Actually when you look at the "log" in 3d you can see the back side is actually there, fully lit. Once you view it in stereo it becomes painfully obvious that it's just the edge of an upturned rock like the others next to it.



Here's a cross-eye stereogram of it. Cross your eyes till the images overlap. There will be 3 images, the middle one will be 3d.

*nevermind, I thought you were saying the "backside of the rock, in shadow" was the same as the top of the slab. Now I see what you were saying.