Two years ago, I was crossing a hilltop on a December afternoon, and I noticed that this line of boulders was aligned with the shadows.
Some of these photos were shown on Larry Harrop's blog at the time. Further exploration of the site revealed that this line of boulders intersected another line of boulders, one of which was propped.
Here is the second line of boulders, with the propped boulder in the foreground. Below is the strangely shaped top edge of the stone in the center of the photo.
The angle of the first line of boulders is consistent with winter solstice. There is no chamber or enclosure up here, so I don't know the observation point. Piecing together the site from photographs, I made this painting of winter solstice.