Small Explorations in Sterling, MA - Quartz in the Wall

When the hill I hoped to explore turned out to be in-accessible because of the clearly posted No Trespassing signs, I found myself again on Upper North Row Rd heading west. So I decided to explore more south of the road along one of the Sterling Conservation Land trails I knew about. As I walked south, following a stone wall I noticed what looked like a rectangular enclosure built out from the stone wall:

It looked like it was not wall damage. On the other hand there is a rock in the middle of the enclosure which would make sense if it is true that a rock is placed in the middle of the enclosure after a ceremony is completed. But then as I took a closer look I noticed this chunk of quartz in the wall at the back of the enclosure.

As I continued exploring in there I found that quartz pretty much comes with the territory out here and I was not able to convince myself that quartz in the wall was deliberate or just the random use of materials. Somewhere in there, along the road indicated by the dashed line, I came to a section of wall that seemed to have one or more pieces of quartz every twelve paces. But I could not tell if this was ceremonial: