I did a bit of research and found quite a bit of information about "Fairy Rings"of which there are three general types - green ones as shown above, brown rings and ones consisting of rings of mushrooms. Each are basically an every-expanding ring of fungi that expand from a few inches to several feet per year. The elevation of the pasture is around 9000 feet [2745 meters]. A detailed description of fairy rings can be found here and a research article can be found here. Fairy rings seem to be a turf problem, especially for golf courses. Below are a couple of pictures of the mushroom type of fairy rings.
Not related to the rings, but along the Peak to Peak I have seen a variety of wildlife, including an especially beautiful black fox, coyotes, deer, bears, and moose.