Llangennith Marshes Slideshow

Red Campion on Llangennith Marsh

I have been experimenting with presentation styles for my photographs and have put together as a slideshow some images previously posted on this blog showing Llangennith Marshes near Rhossili on the Gower Peninsula in South Wales early one summer. Click on the picture above to see the Roxio Photoshow of the flowers and wild ponies on the marsh. Hope you like it.

Caswell Bay Mudstone Formation

These three galleries show pictures of rock texture and pattern in strata of Caswell Bay Mudstone Formation (CBMF) at Caswell Bay on the Gower Peninsula in South Wales. This is the type location for the CBMF – the place from which the rocks were first described and named. The rocks are part of the Pembroke Limestone Group of the Tournasian/Visean epoch of the Mississipian subdivision of the Carboniferous Period. The Carboniferous Period lasted from 359 to 299 million years ago (mya) but the Tournasian/Visean part only lasted from 359 to 326 mya. The CBMF were deposited around the middle of that period. The total thickness of rocks deposited during the Tournasian/Visean epoch was around 750 metres but the CBMF is just a narrow band – with estimates of its thickness varying from 0-14 metres (George, 2008) to 3 – 7.5 metres thick (Barclay, 2011). The CBMF is sandwiched between the Gully Oolite Formation limestone below and the High Tor Limestone Formation above.

Barclay (2011) says that: the Caswell Bay Mudstone Formation is composed of thinly bedded calcitic and dolomitic mudstones and micritic limestones (George, 1978; Ramsay, 1987). The basal bed is a calcrete (Heatherslade Geosol of Wright, 1987b), with beds of algal laminate and oncoid limestone. The rocks are pale grey to greenish grey, buff, brown and yellow, locally with some red staining.

Barclay says there are some but not many fossils. Also that: the rocks formed in shallow water environments referred to as “lagoon phase” by Dixon and Vaughan (1912). They are interpreted as shallow-water, peritidal deposits formed in a tidal flat lagoon complex behind a beach barrier in a humid climate, with abundant evidence of sub-aerial exposure in the form of dessication cracks (Riding and Wright, 1981; Wright, 1986; Ramsay, 1987).

The Caswell Bay Mudstone Formation lies unconformably on top of of the sub-aerially weathered palaeo-karst surface of the Gully Oolite Formation limestone. It means that there was a time lag between the deposition of the limestone and the next phase of deposition of the mudstones. The palaeo-karst surface is full of dissolved pot-holes. These pot-holes are a common karstic feature on Gower south coast beaches – with Mewslade Bay and the Worms Head Causeway exhibiting some good examples.

The Gully Oolite Formation limestone was deposited in warm tropical seas at a time when sea-level was standing still or slowly falling. The extended period of sub-aerial weathering that created the palaeo-karst surface occurred during a significant relative fall in sea-level (George, 2008, 85). The Caswell Bay Mudstone Formation was formed during a subsequent phase of slow sea-level rise.

References

Barclay, W. J. (2011) Geology of the Swansea District: a brief explanation of the geological map Sheet 247 Swansea,  British Geological Survey, Natural Environment Research Council,  ISBN 978-085272581-8, pp 4-6.

Dixon, E. E. L., and Vaughan, A. (1912) The Carboniferous succession in Gower (Glamorgan) with notes on its fauna and conditions of deposition. Quarterly Journal of the Geological Society of London, Vol. 67, pp 477-571.

Geological Society Field Guide to Caswell Bay.

George, G.T. (2008) The Geology of South Wales: A Field Guide, published by gareth@geoserve.co.uk, ISBN 978-0-9559371-0-1,  pp 82- 86.

George, T. N. ((1978) Mid Dinantian (Chadian) limestones in Gower. Philosophical Transactions of the Royal Society, Vol. B282, pp 411-462.

Ramsay, A. T. S. (1987) Depositional environments of Dinantian limestones in Gower, 265-308 in European Dinantian environments, Miller, J., Adams, A. E. and Wright, V. P. (editors). (Chichester: John Wiley & Sons Ltd.)

Riding, R. and Wright, V. P. (1981) Palaeosols and tidal flat/lagoon sequences on a Carboniferous carbonate shelf: sedimentary associations of triple disconformities. Journal of Sedimentary Petrology, Vol. 51, pp 275-293.

Wright, V. P. (1986) Facies sequences on a carbonate ramp: the carboniferous Limestone of south Wales. Sedimentology, Vol. 33, pp 221-241.

Wright, V. P. (1987b) The ecology of two early Carboniferous palaeosols, 345-358 in European Dinantian environments. Miller, J., Adams, A. E. and Wright, V. P. (editors). (Chichester: John Wiley & Son.)

Bishopston Pill at Pwll Du Beach

Video

This short video clip shows two streams of water gushing from the base of the large multi-tiered shingle bank that blocks the valley at Pwll Du Bay in Gower, South Wales. The water comes from the Bishopston Pill river that flows down the valley to the shore, but which has been dammed up behind the shingle. In summer, reduced water flow means that just a trickle seeps out of the shingle base and spreads across the shore. This video was taken in October after heavy rain had increased the quantity of water in the river and subsequently the pressure of the small lake behind the pebble bank. There is a fast and steady flow and the two streams have begun to create channels through the pebbles before converging on the beach. Apparently, in winter, the build-up of water pressure behind the bank means that the river cuts its way straight through to the sea.

