This one can be found at ground level.

This one can also be solved at ground level.

It can be found right after a subtle transition in the terrain.

A transition where your footsteps no longer sound so crunchy.

The transition is marked by a small cluster of rocks.

You should be at the ground-level entrance to the treehouse area.

After the terrain changes, keep looking ahead.

From the obelisk, head down to the ground-level entrance into the treehouse area.  After crossing the small cluster of rocks, you can see the puzzle straight ahead of you as a shadow on the ground. 

There are some spikes blocking you from tracing the path, so you need to walk around so that you are next to the end of the puzzle (rounded tip).  Face the start circle.  Adjust your stance so that the spikes are not blocking you.

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If you don't see it, try looking downward a bit.

Those spikes are annoying. They make it hard to trace the path. Try a change in perspective.  :)