Pebbles with holes made by sea creatures

Pebbles with holes: An assortment of pebbles with holes made by sea creatures from the Jurassic Coast World Heritage Site in Dorset, UK (1) 

Pebbles with holes in them are a common occurrence on the beaches along the Jurassic Coast in Dorset – and lots of other U.K. locations as well. Many of these tunnels, burrows or borings have been made by small marine invertebrate animals such as certain species of  bivalved molluscs, polychaete worms, and even sponges. The arrangement of pebbles shown in the photograph above illustrates a selection of the different kinds of holes created by these animals in soft rocks.

In this post I am just going to talk briefly about the larger tunnels which tend to be about a centimeter or more in diameter. These have been excavated by rock-boring bivalve molluscs. There are several types whose habits result in these borings and it is not easy to say which species has made which tunnel from the shape and size of the holes alone. Fortunately, the shells of these creatures often remain in the burrows. The appearance of the shells is diagnostic for each species. Unfortunately, the shells of some of these bivalves are both surprisingly fragile and wedged securely in the burrow so that they are difficult to extract.

The most commonly occurring rock-boring molluscs are the Piddocks and some other related species. From the Pholadidae family, for example, these include the Common Piddock Pholas dactylus Linnaeus and the White Piddock Barnea candida (Linnaeus). From the Hiatellidae family, these include the Wrinkled Rock-borer Hiatella arctica (Linnaeus). And from the Gastrochaenidae, the Flask Shell Gastrochaena dubia (Pennant).

Pebble with holes: Cobble-size stone with large bore holes made by bivalved molluscs called piddocks on the beach at Charmouth, Dorset, UK - part of the Jurassic Coast (2) 

The softer rocks into which these bivalves bore are generally low on the shore and under water most of the time. When pieces of this rock break away, the stone becomes more rounded and worn and ends up as a pebble on the beach – like the one above which was seen at Charmouth in Dorset.

Pebble with holes close-up: Detail of large piddock bore holes and smaller worm burrows in large stone on the seashore at Charmouth, Dorset, UK - part of the Jurassic Coast (3) 

The photograph below shows a flat platform of soft Blue Lias shale extending seawards at Charmouth. If you look closely at the near-vertical edge of the rock where it is lapped by the water, you may be able to see that the shale has many perforations caused by these rock-boring molluscs.

Rock with holes made by bivalve molluscs: A seaweed covered platform of Blue Lias shale with piddock borings visible along the water's edge at Charmouth, Dorset, UK - part of the Jurassic Coast (4) 

Below is a closer view of the seaweed free edge of the rock platform with the piddock holes.

Rock with animal burrows: The vertical edge of a shale platform showing piddock burrows at Charmouth, Dorset, UK - part of the Jurassic Coast (5) 

The next picture shows empty shells still in situ in the burrows. In future posts I will illustrate the actual shells of these rock-boring molluscs and will describe something of their life in such a unique habitat. I will also discuss at a later date the other smaller types of rock-borings made by worms and sponges; and show how they also colonise and leave physical evidence of their presence on mollusc shells. When this evidence is found in shells recovered from archaeological excavations, it provides clues to the environment that was being exploited for its marine resources by people in the past.

Rock with holes containing seashells: Piddock burrows in Blue Lias shale, some showing the two empty white shells of the bivalved mollusc that made the hole, at Charmouth, Dorset, UK - part of the Jurassic Coast (6) 

Pebble with holes bored by bivalved molluscs - from the Jurassic Coast (7)

Revision of a post first published 4 October 2009


All Rights Reserved

About these ads

46 thoughts on “Pebbles with holes made by sea creatures

  1. I LOVE pebbles made with holes. Especially pebbles in the shapes of faces. Shells work well with holes too. I collected face-shells for many years (during visits to my parents at Fort Myers Beach). Thank you for sharing this information. It’s fascinating! Kathy

  2. Thank you, Kathy. Maybe we could see some photographs of your face-shells and pebbles on your blog sometime, please? Although I have lots of shells and stones with holes, I haven’t noticed any with faces yet – but occasionally faces appear when I take the picture – like the ones I took of small pieces of driftwood.

