Henry Wadsworth Longfellow wrote "The Cross of Snow" as a memorial to his wife, who perished in a freakish accident. The cross that the speaker sees on the mountainside is a symbol of the scar that he has on his own heart from the loss of his wife.
Longfellow's wife had her dress catch fire from the ember from a fireplace, and ended up burning to death. Longfellow tried to put out the fire for her, and the flames disfigured his face to the point that he grew his beard to cover up the scars.
About 18 years after her death, he was reading a book that had pictures of the West, and one picture showed a formation of snow in the shape of a cross. This picture motivated him to write "The Cross of Snow."
The meaning of this poem has a great deal to do with the pain that grief causes. The fact that this image causes such associations 18 years after the death of his wife is a testament to the depth of their love and the pain that her tragic accident still brings to him each day. The scar of pain is represented in the line, "changeless since the day she died."