What do the marshes symbolize?

As much as anything, the marshes symbolize mystery and surprise because so many mysterious and evil things happened there. This is the first place that Pip meets the convict who after having scared him dreadfully, covers for Pip who has given the convict supplies which he forced out of him. The marshes hide "things" and the boats for the convicts are moored there. They are, in some sense, a reminder of Pip's childhood with his sister.

The marshes are a place of mystery for Pip. They are long and misty. They even represent an element of danger which a boy like Pip might appreciate. Early in the book Pip meets the convicts there which certainly are exciting, dangerous and mysterious for Pip. When Pip is in London, he travels to the marshes. This is during his transition to becoming a gentleman. This place in his life is also mysterious to Pip. He feels a certain amount of danger as well leaving his former identity and values.