Geology of Southerndown . 28/10/19. Leader Nigel McGaw

There was a very good turn-out for the final field trip of the season at Dunraven Bay, Southerndown a total of 17 members and guests and a puppy. There was no rain all day, but it was surprisingly cold and few stayed the course which extended deep into the afternoon. Our leader was Nigel McGaw who had prepared a handout explaining the Geology of the area, which he ran through while we were all gathered together in the carpark. We then descended to the beach, called Seamouth in Geology books. The tide was still falling with another couple of hours before low tide so we were able to get a panoramic view of the cliffs and pick out the broad Seamouth anticline before we approached for a close up view whilst also keeping well clear of the unstable cliffs. We mused on the name of the Dancing Stones to the right of the concrete slip (a series of gently inclined limestone bedding planes with conjugate joints ) without reaching a satisfactory conclusion and observed the chert in the Southerndown beds and found some fossil wood.
It was important to keep moving and we made our way across the beach over to the cliffs leading to Trwyn y Witch which had been greatly disrupted by faulting and folding. Firstly we looked at the Sutton Sandstone of the proximal part of the cliff and looked for fossils of gryphaea, pinna and plagiostoma which were numerous.We also found large lumps of tufa , a porous rock formed by the precipitation of calcium carbonate by springs. There is an unconformity between the Sutton Sandstone and the more distal Carboniferous limestone. The complex faulting and folding throughout the cliff will have taken place at different times either side of the unconformity.
We sought the relative shelter of the Victorian Walled Garden, a remnant of Dunraven Castle, to eat our lunch and then continued over to the next beach, our numbers somewhat diminished. Taking advantage of the great views from the top of the cliff, we noted the Blue Lias Formation, the wave cut platform and picked out the obvious fault lines on the beach before descending the slightly precarious path down to Temple Bay. Here we studied the nodular rocks, again sprinkled with chert and found a few fossils. One in particular caused some comment as it appeared to be a broken bivalve with a tiny star, a fragment from a crinoid ,inside it .
The final stop on the tour took us to the other side of the “ nose of the witch” and with caution we stepped down off the path to find a gritty layer interbedded with the more familiar chert containing limestone, which had presumably been laid down in different conditions. By now there were only 5 of us left, but we had an extra treat ,seeing a grey seal in the bay. By the time we returned to the carpark the tide was in. Nigel was thanked for a very full and interesting day.

TJ

THE TRIAS  GEOLOGY OF  THE AUST  AREA  ALONG  THE RIVER SEVERN. Monday 19th   August  2019. Leader  : Dr. Peter Hodges.
                                                                                               
                                                                                                                 
10 members turned out on a lovely sunny day and had an enjoyable stroll beneath Aust Cliffs including a walk under the old Severn Bridge. We looked at the Triassic rocks which ranged from non fossiliferous desert and playa lake red sediments and then up into more contrasting dark shaly and limestone beds from the now marine conditions with plenty of fossils including the well known Westbury Bone Beds with sharks and reptile teeth and bones . Also plenty of bivalve shells which meant people went home happy with some specimens as a momento of the trip.

NM

Builth Wells. 15th July 2019

10 members of the regional geology group , led by Dr. Bob Owens, Research Fellow at the National Museum of Wales , spent a day on July 15 th retracing some of the steps of Roderick Murchison ,pioneer geologist, who in 1831 -2 examined the geology of the Wye Valley in the area around Erwood and Aberedw and established the Silurian System relating to rocks around 420 million years old . We visited several sites covering the Upper Ludlow succession of mainly siltstones and fine sandstones deposited on a shallow shelf sea identified by the shelly fauna , mainly brachiopods, burrows and trails along with disturbed storm deposit features reflecting the environmental changes taking place during this geological period. All this on a beautiful sunny day which made it a great occasion enjoyed by all who came along.

NM

GEOLOGY AND SCENERY OF THE LLANSTEFFAN AREA.

U3A GEOLOGY JUNE 10TH 2019. LEADER DR GERAINT OWEN

It was not exactly flaming June, but the weather was much better than the forecast so the 14 of us who joined our leader, Dr Geraint Owen for a day’s Geology at Llansteffan kept dry for all but one brief episode later in the afternoon. The thunder and lightening held off until the drive home.

