Polygonal Mound (with no hollow) - West of Dan Parker Rd Groton MA

Another small mound from the site I visited yesterday (see here) west of Dan Parker Rd. This mound is conspicuously different from the many mounds to the east of the same road. At first I thought it was somewhat round in outline:
I video'ed it from above to show the outline,

and, now that I am home looking at the video, I can see that where the retaining wall is still visible the sides are straight between angled corners. The part that I was seeing as "round" is more like where the retaining wall has collapsed. So I take it that this is an example of something rarer than a rectangular mound, a mound with a polygonal outline. I know a couple examples of pentagonal piles a bit smaller than this one (Hycrest Pond, Acton Horse Stable) but this one would have to be hexagonal or heptagonal. I should also mention a site in Weston (see here).

Anyway, these are pretty rare and I am confused that 200 yards away on the other side of the hill the piles are rectangular and more conventional. Also, I would expect a hollow at the center, but it is not present or is much less visible than usual.

A couple of poorly formed panorama views, sorry about the blurring:
More to come about this site which I want to call "Horse Hill Left" but which is actually a southern foothill of Horse Hill on Dan Parker Rd.