EMPLOYEE WAS GETTING READY TO PUT THE RIB PIN IN THE XCUT WHEN EMPLOYEE SWUNG OVER TO THE EDGE OF THE PILLAR WITH THE CANOPY AND A PIECE OF ROCK 3'X2'X1"" FELL FROM THE RIB STRIKING THE EDGE OF THE CANOPY. A PIECE OF THE ROCK HIT EMPLOYEE'S LEFT CHECK CUTTING THE CHEEK.