The corridor from Hobbs Brook up to Sandy Pond, Lincoln

In a place you were hoping to find rock piles, you come across a tumble of rocks:

Your eye makes no sense of it. You look at it from another angle:

And...yeah...it does look like there is a straight side (on the left) and maybe some other structure.
There is a stone wall in the background that separates the lowest wet area from higher ground. The piled rocks are on the wet side and are well separated from the stone wall. Also you can see there is a smaller pile nearby:

Taken together, the smaller pile and the setback from the flanking stone wall are "strategic" reasons for thinking this tumble of stones is the mound you are looking for. Another view:

This is about where the 'C' is in 'LINCOLN' on the map:

At the NE end of the valley (under the word 'LINCOLN') is the top of Hobbs Brook, which is a source for the Charles. You go up the valley to the SW and end up at Sandy Pond. It appears Sandy Pond now drains to the south but there must have been a connection down this valley at some time. I know a site at the NE end of this valley and I thought it would be a good idea to explore up towards the SW. After finding the first piled rocks (above) I did not see much. The area is full of things like this:

I turned around and went back via the highest ground, by the letter 'I' in 'LINCOLN', and at the high point there were several damaged rock piles. Some had been recently bulldozed and it made me suspicious of the lot. Still I strongly suspect there was a site here in the past.

Hard to see this as bulldozer damage: