid-Devonian dyke. Finally we looked from our path across to the far side of Burnmouth Bay, where we had a taster for the next part of the story. These cliffs are of Fell sandstone from the Carboniferous, late Carboniferous and early-Permian, plus some igneous material.
Monday 25 May, Morning
Monday morning dawned bright and clear and we were off to look at the Fell Sandstone at Bowden Doors*, a lovely 450m long crag, trending N-NW ~S-SE set high on a hill top with superb views west across the Wooler valley to the Cheviot. From our hill top we had a very colourful panorama of bright yellow fields of rape, fresh spring green meadows and red ploughed fields, even what appeared to be an arena in a wild- western fortified farmstead! Stratigraphically the Fell Sandstone is a basal carbonate in the Tournasian (used to be Tuedian D1b), the lowest division of the Viséan, and lies conformably on the Devonian. It is an important aquifer and the rock provides both water and building stones for Berwick-onTweed.
The outcrop, which we shared with rock climbers from Edinburgh out "cragging", is about 8m high at the S end and consists of two distinct facies. The rock is a pinky-buff-grey sandstone and divided into two distinct units. The lower unit is well-sorted fine to medium, with sub-rounded grains, and is massively bedded and well jointed. It contains some cross stratification and dewatering structures. The upper unit is fine to medium grained, cross bedded with prominent trough cross stratification and some fine laminations, indicative of fluvialdeltaic deposits in a braided river. We decided that the source rock must be the Cheviot granite, which was uplifted and eroded at the end of the Devonian, hence the pink colour from weathered feldspars and the quantities of mica. A superb hill top, with fine views, wild flowers and a morning loud with the sweet songs of skylarks!
(*the word doors means place of wild animals.. .. OUGS? No surely not, more likely the "crag rats" from Edinburgh) More details of this location can be found in the Field Guide to Northumbrian Rocks and Landscape published by the YGS
Monday 25 May, Afternoon
The afternoon visit began with Lesley explaining how the closure of the Iapetus Ocean had resulted in a subduction of the oceanic floor, continental collision and the generation of volcanic activity in the Cheviot region. The granitic intrusion that formed the Cheviot had occurred in the early Devonian and had intruded into Silurian rocks. At the time of the Cheviot granite formation we should remember that none of the Carboniferous or later rocks had been deposited. Weathering during the Devonian was intense so by the early Carboniferous the relief was reduced and lowering the topography. What we hoped to see were Cheviot plutonic rocks including andesitic, granodiorite and porphyritic rhyolite, evidence that had contributed to the understanding and interpretation of the geology of the area.
We planned to lunch at the junction of the Harthope and Carey burns before looking at half a dozen sites at which rock exposures occurred. We stopped briefly in the old town of Wooler for a comfort break and then approached the Harthope Burn on a narrow road through Earle which climbed a very steep slope before descending an even steeper slope to reach our lunch point. From here there were excellent views up the Harthope valley to the Cheviot high point perhaps some 6 or 7 km to the SSW. We could also see features that could be taken as river terraces on the north facing valley face of the Carey Burn which joined from the West. Indeed, we walked on some alluvial terraces as we trekked up the Carey Burn. Looking at some of the boulders in the stream two main rock types could be discerned, some light coloured and identified as granophyres, the other composed of a darker, andesitic material. We pressed on towards an area where there were extensive scree slopes on either side of the Carey valley.
As expected the scree consisted of angular lumps of rock released by ice age weathering (frost action) from an emplacement of diorite. Rock samples, (see photograph 1) showed well developed phenocrysts, whitish and thought to be plagioclase feldspars. Examination with an eyepiece showed white feldspar crystals and square shaped black crystals thought to be pyroxene set in a pinkish feldspathic matrix. Some samples had weathered to the extent that the more mafic materials were degraded and the pyroxene had degraded to green chlorite. Further up the valley we came to a small waterfall which could have been generated by a dyke running at an angle across the valley, certainly there were dyke like features. However it was not possible to confirm the presence of a chilled margin in any of the supposed dyke outcrops.
We were now quite close to the granite and we expected to find some metamorphosed rocks and also some rocks formed in aqueous intrusions, We did find splintery hornfels, samples showing flow bending, epidote and a prize sample of barite. We were able to drive up the valley of the Harthope Burn to our next location where the Hawsen Burn joined some 4 km upstream of our lunch site. The road was a dead end and very narrow so we were in some fear of meeting traffic coming down the valley. But we were not embarrassed apart from taking to the verge and grounding our (Nicole's) vehicle on protruding rocks hidden in the grass. We were hoping to find microgranites, andesites with contact metamorphism and hydrothermally altered andesite.
The Hawsen Burn was a much smaller stream and the route up it was noticeably steeper than in the Carey Burn. From the geological map the rocks on the north side were the andesites which we had already seen but to the south the rocks were granite of the granophyric variety. Lesley explained that the stream more of less followed a fault line and that there were few rock exposures especially of the granophyres. We trekked upwards and looking around we were impressed by the great natural beauty of the area; we could see several tors thought to be comprised of andesitic hornfels. We did not view the last site which had been scheduled; this was the Devonian fan at Roddam Dene as we were advised that the site was in poor condition having suffered some damage in the recent rains. Finally we returned to the hotel vastly pleased with our experience that day.
Tuesday 26 May, Morning
This morning was to be our visit to the Farne Islands. The Farne are an offshore extension of the Whin Sill, and consist of stacks of columnar dolerite. North-west to south-east stria can be seen on the surface of the columns indicating the direction of flow of the Late Devensian ice sheets. Well that's the geology, but the main purpose of this trip was to study the fauna. The islands are home to some 20 different bird species including Puffins, Eider Ducks, Guillemots, Razorbills, and four species of Tern. There are also about 3,000 grey seals on the islands. As we boarded the good ship Glad Tidings Di Clements was heard to remark what good planning it was to have the write-up done by the one person in the group who knew little about birds, (actually she wasn't as polite as that!). I will do my best, but just keep that in mind!
Our captain took us on a tour of the islands, expertly manoeuvring the boat so that everybody had a good view of the seals and the colonies of thousands of nesting sea birds. We also had a good whiff of the guano coating the basalt! We eventually landed on Staple Island where we could see the birds in close-up. As we walked across the deeply dissected surface of the basalt our first encounter was with an Eider Duck on its nest, secure in the knowledge that it was so well camouflaged that we couldn't possibly see it. We were confronted with a huge collection of nesting black and white birds which I am reliably informed were Guillemots, but there might have been some Razorbills among them. On a small vegetated knoll was a collection of birds that I recognised, the Puffins (known locally as Tommy Noddy). There seems to be something wrong with the aerodynamics of the Puffin. The landing procedure is flaps down, undercarriage down, splat! The wing loading is far too high.
Our return journey followed part of the route taken from Longstone Lighthouse by Grace Darling and her Father in 1838 to make their heroic rescue of nine survivors of a shipwreck. We arrived back in Seahouses slightly damp and in time for lunch after a trip which can be well recommended.
Tuesday 26 May, Afternoon
During the late Devonian/early Carboniferous there was a period of crustal extension leading to formation of the Northumberland and Tweed basins amongst others. These two basins, with similar lower Carboniferous depositional histories, later merged as the Northumberland Trough. Lower Carboniferous sediments crop out on the north Northumberland coast where they dip gently and young in a south/south easterly direction. There is folding associated with Variscan and Hercynian orogeny during the late Carboniferous/ early Permian.
