North of Big Sky, the Garnet Mountain Loop is a superb 13 mile mountain bike loop with over 3,000 feet of climbing. It combines FS132, FS3112 and Trail #85.
The trailhead is at NFS Rock Spire Campground. To reach mountain biking at the campground from Big Sky, head north on US191 for 16 miles. Turn right heading east on FS132 for about 2.5 miles.
From the campground, head east on FS132 for 2 miles to FS3112. Proceed south up FS3112 for two miles to the rock barrier. Continue beyond the barrier, up past Rat Lake for three miles. Keep eyes sharp for moose, elk, snowshoe hare, wolves, grizzlies and mountain lion. The climb is long but scenic and the grade is 95% ridable. While atop Garnet mountain, either check out the Lookout (about 600 more vertical) or follow Trail #79 to Pioneer Lake. The crème-de-la-crem is Garnet Mountain Trail (#85) leading four miles down to the Gallatin River. This was the most exhillerating and challenging singletrack descent I ve experienced. Narrow, twisting, rolling, packed, free of embedded rocks, and cut into the cliffside. It also passes through open meadows and pine forests. At the bottom, head east on FS132 back to the campground.
Overall, Garnet Mountain Loop is the 1st most popular mountain bike trail of all 32 mountain biking rides in Montana.