It's in the right position; maybe it's a propped boulder that broke. It has a cavity underneath.
Here's another possibility, a boulder with an inverted triangle shape perched on a rocky outcrop. The shape is similar to the two propped boulders on this hilltop.
The details of this site tell a greater story. Here is a stone wall leading into the site, near the broken boulder shown above.
As I mentioned previously, I wonder if these boulders were used as boundaries in land transfers.
The larger propped boulder has a commanding view from the hill, which unfortunately has made it a target for graffiti artists.
The stone directly to the left of it has an interesting feature: a shallow circular depression on its flat top surface. There are two faintly visible, concentric rings. Unlike natural, eroded depressions, this one is very symmetrical. Maybe this was once a work surface, and its location afforded a good view of the surrounding area.
The best find of the afternoon was directly downhill and 250 feet from this boulder.
This stone is about two and a half feet across, and has a very smooth, slightly concave surface. Note the sharp, bright edge to the left in the photo. This may have been a grinding slick. The proximity of grinding slicks and mortars to propped boulders suggests these were important markers for campsites and villages. Cairns and rock piles are scarce in these sites. The Natives probably did not build cairns in their residential and work areas.