We gathered outside Berwick golf club on a sunny afternoon to have a look at these sediments. Sadly the tide was too high to see them in all their glory but we got an idea. First, we looked north across the bay at what from a distance appeared to be more red sandstone but was in fact part of the lower Carboniferous limestone series stained red by overlying tills. We then walked in a southerly direction along the cliff top, negotiated a highly tensioned and unfriendly fence and descended to a sheltered beach. The limestones here showed bedding varying from 50cm to fine laminations. Finely laminated calcareous sandstone displayed feeding trails, burrows and iron staining which suggests a marine environment close to the shore line.
Walking north along the beach we crossed some slightly anticlinally folded limestone containing crinoids and coral fragments. Some bedding surfaces above this limestone were less competent, distorted and displayed tension gashes. Bird life on this beach provided a great distraction and included eider and shell ducks, oyster catchers, noisy meadow pipits, or were they rock pipits, dive-bombing swallows and martins as well as a flock of 40 or so plovers intent on showing us their group aerial prowess. We left the beach and retraced our steps along the cliff top, carefully avoiding golfers, or rather stray golf balls. We descended to another beach where we had the opportunity to have a closer look at the bedding overlying the limestone. Sandstone passed up into a very bioturbated limestone which passed into some thin beds of coal and mudstone before returning to limestone with iron staining on the joints.
Wednesday 27 May, Morning
I was looking forward to Lindisfarne as I remember tramping over the sands to the Priory in my youth. Memory of those long walks in the area with my uncle was one of the things that attracted me to this trip: it was time to revisit the North East. This time Lesley carefully timed our visit to coincide with a falling tide so that we could safely drive over the causeway. The main purpose was to study the Holy Island Dyke, dated as 295 Ma old, the same as the Whin Sill but not considered to be a feeder dyke. It is labelled as dolerite on the map but it seemed finer-grained and more like basalt without the advantage of a microscope. There were other indications that it is a near-surface feature. We started examining it at the western end on St. Cuthbert's Isle where we immediately found small, round vesicles. We also found ropy lava but only on a lower ledge on the northern side of this outcrop. There was much animated discussion on how vesicles and ropy lava could form within a dyke.
We stopped for lunch at Steel End, further east along the dyke. Here the ropy lava seemed to bow outwards in V-shaped structures appearing to point more-or-less in the same direction (although Lesley felt that if they were plotted onto a rose diagram they would be less directional than we imagined). Here the vesicles were larger and were plugged, forming amygdales of what looked-like quartz (it was hard and did not fizz). At our final outcrop, the Riding Stone at Sandbanks, the ropy lava was much more diffuse and the differences in level very distinct. As before, the ropy lava and amygdales were associated with the lower level, which appeared to be much blockier, and with an abrupt change where it abutted the taller horizon. Here small slithers of the Sandbanks 'Limestone' were found along the junction. Was it possible that this was a separate, smaller dyke paralleling the main dyke? We all went away feeling this was worth more research.
While examining the Heugh Hill portion of dyke opposite St. Cuthbert's Isle we could see quite clearly that it was aligned en echelon. This was even more obvious from the Riding Stone where the Lindisfarne Castle portion was clearly slightly shifted to the north. Apparently this en echelon emplacement was intruded into an existing pull-apart structure. Beneath the dyke at Heugh Hill we found fossils. These are from the Acre Limestone (Carboniferous, Dinantian, Middle Limestone Group). Mary found two nice solitary rugose corals, one showing a good cross-section with septa, the other an elongate cross-section with tabulae. Hand specimens of the colonial, tabulate 'Lithostrotian' coral were also found, similar to the specimen from Berwick, and some nice orthocone nautiloids. These were particularly well displayed on a bedding plane along with large and small productid brachiopods and crinoid ossicles. The next level up had some lovely examples of Carboniferous gastropods. There were also some mineral concretions. Eddie found a nice one where the cubic pyrite minerals were exfoliating.
The weather on Wednesday morning was the least kind but at least the light, intermittent spits of rain allowed us to view the fossils to their best advantage.
Wednesday 27 May, Afternoon
Exposures of the Whin Sill were visited several times during the week, including the Farne Islands and Dunstanburgh Castle. The Whin Sill underlies much of North Eastern England, with exposures seen at High Force in Teesdale, and at Housesteads where Hadrian's Wall sits on the sill. Indeed, the use of the sill as a defensive feature is common to many of the castles of Northumberland.
The Whin sill was emplaced in the late Carboniferous/early Permian (295 Ma) and is associated with regional uplift and extension resulting from the Variscan Orogeny. Updoming allows the vertical migration of magma via feeder dykes and lateral movement of the liquid magma in sills. The magma was emplaced in between sedimentary beds, and emplacement was transgressive, the magma spreading between different layers exploiting relative weaknesses in the beds of the host rocks. The feeder dyke or dykes have not been identified. The duration of the emplacement is also unknown.
Bamburgh Castle sits on an outcrop of the Whin Sill. The castle is of ancient origin, re-fortified by the Normans in 1095 and drastically and inappropriately restored in the 1890s.
Location 1: North side of Bamburgh castle dune level at the base of the sill exposure After a short delay while everyone found the correct location, all members of the group arrived at the rendezvous for the first location at the northernmost point of the exposure at dune level below the castle wall. The exposure at this point is some 20m high. It is not possible to determine what the original thickness of the sill was as the lower surface is not visible and the upper surface, where the castle sits, will have suffered an unknown amount of erosion.
The sill is a grey, fine grained, crystalline rock of basaltic composition. Traditionally, such rocks emplaced in sills and dykes are referred to as "dolerites" although the grain size seen at this location and at location 2 is too small to meet the usual criteria for a dolerite. The grain size suggests rapid cooling. From the base of the exposure up to 4-5m, the rock exhibits pillow-like structures, rounded blocks on a decimetre scale. However, these are not seen everywhere at this level in the sill. Above this point to the top of the exposure, the rock is massive and blocky with no evidence of structure other than cooling joints. As pillow lavas only form in extrusive settings in the presence of water, the rounded structures must be produced by other mechanisms. Here, these are interpreted as evidence of a rubbly basal flow, formed where an initial pulse of intrusive magma entered an open, low pressure space in the host rock. Such structures therefore indicate proximity to the base of the sill.
Location 2: Southside of Bamburgh castle ground level at the base of the sill exposure. At the base of the exposure is a medium to coarse grained sandstone of the middle limestone group of the Carboniferous Limestone Series. The Whin sill lies above the sandstone at a height of approximately 3m above the base of the exposure. The sill rocks here are basalts, as previously seen at the first location. There is no evidence at this exposure of any thermal metamorphism in the sandstone at the contact with the intruded sill. This is due to the resistance of quartz rocks to thermal metamorphism, as compared to, for example, limestone. Clear sedimentary structures can be seen in the sandstone, which suggest a paleocurrent direction roughly from East to West. No fossils or trace fossils were seen.
