Regarding the phenomenon of natural beauty, here's an interesting quote from The Fountainhead:
There were small houses on the ledges of the hill before him, flowing down to the bottom. He knew that the ledges had not been touched, that no artifice had altered the unplanned beauty of the graded steps. Yet some power had known how to build on these ledges in such a way that the houses became inevitable, and one could no longer imagine the hills as beautiful without them -- as if the centuries and the series of chances that produced these ledges in the struggle of great blind forces had waited for their final expression, had been only a road to a goal -- and the goal was these buildings, part of the hills, shaped by the hills, yet ruling them by giving them meaning.
Rand does not deny that there is an "unplanned beauty" in the natural setting of Monadnock Valley. But she seems to think that this natural beauty is only a stepping-stone to the greater beauty of human creation, which gives meaning to nature.
Rand was deeply interested in meaning, and I think it is for this reason that she held aesthetics to deal mainly or even exclusively with art rather than aesthetic phenomena in nature. For Rand, meaning comes from conscious creation, not the "great blind forces" of nature.
The connection here to the idealist tradition of 19th century philosophical thought on art is not hard to trace. Specifically, contra what we could call the "empiricist" tradition in art, which derived from Aristotle and the Greek focus on mimesis, Rand held that art does not represent reality so much as it presents an abstraction. Art instantiates a conceptual body in perceptual raiment. For Rand the raiment is important (thus her opposition to abstractionism in art), but the essence is the body beneath the clothes, and that body is conceptual, not perceptual.
Thus nature can be beautiful, but for Rand nature carries no meaning of the kind art can. We may experience similar sense-of-life emotions in response to art and to nature, but this does not get to the essence of aesthetic experience. That essence, as the essence of all meaning in human life, is conceptual.
(As an aside, we can see at work here Rand's objectivism, since she focuses her analysis on the nature of the art-object and not on the nature of the response to that object. This is not to denigrate sense-of-life responses -- though Rand herself perhaps engaged in such -- but simply to recognize the primacy of object over response.)
At least, this is how I read Rand on art and aesthetics.