chain of mountains vs Sierra Madre Occidental

chain of mountains and Sierra Madre Occidental both are nouns.

Word	Noun	Adjective	Verb	Adverb
chain of mountains	Yes	No	No	No
Sierra Madre Occidental	Yes	No	No	No

chain of mountains (noun)	Sierra Madre Occidental (noun)
a series of hills or mountains	a mountain range in northwestern Mexico that runs south from Arizona parallel to the Pacific coastline