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, firstname.lastname@example.org, 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.
It looked like blood but of course it wasn’t! Red liquid seemed to be oozing from the sand high on the shore towards the north end of Whiteford Sands on the Gower Peninsula in South Wales, approaching Whiteford Point. I have seen the phenomenon before and wrote about a possible explanation in the post “Rusty pebbles at Whiteford”. Only on that occasion the seepage was more orange in colour and clearly ferruginous in nature. Yesterday, the liquid seemed much more concentrated and red, like blood, quite spectacular, but perhaps just a trick of the light. These photographs are as taken and not edited in any way as I am uploading them direct from my i-pad. I believe that the iron responsible for colouring the water draining down the shore is derived from an iron pan caused by decomposition of an ancient peat bed beneath the sand. It is possible to see the layers of peat and clay where they are exposed just below the seepage rivulets. The peat and clay layers are increasingly interesting as more logs and stumps of preserved trees and bushes are weathering out. More about these features in later posts.
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.
Plant fossils are abundant in the Ward Point Member rocks at Cape Enrage in New Brunswick. You do not need to be an expert to find them in beach stones beneath the cliffs. You do need to be an expert to identify all the fragments accurately. I am not an expert. However, as far as I can make out, most of the fossils that I saw were the strap-like leaves of Cordaites, a primitive conifer from upland regions which according to the guide books resemble Amaryllis leaves or corn husks. There were also fragments of Calamites stems; this was a tree-like plant that could grow up to 10 m tall and is related to the much smaller present day horsetails or Equisetum plants (see images below). The stem is ribbed and jointed like bamboo with a diameter of about 10 cm and it would have had narrow whorled leaves at intervals along the stem. It formed dense thick undergrowth in lowland wetter areas. The diverse fossil flora at Cape Enrage represents dead vegetation washed downstream by rivers and stacked up in piles on the banks of many river channels about 320 million years ago in the late Carboniferous Period. The plant debris would become covered in successive layers of sediment brought down by the rivers as they wandered across the flood plain to the sea, and eventually preserved in sandstones and mudstones.
The Cape Enrage Visitor Centre has some excellent examples of fossils on display, and education officers are available to give advice and help with identifications. They are very helpful and friendly. I am sure that, time permitting, a professional guided tour would reveal many more in situ fossils of different types than those illustrated here.
Cape Enrage is one of the many spectacular stop-off points as you travel along the north shore of the Bay of Fundy on the east coast of Canada. The cliffs are composed of thick and thin layers of sandstones, mudstones, and conglomerates that built up along river beds and valleys about 320 million years ago. The ancient river channels can be seen in cross-section in the exposed and weathering strata. The way that the rocks are fracturing and splitting up into their component layers makes many intriguing natural abstract patterns. Plant debris that piled up on the river banks as it was washed downstream in the Carboniferous period has been preserved as fossils.
Cape Enrage in New Brunswick, on the north shore of the Bay of Fundy, is a popular site for visitors because of its fantastic geology. The rocks are composed of Carboniferous Period Ward Point Member sandstones and mudstones of the Enrage Formation and the Cumberland Group. The almost vertically inclined rock layers in the cliff fracture and weather to create wonderful abstract patterns. The sediments that make up the rocks are fluvial in origin, originally deposited by rivers, and the exposed cliff-section shows many in-filled river channels. Large areas of rippled surface can be seen where channels are exposed; this together with chunks of rock with cross-bedding is interpreted as point bars in which the channels were constantly migrating laterally (as in present day braided river channels before they enter the sea). Localities with conglomerate composed of sandstone with rounded pebbles represent the the centre of river channels where the flow was fastest, or where flash floods have deposited heavier loads.