OPERATOR STEPPED OFF D-10 DOZER TO EXAMINE HIS WORK AREA. UNAWARE OF AN OPEN MONITOR WELL, THE OPERATOR'S RIGHT LEG FELL INTO THE WELL HOLE. A ROCK FEL ALONGSIDE HIS LEG, WEDGING THE LEG INTO THE HOLE. MEDICAL OBSERVATION REVEALED A BRUISE ON LEG. INJURED RETURNED TO HIS NEXT SCHEDULED SHIFT.