EE STATED THAT HE HAD POSITIONED HIMSELF IN #107 SHIELD WHILE THE PETITTO MULE UTILITY TRACTOR WAS REMOVING ROCK THAT HAD FALLEN IN FRONT OF THE MRS USED IN SHIELD RECOVERY. A PIECE OF ROCK 1' X4'X8"". FELL ONT HE BOOM OF THE PETITTO MULE, SLIDING DOWN THE BOOM AND OFF TO THE LEFT SIDE STRIKING HIM ON THE LEFT LEG. THIS RESULTED IN A HAIR-LINE FRACTURE TO THE LEFT LOWER LEG.