PL/SQL :: Dates Are Stored In Character Format

In one of the table, dates are stored in character format. I want to find the ID where start_date is greater than stop date have converted the char to date by using to_date function but still I am getting error.  

with t as(select 122 as ID, 'JAN-12' as start_date, 'MAR-13' stop_date from dual unionselect 123 as ID, 'MAR-12' as start_date, 'FEB-12' stop_date from dual unionselect 124 as ID, 'MAR-12' as start_date, 'JAN-12' stop_date from dual unionselect 125 as ID, '01-MAR-1949' as start_date, '13-APR-1947' stop_date from dual unionselect 126 as ID, '01-MAR-1959' as start_date, '13-APR-1970' stop_date from dual unionselect 127 as ID, '11-MAR-1950' as start_date, '13-JAN-1951' stop_date from dual unionselect 128 as ID, '21-JUL-1969' as start_date, '20-JUL-1969' stop_date from dual)select * from tWHERE to_date(start_date) > to_date(stop_date)