It is not inside the Mountain.
Map
River Obelisk
North #4
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It's in the marsh area.
It takes a while to draw the whole thing.
Try the bridge.
Stand near the end of the long moving bridge in the marsh. Set the bridge moving away from the entrance and watch for the two semi-circles to meet. Be careful not to let other shadows including your own get in the way.
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Complete spoiler:
Roll over each black area below for hints.
Each new hint gives away more than the previous one.
Intro
You need to be moving.
Be careful not to block the sun.
This one will probably take a few tries.
The shadows on the bridge.
It needs to be moving away from the entrance (from left to right).
Show hints
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE