Frost line depth in Mineral County, MT
How deep the ground freezes in Mineral County, MT
26 in. Dig to 26 in, just below the frost. Nearest NOAA station: Superior, 10.1 mi.
Your building department sets the legal frost line depth (IRC Table R301.2(1)); footings must go below it (R403.1.4.1). Call them before you dig.