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).

References

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, gareth@geoserv.co.uk, 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.