Just some input on what they WOULD mean, if they were man made. The compass stone is classic Charles Kenworthy example. The turtle is hard to see because it was a little cloudy that day. The skull face and the "mummy" are interesting, but without any info on what they pertain to, they are merely strange landmarks.
It's funny how the compass stone is exactly as Charles Kenworthy described In his book "Spanish Monuments and Trailmarkers..." with 4 stones holding the massive slab above the terrain it sits upon, both to catch the searcher's eye, and to allow water to flow beneath it so it doesn't get washed away in heavy rains.
The compass stone above, as you call it, is as good of a name as any, for the natural formation. Imagine, if you will, as Rod Serling used to say, the physical realities of humans holding the large slab up, while Billy, the stupid one, places the stones below it to prop it up, carefully calculating the right spot, while his buddies scream to hurry up.
I agree with your assessment of the compass stone. It very likely isn't a dolmen. Same logic applies to balanced rocks. Even though these intriguing formations are natural and fairly common, nonetheless they make excellent landmarks and could well be used to help guide a searcher.
