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.
Herne Bay • Hogs Back Geowalk • Pickwick Quarry, Wiltshire • Anglesey • Albury Geology Trail • Bornholm, Denmark • Worbarrow Tout
S260/SXR260 Field Trip to Herne Bay with Members of the LOUGS
23 March 2002
A bright sunny day plus the infectious enthusiasm of our leaders set the scene for the whole of our day. We started out on to Beltinge Beach and due to numbers split into three groups.
There was plenty to see and I felt sorry for the general public using the beach as they had little idea of what wonders surrounded them. Our first clues were the shingle beach itself, a veritable mixed bag of different deposits. Newcomers, like myself, were introduced to the technicalities of grain size, rounding and "chatter" marks etc. and how it all related to events long ago (geologically speaking of course). We could see for ourselves how the coastline was receding, the evidence including the fact that the remains of a Roman Fort, which was once far inland, was now at the cliff edge at Reculver.
Modern man is trying to prevent further erosion by various means. A curved concrete promenade wall has been built along the beach, plus large blocks of imported stone dumped at the top of the beach. These stone blocks gave us further opportunity to try out our skills in typing and dating rocks. Reduction of the angle of the cliffs and drainage were also needed to delay erosion. This is particularly necessary here due to the presence of London clay. We were also told that, tectonically, the area is sinking.
Our leaders encouraged us to take a closer look at the cliffs and their structure. Features such as London clay, parallel bedding and the Upnor formation were pointed out. We were lucky enough to finish the morning session back on the beach, with the tide out, to look for fossils. This was a very new revelation to me. Preserved burrows of long gone sea creatures were pointed out, and shells, which I would have dismissed as just sea shells were in fact incredibly delicate fossils. I believe we all came away with a sample.
As we made our way to Reculver for lunch and our afternoon session I reflected on the fact that I had climbed up and down the cliffs and seen things I had never noticed before. My whole outlook on beaches and cliffs will never be the same again. I shall be looking for that something extra, and I will want to know what that rock is and how those pebbles reached the beach. Truly an amazing day, and a big thank you to the organisers.
Following an interesting morning at Beltinge we moved further along the coast to Reculver. It was a bright, sunny afternoon - most unusual as far as field trips go or so I am led to believe, as this was my initiation. After a pit stop at the King Ethelbert Inn we set about examining the imported rocks that make up the sea defences and looked for evidence of what they were. Thanks to Diana I now know what the twinkle factor is!
We then went on to look at the rebuilt Roman Fort and discuss the building materials used within it. The general consensus was that it is a real mish mash of rock types with no one predominant type. However, they are mainly local rocks. Among those which we identified were:
And there was much excitement when one of the more observant of the students spotted some Oolite limestone. Our thanks must go to all those involved in the organisation for a very interesting day. The enthusiasm of all the tutors was infectious and their patience in answering our never-ending stream of questions was remarkable.
What a Difference a Year Makes
March 2001 OUGS field trip and Im freezing cold in Walton on the Naze looking at London Clay. Im about a month into S260 and Im finding out fast I know next to nothing about geology. The fact is Im stuck on the principal of dip and strike in the first book and wondering whether I should have taken physics instead.
Fast-forward one year. March 2002 OUGS field trip and its a lovely spring morning in Herne Bay looking at London Clay and Ive passed S260 respectably. Now this doesnt mean I know everything there is to know about Geology, in fact far from it. As a geologist I am just starting out. But I do feel much more comfortable with my ability to interpret what I observe and this is the key difference a year on.
Rather than guessing as I did last year (and as some students were doing this year - yes weve all been there) I now have a method to follow, rather like Sherlock Holmes. This enables me to build up a picture of how rocks are formed, what processes were in action, and the possible environment where formation took place. Consequently, the clues that were invisible on last years field trip are now more apparent. This year Im looking back on S260 with a great amount of nostalgia, studying physics which Im finding out I know next to nothing about and thinking that I maybe should have chosen chemistry instead!
Guildford and the Hogs Back Geowalk led by Brian Harvey
28 April 2002
Thirty-five people and a dog gathered at Millmead in the centre of Guildford for this seven mile geowalk. We would explore the influence of geology on topography, land use; building stones, as well as attractive countryside with some interesting local history.
The Hogs Back is a prominent Chalk ridge on the North Downs between Guildford and Farnham, forming part of the northern limb of the Weald anticline. The strata here were deposited under marine conditions during the Cretaceous Period. They young, and, therefore, dip, steeply northwards. Guildford is situated in the gap, cut by the River Wey, through the Chalk and Lower Greensand.
Beginning the walk along the river, Paul Hetherington and Janet Philips talked about the important transport links provided by rivers and canals before the coming of the railways. For example, building stones from the Weald, could be shipped down the Wey Navigation, linking Guildford to the Thames and London. Janet Philips, a representative of the Wey and Arun Canal Trust, talked about the link between London and the south coast, via the rivers Wey and Arun. Both rise in the Weald but flow in opposite directions.
Snaking along the towpath, we looked at building stones in a wall I had walked past many times. I now know that it is built from the Bargate Beds, a calcareous sandstone with poorly sorted grains of very variable size. I had always been curious about St. Catherines Hill where some orange coloured sand slopes steeply down to the river. This comes from an outcrop of sandstone further up the hill: the Folkestone Beds, part of the Lower Greensand formation. The orange colour is produced by iron oxide, derived from chemical weathering of the glauconite.
Leaving the river, we began walking westwards along the Lower Greensand ridge towards Compton, noting differences in land use. The Folkestone Beds, produce acidic soils which are unsuitable for arable farming. Therefore, this land tended to be covered by woodland. The Gault Clay is now cultivated but, being a very heavy clay, it could not be farmed or drained until the invention of the steam plough in the mid nineteenth century. Particularly before the coming of water and rail transport, a local source of building stone was essential. Just outside village of Littleton was a small quarry of Bargate Beds, commonly used in buildings in Guildford and London, including St.Pauls cathedral. It only occurs in this part of the Weald as thin outcrops in the Lower Greensand. Carstone is also widely used. We saw this regularly today, including in the form of cross-bedding in the Folkestone Beds. It is a very hard iron-sandstone. The harder Chalk beds have been quarried for more decorative use in buildings, as clunch.
As the footpath entered the grounds of Loseley house, we moved from the Hythe Beds onto the impermeable Atherfield Clay where there was a lake and some mud. This is the oldest marine deposit in the Weald, laid down when the sea poured in via the Bedfordshire Straits in the north west.
We had lunch in the pretty village of Compton, taking particular interest in the building stones at St. Nicolas church. This dates from Saxon times and has the oldest carved timber in the British Isles. Inside was a font, allegedly made out of carstone (probably not) and walls decorated with clunch. I hadnt realised that Compton also has a memorial chapel, built in 1898, on the hill above the cemetery. This was designed by the potter Mary Watts, the wife of Frederick Watts, the local artist from whom the Watts Gallery, in the village, takes its name. Seventy-three villagers helped build it. People have had mixed feelings about its Art Nouveaux style: the intricate carvings in the red brick outside and the colourfully painted Celtic designs on the walls and roof inside; but we considered it one of the highlights of the walk.
The geese in the farm kept their distance as we left Compton; heading into the woodland on the Folkestone Beds; before reaching the high ground of the chalk ridge. Chalk, a fine-grained limestone, is the youngest of the Cretaceous formations in the Weald. There are Lower, Middle and Upper layers, the Chalk becomes progressively purer with less and less clay content. The Upper layer also contains chert (flint). We tried to discern these changes as we climbed up the hill but were distracted by the heavy showers which had held off until now. Fortunately, the heaviest rain had stopped by the time we reached the clearing in the trees at the top of the ridge. Here, there was a good view south over the Weald. We recognised some local landmarks and tried to visualise the underlying geology.