Shingle Banks at Pwll Du

View looking east across the water's edge at Pwll Du Bay

Long before the beach was actually visible, the thunder of the waves crashing on the shore, and the grinding of the pebbles against each other on the water’s edge, could be heard as I followed the signpost from Pwll Du Head, down the steep path through the tree-clad valley side to the shore below.

Stretched across the mouth of the valley below lies a massive bank of cobbles and pebbles, or more accurately three successive banks. Unusually for this type of beach, almost all the rocks that make up the banks are the waste product of quarrying activity. Certain privileged farming tenants up until the beginning of the 20th century were granted rights of “cliffage” that allowed them to quarry limestone from the valley sides and the eastern cliff face of Pwll Du Head. The quarry on the Head ceased operation in 1884. The workings are now mostly overgrown.

Boulders from the quarries were placed in heaps marked with wooden stakes on the beach. Ships came over from the north Devon coast to collect the stone to be burnt for lime that would fertilise the fields. Apparently, when the boats arrived, they would scupper in the shallow water at high tide next to the marker posts. Once they were in position on the bottom, the sea-cocks were closed again, with the boat remaining full of water as the tide went out. This technique meant that the bottom of the boat would not be damaged because the water cushioned the fall as the first of the quarried boulders were thrown aboard. Once the cargo was loaded, the sea-cocks were opened to drain away excess water, then closed so that the boat could float on the next high tide ready for the return trip to Devon. Small pieces of rock were too bothersome to load and remained on the shore, gradually building up over the years into the banks.

The shingle banks block the flow of the small river known as Bishopston Pill. The barrier of the shingle banks forms a dam. A small lake lies behind the banks. Beyond the pooled water in the pictures below, two white buildings are tucked into the western valley side at the back of the shingle; these were once public houses that catered for the workmen labouring in the quarries – it must have been thirsty work.

In summer when flow is reduced the water seeps gently from the base of the banks and spreads out across the shore, usually at the eastern end of the beach. When I visited in October after heavy rain, the water was emerging in two fast-flowing streams that were cutting embryo channels through the shingle at the seaward face. In winter when flow is greatest it seems that the banks are breached completely by the flow.

The pebbles nearest to the shore, where they are constantly moved against each other by the waves, tend to be the smallest and smoothest. Most of them are the local Carboniferous limestone but some “foreign” pebbles are included and these originated as jettisoned ballast from ships plying trade in the Pwll Du quarried limestone.

As you walk inland across the shingle, the nature of the stones beneath your feet changes. Whereas on the shore small rounded pebbles tend to predominate, further inland the stones increase to cobble size and they become progressively less rounded and more angular. The shingle bank is always a dynamic structure. However, for many years the innermost couple of the three terraces or ridges, storm beaches, were relatively stable. The stability partly enabled and partly resulted from vegetative colonisation.

I have read accounts of lichen-covered pebbles on the inner banks but none were visible when I visited. Although this was my first visit, I have seen earlier accounts of this locality, and I think that last winter’s storms may well have had a profound effect on the shingle banks. It seemed to me that there had been a great churning up of the stones, resulting in a greater mixing of size and shape, possibly a partial reconstruction of the ridges, destruction of some of the patches formerly stabilised by rooted plants, and removal of lichen encrustations or burial of lichen coated pebbles.

REFERENCES

Gillham, M. E. (1977) The Natural History of the Gower, South Wales, D. Brown and Sons Ltd, Cowbridge, ISBN 0 905928 00 8.

Mullard, J (2006) Gower, New Naturalist Series, Collins, ISBN 0 00 716066 6.

Pwll-Du and the quarrymen of Gower The Geological Society Website

Sea-washed Carboniferous limestone pebbles at Pwll Du Bay

 

Pwll Du Rock Textures & Patterns

The small cove at Pwll Du (or Pwlldu) on the south coast of the Gower Peninsula in South Wales has been created by two fault lines extending approximately northwest to southeast. They converge inland, and separate towards the sea, meeting the shoreline more or less at right angles, with one at the east and one at the west of the bay, running through Carboniferous Limestone strata. The fault lines displace the normal sequence of rock layers so that to the west the rocks are Oxwich Head Limestone, in the middle between the faults it is Hunts Bay Oolite, and in the east it is High Tor Limestone.

Images of the rocks at the east end of Pwll Du Bay are shown in the gallery above. Photographs with details of the rocks at the west end of the bay are given in the gallery below.

Overlying the central area of Hunts Bay Oolite is a massive, multiple-tiered shingle bank which blocks the head of the valley and currently dams the river known as the Bishopston Pill. The shingle banks of Pwll Du will be featured in the next post.

The wind blowing sand along Rhossili Beach

Video

A low, thin layer of sand being driven by high wind across the sandy beach at Rhossili in winter. The video clip taken below the Old Rectory or Parsonage on the solifluction terrace at the foot of Rhossili Down, Gower, South Wales, December 2013.

The wind blowing sand by Rhossili dunes

Video

Gale force wind driving sand grains across the beach at Rhossili on the Gower Peninsula, eroding the seaward-facing edge of the marram-covered dunes, and accumulating as new sand drifts (December 2013).