THE EMPLOYEE WAS PREPARING TO INSTALL THE LAST ROW OF BOLTS IN THE CUT. THE ATR JACKS WERE SET AND THE EMPLOYEE STATED THAT THE TOP WAS CHECKED (VISUAL & SOUND). WHEN HE TURNED AWAY FROM THE F ACE, A ROCK (1 FT X 3 FT X 2 INCHES THICK) FELL WITHOUT WARNING, STRIKING THE BACK OF HIS HARD HAT & RIGHT SHOULDER. EX-RAY NEGATIVE, BRUISE TO SHOULDER & LEG.