I can go back to Peter's posts about the area though, with just a simple little search.
I saw this one from not too long ago:
"A familiar shape," Peter writes. "I find it is worth looking at the shapes of single rocks on rocks."
I've been looking at this shape for quite while now, here on what used to be called Turtle Island. It's the Testudinate Key to the Stonework of Turtle Island. It's the cultural clue that separates the older stonework from the New England Stonewalls and the Agricultural Whatchacallits.
A variation of that "turtle head and shell in a single stone" is the “a chunk of crystal in the stone” thing; I don’t know if it is luck alone that results in this, but it’s a ‘new to me” motif I just recognized in the Chicken Yard (in the 12 years I've been looking at these mounds).