I read this book 30 plus years ago... I remember the man, it seems, is on another planet and is trying to climb this mountain which he's having trouble doing. All the while turning into the rock. He reaches the top and falls down the other side but gets stuck because he's now the rock too. Anyone know what this could be?
It might be a story by Barry Longyear called "Adagio".
From this answer:
"Adagio" feels like a Twilight Zone story written by George R.R. Martin. (Why, yes, I am thinking of Sandkings, though there are a number of classic TZ eps that fit the mould as well.) A ship crashes on an alien world, and the survivors are bored stiff waiting for rescue. One crewman discovers that certain rocks in the area are in fact slow moving creatures. The man uses his speed to convince the creatures that he's a supernatural creature and orders them to engage in bloody wars against each other.