Norman Muller writes:

A friend of mine sent me two photos yesterday of a 100 foot wall at the Oley site in PA that cuts across a rocky wetland.  The wall many also have a manitou stone leaning against it. See image below.  The wall is about 100 feet long and not connected to other walls.  It reminds me of a wall on the side of a mountain in Rochester, VT, that cuts across a wet area (2nd image).  It, too, is unconnected to other walls.  Both examples fit.a pattern, and I'm wondering if you and others have recorded similar examples in your travels.