This is the weirdest thing i have seen minecraft do. What you are looking at is the rift between a taiga biome and an extreme hills biome. The giant wall is the inside of a hill. Minecraft sliced it in half and placed a taiga biome. Ohman said that is it completely natural.
^ what tech said, you will always see those at the previous borders of the map when its been expanded. Its not really "natural" lol
Ehh these things always happen with map expansions, I lived on a chunk error like that between plains and the ocean last map, it was awesome.