MECHANIC WAS DOWN IN THE HOLE AT O'FALLON STONE TO REPAIR A DAMAGED FUSE ON A BROKEN DRILL. HE WAS STANDING NEXT TO HIS TRUCK WHEN SEVERAL ROCKS FELL FROM THE HIGHWALL. ONE APPROX 3"" BY 6"" STRUCK HIM ON HIS HARD HAT. HE WAS TAKEN FOR EVALUATION AND GIVEN OVER THE COUNTER MEDICATION AND RETURNED TO REGULAR DUTY. EXPERIENCED NECK PAIN AND UNDERWENT THERAPY 9 SESSIONS.