What is fog?

Fog develops when Chin and Sarah make their way from the camp to Steilacoom. The fog creeps in and obscures Sarah from Chin while he is worried about the noise she makes and the prospect of Indians locating them. The fog is at its thickest when Chin falls and loses consciousness when he hits his head on the gravestone. The fog symbolizes the confusion of the situation and foreshadows that the mystery of Sarah will not be resolved.