10th September 2018. The Geology of the Lower Jurassic at Ogmore -By -Sea.

Ten of us met as arranged on a fine morning and proceeded to have a long discussion about having to pay £6 each for parking and decided to move the cars elsewhere to park for free. Having done that , Nigel McGaw, our leader for the day, gave us the background information and a detailed hand-out. As the hand-out told us, the Ogmore -by- Sea and Southerndown area comprises of an island of Dinantian ( Lower Carboniferous) Limestone which became gradually drowned by rising sea levels. This occurred by a series of marine transgressions followed by standstills which carved out several wave cut platforms in the Carboniferous Limestone.The marine transgression represents the beginning of the break up of Pangaea, the supercontinent.
The near shore deposits surrounding this island which was part of an archipelago, is characterised by the Sutton Stone and Southerndown Beds lithologies. We spent our time examining the rocks in various sites---walls lining the road, quarried areas behind the original car park and in the hillside behind the beach and later on down near beach level. The blackberries growing around our target sites were perfectly ripe and proved irresistible to some members along the way.
The Sutton Stone is white , conglomeratic limestone with pebbles of black chert and of carboniferous limestone , laid down in the early Jurassic times and is seen as a sequence of massive. rubbly bedded cream calcarenites ie a type of limestone composed of >50% of detrital, transported sand size carbonate grains, a clast -supported basal breccia, followed by a shelly bed.
Lunch was eaten in the shelter of the quarry and as we made our way nearer the beach we looked hard for fossils.In particular we were looking for ammonites . These would have lived in the quieter , deeper water and would have been washed in after death. We thought for a short time we had found an ammonite from the Angulata chronozone which would have been a very rare find, but closer inspection suggested otherwise.
Whereas the Sutton Stone represents deposits laid down closest to the shoreline, the Southerndown Beds reflect slightly deeper near shore deposits, a transition zone before the deeper basin, and these are well bedded blue-grey micrites with micritic conglomerates. We found this in the quarry behind the carpark.
By early afternoon, the tide was well out and it was safe to scramble cautiously over the rocks from Pant - y - Slade down to beach level, and get into Fairy's Cave and in this area there was abundant evidence of mineralisation with apricot coloured deposits on the rocks and even mineralised corals. The other striking feature was the presence of stylolites. These are caused by pressure dissolution having taken place about 2 km below the Earth's surface between texturally different lithologies and now appearing as closely zig-zag lines on at least 4 different levels at one point.
The adventure continued as Nigel led us along a sheep track to a mercifully wide platform and pointed out a layer of wavy rocks produced by calcium deposited by cyanobacteria --a sort of bacterial version of stromatolites, and finally on to another dry valley which I had previously thought was only accessible to sheep and climbers.
Nigel was thanked for leading and instructing us and for taking us to places most of us had never been before. Steve Howe was also thanked for his expert knowledge and input.
We look forward to the next instalment from Nigel when he will take us to Southerndown.
TJ

13th August 2018 Ogmore and Southerndown---- This trip has been postponed.

16th July 2018 Cribarth in the Swansea Valley.

Nine people in all set off from Craig Y Nos Country Park on Monday July 16th. In fine weather we climbed the steep slope to gain Cribarth, a summit of 424 m in the Upper Swansea Valley. Led by author of “The Land of the Beacons Way” Dilys Harlow, we set out to examine the geology of the area and its industrial past. Cribarth is a much-quarried upland plateau where Carboniferous Limestone was quarried for agricultural use and Twrch Sandstone for the making of hard silica bricks used to line the iron furnaces lower down the Swansea Valley. The limestone pavements are dipping to the south west and are fossiliferous in some places. We peered into numerous dolines better known as swallow or sink holes, caused by collapses in the underlying strata, examined various mineral outcrops of chert, a hard silica mineral and rottenstone a layer of weathered limestone shale used in the iron industry as an abrasive polishing powder. Dilys fielded many questions very knowledgeably and with great patience. We returned to the Country park around 5pm after a thoroughly enjoyable day.
BD

