Rusty Pebbles at Whiteford

Rusty Pebbles at Whiteford (1) - View looking across to Llanmadoc Hill showing pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

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.

Rusty Pebbles at Whiteford (2) - View looking across to Llanmadoc Hill showing pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Rusty Pebbles at Whiteford (3) - Pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Rusty Pebbles at Whiteford (4) - Pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Rusty Pebbles at Whiteford (5) - View looking across towards Whiteford Lighthouse showing pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer. Ancient waterlogged wood from the submerged forest is also visible.

Rusty Pebbles at Whiteford (6) - Ancient iron-stained log embedded in peat from a submerged post-glacial forest - associated with pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are also covered with a rusty deposit thought to derive from the break-up of an iron-pan associated with the disintegration of the Holocene peat layer.

Rusty Pebbles at Whiteford (7) - Ancient iron-stained log embedded in peat from a submerged post-glacial forest - associated with pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are also covered with a rusty deposit thought to derive from the break-up of an iron-pan associated with the disintegration of the Holocene peat layer.

Rusty Pebbles at Whiteford (8) - Pebbles scattered on the surface of the sand with 'tears' of rusty water, possibly rising from a buried Holocene iron-pan layer below the sand, weeping across the beach.

Rusty Pebbles at Whiteford (9) - Pebbles scattered on the surface of the sand with 'tears' of rusty water, possibly rising from a buried Holocene iron-pan layer below the sand, weeping across the beach.

COPYRIGHT JESSICA WINDER 2013

All Rights Reserved

6,000 year old seashells at Cogden Beach

Fragile remains of a 6,000 year old cockle shell from a peat deposit beneath the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast (1) 

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. 

Ancient peat block thrown up by storms onto the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast - from a peat bed underlying the mobile sediments of the seashore (2)

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).

Mostly gastropod shells found crushed between layers of an ancient peat block found on the shingle at Cogden Beach, Dorset, UK - part of the Jurassic Coast (3)

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.

Paired valves of an ancient cockle shell well preserved (not fossilised) in a block of clay, thousands of years old, thrust up onto the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast (4)

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.

Blue and black pebbles together with cockle shells embedded in an ancient block of clay sediments found on the shingle at Cogden Beach, Dorset, UK - part of the Jurassic Coast (5)

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.

Blue and black pebbles embedded in a clay block found on the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast (6)

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.

A typical ancient clay block (from deposits underlying the present seashore sediments) found on the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast (7)

Revision of a post first published 27 April 2009

COPYRIGHT JESSICA WINDER 2011

All Rights Reserved

Peat ‘pebbles’ with piddock holes

Peat 'pebble' with bore holes made by piddock bivalve molluscs, on the strandline at Whiteford Sands, Gower, West Glamorgan (1)

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.  

Piddock bore holes in a peat pebble: Close-up of rounded and water-worn piece of peat with holes excavated by bivalve molluscs such as piddocks, from Whiteford Sands, Gower, West Glamorgan (2)

Shells of boring mollusc in peat pebble: Detail of peat block showing borings and paired piddock shells in situ, Whiteford Sands, Gower, West Glamorgan (3)

Peat pebble with embedded leaves: Smoothed and rounded lump of ancient peat showing embedded leaves and other vegetable remains, on the strandline at Whiteford Sands, Gower, West Glamorgan (4)

Peat layers beneath a sandy beach: Alternating layers or strata of peat, pebbles, and clay, eroding out at the water's edge on Whiteford Point, Gower, West Glamorgan (5)

Peat and clay on the beach at Whiteford Sands: View looking up the shore at Whiteford Point, Gower, West Glamorgan, over rapidly eroding relatively recent geological deposits of peat and clay (6)

Small lumps of peat braking away from a layer on the beach: Small, cobble sized pieces breaking away from the seaward edge of the clay and peat strata that normally lie beneath the sand at Whiteford Point, Gower, West Glamorgan (7)

Revision of a post first published 22 January 2010

COPYRIGHT JESSICA WINDER 2011

All Rights Reserved