I haven't seen the film yet but it isn't a spoiler to say that it is about a chap trapped by the arm who cuts it off to free himself.
It seems very drastic to self-amputate when he was able to carve his name etc in the sandstone walls of the canyon. Couldn't he have whittled away at the boulder, given that his knife could make an impression on the rock? I see that he spent two days chipping at the rock so why not spend more time doing so?
I can sympathise and empathise with his predicament but I wonder there wasn't some sort of hysteria present and a fixed idea formed as to the only way to escape. Or I might just be a mean person of course.