6th July 2016 - Geology
Bristol Naturalists’ Society.
Geology section Field Meeting to Watchet Saturday 25th June 2016
Ten of us assembled in the car park at the Railway Station at 1100 where Richard briefed us on the day and the risks - cliffs, tides and slippery rocks on the foreshore - and completed the paperwork before giving us excellent handouts on the geology of the area. First we walked along the harbour to visit the interesting museum. There were many artefacts associated with the history of the town on display. These ranged from sailmakers’ and ship repairers’ tools to sextants and many photos of the iron ore mining, mineral railway - including the famous incline - used to export iron ore from the Quantocks to South Wales from the harbour.
We visited the station, complete with an excellently timed train which arrived as we crossed the line. The waiting shelter had many more historical photos for us to see then 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 walked along the platform and across the line back to the car park where we collected our lunch and rucksacks before setting 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.
We could not identify the limestone rocks on the beach with bands of black, hard material. We thought they came from the rock armour and Richard later identified these bands as Chert, using Dorrick Stow’s book, as the rock armour had been sourced from Cannington limestone Quarry at Castle Hill.
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.
References:
The Geology of Watchet, Eric Robinson, Geologists’ Assoc. Guide No. 66.
Geology of the country around Weston-super-Mare, BGS sheet memoir 279
Sedimentary Rocks in the field, A Colour Guide. Dorrick A.V. Stow. CSIRO Publishing.