Pebbles with holes made by sea creatures

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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


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92 Replies to “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. 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?

  7. 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

  8. 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.

  9. 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.

  10. 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

  11. 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].

  12. 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!

  13. 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.

  14. 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

  15. 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

  16. 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.

  17. 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

  18. 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.

  19. 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

  20. 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

  21. 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.

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. 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.

  27. 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.

  28. 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! 🙂

  29. 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.

  30. 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! 🙂

  31. Dear Jessica,
    Many thanks for taking the time and trouble to write this blog and for answering questions.
    I have a collection of over a hundred stones with holes in and yours is the first website I’ve found which offers a plausible explanation of how they’ve been made.
    I’m very much persuaded by your photographs of bivalves burrowing into the clay banks at the shore line, but is it your belief that these same animals can make holes in stone, or is it that their prehistoric ancestors did and then the sedimentary rock layers were formed, subsequently to be broken down into pebbles? In other words the stones with holes we find on the beach are actually very ancient and not being bored currently? One puzzle is the general lack of remains of bivalve shell embedded in the holes – maybe they were softer and eroded subsequently!
    Another theory is that the holes are the result of the repeated freezing of water which makes tiny fragmentations and turns a small crack into a fissure and eventually into a hole? Have you come across this idea and do you give it any credence?
    With best wishes,
    Hamish Pringle

  32. Dear Hamish,
    Thank you for writing to me. Yes, some bivalves, notably members of the Order Myoida, can and do bore into rocks and not just into low shore clays. It is difficult to believe that molluscs can bore into rocks but they use ingenious methods and often have very special adaptations to the shells enabling them to do so.

    According to reference works such as Collins Pocket Guide to Sea Shore of Britain and Europe, there are several species of boring bivalved mollusc occurring on British coasts or just off shore, each species varying in its ability to burrow into harder or softer substrates – but typically able to burrow into rocks and other substrates. In the Family Myidae, the Wrinkled Rock Borer (Hiatella arctica) attaches by a byssus in crevices, holes and algal holdfasts, or bores into soft calcareous rocks. The Flask Shell (Gastrochaena dubia) from the same family is known to bore into sandstones, limestones, and biogenic carbonates (like shells). Whilst in the Family Pholadidae, the Common Piddock (Pholas dactylus) is recorded as boring into peat, clay, wood, and compacted muds. The White Piddock (Barnea candida) bores into wood, peat, and soft rocks. Barnea parva bores into limestones, sandstones and clays; the Oval Piddock (Zirfaea crispata) and Pholadidea loscombiana bore into peat, clay, soft rocks and wood. Then, of course, there are the shipworms of the Family Teredinidae like Teredo navalis and Nototeredo norvegica that bore only into wood. All the rock substrates are sedimentary rocks. I haven’t seen any references to borings into the much harder igneous (e.g. granite and basalt) or metamorphic rocks (e.g. slate).

    Bivalves bored into hard substrates like sedimentary rock in the past and distant past, in the same way that they do today. The burrows and holes made in the past can be discovered today. I found pebbles and beach stones with such holes, cemented into an interglacial raised beach deposit on the Isle of Portland in Dorset, for example. I am sure there must be earlier examples in solid bedrock – I have seen them in huge Jurassic boulders at Winspit high on the shore where they could not result from recent activity.

    It is also true that tunnels and holes made by organisms burrowing through softer shore-side sediments in the past can be preserved in present-day hard sedimentary rocks after millions of years of transformation of the soft sediment by compaction, and diagenetic processes. Most frequently these kinds of bore holes and tunnels, known collectively as trace fossils or ichnofossils, have been in-filled at a later stage by other sediments but retain the outline shapes. These infilling sediments and substances can be harder than the surrounding rock, so that when exposed to weathering processes in the present day, they remain outstanding, while the matrix is worn away. These strange sculptural shapes are sometimes called tafoni. There are some good examples mentioned elsewhere in Jessica’s Nature Blog in articles about the cliffs at Bridport and Budleigh Salterton. However, these particular examples are burrows thought to have been made by crabs. Fossilised worm burrows are found in rocks on the shore at Ringstead Bay. Burrows of boring molluscs are similarly preserved in rocks like those at Winspit.

