Interesting explanation Sky, but no, I am not speaking of the Carlise area, but rather the central region of Massachusetts.
This area is not far east of the Quabbin Reservoir. ( I have always wondered why they chose that particular area to flood and keep underwater, could these strange artifacts in the immediate area have had anything to do with that? ) And the fact that so few know about these remaining "artifacts" is the only reason they have survived this long, no doubt.
But no, these are not glacial dump, drag,or deposit, these are man made there is no doubt about that.
Also, I can not agree with the hollow space in the "monument" having been a place for offerings.
It simply would not work that way. The hollow itself is a kind of "bee hive shaped" opening directly overhead as one sits on the stone bench, there is nothing to hold any offering up, in there.
It truly appears as though it was only meant to accommodate the head of a "human" sitting on the bench!
I should add too, that the internal structure of the entire rock itself has been hollowed out, and there is a shape very much like a long tube, that runs the whole length from front to rear inside this rock.
It is a very peculiar "artifact" and I have no explanation for it, nor do I know of a single Abenaki or Nipmuck explanation, for it either.
And no, I am not speaking of pictographs, petrographic renditions, or any sort of stone pecking techniques in relation to these particular pictures in the rocks.
To people who have not seen them, this will sound silly but, the pictures I am speaking about are more like "modern day" photographs within the natural structure of the rock itself, as opposed to having been artificially placed upon the rock, as if it were simply an artists canvas to be painted on. These are not stick type drawings, symbols, or anything along those lines, these are "real life" pictures or the actual portraits of what appear to have been "living people" at the time they somehow got "into" solid rock!
Can`t explain that either, and our remaining stories do not account for any of these things...but yes, the native presence appears to have been the last of those who may have actually "utilized" these places. In my opinion anyway.