A first rock pile in AZ

I saw a few rock piles in Arizona, none that seemed particularly reminiscent of the ones we have in new England. For example, the first one I saw was in the Tonto Basin. We stopped the car to take a closer look at some prickly-pear cactus fruits. We all tried some and were rewarded with out first cactus pricks of the vacation. In the midst of the cactus was this rock pile. When I saw this one I was a bit confused: those upper rocks on the pile look to have freshly exposed surfaces. How could this be a new rock pile?
Then I looked around a bit more and saw this little bit of buried wall:
The more I looked the more I noticed these vestiges of structures buried in the dirt and realized these were old house foundation - or something. So I concluded that the rock pile was what was left by the archeologists\pot-hunters after they excavated. As I looked around I noticed other little patterns of rock on the ground. An advantage of the rock pile "training" I have is that I notice non-random patterns of rock. In retrospect it seems that there were patterns in every piece of desert I stepped into during the week. Arizona is a place with a deep and complete history. So there were also these outlines:
I was trying to figure out what these were for? I may have read about it somewhere but the idea occurs that these small outlines are little "plant boxes" - I don't know what they housed and I do not know their purpose but I can guess. In fact I suppose that the edible cactus that was all over the place at this site provides an obvious clue: they may have been cultivating the prickly pear. I saw similar "plant box" formations in most of the places where I stepped out of the car.