Iz'zat a rock pile ????

On the way back from Wamesit on Saturday, I was taking Rt 110 through Chelmsford and saw a big hill I did not recognize. It turns out this is the highest hill in Chelmsford, a hill I know from the other southern side. Anyway, I decided to drive up some of the side streets to see if there was any woods up there and I spotted this in someone's backyard.
There was no one home or I would have asked to take a closer look. Here is a zoomed photo.
I could not get a good view from above to see if this might be a water tower base. But it sure looks like a rock pile. It reminds of of that nicest pile at Miner Farm in RI. (see last picture here)

Slightly uphill, closer to the house, was another faint trace of a rock pile like structure.
What is this place? We are deep in suburbia here. Could there be some remnant? I think I can go back and talk to the homeowner, even if it is snowy.