I published a picture of this U-shaped structure in a gully on Snake Hill years ago on the NEARA web page. Back then I thought all "U"s were equivalent. But today I see no similarity between this and the kind of small U-shaped structure which I have come to think of as being a "seat" where someone could sit and watch. Wait, here is what I mean, this picture is from the same general area of Ayer:
That is big enough for one person to sit.
But the larger structure above is probably not a seat at all. Could it be the remains of a dwelling?