Ending the walk on the hill overlooking Guildford and the Tertiary sediments of the London Basin to the north, we thanked Brian for leading a highly informative and fascinating walk.
Pickwick Underground Bath Stone Quarry, Corsham led by David Pollard
18 May 2002
Seven intrepid members assembled at the Pickwick Quarry in the Wiltshire village of Corsham. We were met by David Pollard, the owner. David reopened the quarry in 1999 after it had closed as a tourist attraction (I believe that the Branch visited the quarry at that time). The quarry is now operated under license by the AMEC company. Recently stone from the quarry has been used in the new extension to Buckingham Palace.
David started with a safety briefing and checked we were equipped with torches, hard hats and boots. He then gave us a brief overview of the mine and the underlying geology. All too soon it was time to descend into the darkness.
The actual entrance to the quarry is a steeply sloping adit up which the stone blocks are winched. This is flanked by the 159 steps used by the quarrymen.
Although we visited on a Saturday, the quarry was still being worked, albeit at a reduced level. Nevertheless, two stone cutting machines were in use, plus a fearsome looking articulated loader was roaring around the tunnels.
David, accompanied by his son, led us through the working part of the quarry and explained the process by which the stone is extracted. The mechanics of this method has not changed since the beginning of underground quarrying. A series of horizontal cuts is sawn in the working face, followed by a series of vertical cuts to divide the face into blocks. These blocks are then broken out of the face and hauled away.
The quarry was in production from about 1810 until the 1960's. During the Second World War, the quarry was used to store ammunition. Signs of this occupation can still be seen in the older parts of the quarry. These parts are not being worked but are being maintained for their historical interest. Among other features we saw a wonderful wood and iron stacking crane.
After we had made our way to the surface, David Pollard led us to Box Quarry. This quarry was in use from 1830 until 1960. It has now become the very attractive garden of a private house built on the spoil heap. Originally open cast, the quarry developed extensive underground workings after operations threatened to undermine a road.
After lunch at the "Quarryman's Arms, it was to time to convey our thanks to David for an extremely interesting day and as it was still early afternoon the group decided to pay a short visit to Avebury to look at the sarsen stones (and to finish with a cream tea!)
Jubilee Weekend in Anglesey led by Paul Olver
31 May2 June 2002
Over the Jubilee Weekend The London Branch organised a three-day trip to Anglesey. This is a geologically complex part of the country with outcrops from the Cambrian to Carboniferous, a lot of ground to cover in two and a half days. Paul Olver lead the trip and gave us, as usual, an excellent insight to the geology of the Island. Very good for collecting rocks for the garden. One of the perils of organising a trip so far from home is that booking accommodation can be a bit of a hit or miss affair even with the leaders recommendation. So when I turned the corner in the road to see this blush pink painted Victorian building with fresh vegetation growing out of the roof hove into view my first though was Thats it Im going to be lynched, especially as it turned out I was the last to arrive as usual. However none of the clients complained and we settled down to dinner and the first evening in the bar.
Janet Phillips kindly volunteered to write up the first morning, which dawned bright and sunny.
Anglesey. As soon as I saw that name I knew I had to go. This is an exotic terrane (Monian) that occurs in every OU geo-unit as an example of something or other. I read on, the talented teacher Paul Olver as leader and the dependable Chris Sadler doing all the organising. The modest cost included evening talks and a hotel offering good-sized meals, en-suite bathrooms for washing rocks and tolerance to boots. I went and it was really good fun.
So, on a sunny Saturday, we set off over Telfords bridge, giving us a fine view of the new double-decker Britannia bridge (trains below, cars above). On the NW side of the straits we stopped at Church Island (SH552717). The gravestones were mostly of purple Bethesda slate. They were all blemished by ovoid green marks. These are organic in origin and an imaginary line drawn fro m one to the next along the long axis gives the bedding plane.
The Menai Strait is a NE/SW trending sinistral strike slip fault. Beside the water there was an exposure of mylonites. These enclosed more massive pods, which are interpreted as relict pillow lavas. Paul postulated that these pillow lavas of the Gwna Group were obducted. Prolonged and intense deformation associated with the late Cambrian/early Ordovician had produced these structures. As with other Precambrian rocks on Anglesey they are not related to the rocks found on the other side of the straits but correlate with the Precambrian in Rosslare and in Newfoundland. From the Ordovician onwards, the stratigraphy corresponds with that of mainland Wales.
Our route now took us through Llanfairpwllgwyngyllgogogoch, the name proudly displayed on every business, seemingly increasing the length of the village.
At Porth Isallt Bach, Treaddur Bay, (SH250792) we encountered another Precambrian exposure, this time from the New Harbour Group. This is an attractive shiny green chlorite muscovite phyllite, with parasitic structures superimposed on larger folds which again plunged NE. The chevron folding was so tight that quartz had formed in a crack along the axial plane resulting transpositional (pseudo) bedding.
We took lunch sitting on the cliffs of Holy Island, looking out to sea and the South Stack lighthouse. There were choughs and sea birds hanging in the wind and some of us saw puffins. Memories of the 2001 Symposium came back as we watched the car ferry Ulysses coming in from Dublin and disappearing around the headland in the direction of the unseen ferry port.
After lunch, the geological purpose of our visit was to look back at the cliffs from the South Stack lighthouse island. We left the lighthouse itself to the ordinary tourists (no time, actually!). What we could see in the cliffs was a complex anticlinorium, i.e. an anticline with a series of broad folds containing lots of complex little parasitic folds. These were rocks of the South Stack Series.
There were two lithologies here: white metaquartzites that were relatively competent and green chloritic phyllites that were softer and more ductile. There were quartz tension gashes in the phyllites and the metaquartzites were pinched out into boudins and rotated.
The phyllites were of the same pelitic composition as those of the New Harbour Series in Treaddur Bay, but were older. Both series are believed to have originated as continental shelf sediments lain down on the northern margin of the sea associated with the Gwna Group. This sea was being subducted along its southern margin at the same time that the Iapetus Ocean was opening even further north.
The axes of the folds here were NE-SW like the Caledonian orogeny and so at one time were thought to be Caledonian, but the orientation was just coincidence. The trend for the Cadomian and Caledonian is the same everywhere. The grade of metamo rphism here is higher than would have been expected for the Caledonian and the folds are therefore believed to have been formed by the time of the Cadomian orogeny, about 600 500 Ma. Another aspect of the rocks to be noticed was that the minor folds had the same plunge as the major fold they were sitting on. This demonstrates Pumpellys Law. In order to find the axis of the bigger fold one should look down the plunge and look at the smaller folds. Paul reminded us of the verse Big fleas have little fleas upon their backs to bite em and little fleas have smaller fleas and so ad infinitum. For fleas read folds.
The next stop was an unscheduled one to a quarry in the Rhoscolyn area of south Holy Island where there had been a gabbroic intrusion into the pelites of the New Harbour Series. The original intrusion of massive rock had not been sheared. The local name for it is verdantique and it has been used as a decorative stone for statues and pillars in churches. However, hot fluids had altered the first outcrop that we saw. They had serpentinized the augite pyroxene and plagioclase to bright green epidote and sheetlike amphiboles. The rock was an unusual one, similar to the serpentinite that many of us are familiar with from the Lizard in Cornwall, but with shiny talc shear planes from the two metamorphic events already mentioned. These events had taken it to a higher metamorphic grade. Flaky minerals like talc and chrysotile are generally favoured in metamorphism because they can stand the pressure and occupy the space better than stubby ones, which dont fit.