  3. It amazed me to realise that the holes were made in stone by living creatures; I’d always assumed the holes were effectively the remnant of fossil creatures.

  4. Jessica, you know…I think I’ve given away all except one face-shell. My mom couldn’t believe there were actually faces in shells until I started finding them. Haven’t found too many on rocks lately, either. Sometimes just a single hole is a rare find. I love finding faces! (and in driftwood too…)

  5. I am sure that some holes in stones can be trace fossils, or maybe due to some geological phenomenon. I only know that the ones described in my post are made by various sorts of marine invertebrates.

  6. I will start to look out for faces in shells myself – enough of them seem to pass through my hands to increase the likihood of finding some.

  7. Thanks so much for photos and explaining what created the holes! My grandson brought home (from So. California beach) a fabulous rock with holes and small white, wormy shapes in them. We thought maybe the white strings were bird poop, but now I guess they are some kind of fossilized invertebrate. Could I send you a photo of the rock, so you could verify?

  8. Hi, Terry. I am pleased to learn that you found my blog useful. Please do send a photograph of the rock with holes that your grandson found and I will see if I can tell you something about it. You can e-mail a .jpg file to

  9. Pingback: Benjamin & the pebble full of holes « Jessica’s Nature Blog

  10. I’m re-writing the Seahouses Rocks page and am grateful for your “Pebbles with holes made by sea creatures”. Thus provided with a lead, I looked for Piddocks on Wikipedia – NO LUCK!!!

    Any chance you might make a contribution? (I’ve done a couple and it’s not too traumatic.)

    Kind regards.

  11. Hello, Mike.
    I’ve had a quick glance at your site but need to give it a closer look. Seems as if you have some really interesting geology at Seahouses. I’d be pleased to give you some information about rock-boring sea creatures. I’m assuming that you have found rocks with holes which you think are made by piddocks or similar bivalves. Perhaps you could e-mail me on to give me some more details of what you require for your site, and possibly send some pictures so that I can check on what is creating the rock burrows at Seahouses. You could also post links to my blog from your site. There is actually quite a bit of information in the blog on rock borers in addition to the particular post you refer to – there are different types of creatures tunneling into rocks and seashells other than piddocks – several sorts of bivalves, bristleworms and sponges.
    I look forward to hearing from you.

  12. Pingback: Shells with holes made by boring bivalves « Jessica’s Nature Blog

  13. Pingback: Posts about PEBBLES « Jessica’s Nature Blog

  14. Pingback: Peat ‘pebbles’ with piddock holes « Jessica’s Nature Blog

  15. Thank you very much for the informative site, which more or less solves a question that has been intriguing me for some time regarding a stone that I found in Greece ten years or so ago.
    Mine closely resembles the one at ’3 o’clock’ in your first picture, but has more straight linear pitting. My nephew, who is a geologist referred me to your site, but suspects that (possibly to assuage his uncle’s feelings?), a human hand might have (hubristicly) attempted to embellish natures art.
    Thanks again,
    David Pinnock

  16. Hello, David. I’m pleased that my article has been useful. I’m assuming that the pebble which resembled your own was the white chalk one on the right of my photo. That one may have the holes and marks of more than one type of marine invertebrate organism, probably both sponges and small worms. I have seen shells and stones with more distinct linear arrangements of holes which I believe are more typical of sponge damage than anything else. The holes can be gently graduated in size.

    Would you like to send me a picture of your pebble? You can send a .jpg file to my e-mail address. I am not an expert but I might be able to tell you whether the holes in your stone are entirely natural or if they have been ‘enhanced’. In the past I’ve worked with archaeological material such as shells modified by man. The stones in my own photographs have not been altered in any way, they are just as found on the beach. [The photographs have however been slightly enhanced to sharpen the image].

  17. Hi, I am an artist and regularly paint beach pebbles – they are great sellers! I found a very hard one with round holes – just under 1cm diameter, which resembled either Swiss cheese or the moon surface – so I painted it as a Guinea Pig on the moon. I can send you a photo if you would like to see it!

  18. Hello, Suzanne.
    I’ve just looked at your site – what wonderful pictures! I love your ‘first guinea pig on the moon’. Well done.

