A small rock pile site in Carlisle MA

FFC took me over to visit a friend who has access to a small piece of Carlisle Conservation Fund land behind his property. We joined up with the friend and took a walk, out over the field and past the stone wall into a low semi-swampy area that was more or less surrounded by the curve of a ditch-like water course. At the middle of the curve there was one large rock pile, of indistinct shape. Nearby there were a few low piles. Some were on the ground and some on supports. The whole area was perhaps one or two acres.

Here are some of the smaller piles:Here is a view of the large pile from one side:
And here is a view of the extent of it, with friends on top for scale:
The property owner said that he played in this area as a child. Also, there was part of an old road through there and going right next to the large pile.