I went to topzone.com and looked at the whole range of hills starting with Oak Hill in Littleton and going down through Bolton, looking at where I found sites. Sites occur about once per 1/2 mile and the piles are made from a combination of the bedrock fragments mixed with glacial cobbles. It seems that there are many occurances of burnt rock.
Sunday I went to the Jacob Gates Lane conservation land in Harvard and saw lots of features that fit into the same sort of pattern that I have seen all along Oak Hill. Particularly here, the rock piles occur in clusters of three or so piles and rock-on-rocks are very common.
To me, some of it looks astronomical and some of it looks earth related. Parking at the end of the Lane and walking southwest, you cross a wall with a nice standing stone (above).
I walked straight in and uphill to the right until I hit a site on a little knoll/shoulder of the larger hill. The first pile was the nicest. There were about 10 piles and one at the top had a dramatic piece of quartz. The piles were mostly smothered in dead juniper.
I continued south and uphill and back and forth along the slope for about an hour and kept coming across small clusters of rock piles.Like this (second picture shows a detail of the middle pile):
And like this (note the bedrock):
There were so many rock-on-rock I don't know which to illustrate.
At no point was there a large pile, nor a large cluster of the small ones. This suggests incremental and small scale site creation over time.
One the way back out, lower down, was this possible enclosure.
I'll post more pictures later.