On a river they are believed to form when foam begins to freeze and is sucked into an eddy. The swirling current creates a circular shape as the frozen discs increase in size and are rubbed against each other. 2/2 @ScottishEPA @BBCSpringwatch @BBCScotland pic.twitter.com/GNtFnsF1da

— Scottish Invasive Species Initiative (@SISI_project) December 13, 2022