I realise that the source text does say mountainous, but I think it might be the best solution here. It is not really a "wilderness region" (is it?), or a nature reserve, and rural sound like a lot of farming, but I think mining is the main industry. So you might as well call it what it is!

However big the grand territoire naturel de Santander is, it is but a subset of the region encompassing it.

I agree there is an element of redundancy, but if you can say something like "the Sherwood Forest region", i.e. the region larger than and surrounding Sherwood forest and best identified through reference to the Forest, then you can say la région du grand territoire. The "large natural area" is but a part of the larger region, and much of it might be industrial, polluted, overpopulated, etc. but they don't want to insist on those aspects!

Or, as you way, it might be purely redundant.

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Note added at 2 hrs (2010-10-17 22:04:34 GMT)
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Maybe it's not that the entire Santander area/province is naturel, only part of it, in which case we would be looking at "the region around that large part of the Santander area, in Colombia, which is relatively untouched by modern living".

Much may depend on who XX is and why they might have wanted to go there, and indeed where YY is relative to this location.


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Note added at 12 hrs (2010-10-18 07:28:48 GMT)
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Ah, so it's a person. I was thinking "company", with a mining company moving into an "unexploited" area with mineral resources, a power generating company moving into an "unharnessed" region with hydro potential, etc.

If it's someone who likes hiking, etc., maybe "wild" or "untamed" would be suitable for naturel. Not that that solves your région/territoire issue.

How about "XX turned to the city/town of YY and the region of vastly untamed beauty that is Santander in Colombia" ?



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Note added at 12 hrs (2010-10-18 07:34:08 GMT)
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Or "the region of natural grandeur ".

naturel: important in this case to specify it is completely untouched by humans, therefore I would suggest 'wilderness'

or "the vast natural area of the Santander department"
Some of it is mountainous, some of it agricultural, some used for oil exploitation and transport
so not all wilderness, nor mountainous