  19. So glad you found what you were looking for, Sandra. Good luck with the jewelry. The bore holes created by boring bivalve molluscs make your pebbles ideal for making into necklaces and bracelets.

  20. Hello Jessica.

    Nice pictures did you make, some of them i’f seen here, i remember of a lot of carbon-rocks i found near my hometown.
    in my pebbles was holes from fossilized bivalvia from the cretaceous-periode. Unfortunately they are missing the shells.
    thank you for sharing.

    greets, karl from germany

  21. Hi Jessica – are pebbles with holes in, fossils or are they made recently, by as you say Piddocks. I have found a stone, heavy in mass, that seems to me to have been a soft clay and light in colour. It looks to have been part of a larger piece but the holes have broken part of it away. I am trying to find out when it may have been made so that my Granddaughter may have the fact when she takes it to school. Kind Regards Tom James

  22. Hello, Tom
    You have an interesting question! It’s a bit difficult to tell, even when a photograph is available or the specimen itself can be handled, whether the holes have in fact been made by some sort of marine organism like a burrowing piddock or merely by some other kind of geological process.

    It is also difficult to determine whether the burrows in a pebble or rock have been made relatively recently or in ancient times – and therefore represent a trace or ichno-fossil. I can only say that if the stone is made of a softer material, the holes could be recent/modern. However, even if there is an empty mollusc shell remaining in the burrow, this is not necessarily an indicator that the holes are recent. I have found intact sub-fossil shells in situ in holes in softer substrates like clay that have only just been revealed by weathering.

    Sometimes a broken stone with holes found on the beach can be seen to have originated from nearby bed-rock on the tide-line where live specimens of boring mollusc are still living – you can seen them squirting water from their siphons at low tide.

    As you can see from my attempt to answer your question so far, it is difficult to be certain how old the holes in your stone are. I am wondering where you found your stone? Would it be possible to see a picture of the stone? Perhaps then I may be able to give you a better answer – but no guarantees!You can send a picture of the stone to my e-mail address if you wish.

  23. Hello Jessica,
    This particular stone was found in my garden, when digging out a small tree. The area in which I live was, many years ago, an old brickworks. A location where they took out the clay that came down from Bidston Hill towards Leasowe shore. However having said that it may also have been picked up by myself when out visiting a quarry, English China Clay for instance, down in Helston Cornwall, or any other down south, then droped in my garden.
    For many years I worked as an engineer visiting many mines & quarries and often would collet the odd fossel. Sorry if I cause confusion. But to me the stone, which I believe was a clay at one time, appears not to be of the colour you would expect from this area, more similar to ECC or the chalk stone found further south east.
    With regard to the holes, my opinion is that the sea creatures that bored into it was when it was a clay, or a much softer material and they would use it for food or a mineral that would need to build up their shells. But that would be a complets guess.
    I will try to send you some photo’s
    Regards TRJ

  24. Yes, so do many others. It is the subject most frequently sought out on my Blog. Working out exactly what might have caused which types of holes is an ongoing area of research.

  25. I brought a stone back from Llandudno with full of holes i cant count how many. I just showed my dad he says it could a a bone which I doubt but it’s soooo amazing

  26. I have taken a photo of it I was trying to search what it was on google images. I type stone with holes which i got your website

  27. I look forward to seeing the photograph. Perhaps it will help me to say whether it is a bone or a stone; and what may have made the holes in it.

  28. Thank you. I do love your photos they are amazing. I’m into photographing it’s part of my hobbie but I do love taking photos though. But I’m trying to build my confidence with it at the moment.

  29. Hello, Charlotte. Thank you for sending the photographs of the stone with holes that you found at Llandudno. The images are very clear. It looks as if you have a very good camera to achieve that kind of sharp focus on the details.

    I think the holes in your stone have been caused by a sponge. The sponge starts living on the surface of the stone, maybe in a small crack. As it grows and spreads over the surface it produces acids in its waste products, and it is these mild acids that over time gradually dissolve the stone and form the holes. The sponge extends into the holes it has created so that eventually there is more of the sponge inside the stone than on the surface. The holes can become inter-connected inside the stone to form a kind of honey-comb effect of holes and tunnels. The same kinds of sponges can create holes in sea shells too. I often find old thick oyster shells with these honey-comb like burrows and holes.

