IE was bolting in RM3 XCR at the 120' station. After IE installed 2nd bolt in the 5th row of the cut. The IE lowered the canopy to swing the boom towards the ATRS to install the last bolt. A rock measuring (4'8"" L x 1"" to 4"" W x 9"" T) broke loose from a brow in the roof sliding across the top of the canopy cantilevering over the edge striking the IE on top of the left foot.