Palimpsest

1. a parchment from which writing has been partially erased to make room for some other text leaving the earlier writing sometimes legible
2. parchment from which earlier writing has been removed to clear it for new writing
3. an object, place, or locality that reflects its history

Ex:It was only after they realized that the treasure map happened to be on a palimpsest that they were able to make some headway in deciphering the script.