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.
Crystal Palace • Herne Bay • Butser & The Weald and Downland • The Pyrenees • Isle of Sheppey • Newhaven and the White Cliffs • Harefield and Northmoor Woods • Mam Tor
300Ma in 300m (or thereabouts) - Crystal Palace with Austin Lockwood
20 February 2003
Half Term February 20th 2003 a bright, sunny afternoon and about 20 intrepid time-travellers set off from the Crystal Palace information centre to explore the story of the dinosaurs. (From various accounts, some had already had to prove their intrepidity in order to arrive at the launch pad.)
The renovated dinosaurs and geological trail are not yet open to the public, but permission had been gained from Bromley Leisure and Community Services by our leader, Austin Lockwood of the Ravensbourne Geological Society. Additional information was provided by Di Clements, based on The Crystal Palace Dinosaurs, McCarthy and Gilbert 1994, published by The Crystal Palace Foundation.
The antediluvian reptiles were constructed between 1850 and 1854 by Waterhouse Hawkins in his extinct animals studio at the Crystal Palace, where an enlarged version of the glass and steel structure built in Hyde Park for the Great Exhibition of 1851 was erected in 1852.
The dinosaurs are arranged on and around islands in an artificial lake, originally Paxtons Lake of Extinct Animals, which was tidal because the level of the water rose and fell according to the requirements of the fountains higher up the park. The islands were grouped in primary, secondary and tertiary zones, and this arrangement has been preserved, though there has had to be some reconstruction, in particular the pterodactyls, which had been broken and lost. No effort has been made to alter the life-size models according to more recent ideas, so they remain a monument to the skill and imagination of the scientists of the time. From Paxtons Bridge you can see a cliff of 300 million year old limestone, faulted Coal Measures from Derbyshire, and you could once have visited a lead mine, complete with stalactites and stalagmites. The islands have been planted with tree ferns and other plants, so that the iguanodons and megalosaurus seem to roam through their natural habitat.
On New Years Eve 1853, a dinner was held in the cast of the lower half of the Iguanodon for the scientists involved and the Directors of the Crystal Palace Company, washed down with copious supplies of champagne, prompting a journalist to report: After several appropriate toasts, this agreeable party of philosophers returned to London by rail, evidently well pleased with the modern hospitality of the iguanodon, whose ancient sides there is no reason to suppose had ever before been shaken with philosophic mirth.
Hungry after their wanderings amongst prehistoric monsters, the intrepid LOUGS party repaired to the heights of Chislehurst to emulate the agreeable party of philosophers at the Tigers Head, and afterwards to Bromley High School where Wilf Walker gave us a fascinating talk with slides on his experiences in Antarctica in the 1970s, looking for the position of the ridge just after the idea of plate tectonics had become accepted, with a primitive computer, using dead reckoning when the satellite guidance was insufficient. An exploit rather more heroic than our adventures among the dinosaurs!
S260 Field Trip to Herne Bay with Members of the LOUGS
30 March 2003
This was a field trip organised especially for S260 students, and Herne Bay turned out to be an ideal site for our initial foray into the practical world of geology. The geological features we saw, and the enthusiasm of the tutors complemented the theory of the tuition blocks extremely well and really made our studies come alive.
The day started cold, grey and quite misty, but turned out to be a day very high in twinkle factor and sunshine. After an enthusiastic introduction by Diana, we all scampered down the path to the beach, eagerly clutching our hand lenses and grain size charts. The tide was in, and I quite anticipated being mown over at any second by a jet ski emerging from the heavy sea mist as we examined the many different types of pebbles (predominantly flint) on the beach. Here we saw that this beach was new sedimentary rock in the process of being formed, and we started to understand how to describe and interpret the characteristics of the pebbles in terms of rounding, sphericity, size, grain size, sorting and hardness.
Above the beach we saw a number of slightly dipping sedimentary beds exposed in a cliff face, including a prominent black pebble layer. We could see many of the pebbles from this layer scattered on the beach and at the foot of the cliffs in the landslips. One of the lecturers (who shall remain unnamed) was overheard to wish for a catapult as she spied one of the students bending over in a most provocative fashion at the top of the cliff. Not what youd call a usual application of geology..!
We could see evidence of landslips along the foot of the cliff, and Brian gave us a demo of rotational land slips with a super Blue Peter model he had prepared earlier, and he explained the various means of prevention which we could see around us.
Further up the beach, we looked at some of the sea defences which had been erected to protect against the rotational slips. The defences were remarkable for being comprised mostly of igneous and metamorphic blocks which had been shipped by barge all the way from Norway. The larvikite blocks had huge, interlocking crystals, which we learned was indicative of a melt and slow cooling. They had a fantastic twinkle factor which became even more apparent later in the day, when the crystal cleavages glittered beautifully in the afternoon sunshine. Here we learned about how to identify rocks containing mafic minerals (dark, heavy with iron and magnesium) and felsic minerals (pale, light with quartz and feldspar) and we learned to recite in unison at least one mafic mineral is present. We saw the effect of stress and metamorphism in the segregration of the felsic and mastic minerals to form a texture. The sea defences also contained blocks of yellow fine-grained local concretions, that had fallen from the Thanet sand layer in the cliffs above, and greenish blocks containing evidence of bioturbation and shell fragments from the Harwich formation in the cliffs above.
At the foot of the cliff we practised drawing our own vertical sections to scale, learned how to distinguish between clay and silt (eat it) and even found some fish teeth fossils. At the top of the cliff we were able to examine the upper beds and their boundaries. We saw evidence of climate in desert roses which would have formed by evaporation in a hot climate. We saw evidence of cross bedding, unconformable surfaces, fish fossils, and surface undulations indicative of strong current. We also found selemnite crystals, which prompted Brians story about how iron pyrite oxidises so quickly it can use up all the oxygen and force mining tunnels to be abandoned.
We all learned a great deal from the excellent and enthusiastic guidance of our accompanying tutors. Many thanks on behalf of all of us to all the tutors and organisers for a most rewarding day out. On a personal front, I also learned some valuable tips on field trip organisation:
After a very pleasant lunch (with the option of beer) we gathered by the Saxon Towers of Reculver. Cant say Ive ever taken such a close interest in the materials used to build a wall before, but now I am an initiate, I just might do it again! I wonder what the ordinary tourists who saw us peering closely at the walls were thinking but I bet they dont all know what we know about those walls now. I heard someone (one of the tutors, at a guess) say One could find a much more refined wall than this, but it does have character!