    Some of the stones with holes found on the beach will have originated from rocks that have been recently bored by bivalve molluscs. I think you can only know this for certain where the animal and its shell are still inside the burrow. I found dislodged slabs of shale on Monmouth Beach in Lyme Regis with the recently dead Common Piddocks earlier this year after winter storms. I have also seen them alive in situ in low water shale bedrock at Lyme and Charmouth. Finding the unoccupied shells in the holes in rocks does not tell you how recent the burrows are. The shells can remain in the burrows for long after the death of the animal – how long I am not sure but fairly certain for thousands of years at least – because recently uncovered clay deposits low on Broughton Bay and Whiteford Sands on the Gower Peninsula in South Wales, where overlying peat deposits and glacial clays have been eroded over the last couple of decades, have fragile paired empty piddock shells still in the burrows. It is also possible that some of the bore holes in beach stones are ancient but maybe only a professional geologist could tell.

    You are correct in suggesting that holes in beach stones can result from causes other than the activity of boring bivalve molluscs. Other marine invertebrates that create distinctly shaped burrows in rocks include marine polychaete worms and sponges. Holes can be made by the combined activities of different organisms either individually, in succession, or combined. Acids in metabolic waste products of organisms living on and in rocks, contribute to the formation of the holes. The shells of bivalves are themselves used as grinding tools.

    Non animal-originated erosion/ hole-making processes can occur on both the microscopic as well as the macroscopic level. Holes are also created by non-organic means including chemical and mechanical processes, before, during, and after the rock consolidation. Holes of non-animal origin are commonly found in flint pebbles, for example, and are caused probably by dissolution of the silica during the burial of the nodule. It is also true that pressures in earth movements like faulting and minor movements can result in internal cracks in rock that can widen by solution with percolating groundwater, and also widen by freeze/thawing events. However, these processes would not usually produce the distinctly-shaped holes and channels normally associated with burrowing activity.

    Investigating the causes of holes in rocks is intriguing; I am finding out just how complex a subject it can be. Discovering the causes of the holes found in a particular stone, determining when and how they were made, entails a detailed study and knowledge of the type of rock, its origin, and the context in which it was found, plus a knowledge of all the possible contributory factors.

    I am currently looking into the way that marine organisms erode hard bedrock – rock that subsequently becomes broken and detached to eventually form beach stones. There are some illustrations of the burrows made by marine worms and bivalves (and still being made by them) in near-shore levels of bedrock in cliffs, seashore rock outcrops, wave-cut rock platforms, and in and around rock pools and gullies, in recent and forthcoming postings on the blog. The whole subject of bio-erosion and bio-karst in limestone will be an increasing feature of my writing.

    I hope this has been useful and goes some way to answering your questions.


    Related posts

  33. ​Dear Jessica,

    Many thanks indeed for your comprehensive reply!

    You have certainly looked into this in great detail and I’m looking forward to reading your related posts.

    Meanwhile I wonder if on your many shore walks you’ve found pieces of clay which resemble smooth slate but which are much more fragile? We get lots of these on the beach at East Wittering, especially after stormy weather. Most have no holes, but some have lots, and a rare few have just one or two. I can imagine these are lumps broken off mud banks in the Solent which resemble the photo on one of your posts.

    With best wishes and thanks again,

    Hamish Pringle


  34. I think what you are describing is thin layers of shale; shale is a sedimentary rock that would be a precursor to slate. [Slate is shale that has be metamorphosed and hardened as a result of great pressure and heat]. The softer shale rock is seen in the substrate I photographed at Charmouth in Dorset with piddocks burrowed into it. Could be that your pieces of rock at East Wittering are fine layers of just the same sort of soft rock, that have broken off in stormy weather, complete with the perforations made by the burrowing piddocks or similar organisms. PS Without seeing any images of what you found, I am guessing that they are bivalve holes.

