Underlying the sand at Broughton Bay on the north coast of the Gower Peninsula in South Wales are ancient layers of peat and marine clay. These layers are eroding out of the burying sand, especially low down on the shore, and the newly exposed beds of peat and clay are riddled with thousands of circular holes about a centimetre or so in diameter. The holes have been made by the burrowing bivalve mollusc known as a piddock (Pholas dactylus). You can still see fragile empty shells of the piddocks in some of the holes. The age of the piddocks is debatable. It is not easy to tell how ancient or modern they are but they are clearly not recent, there were no live specimens that I could see.
Holes made by burrowing piddocks are often found in beach stones or pebbles when pieces of infested bedrock have broken free and become rounded by wave action on the shore. There is a lot more information about this particular kind of hole in rocks and stones, and the creature that creates them, elsewhere on Jessica’s Nature Blog and you can access these posts by clicking here.