Solitary platform near Mill Brook, Billerica MA

Because I commute by here, I became aware of a stretch of woods I had not explored - in the triangle between these roads in Billerica. The woods faces northwest towards a brook called Mill Brook. Access is blocked from almost every direction by private property but I found a way in and discovered the whole area is a wet skunk cabbage swamp, so it was rough going after a night of heavy rain. Anyway it is lucky I went in the way I did - because I found one solitary platform pile next to a small rivulet that feeds into the swamp from the east. If I had gone in a different way I would have missed it (and perhaps have seen something else). Anyway I was out and back home in 1/2 hour; making this the shortest site hunt ever.
The pile itself was roughly rectangular and composed of rocks mostly of the same size, built up against a larger boulder on the downhill side. It was perhaps 3 yards from the rivulet.