  35. Dear Jessica,
    I’ve taken some photos at low tide today and posted them in a Dropbox for you – an invitation to join will come direct to you.
    I’m looking forward to having your diagnosis!
    With best wishes,

  36. I really don’t know but I’m fairly sure there must be at least one sensible name for people who collect pebbles with holes in them – and it may depend on what has caused the holes and why they are being collected. Sorry I can’t help you at the moment.

  37. Thanks for the info! Apparently, most people think fairies make rocks with holes. Grateful to find your posts with scientific information. Found a cluster of stones with holes at a beach in Monterey California!!!

  38. I really enjoy reading your blog, and find it incredibly informative. Thank you for putting the time in to do this. I have a question for you if you don’t mind.

    You touch upon this in one of your replies, but I am wondering what forms holes in flint? I understand now that it isn’t piddocks as they work on softer rock, but could it have been another worm or mollusc recently, or in the distant past? Or is it ALWAYS as you say in your reply “caused probably by dissolution of the silica during the burial of the nodule.” I am fascinated by this and just can’t find a definate answer either direction anywhere and just need some clarification. Thank you very much. Mike.

  39. To be honest, not being an expert in the subject, I cannot give a definitive answer to your question. However, I still cannot think of any organism or set of circumstances that would enable some sort of invertebrate to create a hole or burrow, in either the present or the past, in what is essentially such a hard and insoluble rock such as flint. I have found an interesting on-line account of flint formation that sheds a bit of light on the subject. David Bone of West Sussex Geology says how the silica is frequently precipitated in the burrows made by marine organisms in the sea bed, or in the cavities or moulds left by molluscs and sea urchins in the sediments. Sometimes the silica is precipitated on the walls of such cavities, leaving a central hollow. This seems to me to be a good explanation for the presence of holes in flints. These cavities would perhaps be further affected by erosion processes once free from the chalk in which they are embedded. Do have a look at the article and see what you think. I once found a large flint nodule with such an internal cavity at Studland South Beach. I wish I had picked it up because it was so beautiful.

    I hope this helps to answer your question

  40. In a trip to a beach nearby, my son crack open a palm size rock and found a ball-shape seashell about 2-3 centimeters in diameter was trapped inside. It is still half buried in the broken rock. We put the broken pieces back together and examined the rock. It was not a “tunnel” as you find everywhere. It was indeed a sealed cavity inside a rock with a ball-shaped seal trapped intact. The shell does not look like fossil. It looked like it was just dead the day before. Can anyone explain how this happened?

  41. How fascinating! From your description it sounds as if you might have found a sea urchin fossil in the rock. It is not possible to say anything with certainty without seeing the stone and the “ball-shaped shell” themselves, or a some photographs of the objects, and without knowing what kind of rock it is and where it has come from. If you could e-mail me some pictures ( and send further information about the location in which the object was found, that would be very useful in trying to discover what it is. However, sometimes a local museum is the best place for identifications like this because of their detailed knowledge of the area.

  42. Thank you for your wonderful photos and writings. I have collected rocks from the Palos Verdes Peninsula for some years and have always wondered about the holey ones–what would make/cause the holes and so on. What geological processes might produce these effects? The PV area is interesting geologically because three tectonic plates intersect there and it has a complex history of subduction/uplift cycles. I am just starting to explore the geology in more depth, so I am curious what you might make of some rocks. I will take the liberty of emailing you photos with some questions…

  43. Thank you for your comments and questions. I have received and replied to your e-mail with explanations, links and photographs, and hope this helps answer your queries about the rocks with holes from Palos Verdes.

