An Unusual Burial Site at the Gumpas in NH

I was proud of myself for finding rock piles in all those woods. It is a big woods and you can only examine a fraction on one walk. My experience told me there were mounds on the west sides of and downhill of the upland ponds (shown as wetlands on the left edge of map). My experience told me there ought to be some more on the east sides of the pond - especially between hills, at the very highest points of their watershed. And my feet got me there. 
In the end, my nose pulled be back away from the water's edge. First I went to take a quick look at some unnatural piling on an outcrop. From there I spotted a small rock pile. ok. From there I spotted an unusual enclosure built into a wall. OK. From there I spotted a larger mound. OK!!!
Goes like this:

Looking west towards the water. I climbed up to this outcrop to examine the mess (underfoot in the picture). Note the very tall stone wall below. But behind you:

And then, what is this?

 And beyond...behold.

After this I explored outward and took several not-very-good pictures. We will see that most of the piles are like the first one above - a pile on a boulder, near another boulder.
Some observations about the enclosure. It is on the high point. A unique position.

Filling most of one end is a mound with a collapsed center. This is the expected structure for mounds in this area. But the enclosure around it, connecting it to a wall is something new to me. Perhaps added later? There are two things I did not pay enough attention to. There is a cleanly split boulder built into the enclosure wall (back of the first picture and under foot in the second one) and you can see some kind of funny-business at the base of the enclosing wall in the front right part of the first picture. What are these features? I tried to indicate them on a sketch:

What is up with these things? It is unusual to see a split rock directly connected to a mound with a hollow. The mystery deepens when you consider the style of several of the other rocks piles around the enclosure also involves a pile and a separate boulder. We noted "a pile on a boulder, near another boulder". Take a look at some of the others.
Here's one:

Here's one:

Here's one:

Except this one was more complicated and I could not get a good picture of it. Here is part of it:

I have nothing wise to say. I get a feeling the enclosure was added later. I also wonder if the boulder might have been split later? Scattered around among the larger piles were smaller ones, and messy things under the leaves:

A small knoll at the top of two watersheds; I'd say with a complicated history.