And the towers certainly do have character, they are the only remnants of the medieval church of St Mary which was relocated inland in 1809, from this site which was originally a Roman fort when the sea was a mile or so further north. Strengthened in the 1970s, the towers today are used as a landmark for shipping. From a plaque inside the wall: When ancient towers were afterward blown down.
We moved along the sea front as the sun came out, and ooohed and aaahed at the twinkle factor of the huge Larvikite boulders used as sea defences. You know, I think Di made that term up, I really do . But, they were very sparkly. And doing a sterling job of keeping the tides at bay. Mind you, as with the wall, the defences had their own story to tell, the boulders have been brought from Norway whereas the stones comprising the walls of the towers are a wide mixture of local materials. Some of the boulders were decidedly less twinkly though, being made of limestone. Limestones a lot less twinkly than Larvikite, really.
Down onto the beach we paused, briefly, at an old midden containing oyster shells this area is renowned for its oysters and it was suggested that this might be a Roman midden.
Avoiding the Fulmars, (which apparently will stop at no lengths to keep people away from their nests), we looked at the cliff behind the beach which contains lots of shell horizons, whilst diligently donning our hardhats. I wonder how well a hardhat will protect one from the overt zealousness of a Fulmar? (also known as tube-noses). The last thing I looked at was evidence of Boring Piddocks, which sound to me like some kind of dull hand tool but which are, Im told, molluscs.
A very good and interesting day was had, despite the somewhat chilly weather. Do real geologists always have to spend all day outside on such wintry days? Or perhaps it was because it was Mothering Sunday, do real geologists celebrate Mothering Sunday?
Butser Ancient Farm & The Weald and Downland Open Air Museum led by Diana Smith
18 May 2003
Although this was billed as an examination of vernacular buildings and the use of building materials, in reality this was an exhilarating roller coaster ride through 3000 years of history! We started somewhere between 1000BC - 43 AD in the Iron Age, visited the Romans briefly, hurtled wildly forward to the 21st century, plunged back to Medieval times, before returning to our own time.
We met at Butser Ancient Farm near Petersfield, Hampshire. Although I had been past it many times this was the first time I had visited. What a surprise! If one ignored the cars, one could be back in ancient times, standing outside a ditch and bank earthwork containing a large roundhouse fully 15m in diameter, plus three other smaller huts. Soon we were seated on logs around a blazing fire in the hut, whilst Jonathan Edwards, the Education Officer for Buster Ancient Farm explained some of the aims and objectives of the site and the details of the construction the roundhouse.
Butser Ancient Farm is an open-air laboratory devoted to the exploration by direct experimentation of the problems and theories which emerge from archaeological excavations. The focus of the site is the Iron Age (1000BC - 43AD) and the Roman period (43AD - 400AD), plus an interest in the development of archaeological technology. The central feature of the site is the enclosure round the Great Roundhouse has been re-constructed. The Great Roundhouse is most impressive and is based on excavations at Longbridge Deverel Cowdown in Wiltshire. It is built on the double ring principle. The inner circle of posts with a horizontal ring of timber mortised and tenoned into place sustains the weight of the roof of over 25 tonnes! Undoubtedly, the house was the dwelling of a rich and powerful person, but there was little evidence of how the interior was laid out, apart from post holes and the hearth. As we sat around the fire, there was little to remind one of the 21st Century and it was easy to project ones self backwards and to speculate what it could have been like to be an Iron Age farmer listening to an elder of the group explain the passing of the seasons and the working of the land.
Outside, our guide (and lurcher dog) walked us around the site through the Welsh roundhouse from Moel y Gerrdi and the two smallest from Glastonbury Lake Village, then to the ditch and bank that forms the boundary of the enclosure. This ditch and bank itself is one of the experiments taking place on the site. Normally ditches start in a "V" section with the bank on one side, formed of the spoil. The archaeologist normally only finds the ditch, the bank material having been back-filled. The silting and erosion layers found in the ditch can be used to deduce the location of the bank, since they are often asymmetrical. The Butser ditch and bank is been used to test this deduction. Since the plants which colonise the ditch and bank have an influence on the erosion, these are also being studied.
After the Iron Age, it was fast-forward to the invasion of Britain and the Romans. A major experiment is underway to reconstruct a specific Roman building of the late 3rd/4th centuries, paying particular attention to the operation of an aisled hypocaust. The original building was excavated at Sparsholt, north of Winchester in the 1960/70's. Although the walls have been laid out on the original pattern and the walls built to just below first floor level, the experiment is concentrating on the north end of the building to explore the hypocaust.
After a brief return to the Iron Age and a look at the crops being grown to examine the potential yields of ancient farms and farming methods, and hence may give insights to the numbers of people that could be supported, we were brought back firmly to the 21st Century and the Geophysical Research Area. Any fan of Channel 4's Time Team will know the importance of "Geophys" to modern archaeology. At Butser, a testing ground has been built to provide a known range of archaeological features - ditches, gullies, wall foundations, post holes, pits, hearths and kilns, floors and burials. These have been meticulously constructed and recorded, so that new developments in geophysical prospection machines can be tested. We were lucky enough to talk to two of the scientists involved and to see the features that they were working on - we are now sworn to secrecy!
All too soon it was time to leave Butser in our chariots - sorry - cars, and make our way across Hampshire to the Weald and Downland Open Air Museum.
Set in 50 acres of beautiful Sussex countryside, the Weald and Downland Open Air Museum is a fascinating collection of nearly 50 historic buildings dating from the 13th to the 19th century, many with period gardens, together with farm animals, woodland walks and a lake. Rescued from destruction, the buildings have been carefully dismantled, conserved and rebuilt to their original form and bring to life the homes, farmsteads and rural industries of the last 500 years.
We started our visit firmly in our own time, at the Gridshell. The upper part of the building, the Jerwood Gridshell Space provides a workshop where historic timber-framed buildings can be laid out for conservation and repair. The size of the space allows for large frames to be assembled and it will also be used for the museum's growing programme of training workshops and the conservation of large objects. It is also used for exhibitions and other events. The basement store is secure and environmentally controlled and houses the Museum's collections of tools and artefacts from rural life in the region numbering about ten thousand items. Agriculture, domestic life, trades and industries and transport are all represented, and there is a special emphasis on building construction and the building trade.
