Mysterious Ice Circles, Stowe, Vermont

An ice disc, ice circle, or ice pan is a natural phenomenon that occurs in slow moving water in cold climates. Ice circles are thin and circular slabs of ice that rotate slowly in the water. It is believed that they form in eddy currents. Ice discs have most frequently been observed in Scandinavia and North America, but they are occasionally recorded as far south as England and Wales.

source : http://en.wikipedia.org/wiki/Ice_circle