Employee was bolting in x-cut and bent over to pick up bolt, when a 3'x4'x4"" thick piece of coal and rock fell out between atrs and canopy striking him in the back of head, causing a 2""laceration. Cause of injury was safety awareness. Compliance with rules/regulations not a factor, protective equip not a factor, miner proficiency a factor, mining equip/systems not a factor.