EE WAS DRILLING #2 BOLT HOLE IN PATTERN WHEN THEROCK BROKE LOOSE FROM BETWEEN THE RIB & THE ROW OF BOLTS ALONG RIB LINE. THE ROCK HIT THE CANOPYAND BROKE APART. A PIECE OF ROCK 21"" X 24"" X 3"" THICK TO FEATHERED EDGE STRUCK EMPLOYEE ON RIGHTLEG JUST ABOVE THE ANKLE.