Bolting (and screening) in 1East #1 entry, 17-18 block, in an area with exposed shale. The ATRS was set and he was starting to drill first hole when a rock (18""x24""x5"") broke loose outby the drill canopy, swung around and struck him in the lower back. Started missing work on 10/2/15 for contusions. No problems with training, PPE, mining machines or compliance issues.