Wikia

Overlapping Biomes are a semi-common aspect of world generation. This phenomenon occurs when two Biomes intersect with each other. An example is the "Corrupt Jungle," where Shadow Orbs can be found near Underground Jungle caves, or jungle grass may be exposed in a Chasm.

Whenever the player stands at the intersection of two biomes, monsters from both biomes may spawn. The overlapping biomes phenomenon may also be artificially induced via player-made biomes.

NotesEdit

This phenomenon is most noticeable on worlds after the 1.0.5 update.
In Hard Mode worlds, the Underground Hallow and the Underground Corruption almost always intersect or overlap with one another. This is generally caused by the diagonal sections that are generated when the Wall of Flesh is destroyed.
Sometimes the music of both biomes can overlap when the time of day music changes.