While the base of the Whin Sill is clearly visible at this exposure, no examples of the "rubblyflow" structures seen in the sill at location 1 were observed at location 2. There is evidence of a chilled margin in the sill at this point, but none of the group was agile enough to climb up and confirm this. The presence of the base of the sill at the south side of the castle (location 2) supports the suggestion that the "rubbly-flow" seen at location 1 is a feature associated with the base of the sill, and is interpreted as an initial flow of magma into the space. However, such flows are not present very widely as they are not seen at location 2 and can therefore be considered as highly localised phenomena. This was the final location where Lesley Dunlop was able to be with us. In appreciation of her work as group leader, Lesley was presented with a copy of the new edition of North and South 10" Geological Maps. This was followed by a vote of thanks for Lesley and also to Nicole Gay for organising the field trip.
Thursday 28 May, Morning
Today was to be self-led with notes and suggestions of localities given by Lesley, who was not able to be with us today. After a beautiful sunrise, we were soon enveloped in mist, fine rain and low clouds, the so-called Northumbrian "haar", as our now smaller group of 16 set out from Seahouses for the morning's proposed excursion, starting at Embleton, 3 miles to the south. Our first locality was a disused quarry now very overgrown where an exposure of the hard black Whin Sill dolerite had been exploited, most probably for road stone. Along one side of the quarry, the massive jointed face of the Whin Sill could be identified, narrowing out at the edge of the quarry, with an apparent dip of 20o NE. Its base was not seen, but the more rubbly top was noted and there was mineralization with calcite and iron in some joints.
Our next location was a stop at Dunstan Steads, just a short distance further south along the coast. From here, a walk of approximately Ÿ mile along the coastal path in gradually brightening sunshine brought us to the bay and headland on which Dunstanburgh Castle (now largely ruined) was built. It was evident from a distance that a prominent position had been chosen for the construction of the castle on top of a near horizontal massive exposure of the Whin Sill.
Closer inspection of the rocks underlying the sill revealed these to be almost horizontal, finely laminated shales and sandstones in possibly rhythmic successions with some fine sandy lenses in mudstones and occasional bioturbation. The cliffs were also of interest to the birdwatchers amongst us as very vocal groups of kittiwakes, fulmers and razorbills were identified nesting on the cliff ledges.
On the beach beneath the cliff, a slightly dipping wavecut platform was visible beneath large black boulders and further along was a very folded structure dipping out to sea. Here the rocks were dangerously slippery but the more intrepid explorers were able to identify laminated limestones and shales. Much discussion followed regarding the timing of the Variscan Orogeny as the cause of folding and faulting in the region.
My final locality was at Craster, a little fishing village a couple of miles further south along the coast. Here the dark, bumpy and eroded surface of another part of the Whin Sill could be seen flowing seawards. One arm had been put to good use as a wall of the small harbour. A visit to the famous smokeries of Craster to stock up with kippers and other local seafood was the final port of call before the homeward journey and the culmination of a memorable sojourn in Northumberland.
Lecture: 'Sinking Slabs - Moving Plates' by Saskia Goes
18 June 2009
Dr Saskia Goes, lecturer in the Department of Earth Science and Engineering at Imperial College, gave a large audience the benefit of her most recent findings in developing further the theory behind plate tectonics. She and a team from Monash University, Australia, have used computer models and laboratory simulations to discover why, at subduction zones, younger plates descend, when cooler and denser than the underlying mantle, at a much faster rate than older plates, whereas the contrary would be expected, since older plates are colder and denser.
The answer seems to lie in the relative rigidity of the plates. Plates older than 60 million years are rigid; they therefore bend less easily and sink into the mantle at a shallower angle, causing trench retreat. At the 700km mantle transition these older slabs cannot overcome the increase in mantle density and viscosity. Younger slabs on the other hand, bend more easily and sink into the mantle at a steeper angle, 'puddling' at the 700km boundary, before penetrating into the lower mantle. The talk was followed by a lively debate.
Field Trip: Hertfordshire Puddingstone Puzzle presented by Jane Tubb
10 May 2009
On a bright Sunday Morning in May a group from The London Open University Geological Society were welcomed into Ware Museum in Hertfordshire, opened especially for them, to hear the incomplete story with contradictory theories regarding the formation of the highly distinctive yet mysteriously mythical Hertfordshire Puddingstone used locally since Neolithic times, as told by Jane Tubb. In her talk Jane set several questions, but some of the answers proved inconclusive and raised further questions from her avid audience
What is Hertfordshire Puddingstone?
Resembling a fruity pudding, Hertfordshire puddingstone is a siliceous conglomerate which consists of very well rounded, black rimmed, flint beach pebbles varying in colour from ochre through brown to rich red [jasper] and of varying sizes, set in a fine sand and silica cement. Whilst puddingstone resembles concrete, it has a straight fracture because the pebbles, sand and cement, all silica, form a hard coherent structure, unlike concrete, also containing pebbles but bound by a softer matrix that gives an irregular fracture.
Some Puddingstones have been found with layers containing few or no pebbles similar to Sarcen stone, found to the west in Southern England where it was used in the building of Stonehenge and in the stone circles and avenues of Avebury. Ferricrete, another similar concrete looking rock, consisting of iron-cemented, sub-angular and sub-rounded flints with no black rim, set in a dark sandy matrix is used as a durable building stone in Kent, as is Hertfordshire Puddingstone in Hertfordshire.
Where is Hertfordshire Puddingstone found and seen?
The origin of the large blocks of Puddingstone has excited interest for well over a century and is part of a continuing debate, with new exposures noted and photographed by local geologists particularly during local road improvements from 1960's and 1970's. However, recent Geological investigation has shown that Puddingstone is found within the Upnor Formation of the Lambeth Group as indicated by the Dowsett Farm Borehole. This is supported by data from the recent excavation of the A10, Thundridge to Puckeridge bypass, which has led to the discovery of Puddingstone specimens, which appear to be found in situ, in lower Palaeogene sediments east of Colliers End. The pebbles are believed to have been cemented locally, between the Chalk and the London Clay in blocks of up to 6 metres across and 1 metre thick. Other smaller fragments of Puddingstone have been collected locally in glacial till deposits and across some parts of the London Basin, but these are not in situ. The mysterious appearance of stones in fields following hard frosts gave rise to myths as the superstitious tried to account for them. Known locally as 'Breeding or mother stones', people believed the pebbles fell out giving birth to new growing blocks, living, moving and possessing emotions. As they were also believed to grow in the soil superstitious farmers broke up these large blocks to prevent further growth and damage to ploughs and crops. Large blocks can be seen locally on village greens, parks and in walls as well as in churchyards where one specimen was believed to prevent a suspected witch escaping the grave whilst blocks placed on doorsteps were believed to ward off witches. As a strong durable building stone it has been incorporated into walls locally since Roman times and as can be seen by the fine Quern stones in Ware museum, makes a hard grinding-stone for milling grain.
How was Hertfordshire Puddingstone formed?
This proved to be the most controversial question. Theories concerning the formation of Hertfordshire Puddingstone abound but it is generally agreed that following the formation of the Chalk, c. 80 million years ago, hard irregular flints formed chemically from silica in the chalk. Uplift in the South East of England and fall in sea level led to widespread erosion in the area exposing flints on the land surface. Following a subsequent rise in sea level when lowland Hertfordshire is thought to have had a coastal plain, rounding of the flint lumps into the smooth pebbles found in Hertfordshire Puddingstone occurred; a process seen today in operation along many chalk-backed coasts. It is thought, the flints, grey and black at this stage would have acquired their colour and black rims prior to being embedded and cemented into puddingstone, but this is where different ideas have been suggested. The 'Kalahari' Model, suggests that the pebbles rolled around in a lake in arid conditions where iron and silica rich water drawn from below, stained the pebbles and where exposed pebbles acquired 'desert varnish' by collecting iron and manganese from the atmosphere. The varnish, thin and easily scratched, would only form on exposed pebbles so does not completely satisfy the condition of the black rim found round the puddingstone flint which is much thicker and generally found on all flints. Another model suggests the porous pebbles, trapped below sea level in a coastal lagoon, acquired internal staining as the water evaporated leaving a lake, the iron rich water of which formed a coating on the pebbles. At similar stratigraphic levels to the Puddingstone beds, weathered chalk and plant rootlets found in nearby temporary pits and dessication cracks found in Palaeogene muds above 3m of glauconitic sand and pebble beds in Dowsetts Farm Borehole, would appear to evidence exposure of the flint as the area probably experienced a subtropical climate at this time when global temperatures were high.
It has been suggested that the pebbles may have been redistributed by flash floods which would have happened during continued sea level changes following Icelandic mantle plumes and associated regional uplift during Lambeth group sedimentation approximately 56-55Ma, and following staining but prior to concretion. Below a 55Ma surface the sandy pebble beds were covered, and cemented in a fine sandy siliceous matrix, which may have included silica from the Icelandic volcanoes, also found elsewhere in South East England and carried in water percolating from the surface. Whichever hypothesis explains the true origin of Hertfordshire Puddingstone much remains to be discovered and prompts the unanswered question 'Why is it found only in Hertfordshire?'
Pam PettmanAnd so it was that about mid-day the group organised themselves into the minimum number of cars and drove northwards out of Ware. I had Jane as a passenger so I knew we would not get lost.
Keeping to the old road, not joining the A10, w passed through the villages of Thundridge and Wadesmill and on to High Cross turning right at the end of the village into Shutes Farm. The farm entrance is opposite Pest House Lane which, Jane explained was the access to a former Leper Colony.
Having parked the cars we were lead to an area behind the farm house and shown some lumps of rock each about 1m high and having a base of about 1 square metre and I heard my first Yellowhammer in five years ( a little bit of bread and no cheese).
Jane explained that these had been brought to the farm how were building the A10 by-pass and were billeted at the farm. We were told that not all were Hertfordshire Puddingstone (HPS) and were asked to find the genuine article(s). The first two or three I looked at were indeed HPS showing clean fractures across pebble and matrix but then I found one which, although it contained pebbles it also contained irregular clasts and which fizzed when the acid bottle was used showing a carbonate matrix.
All together I found three foreigners out of the eleven lumps. I'm sure if I've got that wrong there will irate letters to the Editor.While we were looking at the stones we were joined by Dr. Bryan Lovell, once with BP and now Senior Research Fellow at Cambridge. He explained that the current thinking as to the origin oh HPS was that, at the time of the Paleocene-Eocene Thermal Maximum (PETM) global temperatures were much warmer than today and with sea water expanding Hertfordshire was at the edge of the North Sea.
In the leaflet that he distributed he explains that the Icelandic hot-spot pulsed irregularly and this disturbance of the sea floor had uplifted western Scotland leading to the deposition of the mud-draped sandstones which form the reservoirs of the Forties Oil field in the North Sea. This disruption of the sea bed is thought to have released Methane Hydrate from shallow depths drastically increasing atmospheric carbon, triggering a rapid temperature rise which, from ice and deep-sea sediments core evidence took some 100,000 years to return to normal. Bryan told us that he and others are collecting current research data for a book to be published later this year.
After thanking Jane, Bryan and the farmer for there time and knowledge we drove back to Ware for a picnic lunch in the grounds of the old Priory by the side of the River Ash.
After lunch on the grass and benches of the Priory grounds, our leader for the afternoon, Dr Steve Perkins, President of the East Herts Geology Club, gathered us together and gave us our assignment: to make up groups of four or five to go round Ware, armed with a bottle of water and comprehensive handwritten instructions, to look at the buildings, monuments, paving stones, setts and kerb stones of the town to identify a huge variety of building materials.
We were each given a starting point, and had to end up back at the Priory in 84 minutes, spending 12 minutes at each point. There was a lot of information and some questions whose answer we were to work out for ourselves. We also had a map and photographs!
Starting at the War Memorial opposite the Museum, made mainly of Portland Stone from The Upper Jurassic, a whitish limestone with ooliths and large oysters, we moved to St Mary's where there's a column of Purbeck Marble and an alabaster statue.
From there we went up and down the streets of Ware, identifying ripple marks in sandstone, York Stone paving slabs stained with limonite, granite setts with mafic enclaves, diorite kerbstones, and walls made of Kentish Rag, a calcareous sandstone.
We visited two other churches, Leaside Church and Christ Church, both of oolitic limestone, and became quite expert at identifying setts and kerbstones of Mountsorrel Granite from Leicestershire with red-rimmed feldspar phenocrysts, SW England Granite with white feldspar megacrysts, and Markfieldite, also from Leicestershire, in which greenish minerals contrast with the pink feldspars.
Our final meeting and summing up took place in the Priory grounds, where we were introduced to Pulhamite, an artificial landscaping feature of limonite bonded sandstone created by James Pulham and his son at Broxbourne, south of Ware.
By this time it was getting quite late, so we didn't go on to look at the Travertine forming in a stream bed in Hertford.
Many thanks to Jane, Steve and Ware Museum for an enjoyable and varied day.
Field Trip: Walton on the Naze led by Gerald Lucy
19 July 2009
We met at the café on top of the cliff at 11, as the tide would not be fully out until 12. Gerald gave us a very informative handout followed by a short talk on the significance of the geology we would be looking at, showed us examples of the fossils we should be able to find (and a few we probably wouldn't) and then briefly showed us some palaeogeographical maps of the distinctive gravels found across North Essex (including the type on the top of this cliff).
The cliffs are receding at such high rates that the exposures are continually refreshed and, as there is insufficient time for vegetation to establish, soft rocks are (unusually) clearly visible. There are two completely different formations here the basal London Clay which is Eocene, 50 Ma old, formed of river deposited mud and silt in a deep (100m) subtropical sea overlying this is the Red Crag deposit which is Pleistocene, 2 Ma old formed in a dynamic, undersea dune environment of a shallow (15/25 m) temperate sea, with a gap of around 48 Ma in between them.
Halfway down the cliff
We then started down the steps, about halfway there is a ledge from which level the contact between the London Clay and the Red Crag can be clearly seen. The junction is uneven and at its base there are nodules formed in the clay that later became incorporated in the Red Crag.This junction is where "boxstones" (hard round lumps of sandstone) can be found which are believed to be the only Miocene rocks found in Britain. Fine bedding layers can also be seen in the Red Crag from this level.
On the beach
We carried on down to the beach to look for fossils. Both the London Clay and the Red Crag are highly fossiliferous, the London Clay Sea containing sharks' teeth and rare bones of early species of birds together with seeds, fruit and wood from the rafts of mangrove like vegetation that sank in it and the Red Crag is virtually entirely composed of shells and shell fragments all of which are iron stained by the pyrite that washed through it from the London Clay.
As we continued further down the beach Di (Clements) pointed out and explained an example of rotational slip (or slump) a landside characterised by movement along a concave-up failure surface and results here from water that has percolated through the Red Crag being unable to move through the impermeable clay.
Continuing northwards along the shore (fossil collecting all the time) we reached the part of the cliff that has fine bands of pale coloured volcanic ash within it. These are believed to have been laid down in the London Clay Sea by the volcanoes of Western Scotland associated with the opening of the Northern Atlantic.From here we re-traced our way back to the steps and up to the café. Over tea we looked at the various fossils collected over the day quite a haul of sharks teeth, fossilised whalebone, pyritised wood, several types of shell including the sinistral gastropod (Neptunia contraria) and some small corals (about 1 cm diameter) that were found from sieving the Red Crag debris on the cliff ledge.
Overall a very enjoyable and informative day and we all went home clutching our various treasures.
Field Trip: Hammersmith Foreshore: a window on the solid geology of London
20 July 2009
Instead of the usual monthly talk, in July it is the custom of London Branch to have an evening event outdoors, often looking at building stones. In 2005 and 2006 we started to explore the best semi-permanent exposure we have in Central London, the Thames foreshore. On the first occasion the walk was purely exploratory and our findings, geological, biological and archaeological are written up in the OUGS Journal (Spring 2007). In 2006 are objective was to find the submerged forest dated at 3350 2910 BC as well anything else of interest. Root balls were found at Blackfriars Bridge and again at Bankside. These and the archaeological finds were written up in London Platform in February 2007.
This year it was the solid geology that was the main focus of our explorations. Fossils have been found associated with the London Clay at several foreshore exposures in West London. Those at Hammersmith have been described by Ed Jarzembowski (1987) and so we set off to look for them, starting under Hammersmith Bridge. The tide was sufficiently low that tell-tale septarian nodules from the London Clay were already well exposed under the bridge when we arrived. Several members spent some time on their hands and knees looking for fossils and Peter Collins was soon rewarded with a newly-exposed, bright shiny internal mould of a bivalve that was later identified as Nuculana bowerbanksii.
Other exposures of clay and septaria were located downstream and several members found small pyrite nodules and pyritised wood, both common in the London Clay. Steve Tracey found a fossil close to the water's edge, away from the exposures, which is a bit of an enigma as it does not seem to come from the London Clay at all and the preservation is completely different. It is a slightly squashed bivalved specimen that faintly resembles a Carboniferous brachiopod.
Several members were curious about the septarian nodules. These are calcareous mudstones found at certain horizons throughout the London Clay (and other clays). Sometimes organic remains are found within them and quite often these horizons are associated with fossils in the surrounding clays. The calcium carbonate content suggests that these horizons were rich in organic life and on decay the nodules nucleated around them. Some of them have preserved moulds of burrows on their exterior surfaces. The word 'septaria' describes the way that many of the nodules are divided into segments or 'septa' by a layer of calcite which related to a second stage in the development of the nodule. Broken nodules on the foreshore showed it quite well. Before the days of Blue Circle and its competitors, the nodules were collected and crushed for use as cement. Consequently they are also referred to as 'cement stones'.
Before setting off Di Clements explained that Hammersmith will be the starting point for the proposed Thames Tideway Tunnel flood relief scheme. A tunnel will run down the centre of the Thames to the sewage plant at Becton and will be linked to most of the flood relief sewers that currently spill into the Thames at times of overflow of the Victorian sewage system. In August 2004 severe pollution of the Thames hit the headlines, caused by a flash flood, but actually spillages into the Thames occur more than once a week on average and can occur after as little as 2mm of rainfall. Jackie Skipper will give a talk on the progress of the Thames Tideway scheme in February 2010.
John Jarvis, our resident archaeologist, talked to us about London's bridges which was appropriate as we were standing right beside Hammersmith Bridge, designed by Sir Joseph Bazalgette and opened in 1877. He was the man responsible for the amazing Victorian sewer system as well as the Embankment through central London. On our previous trips John was enthusiastic about what we might find of interest on the foreshore but on this occasion his preliminary recce had been very disappointing and nothing noteworthy was found by the group either. We did not even find eroded sediment that had supplied a list of sub-Recent fauna on previous trips. Instead, the foreshore was alive with tiny gastropods which were not apparent on our previous trips. These are Lymnaea peregra and they were almost like a crunchy mat underfoot.
For some of us the evening finished in the riverside pub, the Blue Anchor, with a welcome plate of fish and chips. Despite the lack of finds (apart from the 2 fossils) it was another very enjoyable evening on the Thames foreshore.
Reference: Jarzembowski, E 1987. A hundred years of Hammersmith Bridge. Geologists Association Circular 863, 8.
Field Trip: Chilmark Stone Mine
12 September 2009
I have driven along the chalk downland of Salisbury plain and admired the sarsens of Stonehenge as I waited in the traffic queue. The sun was shining. There was a dramatic change on the single track road to Chilmark where I waited for the Royal Mail van at a passing place.
Those fortunate enough to go on the 2006 traverse of the Vale of Wardour (based on Dinton about 4 km to the east) with Lesley Dunlop will know that it is an anticline which plunges to the east and opens out to the west. It originated in the Alpine orogeny. It is in this area that the Cretaceous onlaps the Jurrasic with an unconformity lasting thirty to forty million years during the marine transgression as the Atlantic opened.
The slope changes and fields become smaller as the road drops down as it crosses the Upper Greensand Formation and Gault Formation. At some stage it goes through Lower Greensand Group and then Purbeck Group layers. Chilmark is delightful and I took an unplanned diversion through. The road to the mine continues down the Ravine. It is still narrow and I hoped I wouldn't meet a stone laden lorry.
The OS map shows that the road runs NE-SW with steeper contours to the east. The road goes down a fault and it seems that the downthrow is to the west. A tributary of the River Nadder flowing parallel exploits weaknesses.
A hard-hatted Sheila Alderman showed me where to park and I was in time to eat my banana. Dean the production manager introduced himself whilst Emma, his wife, showed us to the loo. It turned out that Ian Butterworth, the managing director, had to go elsewhere. Dean modestly thought that we would do better with him though. Originally we were expecting to visit Hurdcott Quarry (Upper Greensand) in the afternoon but restoration work meant that this was not possible. Alternative visits in the afternoon were suggested but were unnecessary.
We crossed the road (east) and walked along narrow gauge tracks to the mine entrance where lamps were handed out. The RAF used the mine as an ammunition store during the war until 1995. They put in steel sections as a precaution against roof falls although the mine is known to be stable. The mine reopened and was taken over by Wessex dimensional stone in 2004.
The 8 tonne diesel engine is still used to shift 6 tonne blocks of stone to the yard over the road. As usual in mines the driver sits sideways. We made our way to the second means up a slope and out via the second exit. A third exit has just been reinstated having been filled in the Middle Ages (photo). The bed across the door looks almost horizontal but the angle of the shadows for about 11.45 a.m. 14th September (Courtesy Google Earth) suggests a NW-SE face so this could be caused by both dip of the beds to the north of the anticline and the easterly plunge of the anticline. We took the opportunity to examine the rocks in the light and found fossils in the fine-grained oolitic limestone as well as chert burrows and layers. (NB these rocks were not in situ).
Back inside the quarry the way We returned inside (photo) and then noticed a face carved in the rock wall (photo). Notice how well the rock carves. The surface is quite smooth. See how well it holds an edge. This is a high quality stone. The face was carved by a talented former mine worker, Steve West. That eye inspects you as you walk past.
