A vast circle of ice has been found slowly rotating in North Dakota's Sheyenne river. Perfectly circular ice disks form on outer bends of rivers, where faster-running currents break chunks of surface ice away during cold weather. The rotational shear of the river's current pulls chunks of ice together and spins the resultant mass around, grinding its edges into a neat circle.

Retired engineer George Loegering captured the phenomenon on camera. Some 50-feet in diameter, the Sheyenne disk is larger than those discovered in recent years, including a ten-foot example in the UK. Allen Schlag, National Weather Service hydrologist in Bismarck, North Dakota, told io9 that the larger the river, the bigger the ice circle likely to form on it. Of the new ice disk, Schlag said "that might be one of the better examples I've seen."

From The Verge