Whoever built them, and when, we have a LOT of rocks in New England. New England might seem like it's quite forested, and it is now, but if you look at lithographs from, say, the time of Emerson, it was mostly open meadow. There's the remains of a pencil factory of that era not far from the Nashoba Brook chamber. (I got all excited the first time I was there as I could see evidence of early vertical turbines). Notice all those stone walls running through the woods? Those were edges of fields. Every year more rocks come up, they have to go somewhere, and the edge is the closest spot. My least liked task on a dairy farm was 'picking stones'.

The point here is that if there had been Native American stone structures (and there definitely had been some, evidenced by cache structures in northwestern Maine) the land has gone through a lot of change. Things like chambers wouldn't have been unused or even completely unmodified, so we're unlikely to ever know the identity of the original builders - except in the case of roadbeds. We have the remains of some old farm tracks running through the woods behind our house. It's been interesting to look at 1930s USGS maps to see what was there. You might look for earlier maps for the possible bridge arch.