There is an ancient legend, whose origin escapes me (Welsh, I believe), that tells the story of another legend, how, once every hundred years or so, all the boulders in one area would go down to the stream together and take a drink... One day, this lad is out, larking about, and spends some time carving a cross upon a boulder. As night comes, he lays down to sleep, and is awakened by a loud rumbling noise, as of thinder.
Jumping up, he looks around to see all of the boulders headed to the stream, for their century drink! Seeing the lad, they move to kill him, so that their secret will be safe... All but one; the one that he had carved the cross into... It moves to protect him, blocking the other boulders from killing him, until they all have to go back to rest for the next hundred years.
There are a few adventure ideas, therein. Mine them as you will.