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.
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1$\begingroup$ Please add characteristics of the type of stone. This may help $\endgroup$– Jan DoggenJun 27, 2016 at 9:56
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6 Answers
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Your image looks very similar to rocks we get here on the West Coast of Vancouver Island. We have some rock boring bivalves that will effectively drill into the rock which breaks off and erodes into a shape similar to what you are showing. https://en.wikipedia.org/wiki/Pholadidae or https://the-hazel-tree.com/2013/08/21/piddocks-anything-but-boring/
There are also echinoderms that will bore into rocks, Strongylocentrotus purpuratus, https://en.wikipedia.org/wiki/Strongylocentrotus_purpuratus https://bmscblog.wordpress.com/2013/01/09/holey-rocks/ but the holes from these creatures tend to be larger than the image you present.
I a confident that the holes are created by some form a boring bivalve.
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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.
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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.
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2$\begingroup$ It is not clear to me how an eddy current at that scale would be considered circular? Are you talking about turbulent eddies? $\endgroup$ Jun 27, 2016 at 2:17
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$\begingroup$ Your explanation of a wave drilled rock is perfectly correct I have seen this in action in the Canaries whilst studying wave action. $\endgroup$– Dave DeeFeb 12, 2019 at 3:02
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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.
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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.
!
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Leaning out of the window with:
I doubt the stone in the OP is the outcome of boring organisms. It rather is a cretaceous chert that fell out of the chalk cliffs (if present) or was washed out on the beach by the ocean. It is very abundant in the area (and elsewhere, e.g. the Baltic). I include the second example by @Orgong here, i think it is a chert, too (maybe not cretacous), just because they are so abundant. Both can be checked if they are cracked open. If they only show a white cortex (outer layer) and the inside is amorphous and any shade of gray to black, then it is clearly a chert.
Chert is a sedimentary rock made of amorphous silica from silica secreting organisms. If you crack open a large one you can even find a complete silicified skeleton of a sea urchin in it. They form as nodules in the sediment, sometimes in horizons of decimeters, combine with neighbours in the process, form plates, and anyway can have any shape including concave or with holes. Processes are described in detail for example here.
Sidenote: pretty good stuff for practicing stone knapping. Use a dense rounded sandstone as a pounder. And keep an easy wrist. It is all about the speed and angle, not the force. And do practice on a blanket to collect all the small flakes and chips or people will cut their feet when they wade through the remains of the workshop :-)