coastal plain vs solid ground

coastal plain and solid ground both are nouns.

Word	Noun	Adjective	Verb	Adverb
coastal plain	Yes	No	No	No
solid ground	Yes	No	No	No

As nouns, solid ground is a hypernym of coastal plain; that is, solid ground is a word with a broader meaning than coastal plain:

Other hypernyms of coastal plain include dry land, earth, ground, land, terra firma.

coastal plain (noun)	solid ground (noun)
a plain adjacent to a coast	the solid part of the earth's surface