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I think "The Rocky Mountains is great" is correct. (Rocky Mountains is a proper concrete noun) I think "The rocky mountains are great" is correct. (rocky is an adjective, and mountains is a concrete noun)
Otherwise I don't think Rocky Mountains is a collective noun at all.
I don't think there are any hard concrete rules when it comes to collective nouns. I think you just have to look at the context. From what I know almost always they are singular.
In OG12 SC #1, Glass House Mountains are plural. Any explanation?
I checked the OG and your right. I think Proper Nouns can be singular or plural just like any other noun. I think we have to look at context to really decide. For example Niagara Falls could refer to the actual falls, or it could refer to the city.
So now I think you are right, and we should say "Rocky Mountains are".