EMPLOYEE WAS WALKING BACK TO THE MINER FROM THE SECTION TOOLBOX. AS HE WALKED BESIDE A COAL HAULER THAT WAS STOPPED IN THE ENTRY, TWO ROCKS FELL FROM BETWEEN ROOF STRAPS. ONE ROCK KNOCKED HIS HARD HAT OFF AND THE SECOND HIT HIM IN THE BACK OF THE HEAD CAUSING A LACERATION. THE ROCKS WERE ABOUT 1' X 6"" X 1"" THICK BUT HAD SHARP EDGES ON THEM.