marshland vs fen

marshland and fen both are nouns.

Word	Noun	Adjective	Verb	Adverb
marshland	Yes	No	No	No
fen	Yes	No	No	No

As nouns, marshland and fen are synonyms defined as:

marshland and fen: low-lying wet land with grassy vegetation; usually is a transition zone between land and water

Other synonyms of marshland include fenland, marsh.

marshland (noun)	fen (noun)
low-lying wet land with grassy vegetation; usually is a transition zone between land and water	low-lying wet land with grassy vegetation; usually is a transition zone between land and water
	100 fen equal 1 yuan in China