Employee was plastering the return side of a brattice, a piece of drawrock fell striking them in the head. Rock broke into two pieces, measurements - 27"" long x 24"" wide x 5"" thick & 20"" wide x 14"" wide x 3"" thick. Incident occurred along 2nd Northwest Mains between the #2 & #3 entries at spad numbers 5237 & 5250.