A gridshell is a structure with the shape and strength of a double- curvature shell, but made of a grid instead of a solid surface. The grid can be made of any kind of material, but the Downland Gridshell is made of slender oak laths bent into shape. To prepare the oak laths for use all defects were removed and the resulting pieces finger-jointed together into standard lengths of 20 feet (6m). Six of these pieces were then joined to form 120 foot (36m) laths. The diagonal grid of laths was initially formed flat on top of a supporting scaffold. The edges of the grid were then lowered gradually, and the grid bent into shape, until the full shell was formed and secured to the edges of the timber platform above the basement. The grid is actually a double layer, with two laths in each direction. This is necessary in order to combine the required degree of flexibility with sufficient cross section for strength. A fifth layer triangulates the grid to increase its stiffness. The laths are connected at the nodes of the grid with a patented system of steel plates and bolts.
From the ultra -modern of the Gridshell, we plummeted back into the Medieval past so that Diana Smith could take us on a guided tour through the history of Wealden vernacular buildings. Diana firmly anchored her explanations of the architecture and the materials used in the buildings, in the geology of the Weald. I have been to the museum many times, but I was able to see it through new eyes due to Dianas skill and knowledge. One of the highlights for me was the Watermill from Lurgashall, a 17th century working mill. We were lucky enough to have guided tour from the miller himself and I took the opportunity to buy a sack of flour for home bread making delicious!
All in all a very good day out many thanks to Diana for making it so interesting.
London Branch in the Pyrenees led by Paul Grant
25-30 May 2003
Sunday 25th May 2003
I volunteered to write up the first day of our trip to the Catalonian part of the Pyrenees because I knew that it was going to be interesting and so it was, and complex, and mixed and very enjoyable.
First the weather, when we woke the weather in El Pont de Suert was sunny it augured well for the day I thought! My gloves scarf and woolly hat stayed in the case. Waterproof trousers went into the rucksack along with suntan cream and my lunch. We left the hotel in sunshine looking towards the white tops of the mountains in the distance. They began to become shrouded in mist, O.K. it was still quite high no problem! We travelled on and the mist got thicker and lower. We reached the first stop, close to the tunnel, and got out of the vans. The weather closed in. Bitterly cold and blowing a gale then the snow came down. It was freezing out came the waterproof trousers and I wished that I had taken the hat, gloves and scarf too. Undaunted we went to look at the rocks. It was a relief to go back to the vans. The snow did stop and the sun did come out but it did not get a lot warmer. By the time we had our lunch high on the side of a mountain we were able to sit outside, admire the view and what could be better than lunch surrounded by mountain peaks with snow in the background.
So what of the rocks? In a word - complex. To set the scene. The Pyrenees were formed as a result of the Iberian lands being pushed up towards and skewed against Europe, initially during the Hercynian orogeny between the Carboniferous and Permian, and latterly during the later Alpine orogeny. The central table land of the main craton has been around since the Precambrian and it has been moved around and much altered during its long existence. The oldest rocks we were to see would be the latest Precambrian, sandstones and shales deformed at the end of the Precambrian. The conditions would have been dry and hot with little input. There would have been an abundance of algal mats and dolomitic limestones typical of low input, high energy areas. During the early Ordovician there was uplift with many delta formations. There was a hiatus between the Ordovician and the Silurian with very little sediment until the mid Silurian when there may have been a sea level rise bringing in fresh layers of mud. In this area there is also evidence of later periods of arid conditions with red beds and also limestones from clear seas.
Our first stop, near the tunnel, was to look at the Maladeta granites intruded in an elongate oval trending East West 275 300Ma ago into the Pyrenees at the time of the Hercynian Orogeny. We looked at granite boulders with slickensides (much sensuous stroking), later intrusions showing flow banding, veins with quartz and feldspars and xenoliths these were very easy to see and gave a real sense of the country rock having fallen in and being gradually absorbed they showed up as darker areas with extra ferro-magnesium minerals.
The second stop was back along the road to look at large zoned garnets. A Big Wow factor garnets about 30 mm across with zones clearly visible. As the granites were intruded they formed fluids in hydrothermal zones which then caused reactivation of various minerals. As the conditions changed slightly so the garnets were crystallised with slightly different zones of mineralisation which still exist today millions of years later.
For our next stop we changed our focus from the small to the large 2024m above sea level (as tracked by two GPS sets) we were looking at what had been a glaciated dome of ice between two glaciers. It had been uplifted and incised by rivers and streams, some of which are still active. A truly impressive sight giving a real feel for some of the forces involved in shaping the landscape.
Back to the smaller scale for our next three stops. Glacial moraine for the first and evidence for multiphase deformation in the next two. The lower Palaeozoic rocks were highly deformed by the Hercynian orogeny and display in the field a variety of complex deformation features caused by relatively uncompetent deflection of slaty cleavage against the more competent carbonate rocks. Paul was excited by a piece of rock which showed 3 phases of compression. The rock had a slaty cleavage with a set of crinkles which makes it into a phyllite and another set of crinkles in another direction which indicates yet another phase of deformation. By looking carefully at the small scale structure of the rocks the detail of the larger canvas is painted in. We were also able to put pictures to the words that we read in the text books with excellent examples of carbonate and ptygmatic folds. In a road cutting further down the valley we saw different varieties of tension and extension features including boudinage and evidence of flow banding. This area has been affected by two main phases of orogeny and the rocks reflect many phases of deformation.
We finished the day by thinking in the large scale about the Noguera zone ready for the geology for the rest of the trip. At this point, at the end of a long and exciting day, I got confused. Reading my notes all this time later makes no sense to me now but I am sure that all will be made clear about thrust zones, duplex beds and decollment zones in the write ups from other people. It was a wonderful day because it was so different to anything I had seen before and set the scene for the rest of the week.
Monday 26th May 2003
Today was to be our Triassic day, and it dawned characteristically sunny and hot. Our first exposure was just a short walk from the hotel and consisted of the Keuper (Upper Triassic) Pont De Suert Formation.
The beds here are near vertical and are composed of re-crystallized and deformed gypsum intercalated with red shales. Paul interpreted the depositional environment as one of sea water refluxing into shallow lagoons of high salinity and with flourishing algal mats. It could be seen that the beds had high porosity, and Paul demonstrated that hydrocarbons could be smelt on broken samples. Considerable deformation of the gypsum could be seen, with flow structures and boudinage present. The deformation has been attributed to the Alpine Orogeny.
Our next stop was a roadside exposure of Triassic Red Beds, which here consisted of current bedded very fine sand with mica and quartz pebbles. The fact that these beds are inverted could be demonstrated with bottom structures, graded bedding and truncated current bedding. Again secondary porosity could be seen due to the weathering out of calcretes and mud clasts.
After a view stop to look at the phyllite basement overlying Permian conglomerates we moved on to look at some intermediate ophite volcanics. In these the pyroxene and feldspars had crystallized at the same time forming interesting crystal structures with pyroxene centres and radiating plagioclase.
