fenland vs marshland

fenland and marshland both are nouns.

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

As nouns, fenland and marshland are synonyms defined as:

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

Other synonyms of fenland include fen, marsh.

fenland (noun)	marshland (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