There followed a lesson in stone mining technique with examples of mine-speak. There are early, pre-industrial tool marks on some walls. These are quite short so presumably only small blocks of stone for building were extracted. As longer drills became available holes were drilled horizontally into the face at close intervals. The miner then stood on a ledge on the wall side and drilled down. The tool marks left are as long as the drill. The size of stone blocks increased. Now take a look at this blade (photo) with detail of a ceramic tooth (photo):
The edge itself is the shiny, black circular object set on steel, one per tooth. Individual teeth are replaceable. This cutting machine was made in China. Two metre cuts are now possible. Grooves are cut all around the block to the correct depth. The force due to gravity overcomes the cantilever and the block drops by a blade width. There is a contingency if it doesn't, delightfully called plugging and feathering. Slide in two wedges above the stone and whack them with a sledgehammer. The next photo shows the back wall left behind.
Dean showed us how to test a newly exposed roof. He held a long metal rod in front well away from himself and sounded the roof by banging it the rod. He started at a point where he could stand safely and worked his way along. Mostly there was a solid sound but at one point the sound became hollow. He looked up and thought a crack was developing and there was a possibility of a very small, section falling. You don't take risks when there is the possibility of even a small lump falling out so he stood on guard well away and we were not allowed to approach it. Not that anyone fancied it - even small pebbles hurt. The mine roof is quite stable but the inspector sometimes insists on taking extra safety precautions. Bolts 1.8m long, which have been pull-tested for 24 tonnes of stone, are used. If a pillar side needs stabilising the bolts may have to be knitted by putting them at opposing angles. Sometimes a mesh has to be used. We saw such precautions taken in only one or two places though.
Field Trip: The Stones of Salisbury Cathedral led by Geoff Downer
13 September 2009
Geoff had provided us all with a very interesting account of the history of Salisbury Cathedral, the major building associated with Chilmark stone, and the following day a few of us joined a group from Wessex branch to look at its construction and the stone used.
The Cathedral was built in only 38 years from scratch starting in 1220, when it was decided to move the seat of the bishop from Old Sarum to Salisbury on the plain.
The Cathedral at Old Sarum was constructed in the Eleventh Century, and enlarged in the Twelfth, but now it was dismantled and the stone used for a new building in the early Gothic Decorated style.
We discussed the reasons for the move: space, water, population, difficult relations with the military in the Castle. It is built on the flood plain of the River Avon, but on a gravel bed. It has no crypt and the foundations are nowhere more than 4-5m deep, but it has required little adjustment in succeeding centuries. Originally there was a separate Bell Tower, which housed the world's oldest clock, whose mechanism is still preserved in the cathedral, though the tower was dismantled in the 18th century.
The Cathedral is constructed of Chilmark Stone, though some think it resembles rather Chicksgrove from lower down the succession, but Geoff thinks may actually be Tisbury because of ease of transport by river. The other obvious ingredient is Purbeck Marble, but on the exterior the Purbeck has mostly been replaced with Chicksgrove, because Purbeck weathers badly. The cathedral was renovated by Gilbert Scott in the 19th century and most of the figures decorating the West Front date from that period, but there are several from the 21st century, including St Aldhelm (2001) and George Herbert (2003). The latest is Sudanese Bishop Canon Ezra, carved in Chicksgrove stone in 2008. However there are plenty of niches for more!
Geoff explained in detail differences between the older Norman style of building and the newer, now called Gothic, with its pointed arches allowing the attainment of greater height and light through larger windows. He also discussed stress and the use of flying buttresses.
Some of the most fascinating detail concerned the Spire; (a blue plaque in the Cathedral Close commemorates William Golding's residence). Originally there was no strong central feature, but a square tower was added in 1280, and then the octagonal spire, with the addition of squinches to make the transition. Stone from the castle at Old Sarum was used and the spire was constructed of solid stone round a wooden scaffold. The tower has mediaeval metal ties still in place, and metal bands were placed at intervals both inside and outside the spire. However towards the top of the spire scaffolding had to be constructed on the outside in order for it to be completed, and even today to reach the lightning conductor etc on top, the hardy character has to climb up on rungs and negotiate the 5ft 6inch diameter ball at the base of the cross.
In the grounds is a sculpture by Emily Young: 'Lunar Disc', made from billion year old onyx, and nearby the site of the old Bell Tower, the subject of a Time Team investigation.
After a look at the East End, which is still to be repaired, we visited the cloister and exhibition, said thank you to Geoff for a fascinating tour and went our separate ways.
Lecture: 'Darwin 200: BC AD, Before Charles and After Darwin' by Bob Bloomfield
17 September 2009
As Creative Director of the Earth Galleries at the Natural History Museum, Bob has been responsible for their redevelopment, and also for the Darwin Centre and coordinator of Darwin 200 events.
He started with 'a bit of biography'. Darwin's grandfathers were Erasmus Darwin and Josiah Wedgwood, both members of the Lunatic Society of Edinburgh. The family was Unitarian, and anti-slavery, a position reinforced for Darwin when shocked at the reality of slavery in South America.
He went on to draw parallels between Captain James Cook's journeys on the Endeavour and Darwin's on HMS Beagle, (and also that of one Bob Bloomfield on new Endeavour!)
There was too much to do justice to in such a vast sweep in a necessarily brief report, but he nevertheless introduced us to Linnaeus and Lamarck, the fossils of South America, the impact of an earthquake on Valparaiso, trees in New Zealand, and coral reefs, concluding with the metaphor of the Tree of Life which that Darwin developed on these journeys.
In all a fascinating and enthusiastic exposé, with comments on the economy of nature, ecosystems, the importance of worms in the history of the World, and much more.
Lecture: 'Mantle Xenoliths from Cape Verde Islands' by Liz Evans
15 October 2009
Liz is a researcher in the Mineralogy Department of the Natural History Museum, whose PhD project is analysing these mantle xenoliths from a geochemical point of view to find out what they can tell us about processes affecting the upper Mantle. We were lucky to have her research findings to date.
She started by locating the islands on the Cape Verde Rise in the Atlantic, a product of mid-plate volcanism, with the oldest in the East - early-mid Cenozoic and the youngest, Miocene, in the West. It is highly alkaline volcanism, ranging from nephelinites to phonolites, while five out of the ten islands have carbonatites located in the seamount stage, i.e. very early in their development.
Carbonatites consist of >50% carbonate minerals. They are both intrusive and extrusive, erupt at a low temperature of c.650ºC, and have very low viscosity. With a wealth of photographs, diagrams and slides, Liz talked about the chemistry of the peridotite stressing their enrichment in REE, the two different sorts of clinopyroxene and the processes of partial melting.
She concluded the talk with a film of the Cape Verde Islands, which she has visited twice. After a lively question session, there was a general feeling that a future field trip might attract a lot of interest!
Field Trip: Winter Weekend in Dorset led by Alan Holiday
23 to 25 October 2009
Friday 23 October
The aim of the weekend was to study part of the succession along the Jurassic Coast, a World Heritage Site. The first day was to be spent in Portland, and the second at Osmington Mills to the west.
As we gathered at the hotel on the sea front at Weymouth the rain was starting, and by the time the last of the 29 participants straggled in, having had to park at the station, they were soaking wet and not happy. Alan came to join us at the bar before dinner, and afterwards introduced the Jurassic Coast in general, so that we could understand the context of what we were about to see.
