valley vs rift valley

valley and rift valley both are nouns.

Word	Noun	Adjective	Verb	Adverb
valley	Yes	No	No	No
rift valley	Yes	No	No	No

As nouns, rift valley is a hyponym of valley; that is, rift valley is a word with a more specific, narrower meaning than valley:

valley: a long depression in the surface of the land that usually contains a river
rift valley: a valley with steep sides; formed by a rift in the earth's crust

Other hyponyms of valley include dale, glen, gully, holler, hollow, nullah, ravine.

valley (noun)	rift valley (noun)
a long depression in the surface of the land that usually contains a river	a valley with steep sides; formed by a rift in the earth's crust