Paul had promised that our final stop at Erill Castell would be exciting. Little did he know! After a precipitous climb in the minibuses up a rocky mountain track we had a problem turning the vans round. One of our drivers (who will be nameless) decided to achieve it by driving flat out in reverse and throwing the van onto full lock. The manoeuvre was completed about a foot in front of Pauls van, and the look on his face had to be seen to be believed!
It was well worth the effort though. At our first exposure in near vertical beds an unconformity between the Lower Palaeozoic basement and Middle Carboniferous could be seen with a coarse breccia at the base of the Carboniferous. Moving along the section we passed into pale pyroclastic deposits with large angular clasts. There was some discussion regarding these, ash falls? Pyroclastic flows? Falling into water? Next in line were river channel deposits with carbonaceous sand and shale overbank sediments, coals, log jams and well preserved Carboniferous plant remains. Exciting stuff! Just to finish off those who braved the climb up to the castle could examine basalts of doubtful age. A great day!
Tuesday 27th May 2003
Today, we were to up temporary roots and move from Pont de Suert to Tremp. In fine weather and having achieved the packing in of (too much?) luggage, we set off. Firstly, to the north to view some rocks in a quarry which were supposed to show ophitic texture. This is where feldspars have recrystallised as radial laths within a pyroxene crystal. Some thought they found the state, others werent convinced. No matter, it served to spend time usefully until the shops opened so we could assemble, without a great deal of dither, the necessary for lunch!
On leaving Pont de Suert for the second time, it was some relief to our drivers not to have to negotiate mountain tracks, the road being good. We moved across country to a spot near Perves and examined from across the valley an outcrop of Oligocene Collegiate conglomerates, these being overlain by some more massive material with some folding. The exp osure was about 650 m over about 150m sandy units lying on basement. Sheets of conglomerate showed low angle cross bedding and appeared to come out as alluvial flows. There was coarsening up from quite fine material to some large blocks. This area is thought to have been the infilling of an old valley by a braided system.
We then proceeded to and through Tremp, with a fleeting glimpse of the next hotel. We followed the valley of the Noguera Pallaresa, another river of mountain origin, destined to join the Ebro. As such it had moments of rushing, some braiding and gentle and broadening flow. There was plenty of geology to be seen in passing, including a dramatic gorge. We were able to see the extent and relative flatness of the Tremp basin as we went.
Near to Fontilonga we came to some rising ground and stopped for lunch on a small promontory which gave us a splendid all round view of the landscape. To the south, over a field blushed with poppies, were hills more rounded than in the high Pyrenees, beyond these Paul told us lies the Sierra Marginales, the foothills of the Pyrenees, after which lies the Ebro basin. To the west at some distance across the river we could see the Monsec Thrust against the skyline.
After a leisurely lunch, we tracked back a hundred metres or so along the road to study a splendid roadside section. There was at the top a layer of very well weathered sandy material thought possibly to be an oxidised thrust. Under this were gently dipping massive competent sandstones. Below this and obscuring the face were heaps of loose sand layers. Marks and evidence of road surface repair suggested frequent falls of the harder stuff. Farther down the road more muddy material appeared as well as obvious channel fill. These are thought to be meandering river channel fills with one particular one, according to Paul, could be an oxbow infill.
A short ride further down the road where there was a bend the rocks were more broken with muddy infills in which some fossils were found. We then came across a stunning example of ripple marks, well exposed. These were still sandstones and with some trace fossils and forams being found suggested a terrestrial offshore marine deposit either tidal or as the ripple marks showed oscillation perhaps a long shore drift deposition.
The last thing looked at was a rocky block in which were found plenty of bivalves Thence, the longish ride back to Tremp to take up residence for a few days of further geowonders!
Wednesday 28th May 2003
We left Tremp on a beautiful, sunny morning and headed east. The road we wanted to take was closed for road repairs, so we drove up to the pretty village of Abella de la Concha. Here we had magnificent views. We were on the north-east side of the Tremp Basin. To the south was the Monsec Thrust sheet that we had driven through the day before. This thrust sheet had been carried forward south and upward, and we were standing in the village with the thrust sheet somewhere at depth below us. We were standing right up against the Boixels Thrust sheet. Below us we could see the Upper Cretaceous beds, and Tertiary beds of the Tremp Formation banked up against the thrust and overturned.
As we walked along under the vertical rocks, we could see and touch the thrust fault. The evidence for this is the flat, smooth surface with striations, and deformation of the beds underneath. The breccia was very thin on the thrust fault at this point, but further along we saw three metres of deformed shales, vertical and dipping to the south. Above the fault, the vertical Santonia limestone forms a hanging wall anticline. This is known as the San Cornelli Anticline. We looked across to the beds dipping to the north at the north end of the basin. The Monsec Thrust is pushing south, and the Boixels Thrust is riding over it, the lubricant being the shales. The Tremp Basin is a piggy-back basin riding between the two thrusts. It was awe-inspiring to think of the great forces at work as Spain pushed into the rest of Europe.
We looked at the views, watched Griffon vultures circling overhead, and then drove to Isona to have a refreshment stop. We visited the museum to see a geological exhibition including dinosaurs. While the road works were suspended for lunch, we were able to squeeze past the machinery (only just!) and drive to a viewpoint where we had a very good overview of the San Cornelli Anticline. This made the complicated geology a lot clearer.
We spent the rest of the day looking at some of the Tremp Basin deposits. Further along the road, at Faidella Pass, we examined the gently dipping Santonian Limestone. In one direction these deep-water marine limestones became more marly, and in the other direction, were overlain by sandstones containing plant debris, and cemented with calcareous cement indicating shallowing conditions. At our next stop, back along the road, we found tidal channels with big open folds, which have been interpreted as dune structures, 50-60 metres deep, similar to those seen in the Folkestone Beds in the North Sea. Here we could see large-scale cross bedding, poorly sorted gravels and sands.
We had a picnic lunch in the sunshine and listened to nightingales singing from every tree. Then, to the delight of some of our party, we went to look at some fossiliferous rocks. We found calcareous algae, and rudists. These are large conical bivalves, where one valve acts as a lid, looking rather like an ice cream cone with a lid. We saw some with lids. These specialised bivalves did not survive the Cretaceous, and were confined to warm limestone seas.
The final visit of the day was a visit to Orca Castle, where energetic members of the party climbed to the top, while others took a more gentle stroll, to see the steeply dipping Aren Sandstone interbedded with finer grained rocks. This represents a marine slope with small submarine erosive channels. There was some discussion about sequence stratigraphy, and whether this represents time or sediment lines. This brought us to the end of a very exciting days geology.
