Pile-Space-Pile: a unique site in Westford, MA

by pwax
I spent part of the fall exploring one region in Westford MA that had six or seven different rock pile sites within walking distance of each other. The most spectacular and unique was along the edges of a brook and a collection of vernal ponds. The piles there were really well built and very vertical. Here is a good example.
Note the aperture hole in the middle. This pile is only a single line of rocks thick, it is all vertical dimension and I think it is there in order to create the aperature.

Here is another but with no aperture:
The pile does however exhibit something that I started noticing only after I visited the site a couple of times: it is next to a smaller pile with a gap between the two.
The more I looked at the pictures and the more often I visited the site, the more I was convinced that this is a very specific design being used throughout the site. A pile, a space, a pile. Here are some more substantial examples:
Finally we come to this really magnificent one. This certainly seems to be a deliberate structure of two raised walls separated by a gap. There is no missing rock, fallen down in front or behind.
My guess is that this has something to do with a line of sight. But I am hoping someone will leave a good guess in comments as to what this is for.

I need to mention, about this last example, that if you look closely there is a whitish rock near the bottom in the shadow right at the center of the photo. I didn't look closely enough at the real pile but Bruce McAleer did, and he pulled out that rock to get a closer look and it was covered with small garnet crystals. Afterwards he put it back carefully. All the other rocks in the pile are of the same grayish material. The whitish rock is almost like a battery underneath there, charging up the engine which is this stucture. What is it for?