Speaking of niches: That niche you located on an outcrop with the stone cairn nearby is similar to a that Philip O'Deane showed me in Washington, MA, some years back. Not only was there a niche similar to the one you found, with a cairn nearby, but below it was a single large slab supported at either end by small cobbles; this was not enclosed at one end as was the nearby niche. O'Deane referred to this triple combination (including a small niche under the boulder with the cairn on top) as "three wombs." This combination is unique, and I've never seen anything quite like it since. Within the same town was a marvelous cairn with a niche at the base and topped with a large piece of milky quartz.
I believe Philip passed away a few years ago.