I am attaching a photo of a stone mound in the Finger Lakes region of NY that David Schewe showed me five or six years ago. It seems to have collapsed in a way similar to the mounds from Fitchburg that you describe. You'll notice that a portion of the mound remains fairly intact to give one an idea that the whole mound was once carefully built.
Your drawings also reminded me of the flat-topped cairn at the Oley Hills site, which fortunately has remained intact, although it is oval and not square.PWAX writes: I agree, that first example is exactly what I am talking about. The Oley Hills cairns are so unique, I have never seen anything like them.