Creating a Collision System from Texture?

I’m making a 2D game where the player climbs a mountain. I have one plane that’s the mountain, and an identical plane who’s texture is a black track that denotes where the player can move.

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How do I make it such that the player can only move on the black portion?

FWIW I’ll be publishing something on the Asset Store that would do this easily, should be next week.