Some of the ancient wood that has long been buried in peat and clay deposited after the last ice age has wonderful textures and woodgrain patterns. Whole recumbent tree trunks have been emerging from the peat as a result of recent beach erosion at Whiteford on the Gower Peninsula in South Wales. This wood is not fossilised but is preserved in its original state by the anaerobic conditions in which it was buried – in the same way that the bodies of the so-called Bog People were preserved.
About 10,000 years ago at the beginning of the Holocene Period, sea levels began to rise and drown coastal areas. It didn’t happen all in one go but episodically over time. At the maximum extent of the Devensian ice sheet around 21,000 BP the sea level was about 100 m below present levels (Howells 2007). The melting of the ice sheet and isostatic rebound caused the relative sea level changes. Present sea level was achieved about 5000 years ago. The deposits laid down on the coast during the period of sea rise are made up of layers of soft blue-grey marine clays and silts inter-bedded with freshwater peat. This reflects the way that during the overall time of sea level rise the level rose and fell many times, depositing marine clays and silts when it flooded inland, and allowing salt marsh and peat to develop when the sea receded.
Many coastal areas have these ancient peat beds and boreholes in the UK have shown that they can exist in some places at depths up to 18 m below Ordnance Datum which provides the evidence that sea levels were once lower and the sea level has since risen. However, in the coastal zone near Swansea the peat more typically occupies depths between 2 m above and 2 m below OD, deepening southwards. Offshore the peat lies 16 – 20 m below OD (Barclay 2011).
Here at Threecliff Bay (also known as Three Cliffs Bay) on Gower, South Wales, a bank of storm beach stones and pebbles, now isolated from the shore by a large intervening dune of wind-blown sand, has covered and protected the old peat beds until recent times. The storm beach deposit and underlying peat lie in the final meander loop of the Pennard Pill before it skirts the dune and flows over the shore to sea. Now exposed, the layers of peat and clay are eroding fast. Large lumps are detached around the margins of the bed.
All over the surface of the peat there are random branching patterns of dotted lines. Each “dot” is the cross-section of a stem of salt marsh vegetation preserved in situ. At the edges of the bed, the constituent layers of soft clay and peat are revealed in cross-section. Perfectly preserved remains of plant stems and roots can been penetrating through the strata in their original life position. Rusty staining and deposits in the peat bed are caused by decomposition of the organic matter.
REFERENCES
Barclay, W. J. (2011) Geology of the Swansea district – a brief explanation of the geological map Sheet 247 Swansea, British Geological Survey, Keyworth, Nottingham, NERC, ISBN 978-085272581-8, 24-25.
Howells, M. F. (2007) British Regional Geology: Wales, British Geological Survey, NERC, ISBN 978-085272584-9, 195-196.
Vast swathes of pebbles on the beach at Whiteford in Gower are coloured orange – or at least they were the last time I looked. (The beach sediments there are very mobile so it cannot be guaranteed that you will see exactly the same thing on each visit). These coloured pebbles are found in a band stretching from the base of the sand dunes at the eastern end of the beach towards the disused Victorian Whiteford Lighthouse.
The pebbles seem to be coated in rust rather than rusty because of their intrinsic composition. I guess the first couple of times that I noticed the orange pebbles I vaguely thought that they were stained by rust emanating from the decomposing remains of the old iron causeway that linked the lighthouse to the shore. You can often find pieces of the iron framework of the walkway – sometimes supports still in situ and other times single pieces of the structure lying free.
However, lately, I have been discovering more and more about the Quaternary geology of the Gower – a relatively recent geological period dating from about 2.5 million years ago to the present. This includes the Pleistocene with a variety of glacial, peri-glacial and inter-glacial deposits; and the recent Holocene (from 11,800 years ago) with peat and submerged forests, marsh, dune, beach and alluvial deposits. As I read more, I am gradually reaching something of an understanding about some of the natural phenomena that I observe and photograph on Gower beaches. So I now tentatively consider that the rusty pebbles are not related to the dilapidation of the old lighthouse but are the result of a much older natural geological process.
I have already mentioned in Jessica’s Nature Blog the remains of the submerged forest at Broughton Bay which lies to the west and adjacent to Whiteford Sands. These ancient tree trunks are embedded in peat deposits. While I was reading George (2008), I learnt that the peat decomposes to form a hard ferruginous layer called an iron-pan or hardpan. This has led me to wonder if the iron compounds that coat the pebbles at Whiteford are derived from an iron pan layer.
Supporting evidence for this idea comes from the presence of ancient tree trunks emerging from black peat deposits close to the rusty pebbles – similar to those stumps found at Broughton. The old waterlogged wood is also stained with rust – as you will see from the photographs below. Additionally, slightly higher on the beach, closer to the dunes, the shore is strewn with pebbles around which orange-coloured watery ‘tears’ rise to the surface and weep across the surface of the sand – making me think they might originate from a concealed ferruginous hardpan below.
Then again, I suppose the rust could come from buried decomposing munitions as the beach was used for firing practice in the Second World War!
Reference:
George, Gareth T. (2008) The Geology of South Wales – A Field Guide, G.T.George at gareth@geoserve.co.uk , ISBN 978-0-9559371-0-1, p 70.
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Not fossilised ….. but the actual fragile remains of a 6,000 year old cockle shell in a block of ancient peat flung up onto the shingle bank at Cogden Beach from a layer underlying the mobile deposits on the seashore.
You might not think this a very interesting picture but that illustrates just how easy it is to overlook this repository of environmental evidence from the distant past. Contained within its dessicating layers are the clues from which it is possible to reconstruct the scene on this very spot as it was many thousands of years ago.
Leaves, twigs, stems, wood, gastropod and bivalve shells, and even beaver bones have been recovered fom these peat blocks. Small fragments of vegetation and shells are the most frequent finds as shown in the picture below. The plant and animal remains have been radio-carbon dated to 6,000 BP (before present).
As well as the peat, there are blocks of clay on the shingle, also containing shells. In these sediments the shells are better preserved than in the peat. The peat shells are so fragile that they almost disintegrate on touch. Whilst the acid environment of the peat formation would aid the preservation of plant and animal soft tissue remains, over time this would denature or dissolve the calcium compounds in the shells (or bones). The less acid but similarly anaerobic environment of burial in the fine clay sediments would lead to better preservation of the shells.
The photograph below shows the paired valves of a cockle in situ as they would have been in life.
Included in these blocks of ancient clay sediment are blue and black pebbles which are larger and very distinct from the pebbles forming the shingle bank.
More of these distinctive pebbles in the clay are shown below. The dark colour is caused by burial in anaerobic (without air) conditions in the same way as previously described for black oyster shells from Rhossili Bay.
And lastly, here is a context shot to show location and scale for a typical clay block.
For the real expert explanation of the geological phenomena briefly described in this blog, and for references to original research and publications, please refer to Ian West’s Web Site on the geology of the Fleet Lagoon.
Revision of a post first published 27 April 2009
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These pebbles look like rock but when you pick them up they are light and obviously organic. They are made of ancient peat that underlies the sandy beach at Whiteford Sands. The peat is about 10,000 years old and occurs in layers alternating with clay and interspersed with pebbles. These strata are being eroded away by the action of the waves. You can sometimes see the deposits outcropping during low tide at Whiteford Point – just beyond the Whiteford Lighthouse.
The pieces that break away from the seaward edge of the peat layer can be a metre or more across and these large lumps occur as islands dotted sparsely across the vast expanse of wet sand. Much smaller pieces often wash up on the strandline. The edges are rough and uneven when the peat has recently broken away. Smoother, more rounded, pieces are the result of peat fragments rolling around in the sea over a length of time. It is possible to see preserved leaves and stems incorporated into the peat. These are clues to the past environment. Apparently, no-one has yet studied in depth either these plant remains or the submerged forest timber.
Intriguingly, you can frequently find neat circular holes drilled into the peat. These boreholes have been made by boring bivalved molluscs -although they look regular enough in shape to be man-made. I have written in earlier posts about soft rock and pebbles with holes made by sea creatures. Also shells with holes made by boring bivalves. So it would seem that peat is an additional suitable substrate in which molluscs like piddocks can live equally well.
One of the pieces of peat I picked up on my last visit still had the empty shell of the mollusc in one of the borings. I don’t know whether the shells are ancient or modern. I have not seen any live molluscs in these peat colonies; or in any of the nearby colonies to be found in the clay deposits either. Only empty shells so far.
The shells are very fragile and usually break when you try to extract them. I am going to try and get some decent specimens so that I can make a definitive identification. And I will keep searching for live animals as this will show that at least some of the boreholes in the peat and clay on this Gower beach (and its neighbours like Broughton Bay) are modern. In theory, you could radio-carbon date the shells (for example, this has been done for archaeological oyster shells from Poole in Dorset) but that costs money.
Revision of a post first published 22 January 2010
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