Ancient shoreline peat beds with numerous holes made by burrowing bivalve piddocks

Holes in Peat & Clay at Broughton

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Underlying the sand at Broughton Bay on the north coast of the Gower Peninsula in South Wales are ancient layers of peat and marine clay. These layers are eroding out of the burying sand, especially low down on the shore, and the newly exposed beds of peat and clay are riddled with thousands of circular holes about a centimetre or so in diameter. The holes have been made by the burrowing bivalve mollusc known as a piddock (Pholas dactylus). You can still see fragile empty shells of the piddocks in some of the holes. The age of the piddocks is debatable. It is not easy to tell how ancient or modern they are but they are clearly not recent, there were no live specimens that I could see.

Holes made by burrowing piddocks are often found in beach stones or pebbles when pieces of infested bedrock have broken free and become rounded by wave action on the shore. There is a lot more information about this particular kind of hole in rocks and stones, and the creature that creates them, elsewhere on Jessica’s Nature Blog and you can access these posts by clicking here.

17 Replies to “Holes in Peat & Clay at Broughton”

  1. I’m fascinated by this hidden world exposed by the erosion. And I’ve never heard of piddocks and I’m impressed by their ubiquity and determination. I also love the look of rocks and peat full of holes like you’ve been showing.

  2. Hi Jessica…..you say that below the sand there are peats and marine clays at Broughton.

    What is the evidence for the marine deposition of clays if as you seem to say the piddocks burrowed into a prexisting clay. It would seem usefull to wash some of the clay and see if there are anyforaminifera and ostracods.
    Perhaps I will do that the next time I am back staying the Mumbles.

  3. It’s a fascinating subject area, Claudia. You can find fossilised evidence of invertebrate marine creatures burrowing in rocks as well as see modern representatives doing the same thing on beaches today.

  4. I grew up in Tennessee, my area of which was covered by an ancient sea, and extensive limestone deposits were the result, with lots of fossils and evidence of marine life – we picked up fossils in our yard (the limestone very thinly covered with soil and poking up in outcrops in our yard). So this post really reminded me of that.

  5. It would indeed be very interesting, Rob, to look at the foraminifera and ostracods in the clay. Am I correct in thinking that the different species have different preferences for the degree of salinity or freshness of the water?
    The clay and peat layers lie over the glacial sediments from the Late Devensian that are increasingly being exposed on the upper shore at Broughton. So I guess that makes these blue clays and peats Holocene period. The uppermost peat layers with the embedded tree stumps could be about 4,500 to 3,000 years old, and the lowermost about 8000 years ago, judging by some of the information I have found about Broughton and similar layers in Swansea Bay. The alternating peat and clay layers (the clay may be marine or estuarine) were formed as the sea levels fluctuated up and down so that the clays were deposited when levels were high. The peat (sometimes with woodland) formed when the sea level was low.
    It is interesting to note that when E M Bridges wrote “Classic Landforms of the Gower Coast” in 1987 (republished 1997) he said that “As the climate ameliorated after the Devensian glaciation, vegetation re-established itself on the floor of the estuary, sea-level being lower than at present, and in places peat accumulated. Fragments of this peat are occasionally to be seen washed up on the beach after storms”. This was indeed the situation when I first visited Broughton and Whiteford in 2000, just odd blocks of peat.
    However, since that time, further erosion and depletion of beach sand continues to expose the underlying peat and clay layers. By 2008 Gareth T George in “The Geology of South Wales: A Field Guide” refers to the now clearly visible Broughton peat layers on the upper shore with their in situ stumps and fallen branches of alder, birch, hazel, and oak. The peat has also been known in other places to contain insect remains and mammal bones, even flint artefacts from the Neolithic and Bronze Ages, and also human footprints. In the British Regional Geology series “Wales” (BGS 2007) I have learnt that in other similar situations along the south coast of Wales, these peat layers are “interbedded with marine and estuarine clays, which contain a characteristic fauna that indicates some pauses in the gradual drowning” (of the coast). So it seems likely that some research has already been done on the microfauna of the clays and I will try and track that down.
    More and more expanses of peat and clay have been revealed in the decade since these publications and this important geological and environmental record is fast disappearing.

  6. You were fortunate to find fossils in your own yard – but it doesn’t sound as if the ground would have been very productive for growing vegetables or flowers.

  7. I often have my camera with me and try to capture everything I see so that, when I am stuck at home, I can work with them for posting on the blog.

  8. Yes, in this particular location, it was bare dirt in places and never really even could grow grass. We did not mind (though I am sure my parents did) because it was so much fun to go hunting around there.

  9. Yes, that is is exactly, when we first moved there, there were no other houses, although now it is a huge suburb, 40 years later. We lived on a gravel road and we rode our bikes everywhere on dirt roads through fields. A great place to grow up.

  10. Yes, a funny word. And not to forget that these are only one type of organism that makes holes in rocks – there are lots of others but these ones are larger, more noticeable, and could perhaps be mistaken for something artificially created.

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