Thursday 29th May 2003
This was the day Id been waiting for. Up until now we had seen the most amazing structures form the Pyreneneen Thrust belt but now we had a chance to examine some of the rocks from the piggy-back Tremp Basin between the Montisec and Boixols thrusts. So we climbed up the road running to the west of Tremp and immersed ourselves in the Lower Tertiary.
Our first stop gave us a spectacular view to the east over Tremp in the valley below, and beyond to the San Cornelli anticline and Orcau Castle that we had visited the day before. But the main purpose was to scrabble around amongst the unpromising, rather dirty-looking gravel at our feet. Lentils galore! These turned out to be the single-celled foraminifera (forams) called Nummulites. They were not the cigar-shaped Alveolina that gives the basal beds of the Palaeocene Ager Group their name. Paul G. found some Alveolina further up the section on our next stop. Most of the nummulites were lentil- size but some were as big as I cm across, more like a 5p piece without the rim. I have since learnt that they are probably the same species at different stages in their sexual cycle. Blocks showed them in cross-section, revealing their inner structure. Fay found the biggest and best that I saw. There may have been at least two other groups of forams represented. Eddie found a very enigmatic fossil of concentric circles which got us all puzzled. This turns out to be yet another foram, Orbitolites. Fay also found some echinoid spines and with a bit of eye of faith Gill picked a much-weathered fossil that was probably the beast itself. Gill found an oyster as well but the prize find goes to our American visitor, Rose, from the European Branch of the OU, who found an excellent spiny bivalve that looked very similar to the Upper Cretaceous Spondylus spinosus with its byssal attachments still attached. Paul G. found a nice crab claw.
Oysters were definitely the dish of the day at the next stop, a road-cutting further up the hill. No-one could fail to find them. Big ones, little ones, some in blocks and at least 2 different varieties. Here we stopped at a viewpoint where we could see clearly beds of alternating clays and sandstones in the neighbouring hillside. Paul described these as para sequences of rising and falling sea-level, possibly related to basin subsidence. As we walked up the section others examined the structures in the sandstones, finding lags at the base and dewatering structures. I had my nose to the ground and found occasional patches with gastropods amongst the oyster-rich clays and came upon the horizon where Paul found the Alveolina forams. I also found another of Eddies Orbitolites. Paul S. gave me an oyster encrusted with a serpulid tubeworm which turned out to have a most interesting bryozoan encrusting it as well. A quick glance by Paul Taylor, a specialist at the Natural History Museum, got him very excited, as it appears to be the earliest known record of the genus Electra. This will need more investigating and is definitely one for the NHM collections. Thank-you Paul S! Some of the oysters may prove interesting enough for the collections as well.
As we approached the top of the Montllobar Pass the lithology changed to purply red: we were now out of the marine Palaeocene and into the terrestrial Eocene Montañana Group. With eye-of-faith Paul made us agree we were seeing long root horizons where the red had become de-oxidised. Rose pulled off another coup by finding an excellent piece of horse-tail wood to prove Pauls point.
Our next stop, on the other side of the pass, was a view point where we could look north to the snowy peaks of the central Pyrenees. In the baking heat it was hard to believe we were standing amongst falling snow only 4 days before. Finally we viewed a channel sequence in the middle of the Montañana Group, much loved by the oilmen, as, with its high porosity, it made a good reservoir. Paul told us that one of his students described the trough cross-bedding as smiley beds a very good description!
Back to Tremp for lunch and siesta to conserve our energies for the massive dinner at Casa Lola in the next village. And what a feast! Course after course of wonderful fresh food including local wild mushrooms. Thanks to Gill we could wash it down with good Spanish bubbly. Thanks Paul for another excellent day and excellent dinner choice. Thanks Wilf for organising the bubbly and thanks Gill for providing the excuse!
Friday 30th May 2003
The Day of the Turbidites - a traverse from Tremp to Boltaña via Puente de Montañana - Benabarre - Laguarres - La Puebla de Roda - Torre la Ribera - Egéa - Campo - Foradada del Toscar - Ainsa
We all emerged after the previous night's festivities at Casa Lola and packed our bags ready for the long drive to Boltaña. From Tremp, we retraced the previous day's trip over the Coll de Montlabasto to Puente de Montañana on the Naguera Ribagorçana where we left Cataluña and entered Aragón. After a long drive across the Sierra del Castillo de Laguarres with magnificent views of the mountains and up the valley of the Rio Isábena we arrived at our first stop, just outside La Puebla de Roda. We turned off the main road to a good vantage point looking out over the valley. On the west side of the valley we could see massive turbidite fans created in the Eocene by flows from the east into deep waters in the west. Oil companies have shown a great interest in this area; they have examined many of the channels as type example for the North Sea sands. After leaving, we travelled a short distance where we stopped to fill up the minibuses with fuel and us with ice cream, etc.
The next stop was at Villacarli in the Laval valley, where we looked back south eastwards to observe a landscape that had seen continuous sedimentation through an unconformity. The hills showed the Eocene lying on top of the Palaeocene - a sequence boundary between terrestrial Montañana (top) and Agur (beneath). Nearby, there was a fine example of a hanging wall anticline, where the Cretaceous limestone had been brought over the Tertiary turbidites. The rills and crests gave this the typical badlands weathering.
The journey and exploring the sites had taken all morning, so we found a small road off the main road near Foradada del Toscar, where we could park the buses and sit down to lunch in the shade of some trees.
After we had rested, we continued our drive westward, passing new road cuttings that had exposed turbidites of mass movement downwards in water. This road was in the process of being rebuilt with new cuttings and tunnels to smooth out the old bends. Just before one of the new tunnels we turned off on the old road where we saw a major section through the massive flow. There were examples where the sediment on the slope had collapsed to form a high-speed, turbid, current. There was also convolute bedding, ripple marks and many more features.
After leaving this exposure by the old road, we drove onto Ainsa Quarry, which was billed as the best quarry in the world. The quarry displayed a large number of features that were difficult to explain and had us all putting up different hypotheses as to how they had formed. There were examples of folding, surrounded by no folding; mud volcanoes; slumping of semi-consolidated material. There were fossils, rock types and sediments that seemed to be contradictory; beds with huge boulders of flint, diorite, mudstones, granites, and limestone and shale of Mesozoic origin but microfossils that date it as Tertiary.
