Why did the Big Belts get the reputation of being particularly rough country as noted in the memoir, This House of Sky: Landscapes of a Western Mind?

Big Belts, Montana isn't a town, but rather, a particularly hilly/mountainous part of the state of Montana. Homesteaders and shepherds found that flocks could be moved through here wonderfully in the warm season but often the winters took out more than whatever had been gained in the warmer weather. As a result, the Big Belts have the reputation of being particularly rough country.