    I am always fascinated by holes in stones because they can be caused by different kinds of events. Often, there is a mixture of different kind of burrows and holes and it is difficult to work out whether it is sponge, bivalved molluscs, or even worms that have caused the damage. And, of course, sometimes the holes have been made by natural processes during the formation of the rock in ancient geological times, or by relatively recent weathering/erosion processes.

    ‘Well done’ on spotting the unusual stone and being curious about how it got like that. And ‘well done’ with the photographs which show exactly what I needed to see. In fact, there is one other very small feature that shows up in the second image near the bottom of the stone. You may notice something that looks like a small piece of lace attached to the stone. This is the remains of some Sea Mat or Bryozoa. The Bryozoa is a colony of minute animals that live in box-like compartments arranged side by side. There is some more information about Bryozoa in Jessica’s Nature Blog.

  30. Thank you for that. That’s really interesting to know. I have always been keen on shells, stones and sea life. When I go to place like Blackpool, Southport and Llandudno with a group. I always head straight down to the beach. People laugh at me not in a nasty way though. It’s really interesting what you find on a beach and rocks. When my brothers and I were small we were at the beach looking in rock pool see what we can find. I am so pleased I have found this website now I can officially it will help me on what else I can come across.

  31. I am pleased that you found the information interesting. Seashore life has been a lifelong interest for me and there is always something new to learn and enjoy. I never tire of exploring seashores.

  32. I appreciate the article and pictures–I am in Nome, Alaska for the summer and find rocks with holes in them most visits to the beach.. The big holes I can put my finger through and in other rocks the holes are very small, but often go through the rock.
    I pick up rocks with holes, flat rocks and egg-shaped rocks–and there is a lot of beach glass here
    –the best pieces being thick, worn-smooth, frosted old pieces from the miners of old that lived
    on the beaches of Nome.

  33. It seems that pebbles and rocks with holes in them made by marine invertebrates can be found all over the world – wherever there are sedimentary rocks. I haven’t yet heard of these holes occurring in igneous rocks like granites or metamorphic rocks like slate – those are too hard and cannot be dissolved by weak acids or ground by shell action. As you may have read in the several articles I have written for the blog, the bigger holes are likely to have been made by bivalved molluscs like piddocks, while the smaller holes are generally made by worms or sponges.

    Nome in Alaska sounds an interesting place. It must be fascinating to collect not only attractive beach stones of all shapes and sizes but also sea glass with an exciting historical and social link back to the miners of old.

  34. Thank you for the post! My son and I were walking on some trails by our house in Southern California and found all kinds of rocks with holes in them that look very similar to some of the rocks you have pictured. I was wondering if the holes can be caused by something other than sea creatures? We live about 45 minutes from the ocean, but these rocks definitely look like they belong in the ocean. Just curious! :)

  35. Yes, I think there must be many kinds of holes in rocks which have not been caused by various worms, sponges and molluscs. It’s a bit difficult to say exactly what chemical or physical processes might have been involved without knowing about the specific rock type and exact location – especially if, like me, you are just an interested amateur geologist. Do you have any photographs of the rocks with holes that you and your son found? It would be interesting to see them. I have often thought that the kinds of holes made by sea creatures (like the ones described on my blog) might have occurred in the geological past and been preserved. This would be one possible explanation for your finding them so far from the sea. Many of our sedimentary rocks were formed on the bottom or margins of seas or estuaries; and many of these layers of rocks have been lifted and shifted by subsequent earth movements to locations that are currently far from the sea. There are, for example, many trace or ichno-fossils – including marine worm and crab burrows – in rocks near where I live on the Dorset coast in the UK.

  36. Thank you! Interesting! We are going to be out of town, but I will have to take some pictures of the rocks when we get back home and send them to you. I found it so odd to see these rocks in the hills that looked like they should be near the ocean. Thank you again for your response! :)

  37. Pingback: Common Piddocks – rock-boring molluscs | Jessica's Nature Blog

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s