There are scattered small rock pile groupings throughout at least the area outlined and probably more. The topography is of small knolls and bits of wetland mixed together and, as far as I can tell, small rock pile sites occur at each knoll. To me they look like simple marker pile sites. Most of the piles consist of a handful of smaller rocks built on top of a larger rock with occasional more substantial piles having a well-built vertical face. I got the sense that the piles were visible from and clustered about the little knolls.
Drove to Hopkinton. I parked on Pond St, just west of and downhill from the nearest house. Entered the woods and cut off from the trail (which was marked like this:)I was not too optimistic but saw an interesting propped boulder and was re-assured:After a while I got into the eastern portion of what I had planned to explore and came across a small cluster of piles:Here is a view downhill from above, to give a sense of the moment and place.There are lots of rocks in the picture (click to enlarge). I believe most of them were manipulated. After all, the larger rocks were manipulated:Nearby the first cluster of rock piles, I came across an obvious boundary marker, (the approach:)(The closeup:)Any illusion that such boundary marking might explain the presence of the rock piles and larger manipulated rocks was dispelled not long after when I came to another group of piles, these slightly bigger and better built.
(Two views of a pile)And another nearby. Do you see the vertical face? Imagine viewing these from the knoll...I am thinking something to do with the shadows...? Here was another knoll with a damaged structure on it:This continued, with a couple of further highlights I'll put in separate posts: a larger central pile and some places that stone seats were built into the outcrops.
Update: an Anonymous commenter says they do not see the vertical face of the piles I showed above. Imagine the red outline graphic being in a single plane - not exactly vertical but close. That plane is visible in the pile, which is shown without the graphic above.