Some other ideas to consider....
When we had Madagascar as a "fun" campaign (haven't had one of those in a LONG time), I commented about how rain runoff left behind white paths going around objects. In that locality, the runoff often left behind white heart-shapes.on the ground. Otherwise, the runoff created "paths" around bushes, stones, boulders, etc.
I believe we also saw this in Sudan I.
We also have seen how tree tops and bush clumps look strangely like rocks and boulders with paths around them, especially in the right soil conditions in which white is predominant, as well as when the populace (or animals) take treks along the same paths year after year.
I would say all these ground rings and shapes fit one of these two possibilities. Oh, and let''s not forget the termite mounds in some localities.