Rivers and storm beaches give a significant amount of sorting. Many of the fossils ware non-marine, with 90% oyster (estuarine). There are banks of benthomic forms and offshore muds with bivalve and gastropod traces. Whatever the trigger, we were observing a whole area where the terrestrial side margin had collapsed. As you can see there was much to ponder in such a short visit. We met a PhD student and her research assistant who were surveying the quarry and area around and planned to be there for six months.
As a final remark to the field trip, Paul Grant encouraged us on the purpose of fieldwork, which I paraphrase: - look at anything you can see - your investigation is as good as anyone else's but be self-critical about your conclusions. Think about facts that may destroy your arguments - but not so much as never to make a decision.
After leaving the quarry we travelled beyond Boltaña where we were to stay the night, to one final stop to look at the Boltaña fold. This is an asymmetric antiform pushed from east to west. The thrust had not quite penetrated the surface as we were standing at the easternmost extent of a thrust and nappe structure. Technically, material has been "squished" to the south and west as Spain subducted below Europe.
Finally, all our thanks to the drivers for taking us all safely though some very demanding roads and not so roads. The next day was to be the drive to Pau Airport across the Pyrenees via the Tunnel de Bielsa in what promised to be very good weather - a bit different from the first day from Pau to el Pont de Suert. The big thank you for our leaders was to come the following day.
London Clay on the Isle of Sheppey, Kent led by Fred Clouter
22 June 2003
Happily travelling down the M2 on a lovely midsummer Sunday morning, we didnt stop at the Service Station: were meeting at a camp site: theres sure to be loos! Famous last words! However with the courage and resourcefulness of geologists we survived the day. The idea was to find fossils, and to that end we assembled near Eastchurch, to the bemusement of some of the residents.
For those unfamiliar with the area, I should say that the Isle of Sheppey really is an island, connected to the south shore of the Thames estuary by a single bridge. Our destination was the cliffs on the north side of the island which stretch for some 6.5 km from the outskirts of Minster to Warden Point and along which are up to 50m of the upper levels of the Eocene London Clay formation, overlain by the Virginia Water formation and in places shallow Pleistocene gravels. The section we would be looking at was either side of the Eastchurch Gap at Hens Brook, west of the more famous Warden Point.
We were met by our leader for the day, Fred Clouter, a member of the Medway Lapidary and Mineral Society, and co-author of the superbly illustrated publication: London Clay Fossils of the Isle of Sheppey, (2000). This area has been famous for London Clay fossils for over 300 years and Fred had quite a collection of specimens to whet our appetite from the tiniest fish tooth or vertebra, to crabs, sharks teeth and even a complete birds head, and, armed with his splendidly informative handout, we set off with high hopes down the crumbly eroded cliffs, dominated by a striking arête, to the beach.
This erosion is the reason for the availability of fossils on the beach. Fred explained to us that the erosion at Sheppey is very rapid and cliff failure can produce rotational slips (diagram Clouter et al, 2000), and the nodule bands, originally almost horizontal then slope down steeply towards the sea, allowing them to be washed out by rain and spring tides. The clays date from about 49 to 52 million years, and the fossils reflect a warmer environment than today, and the clays a low-energy middle shelf environment, with water depths of between 20 and 100m, necessary because of the presence of sharks teeth among the finds. There must also have been shallower periods, because of the abundance of semi - pyritised fossil wood and other plant remains.
Having been warned not to fossick around too close to the crumbly bits, we set off on our quest. The smaller fossils are to be found nearer the top of the beach in accumulations of pyritised debris of mollusc remains, fish teeth, vertebrae, tiny worm burrows etc. The larger nodules were to be found near the bottom of the beach on the wave-cut platform, which was extensive as we were on a falling tide, however it was also somewhat treacherous as it was very slippery.
So then we spent a happy few hours working our way east, some of us grubbing about on our knees, sifting accumulations of debris in hollows and ripples, or, for the better prepared, using long-handled tridents to turn over the nodules, going hopefully to Fred with our finds, often to be told alas that this was a chuckit.
Lunch was a quite extended period of dolce far niente in the warm sunshine (we seem to have been lucky on recent field trips) to regain energy for a renewed assault on the beach, with somewhat more success as we got our eye in, finding crabs in phosphatic nodules, and collecting lumps of fossil wood, but the find of the day came near the end, a complete bird bone spotted by Jenny Parry, identified by Fred and kindly donated by Jenny to Fred to add to the collection.
At about 4.00pm we made our way back up to the top, backs bowed by rucksacks and bags rather heavier than on our arrival! Many thanks to Fred for an instructive and very enjoyable day!
Newhaven and the White Cliffs led by Rory Mortimore
7 September 2003
We assembled on a bright morning at the end of the car park on the beach in Newhaven. Diana Clements introduced Rory Mortimore who proceeded to give us a quick outline of the geology of the area and an indication of the days programme. He also gave us a handout together with a blank stratigraphical log that we were to fill in during the day. He also asked us to keep in mind two bands of flints in the Chalk (Castle Hill Flints): one concentrated (band 5), the other dispersed (band 4), which were to be used as markers for the rest of the day.
The group walked slowly to the top of the cliff, stopping to study the Culver Chalk Formation of the Early Campanian and the various beds from the Palaeogene above and to fill in our stratigraphical logs accordingly. Attention was drawn to beds above the chalk containing jarosite and aluminium rich minerals. A cobble bed with unsorted angular flints was identified as a possible river terrace deposit before a marine transgression. Glauconitic laminated sand (marine) had been deposited above, then yellow sand, which got redder as you dug into it and was possibly a soil horizon. We then looked at a cross bedded unit with hummocky/ swaley cross stratification, composed of medium to very coarse sands and the occasional flint pebble, and containing Thalassinoides burrows. Then a silty peaty layer containing organic sulphur hinted at deposition in a low energy environment such as salt marshes. Further up, a blue clayey mudstone was avidly dug up for selenite crystals (in two bands). A pebble bed suggested another transgressive marine episode and this was overlaid at the top of the cliff by a layer of finely broken oyster and gastropod shells identified as the last bed before the London Clay.
We then walked along the cliff path to an area where a new housing estate had been built very close to the edge. All along the path, evidence of the instability of the cliffs was substantial in the form of cracks in the ground. During storms, spray reaches the top of the cliff, saturating the ground and promoting slips. As much as œ metre of Chalk can be eroded away during a year. A discussion took place as to the choice of site for the development and a quick consensus was reached on its unsuitability. We then adjourned for lunch.
