EMPLOYEE WAS OPERATING A ROOF BOLTING MACHINE ON UNIT 3 (MMU-005-0). AS HE REACHED FOR A ROOF BLOCK A PIECE OF ROCK FELL BETWEEN THE ROOF BOLTS (52""X36""X6"") AND STRUCK HIM ON THE LEFT WRIST AN D ARM. THE BOLT SPACING IN THE AREA WAS 48"" TO 51"" APART. THIS INCIDENT IS UNDER INVESTIGATION PENDING MEDICAL REPORTS.