EMPLOYEE WAS ON THE MAINLINE NEAR #3 HEAD DRIVE. A PIECE OF ROCK FELL FROM BETWEEN THE BOLTS STRIKING HIM ON THE BACK. HE WAS TAKEN TO THE E.R. TREATED AND RELEASED. THE DOCTOR INSTRUCTED HIM TO REMAIN OFF WORK A FEW DAYS THEN RETURN ON 10/24/02 FOR FOLLOW-UP PRIOR TO RETURNING TO WORK.