Editor's Note

The newsletter of the South-West Branch of the Open University Geological Society. None of the information in this newsletter constitutes a brochure under the Package Travel Regulations.

Editor’s note (subbing for BO)

Richard, our Branch Organiser, has been unable to make a contribution to Contents this issue, owing to overwhelming work and family commitments, but he has September 2016 asked me to remind everyone that we are looking for a new Branch Organiser, as from the next AGM in January 2017, when he steps down from P1: Branch Organiser’s Bit the role. P2: Cawsand Field Trip We have reports in this issue of the Cawsand, (Cornwall) Field Trip, which P6: Pengelly Caves Field Trip took place in April this year and the Pengelly Caves Study Centre trip which P10: News and Future Events took place in May. The West Somerset trip to St Audries Bay and Kilve will P11: Important Information appear in the December issue. on Membership and As many of you will be aware, the OUGS 44th Annual Symposium took place Committee listing. on our patch this year and was a great success. There is no write up in this issue of Cornubia but I refer you to Alan Holliday’s review of the event in the September edition of the national OUGS Newsletter. I did attend some of the pre-symposium trips, led by John Mather and Jenny Bennet, which were excellent. My only reservations about the event come from my experiences as a car park attendant attempting to deal with a very large coach and a very small turning space! Finally, I am indulging myself by adding this photo of a coastal exposure at Churston Point, Torbay, showing a complex set of beds and folding in a low cliff face.

Martin Broadbent

Fig. 1 The Plymouth/ Cawsand Ferry

Sixteen of us met on Cawsand Beach (SX 434502) First, we walked SW along the beach to examine at 10.45 on a beautifully sunny Saturday and the oldest rocks we'd see in that locality. These observed the pedestrian ferry arriving from The rocks are of the Whitsand Bay Formation (formerly Barbican in Plymouth. The boat drives bow-first called the Meadfoot beds) which is part of the onto the beach and a gangplank is fixed for Dartmouth Group, about 400 Ma in age. They are passengers to walk down onto the beach, just fine-grained, green to purple mudstones and inches beyond the lapping waves (Figure 1). siltstones laid down under coastal-plain conditions Dee Edwards called us to order and explained our when this part of the world was part of the micro- mission for the day. We were to walk to a point just continent Avalonia, about 25° S of the equator, in over 1 km northeast of Kingsand, travelling up a southern desert latitudes. succession of Lower Devonian sediments and encountering en route unconformities in the form of Permian exposures, both igneous and sedimentary.

Fig.3 Whitsand Bay Formation

At this time the Rheic Ocean was closing as continents drew closer, ultimately to form Pangea; Fig.2 one result of this tectonic event was the Variscan Figure 2 is Gordon Neighbour's sketch of the Orogeny, which is responsible for much of the geological essentials of the area. topography of SW England as we see it today. It

© South West Open University Geological Society. 2016. Issue 3. September Page 2 was the Variscan Orogeny that caused the Ma). We reached the first of several derelict 'fish Whitsand Bay mudstones to be compressed into palaces,' the term for the stone buildings erected slates and exploring the beach we could see the for pilchard-processing, an industry now long-gone. slaty cleavage and the extreme angles to which the But we were no longer walking on sedimentary rock strata had been faulted and tilted (Figure 3). - we were on an extensive igneous exposure that Among other features were some nice examples of formed the major part of the beach, (Figure 5). slickensides. The bright sunshine emphasised the deep purple of some of the exposures. Leaving the beach, we walked into Cawsand village and passed into its twin village, Kingsand and across the old boundary between Cornwall and Devon.. Unlike the house on which this sign proudly sits, most of the buildings leave exposed the variety of the local stone from which they are constructed. An interesting variety of material was used to build the seawall adjacent to the Institute, which is famous for its clock tower that so nearly came to grief in the storms of February 2014. The seawall comprises material from all of the exposures we would see on this field trip as well as Plymouth limestone and, inevitably, concrete (Figure 4).

Fig. 5 Kingsand Rhyolite

This is the Kingsand Rhyolite, believed to be the only large extrusive expression of the Permian Cornubian Batholith. N.B. Granite is intrusive, cools slowly and therefore is coarse-grained. By contrast, rhyolite is extrusive, cools quickly and is therefore fine-grained (like basalt).

