Geology Blog

Geology section Field Meeting  31 May 2017

We had a very pleasant evening stroll on Clevedon beach, with a good turn out of 12 BNS members together with Professor Susan Marriott and Dr Shaun Lavis from the University of the West of England and the Geologists’ Association. Being in such a public place, rather than tucked away in a quarry, even meant that other beach users came to ask what we were looking at, with three young, potential geologists, also engaging with us.

After investigating the main fault by the pier, with its associated mineralisation, we studied the unconformity between the rocks of the Lower Carboniferous Avon Group and the lacustrine limestone facies of the Triassic Mercia Mudstone Group.  We were looking at micro/macro scale geology, with miniature faults, folds and slickensides showing well along the beach. There was so much to see that we only covered about 100m of beach.

We saw lead, copper and iron minerals and a few fossils, especially some very delicate undamaged crinoid stems.

Thanks to Richard Ashley for organising the session, David Moore for producing maps to include in the handouts and the other members whose expertise contributed greatly to what we have learned about the geology of this compact and very interesting area.

Geology section Field Meeting to Watchet  Saturday 25th June 2016

Ten of us assembled and first we visited the museum. There were many artefacts associated with the history of the town on display including the famous incline railway – used to export iron ore from the Quantocks to South Wales from the harbour.

We visited the station. The waiting shelter had more historical photos and we inspected the Jubilee Wall which was built by children using exemplars of the rocks, minerals and fossils of the area – all identified on an attached chart. We then set off along the cliff path towards Heliwell Bay.

Steps led down to the beach where the tide was retreating – always approach this area on a falling tide as many tides reach the cliffs. On the right of the steps was a gully which followed the line of the separation between the Red Mercia Mudstone – Triassic and the undifferentiated black mass of the Charmouth Mudstone – Jurassic. This separation is marked by the Helwell Bay / Doniford fault. This is a normal fault with an upthrow of 200 metres, bringing up the Charmouth Mudstone from the upper part of the Lower Lias which is the highest part of the Jurassic rock sequence exposed on the Somerset coast. There are further, more recent Jurassic rocks out in the Bristol Channel, which is an ancient rift feature in which a great thickness of sediments has built up. To the left of the gully were Triassic rocks which had been severely contorted by their proximity to the fault. There were also circular holes in the cliff which we took to be left after evaporites had weathered out. The colouration of the strata meant that the throw of the many small faults could be clearly seen.

The main fault cannot be clearly delineated as it brings together two soft mudstones, but the gully and a line of mud and sand stretching across the bay gives a good general indication. South of this fault could be seen the low cliffs of Liassic strata of the amioceras semicostatum zone. Adjoining the beach were shales and limestones that contained small and large examples of Amioceras sp. and Coroniceras sp. The larger ones reached 32 cm in diameter. There were several beautiful examples.

The succession could be followed across the beach as the dip is about 18 degrees to the NNE.This succession went from the Red/Green/Grey Triassic Mercia Mudstone Group up through the Grey Blue Anchor Formation, the Westbury Formation of the Penarth Group followed by the Lilstock Formation, Cotham and Langport Members. The top, surface rocks on the foreshore are the shales, mudstones and limestones of the Jurassic Lower Lias. We found zone Ammonites from the Psiloceras planorbis to the Arietites bucklandi ( conibeari subzone ) as the tide retreated.

As we walked across the foreshore, we saw part of the cliff that was covered in green moss. Closer inspection revealed that water was oozing out of the cliff face and depositing a tufa layer on the surface of the cliff and on the moss. This showed that the water had dissolved calcium carbonate from the rocks as it passed through. The drop in vapour pressure as it emerged allowed the degassing of the dissolved carbon dioxide resulting in a drop in the carbonate carrying capacity of the water and so it deposited it on the surfaces it traversed.

A very interesting day covering the Triassic and Jurassic succession with excellent explanations and interpretations from Richard with lots of fossils to find on the foreshore.

Proposed Excavation of Temporary Exposure in the Lower Lias

Simon Carpenter has plans to excavate a temporary section through the rarely exposed Charmouth Mudstone Formation during the month of September. It is hoped to arrange a visit for BNS members to the excavation but if any member is interested in helping either practically or financially please contact Simon. Tel: 01373 474086, email simonccarpenter@gmail.com

Report on Geology Field Trip to Trendlewood Woods and Badgers Wood, Nailsea and Backwell, Sat 14 May

Rocks, fossils, a cave, orchids, bird song and sunshine – what more could we have asked for?

Nowhere Wood is an old quarry, now a small nature reserve providing an oasis of calm in the middle of Nailsea. It ceased being worked about 100 years ago and now has a good stand of mature trees which are home to a wide variety of birds which were singing loudly. The best sighting was a Great Spotted Woodpecker.

The Pennant Sandstone rock exposure shows good examples of river bed dune structures (cross bedding), some small faults and some fossil wood. This rock bed is over 600m deep and stretches all across Southern England. It was formed from material eroding from Volcanoes somewhere to the South East, some volcanoes!! The rock was quarried for use in buildings and paving in Bristol and Nailsea. This is the best surviving exposure of the Pennant Sandstone in the Nailsea Coalfield. Further details may be found on the relevant page on the Avon RIGS blog:-

http://avonrigsoutcrop.blogspot.co.uk/2013/01/trendlewood-quarry-nailsea.html

We then moved onto Badgers Wood in Backwell, managed by Backwell Environmental Trust,  www.backwellenvironmenttrust.org. This has been the site of small and large scale quarrying of Clifton Down Limestone and the reserve is now a wooded area with many small rock outcrops. There is a small cave, opened up during quarrying, with very unusual formations of crystals in the walls. We postulated that these were probably formed by Hydro-Thermal action and subsequently surface water has entered and dissolved some of the limestone. Adjacent was a fantastic face of limestone which had formed by a blue-green algal colony, the stromatolites growing through each succeeding layer of limestone, giving a very fine laminated appearance. The visible exposure is around 2 metres high, so consider how many thousand (million?) years that a stromatolite colony lived for. And there are still living examples in tropical seas today.

We also discovered a fantastic specimen of fossil colonial coral, Siphonodendron martini. These corals have daily growth bands, interestingly indicating that a year was 391 days long in Carboniferous times, so we are slowing down.

Walking up to the rim of a large quarry which ceased operating only in 1999 we passed Early Purple Orchids and Lords and Ladies, happened upon a white tailed bumble bee.

We then decamped to a pub for a late lunch and a drink, so all round a very interesting, varied and enjoyable field trip, enjoyed by 8 BNS members and 2 WEGA guests. Thanks to Richard for leading us on the right paths (most of the time) and for his depth of knowledge and clear explanations. David

29 April 2016

Report on Geology Field Trip to Sully Island. Sat 23 April.

This trip was laid on by Bath Geological Society. There was a great turnout of 28 people of whom 10 were BNATS members. The trip was led by Professor Maurice Tucker (Bristol Uni. and BGS) who extensively studied this area when he was a Professor at Cardiff Uni. some years ago. He in fact first identified the dinosaur footprints which are found at Bendrick Rock, and which we visited. The best specimens were lifted and are now in Cardiff Museum, to protect them from “collectors” and the ravages of the sea. We even had on the trip, one of the guys involved in removing them, he said his back still hurt!

Most of the rocks we looked at were deposited when this was the edge of an inland sea which stretched to south of Paris. There is a lot to study in a small area and we saw some great examples of faulting, conglomerates, and of course the dinosaur footprints.

David

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David