I dismissed this as just an out-of-place fire ring, as it was clearly recent. But after seeing Tim's picture here (from here) that shows a similar tight spiral of stones, I suppose it might also have been done with a similar impulse - a bit ceremonial, or perhaps a bit new age. I only saw this kind of thing once before in a pile that seemed old.
All three of these examples -this one, Tim's, and the old one, were next to water. At the time I found this "old" one, it occurred next to fine effigy of a fish, so I assumed the spiral was representing a snail - another local food supply at that spot. But that is just my fantasy and it does not fit what is going on in the other examples here.