Druid's Circle/Boyscout Folly at Spring Hill in Acton

If you follow the trails north from Spring Hill, where Spring Hill and the Boyscout Land touch, you may see this in the woods:

I occasionally see opposing stone "U"s suggesting multi-person ceremony [A Larry Harrop picture, I think from Foxborough?]. But those did not have seats and seat backs like here. Of course you should be suspicious of any strange stone structures found near Boyscout land.