EE WAS TIGHTENING LUGS ON C/H TIRE LOCATED IN ENTRY #3, 1 SE PANEL, X-CUT 3+50. A ROCK 3' X 19"" X 3"" FELL FROM BETWEEN BOLTS, STRIKING EE ON R. SIDE OF HIS HARD HAT. IT CONTINUED DOWN EE'S R. ARM, STRIKING THE BACK OF HIS HAND BEFORE GLANCING OFF HIS R. THIGH. EE RECD. FIRST AID & FINISHED SHIFT. LATER WENT TO ER DUE TO SWELLING & STIFFENING. BROKEN CARPUS BONE - CAST APPLIED.