We finished our lunch on the cliff top, in the lovely sunshine, with the predicted rain staying away. From our excellent handouts we could see that we were going to start our afternoon walk in the Newhaven syncline, looking at the Castle Hill Flints. We made our way back down the steep gully, and all met up at the bottom of the cliffs, where we were treated to the image of large white Chalk cliffs, with many distinctive layers of flint and all topped by Tertiary/ Quaternary sediments.
These continuous layers of flint could be followed all through the Chalk and we were pointed to two particular layers that were going to be our marker beds for the rest of the afternoon. These were flint layers 4 & 5 within the Culver Chalk. They were distinctive because layer 5 was quite solid looking and layer 4 was more scattered.
All the chalks we were going to look at were deposited in the Late Cretaceous, and were extensively deposited all over Europe. We know this because marl horizons found in England have also been identified in France and further (a project that Rory was working on). Within these chalks we could see fault patterns, the faults start to curve as it descends, giving a bigger offset at the base of the cliffs. Another movement feature that could be seen was horizontal laminations seen in the upper chalks. The likely cause was the Tertiary erosion of the cliffs and resettlement of the Chalk.
Within the Newhaven Chalk we saw different types of faulting occurring. These were conjugate faults, where two fault lines cross each other, making a big cross in the cliff face.
The afternoon was getting hotter and the tide was going out. As we walked over the wave cut platform and investigated the Newhaven Chalk underfoot, we could see that it contained large chunks of nodular flint in tubular shapes. Rory explained that these shapes were due to flint replacing the burrows of an animal, probably a crustacean. This sort of burrow is called Thalassinoides. You could follow these casts and could even see passing places.
Then it was a race to see who was going to be the first to find the giant fossil ammonite. It took a little while, I was still looking when it was found so Im not sure who actually found it. It was about a metre across, standing proud of the beach, Rory saying that they could get even bigger, however, they were a bit weather beaten and you had to look closely to really see a shape, but I believed him.
On the walk back I was lucky to find a fossil, an echinoid from the Culver Chalk, this got everyone looking at their feet for the rest of the walk back to the cars. Patrick found a more relevant fossil, Offaster pilula from the Newhaven Chalk. Both were flint fossils and looked quite impressive, Rory said that there were many on the beach but these were the only two found.
The whole day went well the rocks were interesting, the weather was great, the people were friendly and the tutor was fabulous. What more could you ask for on a sunny Sunday? The day finished off in the pub for a well earned drink. I would like to thank everyone there for making my first trip very enjoyable.
Harefield SSSI and Northmoor Woods RIGS led by Brian Cozens
19 October 2003
Harefield pit was one of many chalk quarries in the area which produced lime for the cement industry. The pit was used as a landfill site and was filled by approximately 30 years ago, after which only the top of the Upper Chalk and overlying Upnor and Reading Formations and London Clay are exposed in the remaining eastern wall. It was designated as an SSSI due to the exposure of a section through these deposits and is the stratotype locality for part of the London Clay. Since then the land has been grazed by cattle and the exposures became overgrown. Bryans group, the Harrow and Hillingdon Geological Society, have cleaned up some of the exposures which are now in pristine condition.
At the time of deposition of the top of the Upper Chalk during the Santonian, about 86 Ma ago, Britain was in tropical latitudes and sea level was relatively high. Marine regression exposed the Chalk. There is less Lower Lambeth Group here than further east and south, eg. at Newhaven, as less deposition occurred. More Chalk was also removed here due to prolonged uplift and erosion further inland. There is therefore an unconformity between the Chalk and the Palaeocene Upnor and Reading Formations (formerly the Reading Beds). This was the first of several transgressions during the Tertiary.
The Upper Chalk contains large flints and burrows. Bryan was keen to hear our thoughts on these burrows as they have bee attributed to various species in the past. Originally thought to have been produced by annelid worms and named Terebella harefieldensis, they were later thought to be Thalassanoides made by crustaceans. There followed a lively discussion and a marine annalid worm was thought to be the most likely burrower, there being no fossil evidence as it was soft bodied. The burrows were in various orientations but mainly sub-vertical.
On the very clean exposure the Upnor and Reading Formation is well exposed allowing the fine lamination and stratification of the deposits to be seen. Current bedding and channel deposits are clearly visible. The basal Upnor Formation is shallow marine with glauconite. This is overlain by a pebble bed of glauconite-coated flints. A fall in sea level is indicated by evidence of marshy mudflats which are cut into by river channels containing plant-rich seams including rare leaf impressions. Deposition in lagoonal setting in a warm wet climate is indicated. Channel clays are mottled and contain the only known Charophytes in the Reading Formation. These indicate deposition in non-marine/brackish lagoons.
Bryan told us that calcretes had been noted by the Tertiary Research Group. These were formed by water being drawn up through the chalk carrying dissolved carbonate which precipitated in distinct horizons in the Tertiary beds. Surface crystals of halite were observed on the exposure and the age of them were discussed. It was concluded they were formed by salt coming out of the clean face. Other observations included mud casts which were granular rather than clay, micaceous roots/stems and other plant material, and stone warts representing an algal complex.
Further round the face the London Clay deposits have been covered up by a land slip. The HHGS plan to clear it next year. Coming away from the quarry face, Bryan described the changing direction of the proto river Thames as it responded to glaciations. One million years ago the river flowed NE and several terraces were created. During glaciation ice blocked the rivers path around the Watford area, forming a lake in the tundra south of the ice. The backed-up water forced a way to the SE. During interstadials outwash retreated. There were 3 episodes of this during the Anglian glaciation. This created the Colne river which now flows in reverse direction into the Thames. The gravels that accumulated during the glaciations were extracted during the 1920s and transported along the Grand Union canal.
Bryan also talked about mans impact on the area, particularly the industrial activities which have accompanied the building of the Grand Junction Canal which is close by. The canal was used to transport chalk for cement for building in London in the 19th century and gravels in the 20th. Lakes filling the gravel pits are now used for water sports. Mills on the river Colne ground corn, milled paper and later copper.
We walked along the canal a short way to see Weybeards Pit from a distance before going there. This pit has been filled with houses rather than rubbish. The chalk face has been retained and can be accessed. Here the chalk is blocky in appearance with flint layers and faulting in conjugate sets. Those who had been to Newhaven with Rory Mortimer could see the similarity to the chalk there, but it is not known which part the Upper Chalk is present here.
After lunch, we drove to Northmoor Woods, a RIGS site. A simple geology trail leads past a swallow hole to an old former chalk pit which also shows the unconformity at the junction between the Upper Chalk and the Upnor and Reading Formations. Unfortunately in stark contrast to the exposure at Harefield, this site is in poor condition, having been damaged by quad bikes. We could see massive flints similar to Harefield but no sign of burrows. Above the chalk the gravels and sands of the Upnor Formation can be seen. Back at the car park we thanked Bryan for his very interesting trip.
