<p>What you have found there is a beautiful example of a rock which has been bored by a colony of piddocks. 
Piddocks are little bivalve animals with oval shells edged with fine teeth.  They use their toothed shells to excavate burrows in rock.  The muscular part of their body, called a “foot”, grips the stone surface and helps to rotate the shell, creating a circular scouring action.   Over time and as the piddock grows, its shallow excavation gets deeper. The piddock lives inside this safe tunnel and extends a siphon beyond the edge of the tunnel, which it uses to filter food such as phytoplankton from the sea water.   </p>

<p>Individual piddocks live for around eight years, and so some of the rocks where their colonies settle can be riddled with quite deep hopes.  Their shells are very brittle and once the animal dies the shell is quickly broken by the action of the ocean – which means that you are unlikely to find a whole piddock shell unless it’s still embedded in the stone.
But they leave examples of their handiwork on the rocks they lived on, and over time these rocks get washed up onto beaches. Below are some photographs of rocks with piddock bore holes in them. Some still have the piddocks hiding inside! And the third shows a piddock outside of its protective tunnel.  </p>

CC BY-SA 3.0
2016-11-12T13:03:08.783Z
2016-11-12T20:52:52.867Z
2016-11-12T20:52:52.867Z
Alison Venugoban
<p>There are two processes involved. The first is the formation of any hole, such as erosion of a soft part of the rock, such as flint (silica) forming around something organic, or a softer rock. Also, macrofossils such as tubeworms, gastropods or echinoderms will have a natural cavity to start with. The second process is turbulent (wave driven?) scouring and enlarging of the hole by sand grains. This latter process involves circular eddy-currents which tend to erode an irregular cavity into a rounded cavity. </p>

CC BY-SA 3.0
2016-06-27T02:17:48.500Z
It is not clear to me how an eddy current at that scale would be considered circular? Are you talking about turbulent eddies?
Isopycnal Oscillation
CC BY-SA 4.0
2019-02-12T03:02:35.243Z
Your explanation of a wave drilled rock is perfectly correct I have seen this in action in the Canaries whilst studying wave action.
Dave Dee
CC BY-SA 3.0
2016-06-27T01:49:52.687Z
2016-06-27T01:49:52.687Z
Gordon Stanger
<p>I used to find many of them in gray shale limestone rocks on the lakes hore in the North East USA. </p>

<p>As a kid we were told that they were called "lucky stones".  It was lucky if we found one.  We were told that the Native Americans used to find them and use them for weighting down the bottom of their fishing nets. </p>

<p>I always assumed that an ancient water plant had grown in the mud and over the millions of years, the plant deteriorated and left a fossil.  Over time, the plant fossil got destroyed and left a hole where the plant had been.</p>

<p>Some times we would find small pieces of underwater plant fossils on the shore also. Sometimes we would find plant fossils in a shale rock cliffs as the shale breaks away.</p>

<p><img src="https://i.sstatic.net/GD2z2.jpg" alt="enter image description here">!</p>

CC BY-SA 4.0
2019-05-26T16:28:25.277Z
2020-01-25T05:59:12.177Z
2020-01-25T05:59:12.177Z
Orgorg
<p>This looks like petrified fruit. If you look between the two lobes you will see a lighter round patch that was the attachment area to the parent plant from which this fruit/seed had fallen.</p>

<p>It's probably fruit, as a seed would have had different shape, and it would seem to have fallen at a time of extremely hot weather  becoming quickly dehydrated after being invaded by some form of herbivore. I would discount bivalve activity but a terra based gastropod may have been the perpetrator before the fruit became petrified. </p>

CC BY-SA 4.0
2019-02-12T11:55:43.500Z
2019-02-12T12:13:45.193Z
2019-02-12T12:13:45.193Z
Dave Dee
<p>I picked up this stone form a beach on the south coast of England (lancing). How is it possible it has such round holes? Plenty of stones looked similar. </p>

<p><a href="https://i.sstatic.net/nFjU4.jpg" rel="noreferrer"><img src="https://i.sstatic.net/nFjU4.jpg" alt="Stone with hole"></a></p>

CC BY-SA 3.0
2016-06-27T09:56:35.100Z
Please add characteristics of the type of stone. [This may help](http://meta.earthscience.stackexchange.com/questions/124/a-guide-for-asking-identify-this-rock-questions)
Jan Doggen
<geology>
Cause for round holes in stones
Nickpick
# Cause for round holes in stones

(Asked by: username_0 on 2016-06-26)

I picked up this stone form a beach on the south coast of England (lancing). How is it possible it has such round holes? Plenty of stones looked similar.

