A NON INJURY OCCURED WITH AN APPROX. 400'SECTION OF HIGHWALL, SLOUGHED OFF INTO & UP AGAINST THE COAL HAULER. THE EMPLOYEE WAS NOT INJURED. COAL MINING ACTIVITIES IN THE NORTH PIT. PUMP 3 WAS SUSPENDED & EQUIPMENT & PERSONEL WERE REMOVED FROM THE PIT. EXTERNAL DAMAGE OCCURED TO THE COAL HAULER. MSHA AZTEC OFFICE WAS NOTIFIED THE NEXT MORNING.