There was a large metamorphic aureole of epidotised hornfels around the original gabbroic intrusion. Chris also found a purple and grey hornfels that Paul could not identify without sectioning and further investigation. We wondered whether it was new to science. Somebody suggested the name Sadlerite, after its discoverer! I wonder who?
The final location of the afternoon was a column built in memory of the Marquis of Anglesey, of Battle of Waterloo fame. From the top of the column we could see Plas Newydd, the stately home of the current Marquis of Anglesey. The column itself was made of fossiliferous local limestone, but we had in fact come to see the ridge of glaucophane schist on which it was built.
The schists had been subjected to high pressure, low temperature metamorphism by being taken down a subduction zone. They bore a suite of distinctive minerals glaucophane, lawsonite and pumpellyite. Glaucophane is a blue/grey amphibole that is characteristic of the blueschist facies. Just as the highly deformed rocks that we had seen at Church Island that morning had originally been Mid Ocean Ridge Basalt (MORB), so had these. However, the Church Island MORB had been subjected to shallower, cataclastic shearing, whereas these had been subjected to regional burial metamorphism. Both schists are believed to have once been pillow lavas and we were invited to look for relict pillows; lensoid shapes infilled with minerals with the foliation flowing around them.
After dinner that evening, Paul gave a talk to help put all of what we had seen into the context of the plate tectonics of Anglesey from 1000 Ma to the present, no mean feat!
Still no complaints that evening I think I might have got away with it. Sunday dawned with the smell of fresh rain, electrifying skies and swirling clouds which seemed to be getting and striking closer as we neared our first destination as described by George Gibbons.
A lovely wet morning, entirely suitable for a long beach walk from Newborough Warren Car Park to Llanddwyn Island. This is in Gwna Group territory of Terrane 3 (in Pauls classification) and we were soon gazing in wonderment at some wet and glistening Pillow Lavas that were well weathered but quite undeformed. They were greenish in colour with Jasper-containing red flint in the interstices. Very striking. Absence of chalk-derived material between pillows indicated that they had formed at depth, beyond the carbonate dissolution point. Right now we could see (under Paul's prodding ) that they look upside down, and have been turned through at least 100o since deposition which was in the early Cambrian, according to recent examination of micro fossils in the jasper chert. A further walk across the wet sand brought us to exposures of the Tyfry Beds, vertically cleaved grey green grits or greywackes. Their orientation differed from the pillows the area having been deformed in the Cadomium orogeny. Now the long walk back across the sands to the cars (and the car park loos).
Our second stop was at Porth Trecastell a charming little bay which we could enjoy the more as the weather was by now warm and sunny. This is on Terrane 2 of the Anglesey jigsaw and the exposures here are Penymynydd schists of pre - Cambrian age, in fact, the oldest rocks on Anglesey. These are older than 640ma, as around that time they were intruded by a granite that could be dated. They were definitely wrinkled and also faulted, as befits ancient rocks. On one side of the fault there were black Graphitic schists that were definitely novel in my limited experience. They must have originated in a life-supporting environment, probably marine. But on both sides of the fault there were a variety of schists with both calcite and quartz intrusions and we could have stayed for another age with our little hammers, but Lunch called ..
And is ably described by John Wade
Lunch was taken on the cliff top by the church and graveyard at Llanbadrig, a place of some antiquity, on the North Coast of the island. After lunch, the major structure of the Gwna Melange was examined. This somewhat evocative French word implies a right old muddle! Which it was!
On the western side from the church, there were very large to small blocks of quartzite, sandstone, limestone, phyllite, serpentinite and jasper all set in a fine grained matrix. To the east of the church, the rocks were much the same with one very large quartzite block and a rather indistinct unconformity. This assemblage has caused much head scratching as to origin. Greenly (1919) thought that the cause was tectonic; Shackleton (1969) interpreted it as an olistostrome, a sedimentary deposit caused by large scale submarine slumping. The latest theory, Barber (1998?), suggests that the mixture is a result of a burp of sediments etc rising during subduction. Room for more theories, perhaps!
The next location was not far away, at Penrhynmawr, near Cemaes Bay. This was a coastal disused quarry trending ESE to WNW where there are massive Pre- Cambrian limestones, light coloured and relatively unfossiliferous as far as could be seen, except for some stromatolites seen on weathered limestone surfaces by the shore. The shore side where the stone had not been worked showed heavy weathering. The remains of a limekiln were evidence that the stone was calcined for agricultural use. Of interest was the change at the limit of the workings where limestone became confined with black graphitic schists. The interpretation of this junction suggests that the limestone is one large block within the melange, set in metamorphosed schists. This was supported by the amount of debris of a nonlimestone nature found on the quarry floor including pieces that could have been from volcanic ash layers, bits of jasper and sandstone etc.
After this yet another interesting site, tired and happy, the group made tracks back to Bangor.
All safe and sound!
Monday dawned as wet as Sunday without thunder and lightning but with incessant drizzle and mist. In view of the conditions Paul and I decided it would be better to curtail the trip and miss out the Old Red Sandstones so we headed for the Carboniferous Limestones at Trwyn Penmon just to the north east of Beaumaris. When we arrived the rain was still falling and the lighthouse bell was tolling every couple of minutes. The limestone here is a lime mud with corals, brachiopods and some bivalves. We spent œ hour scrabbling around the rocks looking for specimens in the outcrop whilst trying to stay dry and upright on the slippery surface.
We then moved onto the geological museum and shop, Stone Science, near Pentraeth. The displays include a brief walk through time with rocks and fossils and the environments with collections of fossils, minerals and stones from all over the world. Children were kept amused by a cartoon video about dinosaurs. Having looked at the displays and browsed around the shop, we were made a cup of tea and as the skies had now cleared, we ate our sandwiches in the back garden amongst the large variety of chicken and Indian Tee Pees. By one oclock we said our good byes and started to make our separate ways back home after a very interesting weekend.
I would like to thank the volunteers, Janet, Lynn, George and John who have contributed to this write up and thanks again to Paul for leading the trip. As for the Sadlerite I am still waiting to hear the results of the intensive research being carried out by Sue Vernon.
Newlands Corner-Albury Geology Trail led by Iain Fletcher
30 June 2002
At 10 am on Sunday 30th June, eight fugitives from the World Cup Football Final gathered in the car park at Newlands Corner, a high point on the North Downs, near Guildford in Surrey. The purpose of this meeting was to follow the 5 mile (8 km) Trail as detailed in the excellent and highly recommended Surrey RIGS Group booklet, initiated and organised by our leader, Iain Fletcher. This is on sale for just £1.50 at the Newlands Corner Visitor's Centre.
On still autumn mornings, when mist shrouds the Weald and the tops of the highest hills appear like dark and distant islands, it is easy to stand on this spot and imagine that you are gazing out across a vast mysterious ocean. On this summer's day, however, the neat farmland and wooded ridges were clearly visible, stretching peacefully away towards the South Downs, with barely a building in sight. Considering the small size of the county of Surrey, it has a surprisingly wide variety of scenery, with large areas of uncultivated heath, common and woodland. We were about to discover part of the explanation for this diversity.
To help us visualise the scene during the Mesozoic Era, Iain showed us copies of pictures painted by Karen Carr - Cretaceous Undersea and Coastal Environments. Wondrous animals ruled the world for millions of years but sadly, many became extinct. Fortunately, some left their fossilised remains to challenge and enchant a different species which evolved and rose to power much more recently, in 'the blink of an eye' on a geological time scale.
