On Nagog Hill Rd in Acton there is an entrance to a conservation land, just to the left of a stable. on the east side of the road. If you follow the trail straight back across an open field and into the woods, you will see two very large piles with rectangular retaining walls, each as much as forty feet across.
You look at these piles and can see all different sizes of rocks dumped into the interior. So it seems pretty clear that a field was harvested of rocks to fill these outlines. Are they perhaps ceremonial? Could they have once been ceremonial and then been re-used for dumping field cleared rocks? One slight peculiarity is that both of these piles have their "feet in the water". A brook goes down underneath or along the side of one of them. On the balance, since these piles are right next to farms and old cleared fields it seems pretty likely these are examples of the elusive "agrarian" rock piles. Nonetheless it is fun to speculate that they could be more. If you poke around in the adjacent areas you may see there is a small, third, rectangular pile in on of the lots just beyond the spot of these two big ones.