From Wiktionary, the free dictionary
high country uncountable
Terrain that is at a relatively high elevation — generally, higher than foothills but not above the timberline — consisting of mountainous areas or elevated expanses of plain .
Near-synonyms: highlands upcountry upland plateau tableland mesa 1907 Stewart Edward White , chapter 13, in Arizona Nights :We had topped the high country , too, and had started down the other side of the mountains that ran out on the promontory.
1915 Joseph A. Altsheler , chapter 13, in The Rock of Chickamauga :While Lookout Mountain was the loftiest summit, some of the other ridges rose almost as high. . . . September had now come and the winds were growing crisper in the high country .
2001 August 6, Curt Eidem, “Letters: The War over the West ”, in Time :I grew up in Washington State, and have hiked the high country since I was a boy scout in the '60s.
Used especially to refer to certain Western regions of the U.S. and Canada, and also parts of the Australian state of Victoria. 
