The Acton "grid" is a collection of piles built up with nice vertical faces on one side, oval to teardrop shaped in outline. They seem to be multiples of about 17 feet apart and some of them are lined up in groups of three and at least one group of four. So these are exactly the attributes of a marker pile site. There are more than twenty piles. Some piles have a vertical flake "fin" sticking out of them, which is another occasional attribute of marker piles. I need to emphasize that there are about six very nice other examples of sites with "grids" west of here, especially in the valley of Beaver Brook and Elizabeth Brook in Harvard, Boxborough, and Stow. For example the Hill with 500 Cairns is another example. So I am claiming a basic similarity between this site in Acton and the one we saw on that Hill. We did some new surveying the other day and I am very much looking forward to seeing the maps. Here are some of the nicer examples of piles: (That is a little chit of paper on the pile, to help keep track of them.)
And here are a couple where you can see the "fin": Here is one more of these rock piles. You can see how it is broken down.
These piles qualify as cairns and, being here in Acton, are about the eastern-most place I have found this kind of well stacked pile. There are marker pile sites further east, for example in Burlington, but not with this nice kind of stacking.