"Ice circles are known to appear in the Arctic, Scandinavia and Canada,
but this is thought to be the first seen in Britain.
They occur at bends in the river where the accelerating water creates a force called 'rotational shear', which breaks off a chunk of ice and twists it around.
As the disc rotates, it grinds against surrounding ice - smoothing into a perfect circle."
This is a few lines from the paper article about how they come to be.
This 10' one in Devon was rotating 360º every 4 mins, 10 secs.
BL