Obsessing over rectanglular structures - more short walks from Piper Rd Acton

While I was reviewing the map of Piper Rd (blogged here) I noticed another little landform to the south that also was surrounded by water and looked worth exploring. So I snuck behind houses over there on Christmas day (after getting the roast in the oven) and did the "tour de ridge" from Piper Rd over to School Street. There were little rock piles along the northern side of the landform (middle blue outline) much like at the ones at the earlier Piper Rd site:This was the firsts I found, and I called it my "Christmas Pile", as it was like a present:
More interesting to me was the larger somewhat rectangular pile at the easternmost extremity of the landform, a larger pile that might have been a "Wachusett Tradition" item. It was perfectly placed for it:Is this a pile with a tail? I was more or less visible from houses and did not stick around to take good pictures.

After doing this "sneak" I went over to the west side of the road to the legitimate entrance to the Acton Great Hill Conservation Land. Along the red trail in there there is one curious rock pile:
This could also be a rock pile with a tail. Also along the red trail there is an earthen rectangle with an inner stone lined hollow:
(sorry lousy picture)
Inner hollow:
I also thought I saw another, so faint there is no point showing the picture. So either this little part of Acton is full of (a count 4) rectangular Wachusett Tradition piles - rectangular mounds with hollows and/or tails; or perhaps it was nothing. I know I am obsessing about these sorts of rectangles.