Here we were, standing on a seemingly solid and immovable chalk escarpment, 170m above sea level, which was made mostly from countless billions of skeletons of microscopic planktonic algae which lived in a warm ocean roughly between 100 and 80 million years ago. It took them 50 years to build just 1mm of chalk and they succeeded in constructing a layer hundreds of metres thick. To add to this astonishing phenomenon, the chalk no longer lies horizontal, as originally deposited, but here at an angle dipping northwards at approximately 35 degrees. Responsible for this realignment was the collision of the African and European plates which resulted in the closure of the Tethys Ocean and the Alpine Orogeny between 30 and 20 million years ago. The awesome pressure advanced across the European land mass and on into what is now the south of England, forcing the sediments up into an anticline about 150 km long and 50 km wide, with its long axis orientated east to west.
Since then the forces of erosion, accelerated by periglacial conditions during ice ages, have removed and recycled the top of this impressive structure, more correctly described, as Diana Clements pointed out, as an anticlinorium, as it contains many smaller folds. The remaining edges of this dome are now known confusingly as the North and South Downs. (I think they should be called Ups!) Between these rows of gentle hills, ancient sediments, which underpin the present countryside, have been revealed. I took a very deep breath as we began our 'underwater' journey back through the past 120 million years! Iain gave us all a sketch map of the area on which we were instructed to mark the geological features and boundaries as we followed the Trail. As can be imagined, these became the subject of considerable discussion.
The Trail started, somewhat ignominiously, about 100m behind the only public toilets on the route! At Stop 1 we puzzled over an unexpected exposure of rust coloured sandy clay soil containing pebbles and flints. These originated in the Upper Chalk but here they had been deposited on the top of it. It seems that this formation, very similar to the nearby Netley Heath deposit, was sediment from rivers which once flowed across this area from the south, which must have been higher about 1.5 million years ago. Reappearing from the undergrowth behind the toilets (!), we returned to the car park and walked slowly down the hill, searching for the end of the sandy clay and the beginning of the Upper Chalk. This was difficult as it was covered by short Downland wild flowers and grass. We all chose a position and drew the first line across our maps.
Stop 2 was the Viewpoint and information board where Iain did some arm waving, indicating clues in the vegetation which suggested different strata. Walking diagonally downhill, it was obvious that we were crossing the Upper Chalk which contained bands of tabular flint. The path was covered in broken and nodular flints, probably washed out by rain water as it dissolved the softer chalk. Flint is fascinating and its formation is not completely understood. Possibly, at various intervals, climatic or other changes in conditions caused pauses in the deposition of the Upper Chalk. These promoted the development of flint using silica from the dissolved spicules of types of sponge which were prolific in the chalk sea. It is often found in fantastic, convoluted shapes (burrow infills) or even superficially resembling animals or parts of the human body! Some nodules contain fossilised remains of siliceous sponges or other sea creatures such as echinoids or brachiopods. It is extremely hard and has a characteristic conchoidal fracture. Its uses by humans to make tools, weapons and for building are well known.
Now longer grass, bushes and trees grew each side of the track and we were delighted to see some delicate and beautiful Common Spotted Orchids (Dactylorhiza fuchsii). A little further on, when we began to smell Stop 3 (!), we noticed that there were no more flints in the chalk, so a second line was drawn across our maps to mark the top of the Middle Chalk.
Stop 3 was somewhat fragrant but, thankfully, fairly dry! Here we found Albury Downs Chalk Pit, now used to store farm sewage sludge before it is spread on the fields. By contrast, the exposed chalk was pure and white and it contained fragments of a bivalve called Inoceramus. The obvious cracks in the chalk face looked like bedding planes but they were faults and some had slickensides. Iain pointed out a thin band of marly calcareous clay dipping into the hillside at an angle of about 30 degrees. It has been suggested that particles in this darker band are volcanic in origin.
Escaping from the odour, we continued a short distance down the track until we noticed that the chalk had become darker in colour due to the presence of clay or mud. A third line was drawn to mark the top of the Lower Chalk, sometimes referred to as marl. Further on we found slabs of hard Upper Greensand in the bank on the right. It was slightly grey in colour, calcareous and contained glauconite. To the naked eye it looked neither sandy nor green, apart from the algae growing on the outer surfaces. Another line was drawn and we complained that we needed a larger map! Looking across fields on the left, Iain said that the line of the hedge marked the junction between the Upper Greensand and the Gault Clay. Exposures were hard to find because of the vegetation so we took his word for it and added yet another line to our collections!
Now the Trail gradually flattened out as we approached Stop 4. Ducking under bushes, left deliberately to conceal the entrance, we suddenly found ourselves in a place with an audible 'wow' factor. The abandoned Water Lane Sand Pit is quite small but it has high vertical sides shaded by ferns, trees and bushes, giving the place a rather spooky atmosphere. The clean, bright orange, marine, Folkestone Sand was laid down about 114 million years ago. The vivid colour was staining due to the oxidation or 'rusting' of the green, iron-rich mineral, glauconite. On the south side, bedding planes sloped down to the left, indicating underwater sand dunes and currents moving from right to left, towards the east. Scattered on the ground were curiously shaped chunks of very hard, heavy, dark brown, iron-cemented sandstone, known locally as Carstone. These had fallen from contorted veins or lenses near the top of the exposure. Some pieces were lumpy or flat and rectangular while others were long and curved, resembling guttering. Carstone was highly valued in this area, not as a source of iron but as a durable building stone. Nobody seems to know quite how or why these rocks were formed.
Emerging from the gloom into bright sunlight, the path became wider and the valley opened out. The Bargate Stone was not exposed here and we passed quickly into the Hythe Sand which was more friable and fertile. On the left was the first house we had seen on the Trail and odd sized and shaped pieces of Carstone had been used instead of bricks in the walls. To add to the attractive appearance, little chips of Carstone had been pushed into the mortar at irregular intervals between the blocks. This was called galletting. Iain suggested that, in addition to being decorative, they acted as spacers to support the blocks while the slow-drying mortar, made with Folkestone Sand, solidified.
We crossed the road and walked along the pavement and over a culvert through which trickled the Tilling Bourne stream. Approaching the village of Albury, we paused to admire some pretty cottage gardens and more houses with galletted Carstone or Bargate Stone walls. We found a place where sandstone building blocks were deeply eroded but the little chips of Carstone remained poking out of the mortar in between. The Drummond Arms was a welcome sight and we sat in the garden and enjoyed a refreshing glass of beer and admired Iain's collection of fossils found in the chalk on previous occasions. Here were some he had prepared earlier!
After lunch we headed south again, walking up the steep-sided Blackheath Lane. Sunken tracks are characteristic of the Hythe Formation because it is soft and washes away in the rain. On the right we found an exposure of Hythe Sand which had a 'pepper and salt' texture. The darker grains were glauconite of marine origin and the lighter ones were quartz. Suddenly it occurred to us that the bedding planes were dipping slightly to the south, which meant that we had just crossed the fold axis of the local Peasmarsh anticline. Iain showed us how to mark this east-west axis on our maps.
After the excitement had died down, we continued up the hill to Stop 5 where we studied a good exposure of the Bargate Beds on the left of the track. The southward dip appeared steeper than it really was because of current bedding. Iain said that the dips wedge out to the south. Bargate Stone is mostly confined to Surrey and Sussex with a little in Hampshire. It is an oolitic sandstone cemented with recycled limestone and it is valued as a building stone, for example at Charterhouse School. Through a lens, minute ooliths or ooids could be seen. The theory was that these tiny spheres had been eroded out of Jurassic rocks. Fossils have also been found that originated in the Oxford Clay, a long way to the north, then transported by rivers and deposited here on the site of an ancient delta.
We turned left (east) near the top of the hill and took the sandy bridleway. Here there was a dramatic change in the scenery as now we were walking on infertile acidic Folkestone Sand which had been planted with conifers. On the path I found a very worn but unmistakable flint echinoid fossil (Conulus) which must have been transported over a long time and distance from the Upper Chalk where it lived and ploughed through the soft mud on the sea floor about 85 million years ago. After about 500m we turned left (north) again and entered a narrow pathway going down the hill. We stopped to inspect a small exposure of the Bargate Member beneath the roots of a tree. Here it was non-calcareous, yellow, fine to coarse sand with current bedding dipping south at 13 degrees. The sides of the track became steeper so we recognised that we were once again entering the Hythe Formation. There was a surprising layer of chalk on the path which must have been brought here by people to reinforce the surface.
Stop 6 was next to a large exposure of fine brown Hythe Sand with current bedding dipping north at a low angle. We had crossed the hidden axis once again but in the opposite direction. Iain drew our attention to little round light-coloured spots which were infilled worm burrows. Some darker layers contained small grains of glauconite. A short walk along the main road past Albury Estate houses with their extraordinarily ornate Pugin chimneys, and an old VR post box, brought us to a path up the hill on the left. This led to a forbidding metal gate through which we peered for a glimpse of the working Albury Sand Quarry in the Folkestone Formation (Stop 7). In spite of reassurances and plastic liners, I was not convinced of the safety of using this as a landfill site. We were disturbed by the amount of methane gas burning off and venting from an enormous upright metal tube.
Moving quickly on, we walked downhill (east) along the A25 road and crossed over to Stop 8 at the Sherbourne and Silent Pools, originally dug out hundreds of years ago to make reservoirs of fresh water. These were supplied by streams flowing from beneath the chalk. Sherbourne Pool lay mainly on Gault Clay with Upper Greensand at its northern end. The deep clear water in the Silent Pool above had a bluish tinge and there was an exposure of creamy coloured Lower Chalk beside the path at the stream end. It dipped north at 25 degrees. More lines joined all the others on our maps.
Climbing the steep hill beyond the pools on the way to the final Stop, we stopped briefly to look at a small exposure of white rubbly Middle Chalk with fragments of Inoceramids. A little further on we found the marker post 9 near a brick Second World War 'pill box' built on the base of an old lime kiln. In the past, every farm in this area had its own lime kiln to burn local chalk and convert it into lime. This was spread on the clay and sandy fields to fertilise and break up the soil. Just t hink of all those toasted fossils! We gazed over the fence at the view back down the slope with a new awareness and understanding of the connections between the scenery and geology, and identified the rock sequence with the help of our maps. The path back to Newlands Corner passed over the flinty Upper Chalk once more and on to the crest of the scarp slope where we found more Netley-Heath type sand and clay with flints and gravel.
Thanks to the patient and expert guidance of Iain Fletcher, we had observed and absorbed an incredible amount of geology in one very enlightening and enjoyable day. On 30th June, 2002, Brazil's football team won The World Cup but we gained something of much greater and enduring value.
Bornholm, Denmark led by Richard Bromley and Ulla Asgaard
25 - 30 August 2002
Sunday 25th August
The first questions are what and where is it? Both are easy to answer. Its the Sunshine Island, at least in summer, about 30km by 20km, 1.3% of Denmark, nestling in the Baltic about 130km east of Copenhagen. Next question, how do you get there? You fly from Copenhagen to Rønne in a 45 seater. Third question, Why go there? Because the geology is fantastic!
The island is perched on the Fennoscandian Border Zone, and is the most southerly extension of the Fennoscandian Shield, thats the northern 2/3 of the island! The southern portion is an amazing mélange of sediments, broken into tectonic blocks by a jigsaw puzzle of faults overlying the deep crustal fault zone which can be traced from the Black Sea to the North Sea. These form a series of graben basins. The faults first developed in late Precambrian times and have been active in bursts, up to the Quaternary. It has left Bornholm with relics of their activity in the form of great hiatuses in the stratigraphic record, which means that all the sedimentary blocks are different! They are however, stuffed with trace fossils more later in the week!
On this, our first morning, we set out in the Geobus. It was piloted by Wisty our driver and guide to the historical aspects of Bornholm, to make our greatest journey of the week. We left the converted windmill (sans sails), with Richard Bromley our excellent leader, who claimed to know nothing about granites, but knows an amazing amount about trace fossils! Fortunately, Sue Hay does know something about granites, and she had been there before! We were travelling northwestwards; to cross from the tectonised sedimentary blocks, onto the Basement hard rocks! At first glance, Bornholms Basement is a single fault-bounded granite horst, that has come up through the sediments, but we were to discover that it is in reality, a composite. The Geobus took us through the great Almindingen Forest, which clothes the central part of the island, over the rather flat topography (great for cycling) which is familiar from other old parts of the world, e.g. Anglesey, and we dropped down the winding Vang village road, to the sea. Then we bumped along the seafront track, past the old Vang Quarry, the type section of the Vang Granite, until the bus could go no further. We debussed and climbed to the new quarry, under a monumental Anthony Gormley style, rusted iron bridge, to be confronted by the usual dusty sights. Those familiar with pink Shap easily recognized at least two sets of joints; vertical, because of tectonic movement and shearing, and horizontal overburden stress release joints. At the top of the quarry, the joints and blocks were weathered enough to look like proto-tors!
What was special about the Vang granite, to distinguish it from the other three Younger Granites, (Hammer, Almindingen and Svaneke)? All these granites have characteristics of colour and texture, which means they are easy to distinguish in hand specimen. The Vang itself has a peachy-pink hue to add to its high twinkle factor. With crystals averaging between 5 and 10mm, close inspection revealed quartz, feldspar, amphibole and biotite, quite a high percentage of mafic minerals, but still a granite, in the strict sense. Multiple phase shearing delighted us with the fabrics it had produced within the shear zones, including elongated feldspar crystals with pointed ends, up to 25mm long. Viridian green chloritization was also evident because of water influx within these zones.
The granites were penetrated at late stage by magnificent, crosscutting pegmatites with crystals up to 50mm long. The pegmatites had pinkish feldspars showing their cleavage to such advantage, that quite a few specimens were collected for teaching purposes. I think the sunshine helped emphasize their charms!
Richard decided that we needed a stone anvil, so that people need not feel inhibited and just take tiny specimens, so a suitable sized piece was marked out and two good strong men and true staggered back to the Geobus with it and popped it into the boot! Wisty then took us back to Vang Harbour, to look at the wonderful memorial to the stone workers, or steinhuggerei. The worked granite reject pieces were arranged at the harbours edge, outside their working house, on the place where most of the working was carried out in the open air, even in winter weather. They took it in turns to work inside in the, now restored, building. We were also delighted by smucks (the correct collective noun) of small, gently pulsing, translucent jellyfish, which were thronging the crystal clear harbour waters.
So with one Younger Granite under our belts, specimens clutched tightly for comparison with the next one, we moved off in Geobus, to see the statuesque remains of the largest castle in Scandinavia, which is perched on and partly built of the outcrop of our second Younger Granite, the Hammer. The castle is called Hammershus, the House on the Rock, and its origins in 1250 reflect the opposition of Archbishop Absolom of Lund for the King, another example of Prince Bishopry like Durham! His castle, built of the local material, having high place value, forms the base of a wonderfully striped edifice, which is the result of its importance in the early 1500s. That was when the merchants of Lübeck, members of the Hanseatic League, refortified it using the material they knew and used at home brick! So the Keep has brick upper storeys. The Germans left in 1576. After a brief period of Swedish rule, the local people reclaimed it by shooting the Swedish Commandant and presenting their island to the Danish King. The castle became ripe for scavenging but in 1814 further quarrying was forbidden, and the castle forms a great tourist attraction. Three busloads of Poles, on a day trip, arrived whilst we were in the carpark!
