fenland vs salt marsh

fenland and salt marsh both are nouns.

Word	Noun	Adjective	Verb	Adverb
fenland	Yes	No	No	No
salt marsh	Yes	No	No	No

As nouns, salt marsh is a hyponym of fenland; that is, salt marsh is a word with a more specific, narrower meaning than fenland:

fenland: low-lying wet land with grassy vegetation; usually is a transition zone between land and water
salt marsh: low-lying wet land that is frequently flooded with saltwater

fenland (noun)	salt marsh (noun)
low-lying wet land with grassy vegetation; usually is a transition zone between land and water	low-lying wet land that is frequently flooded with saltwater