Starting with the Triassic sandstones of Budleigh Salterton, through Seatown with belemnite shales and the unconformity at Golden Cap with Cretaceous greensand above Lower Jurassic clays, we arrive at Chesil Beach and Portland, with Portland Stone, sandwiched between Kimmeridge Clay and chalk.
He then talked in more detail about the next two days with dinosaur footprints and fossils, and finally, the weather forecast being poor for the next morning, we delayed the start for an extra half hour.
Saturday 24 October - Morning
Because rain had been forecast, we arranged to meet in the car park at a view point high on the Isle of Portland. The weather was every bit as bad as anticipated and, being well above cloud base, the view of Chesil Beach totally obscured. So after getting equipped with all the wet-weather gear, we left for the short drive to the Albion Stone Bowers quarry for the first close-up view of the rocks.
Alan explained that the surface cover here was the thin Purbeck Beds of the late Jurassic and early Cretaceous, which were of no value as building stone. These thin bands of limestone and clay had to be stripped off to expose the valuable Portland Stone for quarrying. The rocks at the top of the Portland sequence were the highly fossiliferous roach, with gastropods and bivalves. The shell material of these fossils had been dissolved away, leaving cavities and casts which made this oolitic material too porous to be useful for building, but the Whit Bed and base bed units beneath commanded £500 a cubic metre for the raw stone.
At the entrance to the quarry we saw a large ammonite, titanites gigantica, the type fossil for this level, and a large block of tufa flowstone, which had been placed to one side as samples.
Down in the quarry we saw how the stone is currently extracted by mining - cutting into the exposed face of the quarry pit with vertical and horizontal saw cuts and breaking off the stone in large blocks by hydraulic wedging at the front. The entrance to these adits gave some cover from the wind and continuous rain while we examined the thin black chert bands in the stone.
On the way out of the quarry we examined the Purbeck beds more closely. These were laid down in shallower water at the end of the Jurassic, as the environment changed from marine to near continental. There would have been extensive fresh water lagoons with occasional marine incursions.
Leaving the cars where they were, we crossed the road just above St Georges church (just visible in the mist) and examined the section of a large fossil tree which had been uncovered during quarrying and laid by the side of the path. This was probably a conifer from the late Portland/ early Purbeck boundary of the Jurassic/Cretaceous. There would have been islands of muddy sediment in near-continental conditions at that time. Further on, there were quarried blocks of Purbeck showing salt pseudomorphs. In saline lagoons, halite crystals dropped into the sediment and were subsequently dissolved away when fresh water returned. The cavities were then filled with fine sediment replicating the cubic shape of the halite. These are a good indicator of "way up". Ostracod fossils characterised the marine incursion events.
The rain was easing off as we descended to the coastal path on the west of Portland. The exposure at the cliff showed the succession with the Purbeck beds at the top (mostly stripped away by erosion and quarrying), the hard Portland stone and the brown Portland Sand beneath. At the bottom was black Kimmeridge Clay. The coastal path ran along the top of the Portland Stone where there had been some ancient quarrying.
At Black Nor Fort there was a good exposure showing fossil soil horizons in the Portland stone, created during lowstand conditions when land plants appeared. At 12:30 precisely the clouds parted and the Sun came through as we returned to the cars for the short drive to Easton and lunch.
Saturday 24 October - Afternoon
We stopped to park off the public square known as Easton Gardens, and had lunch on the park benches, some of us supplementing the hotel rations with supplies from the supermarket and other shops bordering the square.
After lunch, we moved to the Church Ope car park near the museum. Having left the cars, the intention was to join the upper part of the coastal path overlooking Church Ope Cove by means of a track running past the local museum. However as we made our way there, a kindly bystander, who turned out to be connected with the museum, told us that the way was blocked off for safety reasons. We therefore walked a little way to the north, and took an alternative track down to the Upper Coastal Path. From there we had a view along the east side of the island looking south past a ruined castle, Rufus Castle, which overlooked Church Ope Cove.
Our leader pointed out that the geography of this part of the island was dominated by landslips, especially one large landslip which took place in the 18th Century, the Southwell Landslip. There had been major mass movements, evidenced by blocks tilted out to sea. Indeed, the whole island could be said to be disappearing as a result of mass movements triggered by anthropogenic quarrying. We noticed large jointing cracks in a massive limestone series below the castle, and there was some discussion as to the location in the succession of a line of cherts. It was thought this was not the Cherty Series identified earlier in the day, which would lie below. We walked past the castle, and since the way was not blocked at this point, walked back up towards the museum along the track we had intended to come down on, passing through a gap in the barrier erected by the health and safety enthusiasts.
We saw laid out by the museum some fossil trees and algal growths (upside down!). We rejoined the cars and drove inland to the disused Suckthumb Quarry. The focus of interest here was a collection of blocks with dinosaur footprints. These indicated a lagoonal or mudflat environment, such as might have been expected in these parts at the very end of the Jurassic or the beginning of the Cretaceous. Thee footprints were probably those of iguanodo, and recently some metatarsal bones of a similar creature had been found.
Alan said that Dick Moody, of the History of Geology group at the Geological Society, had recently brought an international group of dinosaur experts to see the quarry, and they had apparently been much impressed.
We then drove to Portland Bill, at the south of the island, to see two raised beaches. There was a fairly brisk wind blowing onto the headland, and some of the party took an early opportunity to take refreshments at the café. Portland Bill is graced by three lighthouses, a testimony to its significance as a navigational hazard, but only one of these is functional. The raised beaches are the relics of interglacial periods when sea level was higher than at the present day. The eastern raised beach represents the peak sea level of the Ipswichian interglacial at about 125,000 years BP. We inspected the accumulation of poorly-sorted material with shell fragments and some larger pieces, mostly of gastropods, with some bivalves.
A smaller number of us walked round to the much older raised beach on the more exposed west side, braving the spray and jumping over gaps in the rock platform, from which there was a dramatic view. The western raised beach rested on the Jurassic rocks of the platform, beyond a security fence marking off military property. The beach consisted of graded layers of material, (rounded, like the material at Chesil Beach, but finer) and Alan told us that it was cemented by a carbonate solution derived from much later periglacial head (presumably Devensian) at the top. The raised beach itself is dated at 210,000 - 225,000 BP.
After an intermission at the café, we returned to our transport and drove down to the New Ground Park, where we enjoyed the view over Portland Harbour (to the right and on the other side of the Mound) which the poor weather conditions had deprived us of in the morning. We looked over at the breakwater which had been constructed for the Marina, and at the Sailing Academy, which will form part of the facilities for the 2012 Olympics (the Marina was previously a naval helicopter base). Alan observed that Portland Harbour was on the Kimmeridge Clay. On the other side there was more Kimmeridge, and the rising ground was Corallian. We also had a good view over Chesil Beach and the Fleet, a large tidal lagoon in which the beach separates from the sea.
The beach is one of the best examples anywhere of a barrier beach. It comprises pebble and shingle, thought to be derived from debris slopes from cliffs eroded when sea levels dropped in the last glaciation. When sea levels then rose, following the end of the last ice age, i.e. about 10,000 years ago, the rising sea carried the pebbles to the new shoreline where they were subject to longshore drift. The beach is therefore of very recent origin in geological terms.
