A Circular Ditch and Mound - Rocky Hill Sterling

On the southern shoulder of Rocky Hill in Sterling (the hilltop itself is in Leominster) you might notice a rock pile and, nearby, a circular ditch with slight mound in the interior of the circle. Here is the rock pile as I first saw it: And here is the circular ditch. Can you make this out on the forest floor in the dappled light?
The rock pile is just to the right of this.

I should have explored more nearby but was in a hurry to get to the top of the hill which turned out to be a disappointment.