The castle crouches on the cliff top, a fine vantage point over the sea. We visited the entrance bridge over the dry moat, the only intact Gothic bridge in Denmark, and walked down the river valley which fringed the south side of the castle. The river had been dammed and 3 mills were originally utilized there as part of the demesne of the castle. Our objective was to examine, very closely, the Hammer Granite exposed under the castle and in the valley walls, and take lunch on the fringes of the Baltic. The Hammer granite is very different from the Vang, a good job wed taken samples for comparison! It is a much rosier pink, with crystals, which appeared to be abraded, and floating in a matrix, it could be likened to an igneous greywacke. Quartz is present, but difficult to see. It is the feldspars which give the granite its colour, elongated amphiboles were also present, but biotite was not readily visible (we needed a thin section)! Again the crystals were large and the texture gave much room for speculation, possibly this part of the granite may have been involved in a tectonic episode, as a shear zone or fault appeared to cut the headland at this point. We enjoyed lunch, waving to the tourists bobbing about on the Baltic, bronzing the limbs and being wowed by a wonderful green frog with golden eyes, female of course, which obligingly sat still on the grasses by a small pool, to be admired.
Feeling refreshed, we set forth along the coast path, under the Hammershus, to see the Lion Rock and other geological excitements. The relationships between the Younger Granites and the Basement are best disentangled by field relationships, but dating them using modern techniques is something which needs doing. Can we find funding for a PhD?
Monday 26th August
We viewed the Rønne granite first from the quarry rim whilst Ulla smoothed the way for our entry into the quarry itself. It was a very handsome black-coloured Precambrian granite, whose feldspars are so clear that the 15% dark minerals dominate the colour. Amongst other things the quarried material is being cut into cobbles, Richard explained that they are being used on the Copenhagen streets as they last much longer than tarmac. Dramatic handsome pink stripes of dilatation pegmatites were visible at the top of the quarry, the result of late stage fluid injection. Returning to the stone yard we admired blocks of the various Bornholm basement granites and gneisses. Sue explained the formation of migmatite by the progressive metamorphism of the country rock or granite to a gneiss. Further heat and/or pressure results in the melting of the lower temperature minerals such as quartz and feldspar which form pink or white granitic stringers leaving behind the higher temperature dark minerals as restite. At this point several muffled explosions without warning suggested that we had been wise to leave the quarry face.
After driving north-east across the island we had lunch on the beach at Årnsdale (translated as Buttock Valley). This beach is composed of gravel sized fragments of decomposed Precambrian Svaneke granite. We then journeyed slightly inland to see a Sprækkedale or fissure valley. These are narrow linear depressions with near vertical sides, probably caused by the erosion of Precambrian dolerite dykes of which there are a large number on the island. Wisti, our coach driver, explained that this one was called Cow Valley because when attacked the inhabitants would hide their cattle in the tree-filled valley before taking refuge themselves in fortified areas. Near the valley we saw the Rokkestenan, one of four rocking erratics, which in spite of years of human interference still rocks slightly. On the way back to the coach we passed a coffin stone where coffin bearers were said to rest their load.
Our next stop was a nearby recently abandoned quarry in the 1700 Ma gneiss at Praestebo, where aligned flame-like bodies of leucosome show the migmatitic nature of the rock. A few years ago an aquifer was believed to have been polluted by the quarry workings so the authorities shut all the quarries in the area. Six years later the pollution is still occurring. The experts were wrong, too late the banning order was rescinded.
On our way back to the mill we visited the Laurentund standing stones, about 50 standing stones of different rock types in an atmospheric forest glen. Finally we viewed the ruins of a Romanesque church beside the new Church at Østermarie. The latter has some wonderful ptygmatic folds in its stone walls. There is also a beautifully laid out and maintained cemetery with simple and geologically interesting head stones with no eulogies to the dear departed. Richard explained that for 25 or 99 years (dependant upon how much you pay) the plot is yours, then it is available for another burial, obviously a practical people.
Tuesday 27th August
Good breakfast, clear sky and warm sun and a prospect of a long walk to the first site. John Wade and I gladly accepted a lift from our hostess, Ulla, in the bright yellow Opel named Ichnos. So it was, having graciously waved as we passed in style, we were able to watch the brightly coloured group happily strolling to the meeting point. Mick Warren was the first of the walkers (two cyclists were thought to have had too great an advantage) but was instantly disqualified for walking too fast.
The first exposure, near a former poor-house at Stroby was like something out of TimeTeam. A trench some 4 metres wide and 50m long with about 1/2m of top soil removed to expose a shallow staircase of sandstone beds gently dipping to the north. At the northern end of this immaculate scrape the gneissic basement is faulted against Cambrian sediments with the crushed zone about 1 1/2m wide. A lasting impression is of one group member standing on the crushed Nexo sandstone and bridging the melange with a walking pole in a graceful arc. Forming the majority of the exposure, down-slope and onto the floor of a quarry, the Hardeberga sandstone showed well-preserved ripple marks. These were not all parallel and initiated a variety of explanations about the environment at the time of their formation.
A small pool at the low point of the quarry was home to a green frog, possibly the edible variety Rana esculenta. Its a good thing France is so far away.
A walk back towards the mill brought us to Lime Street, Limensgarde which gives its name to the mill. Here, in a protected nature reserve, vegetation had miraculously cleared from a quarry face to expose a Lower Ordovician contact between Dictyonema shale below Komstad limestone separated by a phosphatic breccia. The walk into the quarry took over a wooden foot bridge and the Christopher Robins in the party were disappointed with the flow of water in the stream.
After lunch the geo-bus took us to a quarry at Gadeby, Ulla having gone ahead to placate an unpredictable owner. As it happened the quarry had changed hands and the new owner/manager/workman was happy for us to look around so long as we dodged his dumper truck. Here the Nexo sandstone is a wonderful claret colour in places grading to pale cream depending on the haematite staining. The horizontal bedding showed some further ripple marks.
Our fourth location was a beach exposure at Snogebaek where the claret coloured arkosic Nexo sandstone supported the creamy Hardeberga sandstone we had seen before. Here, however, we could see evidence of Skolithos and Diplocraterion as the Lower Cambrian transgression overtook this inter tidal zone. While leader Richard explained the interpretation of this exposure some people snuck away for Ice Creams!!
Our final stop on a warm and sunny day was into the twilight zone. A wood named Gryet (twilight), with the largest collection (56) of standing stones on the island, brought mixed reactions from the group. Some thought it very spooky while others speculated about the lets raise a standing stone parties that might have been. In reality it was not a place to party unless in defiance with gallons of mead.
Wednesday 28th August
This was the day I had been looking forward to: Mesozoic trace fossils with Mr Trace Fossil Himself. After 3 days of brilliant sunshine we woke to a thunderstorm but Richard thought this could be beneficial so we prepared ourselves with the wet gear and set off into the woods at Stampen for our first location, spades, bark knives and all manner of other knives in hand. Under the canopy of trees we hardly noticed the rain but the stream was rather swollen making the OUGS erosion slightly harder than it might have been and giving wet feet to a few members. But it was well worth the effort. Oohs and aahs were emitted in abundance as the pallete knives scraped away the fallen debris to reveal bright green faults and a host of trace fossils in this cross-stratified greensand of Cenomanian age (early Late Cretaceous). This was the Arnager Greensand which lies above the Arnager Conglomerate. But what was that patch of rock to the top right, lying diagonally over the section? The conglomerate!
