This small structure is near a creek branch that runs into the West Branch of the Delaware River. It's the first rock I've found that is propped by 3 supports, but it has me baffled by its small size. Nevertheless, I believe it has several features that are noteworthy. All the more reason to believe it's man made.
The shape of the propped rock and the direction of the elongated rocks around the structure. They run, generally, north/south:
[Update/correction: the direction is east/west]
The 3 supporting rocks underneath are triangular.
The supported stone has lots of character. Intersecting lines and some other interesting chip or notch are in this view.
I went back out today, after a light coating of snow, and the natural line on the top of the rock struck me differently this time. When I first saw it, I thought it was somewhat of a feminine suggestion, but now, with the snow, it looks more serpentine, I think.
I made a rough sketch of the structure as an attempt to illustrate that there are only 3 rocks supporting the top rock, even though there are other rocks nearby and/or underneath that look like they are touching.
[Update/correction: the direction indicated in the diagram is east, not north]