Last week I took a recuperative break in Lyme Regis. Well known for it's unusual geology of course. Wandering on the beach I came across stuff like this:

I forgot to put a ruler next to it, but for scale the biggest hole is about 15mm.The 'stone' is quite soft and may be compressed mud – but how were the holes formed? Googling has so far failed me.

Robin

Arf! Might have a use in the workshop then, hadn't thought of that. Some of the holes are indeed remarkably round, which made me think that there was some physical (eg bubbles in mud – but I couldn't see quite how that would work) rather than organic mechanism at work. But armed with the search term 'piddocks' further googling pretty much confirms that's how the holes were formed. Astonishing, I'd never have guessed that!

It turns out that the same question was asked in the New Scientist a couple of years ago. One of the replies runs thus:

Both shipworms and piddocks exude shell material to line their burrows and prevent them collapsing. The civil engineer Isambard Kingdom Brunel learned from this, and used the same strategy to successfully complete the first tunnel under the Thames in London.

The remnants of (presumably) exuded shell material are visible in my specimen. I wonder if it's true about Brunel though? I can believe it I suppose, he probably drew inspiration for his innovations from many sources.

Thanks for all your replies – I expected that someone here (or their brother ) would shed light.