Wall corner pile from field clearing, but a little peculiar

Went back to the same place as last weekend, just a mile or so northwest of Mt Wachusett and went downhill from BoltonRd into the State Park to the south. Saw some pretty massive piles at the bottom of what were once fields and with retaining walls - something which should be assumed to be field clearing piles. Here was a big one in a wall corner:
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:

• the pile was built with this dip in it
• someone removed material, creating that dip
• the pile was built with a hollow place inside the pile which has now collapsed to form this shape.

If you are dumping rocks, as per field clearing, it is hard to see how you could dump them in a circle to leave this dip. Or if it is a mundane dumping, then why go back afterwards and create a dip? Or if there was a hollow inside the pile - why the heck would it be there as a result of field clearing? I don't feel comfortable with any of those answers.

What does that leave? A manitou stone in front of a pile with a collapsed inner hollow. So what kind of field clearing is this? What about burials that are inside field clearing piles?

Here are some other large "field clearing" piles built into the walls (at the bottom of what was once fields):
There were lots of them.

A bit further down hill a rock-on-rock which I am pretty sure was not from field clearing. [So hard to walk through the woods these days - with all the blowdown trees - that I left it at that.] There certainly were some Indians around here sometime in the past, probably even after there were fields here.