Two distinct types of rock piles at a marker pile site

In the course of exploring last weekend, I crossed a site I knew near the headwaters of Fall Brook in Leominster/Sterling. This is what I call a typical "marker pile site"- with piles tending to line up, more or less evenly spaced, with piles having one good vertical side (but not necessarily good structure overall). Using the term "marker pile" is no doubt confusing as it incorporated my idea that these piles mark lines of site to a horizon. I should just call these piles "line-of-sight" piles - it would be a better term. Here is what I mean, and readers will prehaps recognize the structure here although the piles have seen better days.
Anyway I was crossing this site and noticed one example of a second type of rock pile. Here the rock pile is low, the constituent rocks are smaller (~6 inches across versus ~12). A couple of views:
I looked to see a white rock somewhere in this pile, as that shows up frequently for low ground piles. The central rock was lighter but not much. I could be wrong and this might be just a more broken down example of the same type as the others. But I don't think so. I think that this is actually a typical feature of "marker pile" sites: that there be mixed in a certain number of these low piles as well. One example of a site like this in Stow, MA has the low ground pile at the end of a line of marker piles - connected to the line-of-sight but not marking it in the same was as the other piles.

I walked around a bit and explored the fringes of this site. Saw a nice wedged rock:
Last time I descibed this site, Tim MacSweeney asked about the nearby stone walls. I payed a bit more attention to them in passing this time, noticing that the walls did not enclose the space so much as enlose other areas - with the site outside the walls. But check out the way the stonework is done in this wall:
That is, I believe the way a stone looks that has been split with a flat chisel - the earliest metal tool used for rock splitting around here.

I should mention that here, at the headwaters of Fall Brook I think every little tributary brook had a dam and a small mill - way back when. I'll try to show some pictures of what is left today, little rock piles in the middle of the stream-bed. But this marker pile site was off to the side between brooks and I am sure these piles were deliberate and ceremonial.