Drilling hole near bent bolt to replace it. Drill bit had broken tip out on last hole. Before he could install new bit, a piece of 2' wide, 6""-8"" thick, 6'-7' long shale fell in pieces hitting employee. Bending of previous bolt pryed shale loose creating potential to fall which was unrecognizable. 18 stitches in R hand, & upper minimally displaced fracture upper anterior maxilla.