A platform pile site in Westford MA

Here is a lovely small site off a side-road near the highway in Westford. The site consists of a shallow valley with a small brook, opening southward:
There is a new road going southeast from Hilderth Str. called Monadnock Drive. If you turn in there and cross the low spot and park, you can walk south down along the brook. As you step into the woods you see this: two platform piles facing each other across the little brook:
The one on the right was badly torn up, either by kids making a fort or by treasure hunters. At one end there were two pieces of quartz:

Here is the one across the way:
I did not see any quartz in this one. It is in much better shape and you can see that it has a level upper surface:
About forty yards downstream there was another slightly smaller pile, also in good shape but without any evidence of a flat upper surface:
After taking some photos, and feeling nervous to be only a few feet away from someone's back yard, I went back out to the car. As I was leaving I noticed one more rock pile uphill from these platforms: a linear pile like a low stone wall pointing downhill in the direction of these larger piles.
So here is a sketch of the site: with the rock pile features in gray:
A nice site that reminded me of other sites with platforms along a gully opening in a southerly direction.