18th October 2021. Cefn Onn. Leader Steve Howe

This trip had been postponed for 2 weeks because there was concern regarding the availability of fuel. Nine members turned up, and not one, but 2 leaders, with Phil acting as a back marker, counting us all at appropriate intervals. We were unlucky with the weather. It was a full set of waterproofs and umbrella sort of day, so we were denied the 360 degree views from the top of the ridges. No-one complained. With the aid of Steve’s comprehensive handout we were too busy attempting to appreciate the complexities of the geology of the southern rim of the South Wales Coalfield. There are 2 ridges separated by a valley. The older, outer ridge is formed from Brownstones formation ( lower Devonian Old Red Sandstone). The inner ridge and the valley is of Carboniferous age. The softer, older formations of the Avon group form the valley and the Pembroke Limestone group forms the ridge. Exploring the various exposures sometimes involved scrambling down muddy slopes and limbo manoeuvres under branches, but there was always a helping hand available for those that needed it. That’s not to say the more mischievous weren’t above a practical joke when it came to tackling the linked pair of gates to a ?sheepfold. It was not possible to open one gate if the other was open due to a strong cable between them, and it caused some amusement to watch individuals puzzling over how to open the first when the others had deliberately opened the second.
The plan had been to have a picnic lunch in the shelter of the Cefn Onn Basic Slag Company Quarry. There are dolomotised limestones here, formerly used to supply the Cardiff steelworks. Some of the dolomites contain cavities (dolomitisation causes shrinking) and in places these are filled with minerals (vugs) such as calcite, barites and haematite. There was a modicum of shelter in the wooded areas, but not enough, so the vote was to return to the cars and have a late lunch in the cars.
Having climbed gently uphill through the morning, the return involved a steep and uneven descent which we managed slowly but without adverse consequences. We had learned not only about the geology, but also the local history . Steve was thanked for leading us and his excellent handout , as was Phil , for his help. Despite the weather, it was an enjoyable day, and we were encouraged to return to see the wonderful views on a better day.
This event was the last of the trips for 2021.

TJ

20th September 2021 YSTRADFELLTE. Leader GERAINT OWEN

There was no sign of any mellte ( lightening) when 10 of us met at Ystradfellte for a day of geology . In fact we enjoyed a day of glorious sunshine and stupendous views. Initially, Geraint pointed out the main features in the landscape, in particular the saw-tooth escarpments formed by resistant rocks standing proud where those around had eroded away. Looking from left to right there was the Old Red Sandstone of Fan Mawr and Pen y Fan, the Carboniferous limestone and then the Twrch Sandstone, and underlying the conifer forest, the Bishopston Mudstones.
After a steep climb from the carpark we encountered the limestone pavement and a number of Karst features of clints and grikes , and on a bigger scale, the circular solution features of shake holes and dolines.
Further along we came to Carnau Gwynion which is the site of about 130 lime kilns. This triggered a detailed discussion with input from many members of the group contributing on the use of lime for agriculture, various aspects of chemistry and the history of the Rebecca Riots, thus demonstrating the diverse expertise within the group.
We stopped for lunch at an impressive site where quarrying had taken place. There was an intact lime kiln also and we took the time to find fossils in the limestone and observe the paeleokarst.
The originally planned route was abbreviated as we had used up a lot of time in the above discussions, so we returned along a slightly different path where we could have a close- up look at the Twrch Sandstone, with its pebbles of quartz in a quartz cement making a very hard resistant rock.
We descended to the Mellte river valley to see the cave, Porth yr Ogof. This required a good deal of care. There had been no significant recent rainfall, so the cave was dry, enabling those who wished , to explore it with the aid of torches. A further karst feature was evident ---the karren, formed by a combination of erosion, abrasion and corrosion.
We followed the path of the Mellte noting how it disappeared underground and then re-appeared, on this particular day forming perfect, clear and rather inviting pools. Some of the group spotted fish in the water.
Before we dispersed we thanked Geraint for a quite exceptional day of geology and fine weather , happy to give him credit for providing both.

