It has been speculated that there are literally millions of Tipi (Teepee) rings across the prairies. Most have been disturbed to some degree, but there are still large numbers still in their original state. It is interesting to note that tipi rings in wintering grounds usually had two circles of stones, as the tipi had a second, insulating layer of hides. Tipi rings in windy areas had heavier stones on the windward-facing side.
The Prairie Fiddler 