Never seen a boulder in a tree like that. It does resemble a skull. If it's a true marker it's probably marking the tree for something to line up with another, or maybe even looking down at the spot. My avatar is a skull low to the ground... and it looks toward a spot.
Some things look, some things watch over from the side. Some things look at (point out) something you are supposed to see or look thru, some things are things you look thru.
Here is the eye of Horus, an Egyptian theme. You put your head behind it at ground level and look thru it. It shows 3 white markers on a hillside in a triangle.
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Look closely, not a perfect rendition but you get the idea. The pyramid or wedge shape below the eye is even there. It looks at some white markers, I couldn't get them all on the camera while shooting thru the eye.
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The boulder set up the eye is part of looks like half a face laying on the ground in the field from the front side when you see the whole thing in the field (not shown). When walking over to it from above it looks like a large squarish or diamond shaped boulder, but not really perfect dimensions.
edit: it's very difficult to show what is seen thru these "hoyos" with a camera, lining it up like you see it with your naked eye is very difficult. Anyways, the 3 markers stick out like a sore thumb when you look thru it with your naked eye, it is not easy to show it with a couple pictures, you can;t really align perfectly with a camera thru a small hole in the rocks and still keep things in focus.. the single marker seen thru the hole picture is the bottom marker of where I showed all 3.
Also note on the left side of the triangle of markers , if you look close is a pentagonal shaped rock in the grass... and the hoyo is pointing or indicating to left when you look thru it, so left side of triangle somewhere where things align is probably a pretty good start.