Each has a view of the front or side and one of the tumbled down section follows. There are similarities to the way the break down of the pile looks.
(First example)

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(last example)
This is about rock piles and stone mound sites in New England. A balance is needed between keeping them secret and making them public. Also arrowheads, stone tools and other surface archaeology.

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A large glacial erratic was positioned on a rise. Facing it was a small structure, some 20 yds. off but with a clear view of the stone. A seat? a niche without its top? Stones were clearly shaped and set up, they topped a small low rock pile, the larger flat stones may be part of it. Perplexing.
Definitely a serpent place!
Further along the trail, this perplexing little chamber's opening was 2 ft. by 1 ft high and it went back about 10 ft. A large tree had found it an inviting target. It was along a stream, a nearby ruined dam and mill dated from the 19th century. Sorry for the blurry shot.
A nice rock pile built on ledge! We were now on private property, the landowner, who was with us, did not know what to make of it.
A hot steamy walk, but worth it.
Some clusters were more distinct than others,
and I concluded this was a site made from quarry fragments:
A faint sense of three-in-a-line at a couple of places, suggested it was a standard type of site: 
You can see this rock-on-rock higher up in the last photo and it is featured in this video below also.
Here is a more substantial pile that was certainly not just a coincidental clustering:
A bit of video:
Here is something from the second "site" south of the quarry: This shim of the upper rock, and the shape of that rock with a sharp point on it are both significant. I think exact positioning of that upper rock was important. Why would anyone care? In general, the answer has to be because of a visual property of the pile or its shadow.
Mmm...now the mountain laurels are flowering:
One last pile:
This was off a bit by itself. Its position with respect to the ledge is typical of many of the faint traces there on the west side of the hill. Looking back over my pictures, I see that there were actually several scattered rock piles along similar ledges, all along that western side. Probably worth exploring more lower down too, I did not.
Note the four petals, and the pod-like growths in the lower left. And compare with this buttercup with 5 petals:
Funny how flowers of the same color seem to come out at around the same time, or am I imagining that? And roses:
Getting deeper into the woods, what have we here?
And here we follow the west edge of Yellow Birch Swamp, going north with the swamp to the right, something on the swamp side of the wall:
Ah, it is another split-wedged rock, a little bit more reversible (as in removing the wedge) than the previous example:
And here is another, made from a feldspar conglomerate:
Funny how the Indian ceremonial presence can still be seen here in Concord along these well trodden paths.
But it was beside some signs of quarrying and not connected to the massive piles we saw in the past. Back at the car I got chastised by a state policeman for parking on the side of the road. Wonder if we'll see those Moosehorn sites again?
Given that mundane pedigree it is tempting to ignore that flat vertical rock in the center of the picture. It does not look as if it fell from the pile but as if it was deliberately placed. Huh? A manitou stone on a field clearing pile? Then darned if there wasn't a hollow in the center of the top of the pile:
Let me try listing every way to get a hollow like that in the top of the pile. If you can think of others, please add a comment. So, a hollow could be because:
There were lots of them.
Some of them are nicely built up like "biscuits":
Many of them have a "V" or triangular shape, as seen from above (including the "biscuit" above). Look carefully at the how each of these has two straight sides coming together in a corner:

And this nice one with a piece of quartz at the tip of the "V":
One more:
A very deliberate shape.
One more look, re-enforces the "grid" idea: