The ee was roof bolting. He was installing bolts in 5th row of a 23'cut. While drilling hole for a bolt, drill steel became stopped up with dust, depth of 8 to 10"". When the pressure was released rock measuring 9"" long x 5""wide x 3/4"" thick slid down the steel which spun the rock to the side striking the ee in lower lip requiring 14 stitches.