Circle of Stones
According to this article in Scientific American there is now evidence that stones can self-organize into circles:
The team found, using computer simulations, that the two main mechanisms are lateral sorting, which moves stones and soils to regions that have high concentrations of similar particles, and squeezing, which stretches stones into longer lines by causing movements within a pile of rocks.Freezing and thawing of the ground influence both of those processes, and their relative strengths determine what the final pattern looks like. For instance, polygons arise when squeezing is strong enough to counteract the effects of lateral sorting.
You turn your back on them for just a minute....