TRAVELING FORWARD ACROSS BACKFILL AREA, WENT FROM ONE LEVEL OF BACKFILL TO ANOTHER (APPROX 2-21/2 FOOT DISTANCE) DROPPED ONTO LLOWER LEVEL, DOZER WENT OVER CENTER BLADE FIRST. BLADE HIT GROUND AND DOZER CAME TO ABRUPT STOP. OPERATOR CUT HEADON BRACKET INSIDE CAB. OPERATOR TAKEN TO EMERGENCY ROOM, 16 STITCHES LEFT FOREHEAD AREA.