Frost line depth in Marion County, IA
How deep the ground freezes in Marion County, IA
36 in. Dig to 37 in, just below the frost. Nearest NOAA station: Knoxville, 0.6 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.