Close examination revealed that the section had been overturned and the tops of the traces were truncated. Here the dominant trace was the burrow- lining, vertical Ophiomorpha and we could easily pick out the dark surrounds to burrows where pellets had been placed around the edge. Other prominent trace fossils were patches which, at first sight, I mistook for circular sections of calcium carbonate. These were not body fossils at all but the linings of Palaeophycus. Small strings of white dots were Chondrites. The trace fossils confirmed a marine origin for the rock. This was a loose sand so Richard posed the question: How could loose sand be so faulted? Brians answer was that under pressure even sand can behave as concrete. Richard believed that Tertiary sediments had never covered Bornholm and that these sediments had never been deeply buried. His theory was that the faulting had occurred during the ice age when the sand was frozen solid. The debate continued as to whether the green glauconite had acted as a lubricating fluid or had lined the faults at a later stage. By the time we left the section the rain had stopped and we were in for another glorious day.
At the next stop (Stampen 2) we could see the relationship with the Conglomerate much more clearly. Again, the Conglomerate overlay the Greensand, but overlying that was a brown sand of the Jydegård Formation, dated to Earliest Cretaceous age. So again, the layers had been overturned (albeit obliquely). But where exactly was the junction? A large chunk of Early Cretaceous was missing, but right at the top of the brown sand were numerous traces of Thalassinoides. The animals making these traces do not line their burrows but they have been filled with material from the overlying Greensand to form an ichnoboundary about œ m beneath the unconformity. The phosphatic conglomerate apparently contained ammonites of Albian age (some of the missing strata), so the hunt was on . eye of faith convinced someone of an ammonite fragment not letting on who - others saw turtle droppings in every piece!
We proceeded to the beach to Korsodde headland and an exposure of the Middle Jurassic Bagå Formation. First we set to work cleaning off a section about 15 m long. Spades, brushes and knives came out again and some anxiety was expressed as a plane flew low overhead to land at Rønne airport. But our work was not finished yet. We were divided into 4 groups and while Richard and Sue had a well-earned break, the rest of us logged the section.
The first group had identified occasional tangled traces that Richard identified as Bornichnus. He also spotted some small Planolites in the clays. The next group had correctly identified occasional marine Diplocraterion although none showed the diagnostic U-shape. At the top of their section 100% bioturbation was probably also caused by Diplocraterion. The third group began with a soil horizon beneath a thin coal seam, with roots protruding into underlying sand thus giving us a clue to the depositional environment. Further up their section was a return to the Diplocraterion level indicating a marginal marine environment. The fourth group found possible evidence of burrowing in disturbed clay drapes and strangely orientating iron staining, but not until close to the top did convincing traces appear. Some produced small V-shaped structures and there were others that Richard identified as Asterosoma.
What some of us failed to notice was that a good 3D section had been cut by Richard on a former visit to give an excellent exposure of these beasties! We hope well find a better description of this section in print before too long. The clays above contained Teichichnus a reclining J underlain by spreite. We should have been able to identify it as Richard had drawn it for us the night before during his evening talk. Wood fragments were associated with these upper sediments which Richard interpreted as lagoonal. The species of trace fossil were too restricted for lower shoreface described by other authors.
By now we really felt wed earned our lunch, but after a brief stop on the cliff top we were off again. This time, a long hike around the airport perimeter took us to our next location. This was the youngest non-Quaternary deposit on Bornholm, the Bavnodde Greensand of Late Cretaceous, Senonian age. This proved to be the least interesting section of the day, but apart from the wood, it did turn up the first body fossils. Di Smith found a section of belemnite in the storm bed and there were fragments of bivalve but it was our geobus driver, Wisti who came up with the best specimens. He found a nice sponge and a woodboring pholad, Martesia. Well have to come back in the spring when the section is cleaner!
We made up for it with the final geology of the day. This time buckets and brooms were employed to clean the wave-cut platform exposures at Arnager. Excellent Thalassinoides were eroded out in 3 dimensions. We were now back to the Arnager Greensand that we had seen earlier in the day, but instead of the Greensand being transported to lower layers by burrowing, in the cliffs we were seeing the white chalk from above being piped into the Greensand (this time lying horizontally, the right way up). The burrows also contained phosphatic nodules from the conglomerate layer at the base of the chalky Plattenkalk of the Arnager Limestone. As before, the phosphates signalled a hiatus in deposition. Our next scrubbed section showed Zoophycus winding in between the phosphatic nodules. This was a bit of an enigma as Zoophycus is characteristic of quiet, poorly-nutritious, deeper water sediments (probably 100-200 m during the Cretaceous) and here they were within the high-energy environment of the phosphatic conglomorate. The explanation was at hand: they had been scavenging the layers through the younger Plattenkalk, reaching the conglomerate long after it had been deposited. We finished the day with a spectacular display of sponges well worth all the cleaning.
Today was a rare opportunity to see a wonderful display of trace fossils with not too much distraction from the body fossils. Thank-you Richard. There was just time for another Wisti mystery before our return to the Mill for tea and cakes. This time we visited the chambered Passage Graves at Arnager. Despite the traces of our ancestors in the form of cup marks, geology overrode the mysteries of the Ancients as we admired the onion-skin weathering on the erratics used to line the tomb.
Thursday 29th August
Rønne - Today was spent mixing a bit of geology with some interesting features of the island. First was a visit to the capital, Rønne which is the largest place with a population of about 14,500. It is linked to mainland Denmark and Sweden by ferry and to Copenhagen by air. Wisti, the driver took us on a geobus tour round the city followed by a walk in the centre and along the harbour. The houses are mostly simple wood framed buildings, very characteristic with many coloured in the distinctive Bornholm Red, others in bright colours. There are very few tall buildings here or indeed anywhere on the island. The commercially active harbour is quite large. One landmark within the town is a tall lighthouse, another being the church within which hangs a large model sailing ship. Wisti told us that a similar model (also in the church) is carried down to the sea once a year for a blessing ceremony. After a break for coffee and some shopping then we went on to Hasle, on the coast north of Rønne.
Hasle - At Hasle we were introduced to a common small industry in most of the seaside villages, that of fish smoking. After a look in the museum we were treated to a splendid special smoked fish lunch of herring, mackerel, salmon (a big thick slice of this!) and prawns, accompanied by a salad, dark bread and naturally, Bornholm Beer. We were impressed by the finger de-smoking facilities!
Madsegrav - Following this treat, a beach exposure of Cretaceous sediments followed. Access to the beach was at the poorly exposed junction of the Arnager Greensand and the Robbedale Formation. Farther along the beach, the muddy sand in the upper cliff contains the trace fossils Teichichnus and Thalassinoides. This overlies some pure white sand in which long thin Ophiomorpha nodosatrace fossils were evident, unbranched here, but, although not seen on this occasion, in other places giving a box like appearance. Phosphate nodules are present. Walking farther, we come across the Rabekke Formation, basically dark clays with scattered quartz, kaolin and some fossil logs and wood.
More Churches - During the day we visited two more churches - that at Åkirkeby, the largest church on the island with 17th C. fittings, a decorated canopied pulpit and an extremely old sandstone font. The other church was the round church of Nylars, a most fascinating building. In times of strife, the churches doubled as forts. Of note here and at other places were the wonderfully kept graves with superb examples of all the indigenous stones of Bornholm.