Fig.4 Seawall at Kingsand

The twin villages lie on the Cawsand Fault, part of the extensive NW-SE strike-slip fault zone known as the Plymouth-Cambeak Fault Zone. It is believed to be pre-mid-Carboniferous in age. There are many minor faults in this area and we spotted several during our field trip. Back on the beach at Kingsand we began to walk NE, towards Fort Picklecombe, one of several forts protecting Devon from seaborne attack from the French. These forts were built, however, well after the Napoleonic wars and are often known as Palmerston's Follies, after the Victorian prime minister whose idea they were. Like Fort Cawsand, high above the village, Fort Picklecombe has been converted into luxury apartments. Fig. 6 Vesicular rhyolite in seawall, Kingsand As we walked, we were progressing up-sequence, leaving the Whitsand Bay Formation for the Rhyolite is of granitic composition and has a silica Staddon Formation, late Emsian in age (ca 390 content greater than 70%. Gases escaping during

© South West Open University Geological Society. 2016. Issue 3. September Page 3 the rapid cooling result in vesicles and much of the We continued NE towards Fort Picklecombe but rhyolite we examined was vesicular, like the adjacent to Sandway Cellar and before we reached handsome example in the seawall at Kingsand, the Staddon Formation we found ourselves (Figure 6). The deep red colouring is the result of confronted with a striking outcrop of conglomerate, oxidative weathering. Along some joints the colour (Figure 8) is altered to green-grey. Crude columnar jointing . can be seen, although for this author, who's been to the Giant's Causeway, it took the eye of faith to see it. The Kingsand rhyolite unconformably overlies the Lower Devonian sequence and is in places up to 100 m thick. A lack of internal brecciation (that would indicate a flow surface) suggests that the deposit was formed in just one flow. Geochemical assessment of the rhyolite indicates that it is closely related to Bodmin Moor granite and it's of similar age; (K-Ar biotite dating places it at 289 ± 4 Ma and Bodmin Moor granite is 291 ± 0.8 Ma). This is at least 10 Ma younger than Dartmoor granite and is coincident with the start of the Permian period (290 Ma). So the Kingsand rhyolite is 100 Ma younger than the Devonian sediments on which it lies. It was almost certainly extruded around the Fig.8 Conglomerate on Kingsand beach. same time as Bodmin Moor granite was intruded, This was where we stopped to eat our packed so the two events are very likely closely associated. lunches, a most congenial interlude in warm The intrusion of the Cornubian Batholith was sunshine and at a spring-tide low-water there was actually a series of individual events occupying plenty of space for us to spread out and still leave nearly 20 Ma - Carnmenellis being that much room for a party of students from Plymouth younger than Dartmoor. University embarked on a similar mission to ours. Dee invited us to examine the conglomerate and to look especially for signs of long-vanished life. There are supposed to be burrows. The conglomerate and rhyolite seemed to be arranged haphazardly on the beach but in several places were in contact and I saw at least one place where the rhyolite overlay the conglomerate; these were no pieces of rock tossed together by the waves - they were in situ. During the Permian Britain was about 15° N of the equator and more or less in the middle of Pangea. The Variscan Orogeny had produced high mountains from which, during occasional flash-floods, would be discharged vast amounts of loose material and this breccia would be deposited on the plains as alluvial sediments. This is what we were looking at. There were boulders easily 0.5 m big in the conglomerate, clasts of varying shapes and sizes and all in various shades from orange to purple, via yellows and greys. It is thought that the torrents of breccia that form the conglomerates near Sandway Cellar are coeval with the rhyolites. Beaconites is the name for the burrows that have Fig. 7 Channel blasted in rhyolite been seen in the conglomerate. Unfortunately, It was interesting to note a number of places where nobody seems to know what the creatures that fishermen had blasted channels in the rhyolite to made them looked like, nor whether they were allow their boats easier access to the fish palaces, worms, reptiles or who knows what. And if any of (Figure 7). our party spotted any burrows they didn't tell me.

