Hills along the Rt 495 corridor

Rt 495 out west of here runs north/south through the valley of Beaver Brook (flowing north into the Nashua River) and Elizabeth Brook (flowing south into the Assabet River). There are wonderful rock pile sites down throughout this valley. The Boxborough Esker, made famous by Mavor and Dix in Manitou is here, as are some of the stone chambers they discuss. If you visit every hilltop out here, you'll find the rock piles sites. Hilltop sites don't happen over here in central Middlesex County but they are the standard out there along the border with Worcester County. Towns of interest, I call it the "Bobo corridor", include Boxborough, Bolton, Harvard, Littleton, and Stow. These are USET protected towns (except Harvard which missed the list).
I noticed an unexplored hilltop on the topo map last week and today, it being Sunday, I had a chance to drive over there and check it out. Well, all the hills near this have sites that seem related to sky watching. The sites are always on the southern and/or western side of the hilltop. Here are examples of one type of structure I found, similar to some things I saw on a hilltop in Groton: crossed-pairs of stones, like "rock-on-rock" but seemingly designed to support the upper rock in a particular position.
Later in my walk today, I actually capture some rock-on-rocks in the act of forming an alignment. Usually they are not so easy to photo. Here they are:

This may be more of the same from nearby:
I'll continue the site description in the next entry.