"It was stan that told me, over the phone, that these things are called micro-cups."
Another term is "pit marks", these are found on Holy Shrine of the Mnajdra temple in Malta, curiously they can be found on at least 5 Cornish crosses.
http://www.themodernantiquarian.com/site/1754#post-4030
The holes that Miners filled with gunpowder at midsummer would have to be much deeper.
The gunpowder could have been stuffed down hard into a hole (with some wadding) and make a much more spectacular pop!
The practise was popular in the St Just area of Cornwall (I believe).