25th June 2018 PENWYLLT
On the hottest day of the year so far, 10 of us including our leader Geraint Owen met at Penwyllt to explore the karstic landscape and the industrial past of the area. From the car park Geraint pointed out various landscape features including, the assymetrical syncline, the North side gently dipping to the South and the South side nearly vertical; the big escarpment of Fan Hir with old red Sandstone on the top. He reminded us that the Maros group used to be called the millstone Grit and consists of the upper Bishopston Mudstone and the lower Twrch Sandstone.
As we left the car park we inspected the boulders marking the margins of the track , and these were full of fossils—big colonial corals, solitary corals and brachiopods, and the occasional jagged lines of stylolite caused by pressure solution.
We passed the long since abandoned personal railway station of Adelina Patti and at Fan Gyhirych to our left with a steep scarp on its North side(Maros Group) and a gentle slope to the right (Carboniferous Limestone). Loose rocks on the ground were tested with HCl acid –most reacted (Carb L/S) but there were also non-reactive brownish sandstones.
The contact between the sandstone and the limestone was very irregular, and this was attributed to the big fluctuations in sea level having eroded away the limestone ,forming caves with the thin beds of sandstone deposited later filling in the gaps. There were beds full of brachiopods nearly all the same way up and some areas of rubbly rock which were less reactive to the HCl acid. These were areas in which the limestone and the sandstone had undergone mixing presumably by bioturbation prior to lithification.
From the top of the ridge we observed the landscape again. U shaped valleys carved out by glaciers, and two very straight valleys (Swansea and Neath valleys) trending NE → SW caused by faulting. Cribarth was in front of us, with Carboniferous Limestone on it’s top instead of being hundreds of metres below us because it is the top of an anticlinal fold as a result of the Swansea Valley Disturbance where there had been reactivation of older faults in the Silurian and Devonian rocks which lie below but are not exposed. Cribarth will be studied in more detail at the next meeting.
We meandered down the slope and passed various buildings including what used to be the pub, and the remains of lime kilns. The Penwyllt villagers quarried the limestone and the Twrch Sandstone using the lime as a soil improver and grinding the sandstone to make inert silica bricks for use in iron smelting furnaces. There were plenty of these black bricks to be seen ,leaving little doubt as to where they had been made as they each bore the name of Penwyllt .

After taking lunch sheltering from the heat and the sun ( how often does that happen?) we tackled the opposite , steeper side, walking along an old tram road , making the ascent gradually. Along the way we found the shake holes or dolines, formed where the surface has collapsed having been undermined by the kilometres of cave systems beneath. Following the undulations of the old train track it was obvious that the collapse features are still active.
On our hot , slow ascent we happened upon a boulder of Old Red Sandstone, the nearest source for which would be either Fan Hir or Fan Gyhirych , both some distance away. This was therefore a glacial erratic.
We took a short rest at the main entrance to Ogof FFynnon Ddu cave system, and on moving on observed some honeycomb sandstone and small samples of chert, and eventually reached the top of Carreg Llwyd from where we could see Fan Fraith next to Fan Gyhirych and the Swansea Valley Disturbance in between.
We then turned our attention to finding burrows---pairs of holes formed by burrowing worm-like creatures which lived in a U-shaped tunnel. Remarkably, one loose rock had fractured through such a tunnel so we were able to see one of them in cross section. There was evidence of peri-glacial frost fracturing and on one well polished rock surface on close inspection in the bright sunshine we were able to see (and feel) glacial striations.
Carefully descending Carreg Llwyd we stopped at a very large doline, 20 or 30 metres wide and much bigger than the previous ones we had seen . Geraint climbed down to the base and tested the rocks and found that there was no limestone. Here, the karstic landscape had had a bridge of Twrch Sandstone on top which is very durable, but when this was further undermined the Twrch Sandstone will have eventually collapsed and left a very large hole.
Back at the car park Nigel thanked Geraint on our behalf for taking a day off work to lead us on an excellent day which was remarkable both for the weather and the Geology and we all wished him well for his forthcoming retirement.
TJ

14th May 2018 The Precambrian Igneous Rocks of the Johnston Area near Haverfordwest.