The Læså Stream Bed - An afternoon stroll of some two kilometres was undertaken along this stream bed, which is in fact a reserve no hammering! Fossils are not plentiful although the loose material in the stream bed was examined for trilobites etc. However , we were able to progress up the stratigraphy along the way. At the start was Lower Cambrian Broens Odde silty sandstone, well bioturbated, then a well exposed boundary with the Middle/Upper Cambrian Alum Shales, showing the thin Rispebjerg Sandstone and Exultans Limestone. The sequence up to the latter is interpreted as a single transgressive/regressive cycle which is considered to be a global sea level change. A short distance up the shales there is an outcrop of Ordovician Andrarum Limestone, in which were seen some large concretions of anthraconite. Finally in the stream bed the Komstad Limestone appears and further upstream are low cliffs of Dicellograptus Shale, named after a graptolite. This walk was great in the sunshine and further reward was had in examining a long barrow, not to mention seeing a wonderful bright yellow bracket fungus on the way. Another satisfying day.
Friday 30th August
This was our last day in the field, and the first stop was one of Richards "mystery" locations. Our geobus with Wisty at the wheel set off north to Aakirkeby and then edged its way across towards the NE coast of the island until it reached Listed, a very pretty seaside village. We finally stopped about a kilometer up the coast, and made our way down to the beach where large exposures of Svaneke granite gave way to shingle and the sea. However, Sues instructions were not to admire the view, but to measure Dip and Strike and to look for variations in the rock which is a "younger" granite at only 1400 M yr. We were not surprised therefore to find bands of migmatised Gneiss of mixed origin in contact with the Svaneke which itself we estimated to dip at 30 and strike 200/020. Our duty done we moved nearer to Listed to find a splendid dyke of dolerite, with very fine olivine crystals, intruding the granite, not surprisingly at 185/005. It seemed that the dyke had chilled out against granite that was already cool but the origins of the dyke were long gone so we dont know which way it flowed. But we did recognise the "pepper and salt" sediments that occurred much later, forming a neptunian dyke, possibly in the lower Cambrian.
After the serious work, a bit of fun at Arsdale mill where the sails were whizzing round and the main shaft in amazingly silent motion. We climbed to the top enthusing over the wonders of the engineering, and then descended to the mundane matter of buying souvenirs. Off then to Østerlars Kirke, the largest round church on Bornholm, for a bout of picture taking, and finally back to the mill for lunch and to say farewell to our friend Wisty, who had other duties in the afternoon.
Refreshed by lunch we strolled down the road to Skelbro Quarry where is exposed the lower Ordovician Komstad Limestone, 5m thick and taking 5M years to form, -nothing rushes in this part of the world. We busied ourselves looking for trilobites, having been assured that 3000 had been found hereabouts, but well, perhaps it was the lunch, perhaps they had scuttled away, some of us found fragments. With a glance at the glacial striations on the uppermost surface of the exposure, we moved off to the beach once again.
Definitely lower down, but also a lot more recent, in fact Triassic, the Kagerod Formation, coloured clays and cream sandstone and occasional caliche concretions. Where the Risebaek flows down, we turn off and climb up again to the level of a waterfall, crossing a fault to arrive at Upper Ordovician exposures, the Dicellograptus Shales. Here graptolites were found by several party members, making up for the disappointments of the absent trilobites. And returning to the beach, in a sandstone boulder Richard pointed out a whacking Diplocraterion parallelum in 3 dimensions with 10cm long U-tube branches and 3/4cm spreite wonderfully preserved.
Moving N up the beach we came to Richards second Mystery of the day. With the aid of trace fossils we were put to work to try and match up two small outcrops either side of a headland. What Richard hadnt anticipated was that, well-trained as we had become in the art of surface scraping, we would uncover an outcrop of black sand with a sharp contact with the overlying white sand above. Trace fossils of the white sand had been piped down into the black, but the origin of the colour remains a mystery as the sand itself seemed to be the same. Richard had never seen this black sand before.
The section was a cacophony of colours with reds and greens in a rather chaotic way and some of the white sand was soft and some hard. Looks like the whole section will need cleaning up and another paper produced... Eventually we did get down to the exercise of the afternoon and matched the right-hand base of the section with the one we had first examined round the corner. It seems that there is a whacking great unconformity at this point, probably caused by faulting, with the white, red and black loose sediments, probably of Triassic age, overlying the green sediments with trace fossils. These were best examined in the undisturbed section where we could find excellent examples of Teichichnus. Characteristic well-cemented brown stormbeds helped date this rock to the Lower Cambrian Green Shale (actually a sandstone also known as Broens Odde Sandstone) which we first encountered on the beach at Snogebæk (what a language).
The day finished with a reception and presentation to Richard and Ulla for their wonderful hospitality and brilliant geology. Sue had also bought a present for Line, a geology student, who had spent the week busily in the kitchen providing delicious meals. We had a splendid evening under the stars finishing with singing, vigorously conducted by Mick. Our rendering of 'The Music Man' would surely have brought the neighbours round if there had been any within a couple of kilometers!
Worbarrow Tout led by Paul Ensom
22 September 2002
16 of us met at Tyneham Village in the Isle of Purbeck, Dorset, glad that, unlike last year, the walks were open and the army would not be using us for target practice. We were met at a viewpoint overlooking the splendid expanse of Worbarrow Bay by our leader, Paul Ensom from the Natural History Museum, who gave us our safety briefing very necessary and explained the geology of the area. Because of faulting and folding of the Mesozoic, we could see a succession of formations spanning the Jurassic and Cretaceous, nearly 100 million years. Here, overlying Kimmeridge Clay, a source rock for oil, were, in stratigraphical succession, Portland Beds and Purbeck Beds, Wealden Beds, Gault and Greensand, and finally the Chalk. Worbarrow presents a basinal facies, open marine conditions alternating with hypersaline episodes, brackish lagoons, swamps, estuaries and rivers; a marginal marine environment where we could find freshwater and marine fossils and, if we were lucky, dinosaur footprints.
On the map Worbarrow Tout is a little finger curving southwards out from the Dorset coast at the eastern end of Worbarrow Bay. As we descended in glorious weather towards the beach, we found it was a hill, about a hundred metres high jutting up out of the sea. As we scrambled over the boulders along the inside of the curve, looking westward towards Lulworth Cove and Portland Bill, we could get a general view of the formations we would be examining in detail later. Strata, dipping steeply to the north-east, have been cut through to the west forming an upended cliff whose base is Jurassic and whose crest is Cretaceous.
The boulders themselves, which had either fallen from the cliff or had, in the opinion of our expert guide, been washed ashore from further round the point by the storms that batter this coast, were specimens in their own right, but a formidable obstacle to progress: it took something like an hour to get us all assembled in a little cove just short of the point. Many thanks from the writer of this piece for help and encouragement received! Here we were near the base of the succession and as we made our way back to the beach we would travel stratigraphically upwards and forward in time. This section had been the focus of research done by Paul in 1983 and 1984, and our field notes were a copy of his paper in which 12 members had been sorted into 209 beds.
The cliff was spectacular, deeply undercut at the base and rising in protruding limestone shelves where softer sediments had been eroded away. Paul pointed out one of the Broken Beds (9b) that give their name to this member and we saw evidence of evaporites and micritic shales. Lunch, which the writer had cleverly forgotten to bring, (more thanks! geologists are a kindly lot) was taken in the vicinity of the Soft Cockle member, where there were halite pseudomorphs and huge lumps of gypsum in complex folds. And so we made our slow way back, looking at stomatolitic beds, chert formations, tetrapod tracks. Somewhere just beneath the Cinder Member (beds 119 120), within the Cherty Freshwater Member (beds 100 118) lies the boundary between the Jurassic and the Cretaceous (according to recent research in the North Sea).
Back near the beach, where the cliffs are lower and rubbly, were pyritic patches at the top of the Unio Member (beds 200a and b). There are clasts and bits of bone and pale green argillaceous micrites, and Paul performed his last trick by upending a very large boulder to reveal a splendid cast of a dinosaur footprint.