© South West Open University Geological Society. 2016. Issue 3. September Page 4 When standing on the beach with conglomerate a weathering. metre to my left and Staddon Grits a metre to my We'd almost reached as far along the beach as right I assumed that the contact between the two we'd planned to go but there was more igneous units wasn't far beneath me, so I was hovering over activity to explore just a little further on. This time it an unconformity of 100 Ma. So, back to the Lower was intrusive and indicated in Figure 2 as marks Devonian. The Staddon Grits seem to have been numbered 1 to 4. These represent 'elvans' (dykes) deposited on the outer fringes of a river delta, with that are thought to be related to the rhyolite and ripples, cross-bedding and infilled channels. We therefore coeval with the formation of what is now saw some interesting sedimentary structures, e.g. Bodmin Moor. ripples (Figure 9)

Fig.11 Elvan dyke

We were able to find at least two of these features. Fig. 9 Ripple marks One is about 1m wide and is the band of rock with yellow lichen in the foreground and Dee is looking at the same feature in Figure 11. The elvans aren't easy to spot and don't run straight and true, like a textbook dyke. Nonetheless, it was interesting to see these features. The party turned back for Cawsand, retracing our steps and looking again at some of the features we'd examined earlier and gazing across Plymouth Sound to Staddon Heights.. One of our number leapt into the sea for a quick swim. Arriving in Kingsand in time for tea, we all agreed it had been a splendid day, in glorious, springtime sunshine throughout. Our thanks go to Dee Edwards and Dave Williams for leading the trip, for the comprehensive hand out and for giving us such a worthwhile and enjoyable day. Fig. 10 Flute casts Reference and flute casts (Figure 10). Everywhere the strata Bristow, C.M. (2004),Cornish Hillside Publications, were steeply inclined (Figure 13) - that old Variscan St Austell, Cornwall's Geology and Scenery Orogeny again. There were good examples of distorted cleavage caused by tectonic movements. Words and Images The rich red-purple colouring is due to Permian Trevor Lockwood

A group from OUGS South-West visited the mountain building period. The faults formed William Pengelly Cave Studies Centre in fissures which were were sometimes filled with May 2016. The Centre is a trust dedicated to lava to form dykes, or had minerals deposited the pursuit of cave studies, programmes in them from hot brines. Here the surrounding designed to educate and inform about caves limestone has become crystallised on both and the promotion of cave conservation. The sides of an iron-rich intrusion. This rock was centre is located at Higher Kiln Quarry on not quarried so has not been touched and, now Buckfastleigh Hill, between Buckfastleigh and covered with vegetation, stands high above the Buckfast. The site is a nature reserve owned path. and operated by the Devon Wildlife Trust. http://www.pengellytrust.org/ Beyond the dyke we came to the medieval lane (possible a coffin path). By turning right here we would have seen some of the oldest rocks in this part of South Devon, namely the mudstones, shales and slates of the Lower and Middle Devonian Period (400-350 Ma) which underlay the tuff and limestone we were to investigate further. These older rocks were deposited in the deep, tropical ocean basin (with the Devonian continent far to the north) when this part of the world straddled the equator. Most of these rocks can be seen towards the bottom of Buckfastleigh Hill by following the footbath down towards Dart Bridge. We looked at a laterally compressed mudstone Fig. 1 Geo-garden, Caves Study Centre on the pathway, which Stephan Harrison

(Exeter University) has suggested may be part Outside the entrance of the centre there is a of a meltwater channel from a glacier, (a group of large rocks set out as a geological subject discussed at the OUGS Symposium in garden. (Fig.1) They give some idea of the July). More evidence of glacial movement was range of different types of rock to be seen on the erratic granite block on the path whose and close to Dartmoor. Some time was spent crystal structure is the same as granite 15 by a number of us attempting to identifying miles inland up on the moor. them all (without looking at the key), slight confusion arising from the varying granites on Buckfastleigh Hill is primarily known for its show (namely biotite granite, porphyritic limestone. During the middle part of the granite, two microgranites, big feldspar granite, Devonian Period volcanic eruptions (caused as well as a quartz rich rock). by the ever-closing sea bed) formed a chain of

volcanic islands. Around these islands coral In the small museum before setting off, Sheila reefs flourished which eventually formed the Phillips, the long-serving education officer limestone seen in Buckfastleigh Hill and entertained us with her knowledge of the around Devon today. Volcanic eruptions would history of the cave complex, and her periodically kill off the coral reefs by depositing involvement with it over the years. more ash, upon which new reefs would grow.

The ash consolidated to form tuff and three We started our walk to the north of the centre. layers of the interspersed beds of coral As we exited the gate we were shown the limestone and tuff (of variable thicknesses) remains of a lamprophyre dyke formed have been identified making up the Hill. (As an through faulting as a pre-cursor of the aside we were told that Pillow lava is also Dartmoor intrusion during the Variscan

© South West Open University Geological Society. 2016. Issue 3. September Page 6 evident in the A38 cutting at the Ashburton bypass) Sheila pointed out where the granite of Dartmoor meets limestone just behind the Abbey. The Hill has been extensively quarried for limestone; the stone from Bakers Pit (now filled The mountain range that existed above the in) and Higher Kiln Quarry was largely used for moors would have been as high as the Alps at lime-making, while that from Bullycleaves this time. The continual processes of river Quarry (towards Buckfast Abbey) was used for cutting and terracing, uplift and sinking of land, building and roadstone. Tuff was not ice ages and flooding has resulted in the considered useful enough to be quarried. landscape we can see now. It is suggested that the first river terrace was brought about by We inspected evidence of marl on the sides of a river flowing off Dartmoor which discharged the pathway as we continued up the hill. Marl into the sea at Teignmouth. More recently a had been collected by local people in the 16th river which discharged at Dartmouth back century to be used as a soil conditioner, we eroded and ‘captured’ a lesser outflow at Dart concluded that this must have been very Bridge. The remaining river is now the River laborious. Lemon (Saxon lemon meaning small river) which joins the Teign at Newton Abbot At the top of the hill we looked over the field below which were the limestone caves we We visited the Cabell tomb (of Hounds of the were to visit later, and above which fly (at Baskervilles fame) built immediately above the dusk) the Horseshoe Bats that the area is Little Man, a calcite growth in Reeds Cave (see famous for, (the field and livestock are farmed below) which, it is suggested, was split by an so as to encourage a perfect life cycle which earthquake causing the crystals to grow at an includes dung beetles and the like). angle. We walked through the remains of the 12th century church which had been burned in The network of caves below us may have an arson attack in July 1992. formed during the Early, or early Middle Various rock could be seen to have been used Quaternary through solution. The dissolved in the building as well as in the chapel beyond carbon dioxide in the rainwater reacting with – 55 types of spilitic rock, (a basic fine grained the calcium carbonate of the limestone and igneous rock) can be identified making up the passages opening up in the rock particularly at wall, and alternate window stones including old a bedding plane, joint or fault. The tuff, being red sandstone from Torbay which has been impermeable tends to form the floor of the used in the chapel window arches, (one of caves. these was robbed out and has been built into the courtyard wall at the Centre. The level of the River Dart has been a major influence in the formation of the caves. When the river was 85 m above sea level, (the top of So much of what had existed as part of the present hill), water-worn granite pebbles Buckfastleigh Hill has been either quarried that once lay on the bed of the river have been away by man or dissolved into caves, so we deposited and found by grave diggers. More returned to the William Pengelly Cave Studies pebbles appear at a 55 m level on a terrace Centre to don helmets. In the rock face of we were able to see on the western edge of Higher Kiln Quarry are the quarried openings Bullycleaves Quarry, and Buckfast Abbey to six caves (Partition Cave, Spiders Hole, stands on another terrace at 43 m (some 5 m Joint Mitnor, Rift Cave, Disappointment Cave above the river’s present floodplain). Cracks in and Reeds Cave) all of which form part of a the limestone would have been flooded by single system that runs through Buckfastleigh water from the river which gradually dissolved Hill to Bakers Pit and Bullycleaves Quarry. the rock. As the River Dart cut downwards in (This has been surveyed in the past - see Colin its valley, small streams would have flowed in Bristow, ‘A New Graphical Resistivity the cave passages and eroded the rock Technique for Detecting Air-Filled Cavities’. Vol further. 4.1. pp 204-227, 1966., and P Grainger, P.G.

© South West Open University Geological Society. 2016. Issue 3. September Page 7 Kalaugher & T S Flaxman, ‘Geophysical Rift Cave is gated and the entrance is viewed surveying at Joint Mitnor Cave, Buckfastleigh’, from the quarry boardwalk. From where we Vol 6, pp 51-61, 1985). stood we could see that this solution cave Three caves were visited by us: would have been formed by water trickling down the fault. It is one of several that Reed’s Cave (discovered in 1938) intersect the quarry face, and is clearly visible above the Rift cave entrance

Fig. 2 Reed's Cave; limestone and volcanic ash Fig 3 Rift Cave with fault above

We were allowed as far as the entrance to (Fig.4), with stress lines evident in the Reed’s Cave which is a former underground limestone above the fault. This is part of the chamber exposed by the quarrying operations faulting and displacement of rock that occurred in the 19th century. Here we could see that the when the Dartmoor granite was intruded some volcanic ash had infiltrated the coral reefs, 290 Ma as part of the Sticklepath Fault. There presenting a definite line before the limestone is a small amount of graphite in the contact is evident (Fig.2). zone here, but the large workable deposit was in Bully Cleaves Quarry (next door). Very limited access is allowed beyond the entrance allowing this phreatic cave (formed Rift was one of the main hibernation caves from phreatic water-filled tubes below the until warm air (from the decomposing rubbish surface source of water), to be in an excellent in Bakers Pit landfill) flowed through the cave state of preservation. The Crystal Cave is the system, this upset hibernation patterns and most richly decorated cave; there is a good now bats have moved to isolated caves. example of a flowstone floor dipping up to the left from the door, flowstones being thin sheets Joint Mitnor Cave – contains one of the riches of calcite precipitated from flowing water in a deposits of interglacial mammal remains in thin sheet resulting here in a mirror with a three England. dimensional effect. There are also a number of Originally the cave had an entrance above in nice fins and crystal pools as well as the the surface of what is now the field that we unique ‘Little Man’ formation mentioned above. looked at earlier; this entrance was a shaft in the ground into which animals must have The cave provides an ideal winter hibernating fallen. Their bones were discovered in the cave spot for the local horseshoe bats. in 1938. Just inside the entrance is a fair sized chamber to the left of which a talus cone of

© South West Open University Geological Society. 2016. Issue 3. September Page 8 earth, boulders and bones covered by Much of the finds are now in various museums, stalagmite slopes steeply upwards (Fig. 4). luckily as it turns out as, very unfortunately, This talus once extended across the floor of thieves broke through the locked gates of the the chamber, but has since been excavated to cave less than a year ago, and stole the reveal over 4000 bones of animals such as beautifully preserved bones of a young hippopotamus, bison, hyena, and straight- straight-tusked elephant and other bone tusked elephant, highly characteristic of the deposits forming the talus cone. Post theft, warmest part of the last interglacial similar bones from the cave held by the (Ipswichian) and dating from around 125,000 National History Museum were 3D scanned. to 120,000 years ago. These are to be made into models by Prof. Robert Stone (Birmingham University). Casts made from these at GeoEd and authentically coloured ready for installation by Dr Andre Chamberlain (Manchester University).

Sheila Phillips was thanked for a very interesting visit and she encouraged some of us to visit the limekilns below the centre: There are three limekilns on site below the carpark. In the past they maintained a continuous production of lime. Crushed stone from the quarry and coal from South Wales were loaded into the top of the kiln in layers and the whole burned for several days. Once cool, white lime was dug out of the bottom of the kiln to be used for agriculture and as a whitewash.

Report and photos by Celia Hadow

Fig.4. Joint Mitnor Cave; talus cone

Advance notice of OUGS South West Branch AGM for 2017.

The next Branch AGM and Geo Weekend will take place in and around Falmouth on Saturday 28 and Sunday 29 January 2017. The National Maritime Museum Cornwall is the venue for Saturday, where we have a large room dedicated for our use. As well as the AGM we will have a series of talks, lunch and plenty of time to explore the Museum. Falmouth Town Station is a 3-minute walk from the Museum, as is a long-term car park. Rick Stein's Fish Café, on Discovery Square right beside the Museum, would make a suitable place for an informal supper afterwards.

