A very large rock pile - more from "out west of here"

Last weekend and two weekends ago I have been exploring around the edges of a region of multiple rock pile sites, hoping to see new piles and hoping to get a better sense of the scope of the sites in the region. The piles here are what I call "monumental" because they use large rocks and are large piles and are often in groups with a variety of shapes and sizes - suggesting complex use and, at least in my speculation, suggesting use by multiple people at the same time, performing different roles in more complex ceremonies. As far as I know these sites do not occur in eastern MA but are rather a "central Mass." phenomenon. I have compared this "out west of here" site with the ones we saw near Moosehorn Rd in New Salem.

Last weekend, exploring around the edges of one of the known groups of piles, I came accross what, for me, is a really big rock pile. Although badly damaged (in my opinion) it included all kinds of rectalinear structure: extra corners around the edge, evidence of internal walls, etc. Here is a view from a slight distance: It forms a hill of its own. I only know two piles of similar size: one at Whipple Hill in Lexington and one in Hayden Woods in Wayland.

Here are views from above the other side, where most of the retaining wall is still visible:
In this last picture, note the extra corner in the retaining wall. Note also what looks like damage on the right.

Here is one more view from above of a long trench in the pile. Note how the piled up rock are higher to the left than the right. I think this trench was dug into the existing pile and the rocks were moved from inside the trench to the left in the picture. So I think this is vandalism. Walkind around and over the pile it looked like it was dug into in multiple places. If this is correct then I can say that with only one exception, every single large pile in the region was excavated - maybe ten piles in all.
It is all the more surprising to me that I did find one un-damaged pile (shown a few days ago).

If all these piles were worth excavating then someone must have been finding something - cuz it is a lot of work and would not have been continued if it was not rewarding. So I know this is just a house of cards but it makes me wonder: where are the private collections of artifacts around here? What do they contain?