EE WAS INSTALLING THE OUTSIDE BOLT ON 3RD ROW OF BOLTS IN THE FACE OF NO.1 ENTRY. EE STATED HE DID NOT SEE ANYTHING. ALL HE COULD REMEMBER WAS THAT HE WAS HIT BY A ROOF ROCK. INVESTIGATION FOUND A PIECE OF ROOF ROCK 65""L X 13-28""W X 3-5""THICK FELL STRIKING EE ON HEAD & SHOULDER AREA.EVIDENCE INDICATES FORCE & WEIGHT OF ROCK CAUSED INJURY TO RIGHT KNEE ALSO.