  44. I have several tons of beach pebbles in my garden here in Berkshire UK as a weed suppressor to form a beach scape garden. They were purchased as graded beach shingle 25mm also sometimes called beach pebbles so I don’t know where they originated. They are all rounded with no sharp edges as you would expect and most are of flint and some have very neat holes about 5mm diameter in them none larger – In amongst this majority of flint stones are the shells of bivalves of the sort that you traditionally associate with beaches. There are also what appear to be bone fragments frequently also with many holes. There are also what appear to be large molar teeth – definitely not made of any stone – and pieces that look and feel like hard coral. Although these pebbles have been through a grading process both in the sea and in processing I am amazed at the proportion with holes! I assumed that most were made by once living creatures – why would any human make these when they are so plentiful? As to their uses – they look as though they would be perfect to weight a throw net for catching fish that frequent shallow waters near beaches such as mullet. Everyone knows you can’t catch mullet with a hook and line! Given the large proportion with holes – about 0.5% – whatever made them must have either been plentiful or had a very long time to do their work! I find your blog fascinating.

  45. How fascinating, not only the stones with holes but also the presence of bones and teeth. It might be worthwhile taking some specimens to a museum to determine what your deposit really represents. The bones and teeth may be from Quaternary deposits containing remains of animals now extinct. It is difficult to say whether this is the case or not from just a description. An expert would have to handle the pieces and perhaps compare with reference material.

  46. Interesting stuff! I have found a small pebble on Aberconwy beach that has been totally hollowed out by burrowing beasties! About the size of a 1p piece, Pictures available if anyone is interested…

  47. The pictures have now arrived safely. Thank you. What I think you have there is a pebble derived from a piece of rock that has been colonised by a sponge such as Cliona celata that has eroded/dissolved the stone as the organism penetrated the matrix. You can often find seashells as well that are riddled with similar holes – to the extent that a honeycomb effect is achieved. There are several posts on this blog that illustrate such sponge holes. The sponge can only take over limestone based objects that can be dissolved by the acids that are incidentally generated as a by-product of its metabolic processes. The stone or shell provides a safe habitat for the sponge.

  48. Hi there,a whole new world of information is held within these blogs and I am truly fascinated.I recently found a flat dark coloured rock that was lying on a shelf of a rock pool on the Fife coast where I stay.I was baffled as to how the holes could have been made as at the time I thought it must have be man made somehow possibly to do with the Fishing industry.Who would ahve thought!.

  49. I found some beautiful stones on the beach of dawlish in south Devon made a beautiful necklace out of one. Am trying to find out also how some of the rocks get the patterns on them. I found one rock bit bigger than a fist white in colour with swirling black lines almost like a tribal tattoo. Any ideas?

  50. Hi, Thanks for a wonderfully informative, credible and scientific post. There is a beach near Bolinas, CA riddled with rocks riddled with holes. People seem to be unwilling to believe that these holes are made by creatures to live in, that they then extend a siphon to filter seawater for food (“explanations” range from “natural process of erosion” to “rock is eaten by sea creatures in search of food”), now I can point them at your blog or give them a term they can google – “piddock”.

  51. I am really pleased that you found the information that you were looking for. It is amazing how many people are intrigued by the origins of the holes in the rocks that they find on the beach. Piddocks are responsible for some of the larger effort holes and tunnels but there are lots of different types of organism and many species that in effect create places in which to shelter by dissolving the stone or grinding it away.

  52. Hi i found a stone in a stone i keep thinking how is this possible?? My brother called it a ‘rattler’ any information on this will be great

  53. How curious! A stone inside a stone. I don’t know how that could have happened – I think I would need the examine the stone for myself in order to suggest some possible answers to your question. Might the internal object be a small fossil instead of a stone, or perhaps the shell of a rock-boring mollusc? Once rock borers like Piddocks and other smaller species like Flask Shells or Wrinkled Rock Borers have worked their way into the stone they are trapped there for ever and cannot get out.

  54. I have a 1-2lb rock that has holes in it. It looks like it could be a fire starter stone; or a spice grinder. How do I find out about this rock??

  55. Plz can someone help me I been to the beach today and my kids have picked up some pebbles I don’t know if they are rare or not half white half green and a white one with what looks likes part off a blue pic on if plz help

  56. Hello, Rebeca. I am very sorry that I am unable to help you identify the pebbles you found today but would like to suggest that you take the pebbles to your nearest museum where someone with a knowledge of the local geology might be able to tell you what they are after examining them. It is too difficult to do with just a brief description to go on. Hope this helps.

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