Why does fitzgerald contrast “hard rock” with “wet marshes”? what does he mean?

These contrasting metaphors were used by Fitzgerald to convey the message that tolerance has a limit and that morality is always at stake despite having a strong foundation. The end justifies the means most especially when there is a lot to gain. Ethics can be powerful as described by a "hard rock" or weak as referred by "wet marshes".

Idek111111111111111111111111111111111111111111