To set the Geological scene, Llansteffan is currently a sleepy seaside town off the A40 on the way to Carmarthen. It has a number of cafés around the car park, all of which were closed, at least on a Monday. The rocks exposed on the beach are of Old Red Sandstone and record the presence of ancient rivers systems
The mountains of Scotland were formed during the Silurian period , roughly 425 million years ago, (the Caledonian Orogeny ), and the products of erosion of these northern mountains were transported southwards providing the sediment to form the Old Red Sandstone (ORS) which was deposited mainly in the Devonian period (400 Ma).
At that time, the land mass which included what was to become Britain was still situated South of the equator with a climate similar to the present day Kalahari Desert with hot ,dry and warm ,wet seasons .
We started at the east end of the beach where there was a fine example of a well developed calcrete paleosoil. Calcrete is a an accumulation of calcium carbonate in the soil as a result of climatic fluctuations in arid and semi-arid regions. Dissolved in the ground water it precipitates out as water evaporates at the surface, as though it had been sucked up, and then may re-dissolve in wet conditions. The calcrete here was in the form of lumps in a fine grained flood plain sediment which may have taken hundreds, thousands or even tens of thousands of years to form. At different sites the calcrete is in the form of rods, and one theory is that this type of calcrete formed around roots.
Westwards along the beach were mature calcrete paleosoils 3m thick, the most recent at the top showing signs of disruption , a well formed (mature) middle section and a a less well formed (super -mature) section at the base with curious disc-shaped ,curved pseudo-anticlines . These are fracture surfaces accommodating swelling and contraction in the wet and dry seasons.

Walking across the beach we had a live demonstration of how ripples are formed by observing the effect of the stream running over the sand, aided also by Geraint’s drawing on the sand.

Examining the rocks is all about seeking and finding various clues , putting together a 3D jig-saw puzzle like a geological detective , allowing the story of the past to be assembled . At the point on the west end of the beach the rocks were different and provided evidence of a river channel.
Hence, mud cracks, trace fossils (beaconites) of unknown soft bodied animals which once made their homes in the mud, become evidence of a flood plain. Small scale and large scale cross bedding are the effects of wind and water currents forming ripples and dunes . Trough cross bedding indicates point bar surfaces where sediment deposited at a meander in a river and an oxbow lake is detected by flood plain sediments with lots of beaconites, a sharp base to the channel and interbedding with sandstone and siltstone.

Geraint was thanked warmly for giving us an excellent calcrete -themed day and for generously giving of his time and expertise .His patience and teaching skills were much appreciated by all.

TJ

15th April 2019 Marloes Peninsula. Leader Sid Howells

Eleven members turned up on the day to this wonderful part of Pembrokeshire, meeting at the National Trust Car Park in Marloes. We had driven through grey ,wet and misty weather for much of the way so it was an improvement to find it was not actually raining when we arrived but we could hardly stand up in the wind at times and it was cold enough to need every available item of clothing. Ideally, we would have started with the oldest rocks and worked our way along through the ages to the youngest, but that would have meant battling head -on against the wind for most of the day and so we reversed the route making our way past the Marloes Mere Nature Reserve to overlook Marloes Sands ,observing Gateholm Island , following the coast path along to Renny Slip , via Rainy Rock and Deadman’s Bay , then cutting across the isthmus to Martin’s Haven and down to the shelter of Wooltack Point where we ate our lunch out of the wind, overlooking Skomer Island ,while swallows swooped overhead , cetaceans , probably porpoise, circled in the sea currents and sea squill bloomed beneath our feet. After lunch, it was into the wind to complete a figure -of -eight around to see Jack Sound and The Anvil , back to Martin’s Haven and onto the little beach to find some interesting pebbles including some containing the semi-precious epidote. A few drops of rain were being felt which prompted us to continue to the north coast to High Point and finally to pick up the footpath back inland to the car park arriving just as the rain started in earnest.

Sid distributed several Geological maps of the area at the outset with the rock types marked and he stopped at various sheltered spots along the way to explain the conditions in which the rocks were laid down, the effects of rising and falling sea levels, basaltic extrusions and volcanic eruptions from nearby , and the many faults some of which may have been too small to feature on a map.

Pembrokeshire is geologically complex. Near the end of the Ordovician Period there was an Ice Age which resulted in global sea levels being 100m lower than previously. In the late Ordovician/Silurian ,sea levels rose again with the ending of that Ice Age .