[![Stone with hole](https://i.sstatic.net/nFjU4.jpg)](https://i.sstatic.net/nFjU4.jpg)

**Tags:** geology

> **Comments:**
> **username_1**: Please add characteristics of the type of stone. [This may help](http://meta.earthscience.stackexchange.com/questions/124/a-guide-for-asking-identify-this-rock-questions)  

---

## 4 answers

### Answer by username_2 on 2016-11-12. Score: 3

What you have found there is a beautiful example of a rock which has been bored by a colony of piddocks.
Piddocks are little bivalve animals with oval shells edged with fine teeth. They use their toothed shells to excavate burrows in rock. The muscular part of their body, called a “foot”, grips the stone surface and helps to rotate the shell, creating a circular scouring action. Over time and as the piddock grows, its shallow excavation gets deeper. The piddock lives inside this safe tunnel and extends a siphon beyond the edge of the tunnel, which it uses to filter food such as phytoplankton from the sea water.

Individual piddocks live for around eight years, and so some of the rocks where their colonies settle can be riddled with quite deep hopes. Their shells are very brittle and once the animal dies the shell is quickly broken by the action of the ocean – which means that you are unlikely to find a whole piddock shell unless it’s still embedded in the stone.
But they leave examples of their handiwork on the rocks they lived on, and over time these rocks get washed up onto beaches. Below are some photographs of rocks with piddock bore holes in them. Some still have the piddocks hiding inside! And the third shows a piddock outside of its protective tunnel.


---

### Answer by username_3 on 2016-06-27. Score: 1

There are two processes involved. The first is the formation of any hole, such as erosion of a soft part of the rock, such as flint (silica) forming around something organic, or a softer rock. Also, macrofossils such as tubeworms, gastropods or echinoderms will have a natural cavity to start with. The second process is turbulent (wave driven?) scouring and enlarging of the hole by sand grains. This latter process involves circular eddy-currents which tend to erode an irregular cavity into a rounded cavity.

> **Comments:**
> **username_4**: It is not clear to me how an eddy current at that scale would be considered circular? Are you talking about turbulent eddies?  
> **username_5**: Your explanation of a wave drilled rock is perfectly correct I have seen this in action in the Canaries whilst studying wave action.  

---

### Answer by username_6 on 2019-05-26. Score: 0

I used to find many of them in gray shale limestone rocks on the lakes hore in the North East USA.

As a kid we were told that they were called "lucky stones". It was lucky if we found one. We were told that the Native Americans used to find them and use them for weighting down the bottom of their fishing nets.

I always assumed that an ancient water plant had grown in the mud and over the millions of years, the plant deteriorated and left a fossil. Over time, the plant fossil got destroyed and left a hole where the plant had been.

Some times we would find small pieces of underwater plant fossils on the shore also. Sometimes we would find plant fossils in a shale rock cliffs as the shale breaks away.

![enter image description here](https://i.sstatic.net/GD2z2.jpg)!


---

### Answer by username_5 on 2019-02-12. Score: 0

This looks like petrified fruit. If you look between the two lobes you will see a lighter round patch that was the attachment area to the parent plant from which this fruit/seed had fallen.

It's probably fruit, as a seed would have had different shape, and it would seem to have fallen at a time of extremely hot weather becoming quickly dehydrated after being invaded by some form of herbivore. I would discount bivalve activity but a terra based gastropod may have been the perpetrator before the fruit became petrified.


---

