I was shown a number of rock piles this weekend in Stow, MA. One of particular interest was built as a curve between two boulders. From one side, it looked like there could have been a space inside the pile, now fallen in.
From the other you can see the curve of the pile.
The discoverer of the pile pointed out a small standing stone placed a few yards from the boulders along the axis that bisects the two boulders. Then he also pointed out a slightly larger then an slightly larger pile back further along the same axis. Sort of like this