One rainy morning I walked the coastal path along the terrace at the foot of Rhossili Down in Gower. Clouds shrouded the slopes of the Down above me. Ahead of me, Burry Holms and Llangennith Burrows were part concealed by mist. Below me, the tide was going out – far out. The sand on the beach was still wet from the waves and the rain. Slowly the cloud cover thinned and allowed a filtered light to penetrate. The pale light was reflected by the shore, high-lighting the sea-sculpted rows of ridges and ripples in gentle gleaming silver. A stream cascading from the height of the Down worked its way down to the beach, where it spread out in a fan of interweaving channels that cut across the parallel ridges on its way to the water’s edge. Coloured sediments carried by the stream tinted the silvery patterns and made them seem opalescent.
Dark streaks drawn into abstract designs by the swash and back-swash of waves sometimes decorate the succession of driftlines on the sandy beach at Rhossili Bay as the sea recedes. The blue tinge of the darker areas makes me wonder whether they are composed of comminuted fragments of blue-black mussel shells.
I never cease to be amazed by the new variations of natural patterns in the sand revealed with each ebbing tide on the vast sandy shore at Rhossili Bay in Gower, South Wales. These photographs were taken in March 2015 and add to the continuing documentation of the array of topographical changes that affect this wonderful beach.
Extreme low tides at Rhossili on Gower allow access to parts of the cliffs less frequently inspected. The distal north face of the Rhossili headland shows a complex mixture of natural and man-made features, on both a minor and major scale, often impossible to distinguish. The Carboniferous limestone was at one time quarried and this has resulted in changes of shape in the cliff face. However, most of the characteristic cracks, crevices and caves are the result of the waves and the weather working upon naturally-occurring fissures and cavities in the exposed bedrock surface.
On a small scale, it is possible to see where fractures in the Black Rock Limestone, sometimes infilled by white crystalline calcite and red haematite, are the lines of weakness along which erosion is progressing. These natural clefts are typically wider towards the outside, narrowing down to the width of the crack. The wearing away and widening of the break in the rock is initiated by mechanical and chemical action of waves and rain. Once the crevice has started to widen, it becomes a habitat for small organisms which add to the erosion process, again by both mechanical action and acid erosion.
In the photographs accompanying this post, it is possible to see some conical limpets, small acorn barnacles, and spots of bright orange sponge taking advantage of the relative shelter of these natural clefts; while minute blue-black mussel spat frequently settle (at least temporally) in the narrow confines of the crack itself. The clefts tend to retain moisture to a greater degree than the main cliff face, and may act as channels for water to drain downwards to the beach. These damp locations make ideal places for microscopic algae and bacteria to grow, and for mud-tube dwelling marine worms to thrive, especially low down, near the mobile beach sediments. Mostly, it is the former presence of the worms that is indicated. The level of the sand and pebbles fluctuates and affects the survivability of the worms. They leave a strange texture on the rock surface – composed of thousands of burrows or tunnels, just a few millimetres in width, that were incidentally etched by their acidic metabolic waste products.
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.
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.
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.
George, G.T. (2008) The Geology of South Wales: A Field Guide, published by email@example.com, 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.)