DOZER WAS PUSHING OVERBURDEN OVER HIGHWALL. DOZER GOT TOO CLOSE TO HIGHWALL EDGE. DOZER STARTED TILTING OVER HIGHWALL. HIGHWALL HEIGHT IS APPROX30FT TOTAL. FIRST DROP APPROXIMATELY 15 FEET TO RED DIRT THAT HAD BEEN PUSHED DOWN. DOZER ROLLEDAPPROXIMATELY ANOTHER 15 FEET ONTO LEFT SIDE.