Seashore Rocks at Manorbier

The red-rock wave-cut platform on Manorbier beach in South Pembrokeshire is pretty spectacular. Row after row of vertically aligned strata stretch from the shore across the bay and up the cliffs in the distance. They form an incredible backdrop to the expanse of sandy beach topped by colourful pebbles at high water mark. They make an ideal place for clambering around and exploring rock pools.

Up close, each layer of rock looks slightly different – darker or lighter; harder or softer; coarse-grained or fine-grained; smooth, rough or sculpted; homogenous or with inclusions. They are different colours – red, cream, orange, purple, and green. They sometimes have internal patterns. Every layer of rock represents a particular phase of activity during the deposition of the sediments in what is known as the Freshwater West Formation between approximately 416 and 410 million years ago in the Lower Devonian Period of the Upper Palaeozoic Era. These rocks are also referred to as the Lower Old Red Sandstone.

The 580 metre thick rocks of the Freshwater West Formation are made up of clastic sediments. Clasts are particles of broken-down rock, and the size of the fragments may vary in size from boulders to silt-sized grains and are invariably the products of erosion followed by deposition in a new setting. So clastic rock is consolidated sediment composed of fragments of pre-existing rocks (Allaby 2008). Examples of clastic rocks would include conglomerate, sandstone, siltstone and mudstone. Sediments for the rocks at Manorbier were derived from the newly uplifted Caledonian Mountains and subsequently deposited in a variety or arid to semi-arid continental environments, including estuaries, broad alluvial plains, ephemeral braided-meandering rivers and alluvial fans (George 2008).

The sediment size in the rock layers depends a lot on the speed and volume of the water transporting the fragments from the mountains and across the land. The greater the speed and volume of water, the larger the particle size that can be carried – but the greater weight means that the fragments are deposited sooner than the finer particles which can travel further even when the water velocity is decreasing. The Manorbier rocks illustrate the cycles of deposition during this period, following greater and lesser flows. Each phase is marked first by a layer of the coarsest sediments usually with cross-bedded sandstones which can be red-brown, purple or even green resting on an erosion surface (Howells 2007) followed by layers of increasingly finer sediments like mudstone in a phenomenon known as upwardly-fining. The fining-upwards cycles are interpreted as the fills of ephemeral fluvial (river) channels with sinuous profiles (George 2008).

Pale-coloured nodules often present in the red mudstones and calcareous siltstones are calcrete. Calcrete is a concretionary carbonate horizon formed in the soil profile in arid to semi-arid environments. Calcretes are a feature of a palaeosol or fossil soil in which calcium carbonate is precipitated as root encrustations (rhizocretions) and as small nodules (glaebules) from water flowing through the soil profile. The glaebules grow and coalesce to form a calcrete or dense layer of calcium carbonate near to the surface (Nichols 2009).


Allaby, M. 2008, Oxford Dictionary of Earth Sciences, Oxford University Press, 3rd Edition, 978-0-19-921194-4.

George, G. T. 2008, The Geology of South Wales: A Field Guide,, 978-0-9559371-0-1, pp 22 and 132-135.

Howells, M. F. 2007, Wales, British Regional Geology, British Geological Survey, Nottingham, Natural Environment Research Council, 978-085272584-9, pp 99-108.

Nichols, G. 2009, Sedimentology and Stratigraphy, Wiley-Blackwell 2nd edition, Sussex, England, p148.

All sorts of colourful & interesting rocks

This gallery displays a selection of the most colourful and interesting rocks that have been featured in posts here at Jessica’s Nature Blog over the past couple of years. While I am out walking on beaches, I am always drawn to the colours of the rocks, sometimes bright and other times more subtle, and the many different patterns and textures. Initially it is the way that the rocks look that is so appealing. So much of what I see seems like amazing natural abstract art. I try to frame the composition so that it stands alone as an attractive image in its own right. But then I get curious and lots of questions come into my mind. I always want to know what kind of rock is it? What is it called? How old is it? What is it made of? How did it get to look like that? What happened while the rock was buried? What are the elements doing to it now that it is exposed?

As an amateur with a keen interest in geology, I start by looking at maps. I try to pinpoint the exact location where I photographed the rock. Then I try to get hold of the correct geology map. Geology maps have a lot of information about the age of the rock, the type, the period in which it was laid down or developed, as well as the distribution of the different rock types in the locality. Often there are references to special papers, memoirs and so forth that discuss the geology of the area. Sometimes these publications are available on-line. I do a lot of Googling. Sometimes a visit to the library is needed. Libraries and the internet don’t always have the information I am seeking so I buy books too. Sometimes books about a specific place, and sometimes more general textbooks. I need those too because it is quite difficult to understand everything. Geology is a complex subject with a great deal of specialist terminology.

Once I am fairly certain what the rocks are, I try to write a bit about them in a straightforward way so that anyone else who is truly interested will be able to understand. It is fascinating. Slowly I learn more about the rocks and can fit the pieces together into the bigger picture. Walking along shorelines becomes a whole new experience when you are able to visualise the former environments in which the bedrock originated, or the drift geology was created, when you begin to understand what has happened to the strata over the millions of years since they came into being, and when you first begin to grasp what processes are affecting them once they are exposed to air. I love it when I can recognise strata belonging to the same geological period in different parts of the world, and see their differences and similarities, whether in situ or in buildings, walls and other structures. I begin to feel an enormous sense of wonder and awe, as well as an enormous feeling of humility, at this hugely significant part of the natural environment, a part on which everything else in nature depends or by which it is affected.

Rocks at Fermoyle on the Dingle Peninsula

Red Devonian sandstone rocks at the beach with fucoid seaweeds

The place where I took these photographs is marked on the map as an island but it is actually just a tiny promontory near to the village of Fermoyle, along the Dingle Way, on the north coast of the Dingle Peninsula in Ireland. I am sure that most people visit the location for its wonderful long unsullied sandy beach. However, I was drawn to this particular part, at the extreme western end of the beach, because of its fascinating geomorphology. The rocks are sandstones and conglomerates (mostly but not exclusively red) of the Glengarriff Harbour Group from the Devonian Period. The bright olive, lime, yellow and orange colours of the seaweeds, and the black, yellow and white of encrusting lichens, clash garishly with the red rocks. The rock strata are clearly defined: sometimes on-end, sometimes as flat bedding planes, and in one place a dome of strata lies cut-away and exposed. Beach stones rather than pebbles cover a portion of this area; and there are also occasional huge boulders composed of conglomerate scattered along the shore nearest the inlet from Brandon Bay.

Rock Textures at Fall Bay (1)

Limestone rock texture on the coast

The rocks at Fall Bay are arrayed like the riffled pages of a book. Layer after layer of Carboniferous Limestone is sequentially spread out across the west side of the bay. Each layer has an observably different texture; some are bioturbated with bioclasts and fossils such as fragmentary crinoids and corals. The bedding planes of some strata have deeply sculptured surfaces from weathering and bioerosion. Lichens, barnacles and limpets colonise the rocks and take advantage of the meagre shelter offered by cracks, crevices, and solution hollows.

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.


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, 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.)

Rocks at Clogher Bay 2

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

This is the second in a series of photographs of rocks at Clogher Bay on the Dingle Peninsula in the West Coast of Ireland, and they belong to the Dunquin Group from the Silurian Period. Clogher Bay is just south along the coast from Ferriters Cove which has featured in earlier postings.

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

Silurian Period rocks belonging to the Dunquin Group on the Irish Coast.

Elephant Skin

Specimen of elephant taxidermyIn an earlier post about rock textures and patterns at Tenby in South Wales I said that some rock surfaces reminded me of elephant hide. So, shown above are a few photographs that I took of the elephant skin on a prepared specimen exhibited at the Natural History Museum in London to show you what I meant – while below are a couple of examples of the textured limestone from Tenby for comparison.