Spanish /ll/ --> /ly/ --> /y/

[Bill Smith]

I just got around to following this thread.  When I was a graduate student
at the Univ. of Georgia, about 25 years ago, I attended a presentation on
American dialects of Spanish.  I have no idea who the speaker was (Spanish
is not my field), but he argued that Latin American dialect boundaries are
based on altitude:  higher altitudes tend to be more Castilian, while lower
ones are more Andalusian.  This idea hasn't been mentioned; is anyone out
there familiar with it?