I’m very cautious about following links from Facebook, probably as the result of running a series of seminars in my department about information security and reminding colleagues to be cautious about what they click on. If I see something that piques my interest, I’ll see if I can track it down by running a search and that’s what I did this morning to find out what on earth piddocks are. It turns out (either trust that link or do your own search) that they are molluscs found along coastlines, including round the UK, which can bore holes in stone to form a refuge. Stones which look like someone has had a go at them with a drill bit are probably the work of piddocks or a number of other stone-boring species.
I’ve definitely seen the kind of markings illustrated in the article on shorelines. I’m looking forward to next time I get down to a beach to take another look, hopefully with the word piddock lurking about near enough to my conscious brain to produce it when needed.