The pilot of the tundra tire–equipped, tailwheel airplane reported that he made an off-airport precautionary landing due to icing conditions; the airplane encountered soft terrain during the landing roll and nosed over. The airplane sustained substantial damage to the left wing and empennage. The pilot reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation. A review of recorded data from the intended destination's automated weather observation station about 19 miles to the south west, revealed that, about 37 minutes before the accident the wind was 120 degrees true at 14 knots, visibility 1 statue mile, light snow, mist, temperature 34 degrees Fahrenheit, dewpoint 30 degrees Fahrenheit. And, about 23 minutes after the accident the wind was 110 degrees true at 13 knots, overcast 1700 feet, visibility 1 1/4 statue mile, light snow, mist, temperature 34 degrees Fahrenheit, dewpoint 32 degrees Fahrenheit.