Slag and Slipper Limpets in Swansea Bay

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Up against the rusty iron panels of the West Pier at Swansea Harbour lie drifts of seashells, mostly of slipper limpets with oysters, whelks and the occasional variegated scallop shell. Intermixed with the shells is evidence of the area’s metal manufacturing heritage in the form of dark lumps of clinker and slag from furnaces and smelters used in the past production of metals such as copper, zinc, steel and tin. Clinker and slag has been extensively used along with domestic waste for land reclamation and landfill in the region. This is documented by Waters et al in a report on the Urban Geology of Swansea and Port Talbot. Some of this industrial waste material has ended up amongst the natural debris on the shore of Swansea Bay, most notably in large accumulations against the pier structure but also along the top of shore strandline on the beach for hundreds of metres.

6 Replies to “Slag and Slipper Limpets in Swansea Bay”

  1. one of my first geological trips, from school, was to look at the exotic pebbles on the shore, near to the West Pier. It’s a long time ago but at the time there were common pebbles of various igneous and metamorphic rock, which were explained, to us, as from the ballast jettisoned from ships before they loaded with coal.


  2. There are a lot of worn and blackened oyster shells along Swansea Bay?
    Do you believe that they are from
    1. Recently dead oysters
    2. From the 19th and early 20th century when oysters were abundant
    3. Reworked from the subrecent clays


  3. That is very interesting, Robert. I will have to keep my eye open for ballast pebbles when I go there again. I wonder where the ships’ ballast came from. Do you think it could be from Cornwall or further afield?


  4. I have often wondered about these blackened oyster shells myself. They wash up frequently on Gower beaches such as Rhossili and Whiteford as well as Swansea Bay. I know how they became black but I have no way of knowing how old they are. What are your own thoughts about it? I think most of the black oysters are not recent. I think it could take a long time to get that colour, certainly decades of burial in anoxic sediment and maybe centuries or longer. I have not found any literature about oyster shells in sub-recent clays in that area, or seen any shells actually embedded in clays in Swansea or Gower – but my own experience and knowledge is very limited. Do you have any thoughts on this yourself? I find the geology of the area, particularly with regard to the more recent and superficial deposits, complex. The British Geological Survey booklet Geology of the Swansea district: a brief explanation of the geological map Sheet 247 Swansea by W J Barclay (NERC 2011) gives a good overview but I am unable to see any mention of the oyster shells in Quaternary deposits of any kind. I need to do more homework.


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