A select few met the Leader , Sid Howells in a car park in Broad Haven on a beautiful sunny day, and having checked that everyone was supplied with hard hats, high visibility jackets and steel toe-capped boots we climbed into Sid's vehicle and set off for the nearby quarry at Bolton Hill. This is a working quarry on a working day and strict health and safety procedures had to be adhered to. Walking along in single file, Sid explained various terms like rockhead which is below the superficials, pointed out the limited overburden and that most rocks were the same as the bedrock with the addition of some erratics which were associated with glacial melt out till. Weathered granitic rock became whitish and crumbly similar to china clay. Further along the exposure was heavily altered shale which is part of the coal measures. Associated with the Variscan fold belts are the regionally important Johnston faults where precambrian diorites are thrust over the coal measures.
Moving to the bund, we looked down into the quarry to see the various levels 0-3 , observed the apolite and picked out the basaltic dykes. After a picnic lunch away from the work site, we moved on to see a large collection of core samples stored in sheds and then onto a second quarry. Here Sid explained that the faults tend to shear up a coal seam which acts like a lubricant and he showed us a superb exposure of the 300 million year old coal measure contact with the 650 million year old diorite.
We finished after two brief stops at the beach mainly to see where the left over quarry rocks were used . Having been supplied with a detailed handout to study at our leisure, Nigel thanked Sid on our behalf for a very informative day and expressed our appreciation of his having taken a day off work and our thanks for driving us around.

19th March 2018. Geology of the Lavernock Point Area

It was always touch and go that this trip would go ahead because of the weather. The forecasts had been very accurate and the weekend snow was due to thaw. Sure enough 12 of us managed to get to the car park near Lavernock Point after we had had the last minute thumbs up.There we met the leader Peter Hodges. He explained his background and then we followed him down to the beach. The temperature may well have improved, but there was still a bitter wind and it was cold enough for many icicles to be evident on the cliffs, many hanging vertically and many others on more exposed cliffs frozen at an angle of about 30 degrees from the centre . We looked at the Mercia Mudstone of the Trias to the advancing marine conditions of the Blue Lias of the Jurassic. Pete described the different environmental conditions which would have prevailed at the times the rocks were laid down and we spent much of the time hunting for fossils in the Jurassic, finding mainly ammonites and bivalves. Ammonites are excellent markers for different ages within the Jurassic as they evolved very quickly and then became extinct. The site of the most recent dinosaur find -- Dracoraptor hanagani
--was pointed out to us. From the earliest Jurassic, it is believed to be the oldest dinosaur find in Europe. About 3/4 of the skeleton has been uncovered so there is always a chance that the next cliff fall will expose a bit more. There was a fairly constant stream of shale falling from the cliffs as we walked along, so it may not be too long to wait.
While looking for the fossils we observed the fissured, hummocky limestone which was caused by stormy conditions in relatively shallow water and which later gave way to dry conditions resulting in the fissures.
Gryphaea, commonly known as Devil's Toenails are extinct oysters which occurred mainly in the Triassic and Jurassic.Some samples of the same species of Gryphaea demonstrated differences in anatomy in that they had different sized muscle attachments, presumably showing an adaptation for living in different conditions.
We had a picnic lunch on the beach in a mercifully sheltered spot in St Mary's Well Bay not far from the caravan site and finished the walk at the Lavernock Fault. We then picked our way over a thick bed of seaweed to a path up the cliff and joined the Coastal path to make our way back to the carpark having first traversed a field still ankle- deep in snow.
Pete was warmly thanked by Brian for his time and expertise.
TJ