More AZ rock piles - broken down plant boxes

The most impressive collection of rock piles I saw was off Rt 60 coming south from the Apache reservation(s) entering the Tonto National Forest. I pulled off to the right and stepped into this scene:I was not clear what these might be, until I found one that was in better shape: If all of the piles originally were like this one then we are seeing another structured cultivation area. What were they cultivating? I noticed an unusual type of yucca\agave, which you can see in the background of this picture. Why use plant-boxes? I am wondering about moisture retention and the small bit of shade provided. As I mentioned these types of structures are pervasive in southern Arizona. I think I saw such things just about every time I stepped into the desert.

It is ironic that I am always putting down the "agrarian hypothesis" for rock pile around here in New England and then in Arizona, no sooner do I spot rock piles than I am quick to bring up exactly that kind of a rationale. What can I say? I was hoping to see a spring with some effigies, as I have heard rumors of. But I did not.