Trying to identify a story I read in a collection in the late 70s. My memory is sketchy but I think the story starts with two men, possibly astronauts. I think they are somehow separated (either one dies, or they split up). The main character comes upon the ruins of a city. He is dying due to lack of food/water and collapses on a marble-like (?) slab. A small pool of water emerges from the stone, enough to save him. And that's all I can remember.
Any suggestions much appreciated!