This is the thimbleweed flower. Taken at Dolly Sods National Wilderness Area in Pendleton Co. WV, 7/16/15. There are other plants called thimbleweed, so the Latin name is important with this one. Anemone cylindrica, in particular, is very similar to the greenish flowered variation of this species.
Variation within a species can be baffling. The common local variation of thimbleweed is greenish flowered. Occasional plants with white flowers also occur. Farther north, there are plants with larger white flowers, and smaller greener ones. The largest and smallest have been called var. alba, and var. riparia. But there is a lot of integration. The named varieties seem to be a good way to talk about the extremes, but not so good to describe discreetly different populations.