Oval Pile with "satellites" - Concord, MA

Here is an unusual configuration of rock piles on a knoll looking over a swamp and pond in Concord.A knoll overlooks the top of a wetland, next to one of the brooks that feeds the wetland. At the top of the knoll is a boat-shaped or oval pile about 15' long and there are four smaller piles arranged slightly below it on the water side. The knoll has a natural view to the southwest over the water.

Here we are looking towards the knoll with the water to the left.
The pile in the foreground was made of light colored rocks:closer:
Here are some views of the main pile. It is an oval or boat-shaped rock pile:The impression is of a messy pile with larger rocks around the edges and small ones filling in the outline. I was suspicious of this pile until I went hunting around for others. I first noticed the one with light rocks but, looking closely, also saw three others on the slope down to the water:
These were pretty well buried:These also seem to incorporate lighter colored rocks.

Update: Compare with the 2nd photo from the end here.