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 © South West Open University Geological Society. 2016. Issue 3. September Page 1 Field Trip to Cawsand Bay, 23 April 2016 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.