MOBILE EQUIPMOPERATOR WAS IN #25 END DUMP AND WAS BEING LOADED AT THE CAT 5230 SHOVEL, THE SHOVEL OPERATOR HAD PUT A COUPLE OF BUCKET LOADS OF DIRT ON THE TRUCK WHEN A LARGE ROCK BECAME LODGED IN THE BUCKET. AS THE OPERATOR SWUNG TOWARDS THE CUT, THE ROCK CAME LOOSE, STRIKING THE END OF THE TRUCK BED.