Sierra Nevada Mountains vs High Sierra

Sierra Nevada Mountains and High Sierra both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sierra Nevada Mountains	Yes	No	No	No
High Sierra	Yes	No	No	No

As nouns, Sierra Nevada Mountains and High Sierra are synonyms defined as:

Sierra Nevada Mountains and High Sierra: a mountain range in eastern California; contains Mount Whitney

Other synonyms of Sierra Nevada Mountains include Sierra Nevada.

Sierra Nevada Mountains (noun)	High Sierra (noun)
a mountain range in eastern California; contains Mount Whitney	a mountain range in eastern California; contains Mount Whitney