Ashburnham Hill Rd

A couple of turns off Ashburnham Hill Rd in Fitchburg takes you to the end of Scott Rd where it turns into a dirt road. I walked in from there and stayed to the south, straying downhill to a place a bit north of the "O" in "Overlook". Here I saw a curious disturbance:
Take a closer look at this structure to the right:Isn't that purdy?I did not know what to make of this thing - it combined the look of a sandpit with the look of a rock pile. Again:It got worse, downhill from there, behind the parking lot for the Audubon Soc. "Flat Rock" parking lot. Things got really confusing.What's this?Some little terraces:Some stone support structures (not "bays"):Some rock piles heading off into the woods:Does that look practical? You tell me.What about this one, built into a stone wall?Another view,A closeup of the quartz, visible in the first picture:
Here is the length of the wall:At this point I recalled seeing something similar across Ashburnham Hill Rd on the drive in. So I was not far, walked over, and I snuck in to what seemed private property in order to get a photo:
Not quite the same thing as by the "Flat Rock" parking lot but close enough. I guess that, here, there are older ceremonial structures overlain by the more modern. Back at the parking lot, there was a house foundation,and a stone lined brook.And other signs that this could have been a mill. All in all, this site was as ambiguous as a place could be.