In the St Mary area, there are several ungroomed cross country skiing options starting from the 1913 Ranger Station southwest of St Mary.
The Red Eagle Lake Trail is a nice 8 mile round trip. Start by following the first park of the Eagle and Elk Loops to the old fire road (aka Red Eagle Lake Trail). The route rolls over terrain to the scenic bluff overlooking Red Eagle Creek with great vistas of the mountains along St Mary Lake and Red Eagle Valley.
Overall, St. Mary - Red Eagle is the 33rd most popular cross country ski area of all 37 Nordic ski destinations in Montana.