Translation:These are the mountains of the American South-West.
Actually the main topic is the American South-West, not the mountains. There's a difference with regard to the focus of the sentence. Imagine you are talking as a geography teacher about the South-West and after you say everything that you need to say about rivers, people, animals, birds, agriculture ... you need to introduce another element of this South-West.
"These are south west american mountains" should translate as "Aceștia sunt munții americani sud-vestici." - with focus no the mountains.