marshland vs fenland

marshland and fenland both are nouns.

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

As nouns, marshland and fenland are synonyms defined as:

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

Other synonyms of marshland include fen, marsh.

marshland (noun)	fenland (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