There will be plenty of overnight accommodation in Falmouth and rather than attempt to negotiate for reduced prices in one of the big hotels it would be preferable for members to make their own arrangements.

On the Sunday it is hoped to offer a field trip to a locality quite close to Falmouth and a visit to Camborne School of Mines (CSM), which has state of the art facilities and is on Tremough Campus near Penryn, just 3 miles from Falmouth.

Please contact Trevor Lockwood with your expressions of interest.

Branch Events

DATE Title LOCATION LEADER DETAILS 02 Oct Introduction Torbay Pat Wilson Looking at local rock types and structures, moving 2016 to Geology on to basic mapping techniques. (Unfortunately, this trip has had to be postponed. It is hoped we can arrange a new date in 2017.) 28-29 OUGS National South West Branch Annual General Meeting, Jan South West Maritime followed by lectures on 28 Jan, and field trip on 29 2017 Branch Museum, Jan. AGM. Cornwall.

Regional and National Events

7-16 Oct 2016. Sidmouth Science Festival. www.sidmouthsciencefestival.org

5 Nov 2016. Festival of Geology, Geologists Association, University College London, http://www.geologistsassociation.org.uk/festival.html

8 Dec 2016. Burlington House, London. One day conference to celebrate the life of Robert F. Symes OBE. Contact [email protected]

3-6 Oct 2017. OUGS Wessex trip. The Geology and Archaeology of Jersey, led by Societe Jersiase. Contact Kathy Stott, [email protected]

At the Society AGM, held on 16th April 2016 in Scarborough, the Members voted to increase the membership subscription rates from the 1st January 2017. The increases are: Single membership from £18 (€21) to £22 (€27) Joint membership from £26 (€30) to £32 (€39) Family membership remains unaltered at £2 (€2)

MEMBERS ALREADY PAYING BY STANDING ORDER Now is the time to change your Standing Order, do not leave it till later, you may forget! If you use electronic banking, you can do this on-line or otherwise visit your bank branch. a) Calculate your new subscription amount b) Ensure that the payment date is 1st January – not in December, or later in January, and definitely not in February or March (see Note 1 below) c) Ensure that your membership number (or membership numbers in the case of Joint Members) is in the reference field (see Note 2 below). MEMBERS WHO DO NOT PAY BY STANDING ORDER Why not take this opportunity to set up a Standing Order? You can find a Standing Order Mandate form on page (??) of this Newsletter. If you do not want to set up a Standing Order please remember to make your subscription payment by 31st January (see NOTE 1 below) and ensure that your membership number is quoted (see NOTE 2 below).

Note 1: At the AGM it was proposed that a Constitutional change should be raised at the next AGM (13th May 2017 in Milton Keynes) to change the final subscription renewal date from 31st March to 31st January and that this would come into force in 2018. This would mean that in 2018 any member not renewing by the 31st of January would cease to be a member on that date, and therefore, would not be entitled to a copy of The Proceedings; which is generally sent to the printers in February. Note 2: The membership number is very important as some subscription payments are made from non-members accounts and the actual members cannot be identified. There have been numerous occasions in the past where this has occurred and it is a lengthy process to identify the member.

If you cannot remember your membership number, please contact the Membership Secretary at: [email protected] Please advise the Membership Secretary of any change to your name, address or email address. This is important because undelivered Newsletters and OUGS Proceeding will not be forwarded at the Societies expense. OUGS SW Branch Committee

Branch Organiser Richard Blagden [email protected] 07530 502837 Events Organiser North Devon Michelle Thomas [email protected] 07919 893644 Events Organiser South Devon Celia Hadow [email protected] 0755 1900 240 Events Organiser Somerset Garry Dawson [email protected] 01823 282305 Events Organiser Cornwall Trevor Lockwood [email protected] 01326 231399 Web Manager Liz Williams [email protected] 07530 502837 Treasurer Tim Legood 01392 274387 Librarian Linda Luckhaus [email protected] [email protected] Newsletter Editor Martin Broadbent [email protected] 01803 865544 Ordinary Member Liz Williams [email protected]