Beach Stones with Holes at Worms Head Causeway

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Beach stone with holes mostly made by boring bivalved molluscs.

Larger than pebbles, and less rounded, these beach stones are more cobble-size than small boulders. They were lying on the shore at the landward edge of the Worms Head Causeway – which lies at the very tip of the Gower Peninsula in South Wales. Like the pebbles and shells previously described in Jessica’s Nature Blog, they are riddled with holes and burrows caused by various seashore creatures such as certain bivalved molluscs and mud-tube living marine polychaete worms.

The larger rounded holes were made at some time in the past by molluscs such as piddocks or rock-borers. There are still the paired empty shells of the mollusc in some of the holes. It is not possible to day how old these shells are but they could be very old indeed – perhaps dating from the time when the sea level was higher and when the now raised beach was formed. The much smaller, often paired holes leading into U-shaped burrows being made by worms such as Polydora ciliata.

Beach stone with holes mostly made by boring bivalved molluscs

Beach stone with holes mostly made by boring bivalved molluscs

COPYRIGHT JESSICA WINDER 2013 – All Rights Reserved

5 Replies to “Beach Stones with Holes at Worms Head Causeway”

  1. are these fossils of worm burrows in sedimentary rocks, or did the worms burrow through the rock itself? Seen similar stones at Durness, NW Sutherland, and on the East coast, N of Newcastle..


  2. That’s a question that I am always asking myself! These rocks with worm burrows seem to occur all over the world. They are very common but there is not a lot of literature about the phenomena. I have been collecting and pondering about them for decades. Basically, I think it can be very difficult to tell how recent the holes are; and each case has to be considered on its own. The organisms that create the holes and burrows in stones, shells, and wood are still at work today. I have seen actively occupied burrows in vertical surfaces of rock outcrops (including cliff faces) at high tide level as well as on horizontal rock surfaces that spend much of the time below water (on the Gower Peninsula and Dorset Beaches). When you see the living worms or boring bivalves actually in situ, in the holes, then you know for certain that the holes are current and not ‘fossil’. However, the shells of piddocks and borers can remain in the burrows for long after the death of the animal – I have, for example, seen these in recently exposed clays at low tide at Whiteford Sands and Broughton Bay – and this phenomenon can be misleading; the burrows are indeed old but it is not easy to tell how old. The shells are real, their original structure, but too fragile to remove. They probably date back to the end of the Ice Age in this particular case. I have also seen holes made by both mud-tube worms and boring molluscs in rounded limestone pebbles embedded and ‘cemented’ into raised beaches dating from about 10,000 years ago, for example, on the Isle of Portland. So these are very old holes in old pebbles but not actually fossilised burrows. They remain intact but unaltered in the beach deposits. The holes have not been in-filled by other sediments. Yet worm burrows and tunnels made by crabs do occur fossilised as trace fossils or ichnofossils and are a feature of the sedimentary Jurassic limestone rocks on some Dorset beaches with which I am familiar; all these tunnels and burrows have become filled with later sediments subsequent to burial so that the shape is preserved but not the cavities.


  3. Lovely to see this! I have found countless piddock-holed rocks myself, over the years, mostly on the west coast of Scotland. I wrote about them recently on my blog. Great post and photos! 🙂


  4. Thank you, Jo. People are endlessly fascinated by the holes found in rocks on the beach and the many types of organism that can be responsible for them. Nice to hear from someone with such similar interests from such a beautiful part of the world.


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