The Skomer Volcanic Group is Early Silurian in age and provides evidence of sustained volcanic activity, mostly on land and geochemically alkaline and believed to be part of an extensive province formed as a result of intra-plate rifting and is represented by basaltic flows, ash flow, rhyolitic tuffs and conglomerates , ignimbrite and sediments. Most of the basalt flows and the rhyolites would have been extruded subaerially prior to burial by subsequent flows. The tuffs and ignimbrites were the result of explosive eruptions. The two major faults locally are the Ritec and Benton faults , trending WNW between which is a graben, and it is through either or both of these faults that the lava would have found a route to flow.
At the end of the Early Silurian the local crust was uplifted, tilted and eroded and the Coralliferous Sandstone Group began to be deposited which encompasses Deadman’s Bay for example, and which transitions into the Gray Sandstone Group representing a change in sea level possibly caused by a tectonically forced regression . This Gray Sandstone Group comprises interbedded flaggy sandstones, siltstones and mudstones with rotten stone bands.

There was much to see and discuss in difficult conditions and it was definitely a case of absorbing what the conditions allowed at the time and reading up afterwards with the help of the hand-outs. Sid was warmly thanked for giving of his time and expertise and everyone appreciated being shown this part of Pembrokeshire at its wild and rugged best.

TJ

Field Trip to Whiteford Bay and Broughton, North Gower. 25th March 2019

There was a very good turnout, 20 in all including some guests and first -timers, on a lovely day filled with not only Geological interest for all levels beginners and professionals , but plenty of Archeology and Natural History as a bonus too. We were to hear a story of ever -changing landscapes as ancient rocks are constantly exposed and obscured by the present day shifting sands.
We met Stephen Howe the leader, in the car park in Llanmadoc and set off for the beach. Passing the church, the smallest in Gower, Steve pointed out the blocks of local Old Red Sandstone (ORS) conglomerates and sandstones and Carboniferous limestones with Bath Stone dressings with which it was built.
We admired from a short distance Llanmadoc Hill, an east- west anticlinal ridge composed of ORS quartz conglomerates, sandstones and siltstones. At the east end of the crest is The Bulwark , an Iron Age hill fort.
Broughton Bay lies between the limestone headlands of Burry Holmes in the west and Prissens' Tor in the east. The floor of the Bay is pebbly basal till, with sand and clay layers deposited by the Welsh Ice sheet. Movement of the glacier resulted in the layered appearance of the till at these glacial margins. A raised beach at the west end is evidence of an earlier temperate interglacial climate.
We walked along the beach and observed the change in the rocks from the Gully Oolite through the High Tor Limestone up to the Hunts Bay Oolite, noting minor faulting and slippage movements along bedding planes associated with the folding. (Slickensides). The constant and dramatic movement of the sand dunes was well illustrated by a series of photographs on the comprehensive field notes . Some things we were able to see, such as the Caswell Bay Mudstone Formation , were notably absent from the BGS sheet as it has only been exposed since the 1980s. It is about 12 m thick and contains a range of lime mudstones and dolomites with desiccation cracks and nodular beds, indicative of deposition in shallow water with air exposure at times.
The High Tor Limestone Formation contained Crinoids, corals and Branchiopods.
Out of our reach, but visible above us in the cliff was the Prissen's Tor Cave which on excavation in the 19th century revealed hyaena and rhinoceros teeth and other evidence associated with a cold climate. A midden was exposed in the 1990s.
As we walked towards our picnic spot we were delighted by the calls and flights of lapwings, infrequently seen these days, and a red kite glided silently overhead. The very keen -eyed could even make out eider on the estuary.

There was repeated evidence of the sand dunes migrating back and forth. The dune system developed between the 12 th and 16 th centuries when there were long periods of stormy weather resulting in large- scale sand movements along the South Wales coast . (The abandonment of the coastal village of Kenfig after it was engulfed occurred at this time.)
One of the highlights of the day, not least because it is not often seen even by those of us who are frequent visitors to the area , was the fossil forest, exposed by the low tide which we reached after walking across the peat beds. There were oak and hazel tree stumps , logs and fallen tree trunks , largely orientated in the direction of the prevailing winds and tides of the time. This triggered a long debate about the definition of a fossil which was sadly inconclusive.
By this time the iron lighthouse at Whiteford would have been accessible by foot, but we made our way back to the car park via Cwm Ivy Marsh noting as we walked through the woods the call of a chiffchaff on the way.
Stephen was warmly thanked for an excellent day, enjoyed by all. What a great way to start the 2019 programme!