TJ

FIELD TRIP TO PENARTH. LEADER DR PETER HODGES. 9th AUGUST 2021

Some of us drove through a deluge to get to the Cliff Parade carpark in Penarth but the weather was dry at the destination and remained so for the rest of the day for which the 12 members and our leader were very grateful.
We descended to the foreshore and made our way towards Lavernock noting the changes created by the arid conditions of the Trias through the marine transgression of the Jurassic. The sequence involves the Mercia Mudstone--- green layers (anaerobic conditions, ferrous oxide) or red layers (aerobic conditions, ferric oxide) overlain by black shales forming the Westbury Formation occuring in 6 cycles, with fossils occurring mainly in the lower part of the cycles. These rocks were formed at the time of Pangaea when there was a great deal of volcanic activity. The sediments were laid down in a lagoon which was salty at the bottom , with plenty of shellfish and fewer at the the top in the fresher water. Further along we also saw the White Lias Formation, the Watchet Mudstone Formation and the Lower Lias. Along the way were dinosaur footprints----big squelchy ones left in the mud, the location of which is indicated on Google Maps! There are many fossils in the rocks as well as ripple fossils and sun cracks . We looked for fish teeth and scales in the black shales but could not find any, but Peter did show us fish coprolites which was a first for many of us.
The walk ended at roughly the point where the dinosaur Dracoraptor hanigani was discovered, but we did not spot any bones ourselves, but were content to climb up the bank and make our way back along the cliff top to the carpark where Peter was thanked for a very good day of Geology.
TJ

CASWELL BAY TO BRACELET BAY 6/7/21 Leader Ian Prothero

Ten of us met at Caswell Bay armed with copious notes previously supplied via email by Ian. It was just before low tide so there was ample opportunity to get onto the beach and look at the rocks. Ian explained that we were looking at an E-W trending anticline, cut by roughly N-S faulting. The aim of the day was to take in not only the geology but the history , human geography and art associated with the area. At Caswell this encompassed discussion of the Iron Age Fort and a windmill as well as the Caswell Thrust formed in the Variscan Orogeny, and the beach springs.
We drove to Langland where we were joined by another group member and spent some time observing the former Crawshays’ Summer residence with its flamboyant display of wealth, built largely of limestone with decorative additions of Bath Stone and old Red Sandstone, all topped by a slate roof.
The beach itself held an unexpected fascination. Following heavy rain flooding down the steep road a model replica of the Grand Canyon had been created cutting into the sand with layers of shell and the occasional boulder/ large pebble. Modern day geology, demonstrating Hutton’s Principle of Uniformitarianism –‘the present is the key to the past’.
Having crossed the beach, we reached Rotherslade---another dry valley ,and talked about a cave where the bones of an elephant were once found. There are a number of caves in Gower where animal fossil remains have been discovered including mammoth, rhino, hyena and bear, not to forget the Red Lady of Paviland. At this time the sea levels were much lower than present day.
We sat on the steps to eat lunch, overlooking Donkey Rock aka Storr Rock (Gully oolite).
Post lunch we continued along the coast path to Limeslade, observing the effects of glaciation. Solifluction and talus from the last glacial episode and the polymictic boulder clay (derived from various sources) and loess-- the windblown dust from the end of the Ice Age.
Some coral and bivalve fossils were found and fine scale cross bedding in ORS rocks. There was also a wide variety of wildflowers along the path.
At Limeslade, the fault line marks where the Romans mined iron, derived from Triassic desert sediments which were subsequently eroded.
Continuing to Bracelet Bay, proceedings were interrupted by a very heavy downpour, so we cut our losses and retreated into the Castellmare café and had a tea break, and rounded off the trip when the rain stopped (great timing) when we heard about the Shit Caverns built to dispose of sewage from Swansea, and finished the day overlooking Mumbles pier, Mumbles Hill formed of Oxwich head Limestone Formation, the huge sweep of Swansea Bay and Port Talbot steelworks in the distance .
Ian was warmly thanked for a very enjoyable day, and we dispersed by various means to reclaim our cars in Langland.

TJ

PORTHCAWL. 24/5/21 Leader Greg Nuttgens

There was a palpable air of excitement when 17 of us met in the car park at Rest Bay for the first field trip for a very long time. Greg Nuttgens had kindly volunteered to lead the trip and contributions from others present were invited and gratefully received for everyone’s benefit. The weather was not perfect , but we managed to avoid getting too wet or cold. We set off first turning left for Locks Common and looked at the Carboniferous Limestone laid down about 350 million years ago in a warm, shallow, sub-tropical sea when our part of the Earth was situated just south of the equator. We saw the effects of faulting, which had caused deep gullies, palaeokast surfaces and, on the cliff top a limestone pavement with its clints and grikes. Dissolution of the limestone along joints have produced bare blocks of limestone (clints) separated by deep clefts (grikes)with rich plant growth.
Other karst features seen were solution pipes—semi-circular holes in the limestone formed by running water.
We headed back towards the carpark and had our lunch, after which we went in the opposite direction, turning right, walking along the boardwalk beyond Rest Bay to Pink Bay—named for the Triassic Conglomerate which had flooded into the area forming a fluvial fan. It was possible to see evidence of a range of conditions from high energy to low where a flash flood had had sufficient power to carry many rocks within it, to the very edge where there had been insufficient power to carry anything more than sand. The itinerary had been planned carefully around the low tide so it was possible to see another feature which was similar to a large geode in the rock, locally known as Porthcawl Marble.
Greg was warmly thanked and presented with a small token of our appreciation.
TJ