Hopping Brook in Holliston - part one

Jim P took me to this place long ago but I had forgotten where it was. From the modern and recent flagging, it appears it is known to the public now. A very significant site. 
I remember when we first looked at it, my father, Mary Gage and I. And this pile did not make any sense:

 (other side)

I set out on Saturday to explore this hill because it looked worthwhile, down there in Holliston near a large brook. 

I parked where the cross-hair is on the map, just left of the lower blue outline, at the end of the highest office park road; and walked east beneath the power lines and stepped into a woods thick with small saplings. Saw the first of several small, double chambered rock piles and...

... had just spotted another when I got distracted by the large one shown above. I'll come back to the larger pile later. Here was what I spotted:

  

Note the orange flag on a small pile, and the larger one behind. [I think this might be an example of rock pile enthusiasts surveying and not cleaning up after themselves. Does any one know anything about surveying at Hopping Brook? Flagging at a small pile is suspicious.] 
Closer up:

 There are several hollows, I cannot remember the arrangement.

A small pile nearby:

A"satellites" of the larger mounds. Back to the larger mound. Next to it was some water and other satellites:

and here is a closeup of the large pile:

 Again I see an orange flag.

Today when I look at this it is not so confusing. Although it is messy, you could make out a number of smaller enclosures around a central rectangle within a retaining wall. In one place the retaining wall was as wide as the separate enclosures. Something like this:

I think those hollows are from the collapsing of inner chambers. This big mound would be large enough to hold a hundred people. It is interesting that it is only a few feet high. It is pretty unique. To put speculation on top of speculation: what causes a hundred people to die? 
Let's look at more of the mid-sized mounds with two or so hollows. These are a more common sight north of here.

 

Another

Ah! to see this looming:

A little higher on the hill, this beauty:

or:

There appear to be little hollows all the way around a central mound. It seemed the piles were taller now. I was higher on this hill. I am sure thorough exploration of this whole valley would find many others in different conditions. It would be a great place to study. 
Here was a square wall bulge:

 Here was an isolated cluster of two or three ground piles (meaning they are lost in the soil):

Then back to the power lines and a wall with a couple of smaller piles up against it:

You could think someone had been here goofing around.