Longshore drift resulted in the grading of the pebbles, which are pea-sized at West Bay, but the size of large potatoes, suitable for baking, at Portland. It is said to be possible to determine your location along the beach (which extends for some 28km), from the size of the pebbles.
Leaving the car park, and going past the Heights Hotel, we noticed a piece of tufa, and a nice piece of fossil wood (a conifer or monkey-puzzle tree). We then went for a brief visit to the abandoned King Barrow Quarry, which was now a Dorset Wildlife Trust site. Alan said that unfortunately the Trust thought that the wild flowers and plants at the site were more important than stromatolitic limestone. Algal limestones were deposited around the trunks of trees. Alan remarked that geologically we were at the fossil soils level found in the area to the east of Lulworth Cove, at the top of the Portland, base of the Purbeck, around 140 Ma.
The final stop for the day was the Masonic Car Park at Chiswell, on low-lying ground to the north of the island. This had been the subject of severe flooding in 1978 and 1979, in consequence of which a number of buildings had been badly damaged. After this, flood defence measures had been put in place. These included slightly raising the beach (i.e. Chesil Beach itself) at that point, and placing behind it a trough to act as a drain and a collection point. The road which led north-west from the island to connect it with the mainland had also been raised and provided with drains. A protective bund had been constructed to protect new development in the area. Nevertheless there were other properties, in the parts which had been flooded in 1978 and 1979, which still remained unprotected. Memories were short, and there was considerable interest in property there, in anticipation of the 2012 Olympics. Many unprotected properties were now filled up with furnishings and chattels, and these were clearly at risk.
Some of us then climbed up the berm for a view along Chesil Beach in the dusk, before we returned to the hotel.
Sunday 25 October Osmington Mills
On Sunday, we visited the north arm of the Weymouth anticline (see sketch). Standing on a windswept car park at Osmington Mills on a brilliantly sunny day, we could see south to Weymouth and Portland (where we had been the previous day) which represented the centre and south of the anticline. Looking west we could see the small Ham Cliff anticline beautifully picked out by a clear band of Corallian sandstone (Preston Grit) set within the cliff, rising then falling over Oxford Clay. At that point, a kestrel hovered at eye level some 10 meters away, and carried on hunting along the cliff in clear view. In addition, Alan gave us a free hand-out on trace fossils it was going to be a good day!
We walked east towards Bran Point. Here were rocks of the late Jurassic, representing various marine environments caused by sea level changes - clays, sandstones and limestones. Initially we encountered Nothe Grit (base of the Corallian), and from here on there was plenty to see - bivalves (eg Myophorella), trace fossils (eg Rhizocorallium) and sedimentary structures (cross bedding, ripple marks and round boulders of calcareous concretions). Having stopped to save a dog stranded on a cliff we continued eastwards. In the Bentcliffe Grit we saw hummocky- and swaley- cross stratification beautifully preserved. These represents structures formed below fair-weather wave base and storm wave base. This was exciting in three respects: it enables us to be very specific about the environment these in which these structures were formed; it was the first chance for us who had just taken our S260 exam to actually see these in the field; and Di passionately argued that the two features ('swaley' and 'hummocky') are products of the same processes, rather than separate structures. More superb trace fossils (eg Thalossinoides) and bivalves (eg Pleuromya), then back to Osmington Mills for lunch.
In the afternoon we walked westwards along the beach in lovely sunshine. Here we saw the top of the Corallian (Sandsfoot Grit) and up into the Kimmeridge Clay. As usual, the Kimmeridge Clay was poorly consolidated with very fragile flattened ammonite traces. There were hardened nodules (carbonate mud and crystalline calcite) within the clay, and we did find a more consolidated seam that contained a variety of shells (including some very large oyster shells). Further west we returned to a long stretch of highly fossiliferous strata , especially for trace fossils. Perhaps the best find of the day was a slab of rippled rock with trace fossils across it wonderful.
The afternoon ended with a presentation of Malt Whisky to Alan in thanks for his tour leadership of the weekend, and we returned home happy in what we had found and pleased that the weather had been so warm and sunny.
19 November 2009
We were treated to 4 very different, most interesting illustrated talks by our own members:
Hazards Alison Mathews
Alison described the work she had done during the course of her project on Hazards, and her study of Hilo on Hawaii which has regularly suffered from the effects of tsunamis. Hilo's position is effectively a tsunami 'hot spot'. Alison reported how recent developments in technology aim to predict and understand the impact of tsunamis. The DART (deep ocean reporting of tsunamis) system has recently been installed to provide an extensive warning system.
Glasshouse Mountains Eddie Yeadon
Eddie began by relating the aboriginal version of the story surrounding the birth of these mountains in Queensland, Australia, and then went on to explain the geological aspect of his trip to this region. The Glasshouse Mountains are steep-sided volcanic plugs of rhyolite and trachyte. Over the 25 million years since their subsurface intrusion, weathering has caused valleys to erode around them and they are now prominent mountains in the landscape.
Himalayas Kwame Ofori
Kwame related some of the most momentous aspects of his trip to the Himalayas, illustrated with exceptional pictures of the geological structures visible in the rocks. Kwame's trip was not organised as a geological trip, but he was able to produce a most interesting report on the geology of the Himalayas as he saw it.
Meteora Roger Lloyd
Roger told us about his visit to Meteora, which is a complex of monasteries built on sandstone rock pillars in an area of North West Greece. The sheer-sided pillars are unusual layered sedimentary structures, some of which reach 1800 ft above the plain. Faulting from earth movement about 60 million years ago, followed by continuous weathering, eroded the layers of sandstone into pinnacles. The region's fantastic scenery has been put to use more recently by James Bond!
Lecture: 'Large Igneous Provinces, Meteorite Impacts and Mass Extinctions: Timing is everything' by Simon P Kelley
6 December 2009
Simon is Professor of Isotope Geochemistry in the Department of Earth and Environmental Science, Open University, and also Director of CEPSAR (Centre for Earth, Planetary, Space and Astronomical Research), a multidisciplinary group at the OU.
Thus we were delighted that he found time to come and talk to us at our pre-Christmas meeting on the subject of Geochronology, relative and absolute.
We started with a scare in 1937 when an asteroid came close enough to cause alarm and we concluded with a scary moving image of the solar system showing how frequently this must happen. In between Simon discussed the question of mass extinctions and their relationship to HVIs (hypervelocity impacts) and LIPs (large volcanic provinces), and whether it is possible to prove a link, as has been proposed for the K/T boundary, a mass extinction event associated with the Chixculub crater and with the Deccan Traps at roughly 65Ma. But that 'roughly' is the problem.
Whereas with the use of the mass spectrometer and Ar/Ar dating more and more accurate estimations of the age of peak eruption in LIPs can be made, since there is a huge volume of dateable rock, it is more difficult to give an absolute age to HVI craters, except for the largest because the volume of melt suitable for isotopic dating is small, and in addition we have so far located only a small proportion of impact sites. He took the example of Boltysh, a 12km diameter crater in the Ukrainian Shield, perhaps 2Ky before Chixculub, which has been drilled and yields evidence of pollen count before the impact and a barren period after it.
Many interesting problems were touched on, including clusters of HVIs, perhaps reflecting collision in the Asteroid Belt, but so far though the record of sudden environmental change coincides with 6 or 7 of the 12 large volcanic provinces studied, the case for HVIs coinciding with stratigraphic boundaries is unproven.