BISMARCK, N.D.—When George Loegering saw a large spinning circle of ice in the Sheyenne River while out hunting with relatives, the retired engineer couldn’t believe his eyes.

“At first I thought, no way! It was surreal,” Loegering, 73, said Tuesday of the large ice disk he witnessed Saturday. “You looked at it and you thought, how did it do that?”

Then his engineering background kicked in. He calculated the disk’s diameter to be about 55 feet, took photos and videos of it and then turned to the Internet for more information about what he, his brother-in-law and nephew had seen.

“It’s not an unknown phenomenon, but it is relatively rare,” said Loegering, who lives in rural Casselton, about 20 miles west of Fargo.

Allen Schlag, a National Weather Service hydrologist in Bismarck, and Greg Gust, a weather service meteorologist in Grand Forks, said a combination of cold, dense air last weekend and an eddy in the river likely caused the disk.

“It’s actually quite beautiful,” Schlag said.

The cold, dense air—the air pressure Saturday in nearby Fargo was a record high for the city for the month of November, according to Gust—turned the river water into ice, but since the water was relatively warm it didn’t happen all at once. Floating bits of ice got caught in the eddy and started to spin in a circle.

“It’s not a continuous sheet of ice,” Schlag said. “If you were to throw a grapefruit-size rock on it, it would go through. It’s not a solid piece of ice—it’s a collection of ice cubes.”

Loegering said the spinning disk had frozen up but was still visible in the river.

“I’m not sure how long it was there (spinning),” he said. “It had to be quite a long time. If you look at the picture, you can see growth rings on the disk.”

Schlag said he was surprised by the size of the ice circle, which he said would be more likely on a larger river such as the Missouri.

“That might be one of the better examples I’ve seen,” he said. “It’s a pretty cool one.”

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