EMPLOYEE REPORTED: OPERATING ROOF BOLTER IN FACE AREA, INSTALLING RIB LINE BOLT. WHEN STARTING THE DRILL HOLE, A PIECE OF DRAW ROCK FELL, STRIKING THE ROOF OF THE CANOPY. A SMALL SECTION OF THE LARGER ROCK BROKE OFF (2' X 2' X 2-3"") AND FELL, STRIKING HIS MID RIGHT CALF, RESULTING IN A SEVERE LACERATION TO THE CALF.