Mam Tor Winter Weekend with Chris Arkwright
1 - 2 November 2003
Saturday 1st November
On a clear sunny day our party set out for our first stop at the back scarp of the Mam Tor landslip (SK131835). We looked out over the exhumed topography of the Lower Carboniferous as Chris gave us a brief history of the geology of the area. Uplift and erosion in Tertiary times has revealed the southern edge of the Lower Carboniferous platform with lagoonal and basin sediments interspersed with various reef types. Evidence of marine volcanism in the form of ash and lava can also be found.
The steep back scarp of the Mam Tor landslip can be inspected at this location, with Mam Tor beds overlying Edale Shales. The back scarp is very impressive here, a near vertical cliff about 100m. high. The Mam Tor beds are down faulted against the Shales at this point. Inspection of the exposure indicated that the Mam Tor beds show repeated coarsening upward sequences with flute marks on the bases characteristic of turbidites. The Edale Shales could be seen in a mudflow to the left of the scarp. These are organic rich marine mudstones containing pyrite and gypsum.
Our second stop was at the small quarry at Windy Knoll (SK126830). Back reef fossiliferous limestones overlain by a boulder bed are exposed at this site which is famous for the discovery of Pleistocene bones in a cave in the 1870s. Many fossils could be seen in the limestones here. Solitary corals, bivalves, crinoid debris, and algal mats were all identified. At the top of the quarry hydrocarbons could be identified.
We walked on through Winnats Pass (SK136827). The pass is an exhumed channel in the reef complex of the Carboniferous platform edge. Mine adits here indicate that the reef has been heavily mineralised. Beds are almost horizontal at the west end of the pass, but dip sharply to the east as the edge of the reef is approached just above Speedwell Cavern.
Our next stop was a well-deserved lunch stop at Treak Cliff Cavern (SK137830). From here we walked on to Odin Mine (SK134834). The mine is a 2m. wide strike slip fault, which was worked for galena, barite and fluorite from the 13th century until 1869. Slickensides on the sides of the mine indicated movement on the fault, and traces of mineralisation could still be seen on the walls. Chris explained that the source of the mineralisation was from late Carboniferous hydrothermal fluids picking up minerals from anoxic basin clays and depositing them in up-dip fissures along the edge of the carbonate platform.
The remainder of our day was spent examining the Mam Tor landslip from the toe back to the scarp (SK136835 to SK132835). Passing the remains of an old crushing circle where ore was crushed by horse power we came to the toe of the slip, clearly marked by a change in vegetation from grass to bracken on the landslip. The toe of the slip here is nearly a kilometre from the back scarp. Elevated ponds formed by the seepage of groundwater are present along the toe, and contain precipitated iron oxide produced by the chemical weathering of the Eden Shales. Chemical weathering involves the oxidation of pyrite and the removal of diagenetic carbonate cement, thus increasing porosity and permeability and reducing the strength of the shale.
Passing along the lower part of the old A625 Manchester to Sheffield road the true magnitude of the slide began to be appreciated. This part of the road is still used as access to Mam Farm, but is ridged and cracked by continual movement of the earthflow. Chris explained the purpose of the many survey points seen along the route. These were installed in 1996 and have been read in the spring of each year using electronic measurements of distance and angle. The results of the survey indicate that sections of the landslide move at different rates and directions, with maximum movements of approx. 1m. occurring every four years when the water table is high and the main slip planes are reactivated.
We continued to climb up to the upper level of the A625, now closed, where we were met with a scene of utter devastation! Rotated blocks of Mam Tor beds dipping back towards the scarp have displaced the road surface by several meters. Evidence for many years of failed remedial measures was seen in the form of herringbone drains and successive layers of road construction, in places over a meter thick. Lots of photographs taken here!
We ended the day with a view of the Lafarge Cement Quarry, courtesy of one of our local members who works at the quarry. We had had an excellent day thanks to Chris, including a detailed inspection of one of the most spectacular landslips in the UK as well as a splendid overview of the geology of the area.
Sunday 2nd November
As a complete novice to the study of geology and living close to the Peak District, I hoped that a weekend field trip with the London branch of the Open University Geological Society led by Chris Arkwright would help throw some light on the mysteries of geology within the context of the Carboniferous period in Derbyshire. The experience was indeed illuminating and surprisingly enjoyable in a number of ways. The following account describes the groups visits on the Sunday to 2 sites in the White Peak. The sites are of interest because they are good examples of relatively rare pockets of volcanic activity in what is literally a sea of limestone covering the whole of the White Peak District. OK, so what did I learn?
The first site, Calton Hill quarry (SK112709), now designated a SSSI is of particular interest because of the dolerite found there and which was quarried extensively between the 1920s and 1990s. The dolerite at Calton Hill is specifically described as basanite, an analcitic olivine dolerite which displays clear hexagonal columnar jointing (both vertical and combajoint). In fact there are 3 types of igneous rock which can be identified in the quarry from what is generally thought to be 2 main phases of volcanic activity. The first phase, during the Visean period resulted in a mixture of tuffs, agglomerates and basaltic lava flows of the Upper Millers Dale Lava series on an Asbian Low Limestone surface. The later phase during the late Westphalian period involved the intrusion of the basanite through old vents into the previous eruptions. The basanite contains olivine-rich (ultramafic) nodules from the upper mantle which are difficult to find due to weathering which has left small angular cavities up to 5 cms in diameter. Silica rich, hydrothermal late-stage mineralization is clearly indicated in vesicles and fissures. The lavas contain quartz, calcite, barite, and pyrite. The basanite contains analcite and chlorite, the latter a greeny-black mineral leading to the description of the rock as toadstone.
There are several good examples of spheroidal weathering of the dolerite, a reddish-brown colouration indicating high levels of iron and greenish colouration indicating fluorite.
The second site at Tideswell Dale quarry (SK155738) is also a dolerite site, interpreted as a sill, known as the Tideswell Sill. The rock is about 12m thick, a coarser and younger variety than the Calton Hill dolerite. In a gulley adjacent to the picnic site can be seen an interesting layer of clay marmorised by a later intrusion of lava.
The weekend was very informative, the group very knowledge, everyone was only too happy to help answer a long list of queries, and the experience was extremely useful. The weekend was also great fun. Very pleasant and interesting company, in a beautiful location with good food (and drink) thrown in. Im looking forward to my next field trip and hope it lives up to my first experience with LOUGS.