As temperatures fall along the River Dee in Aberdeenshire, Scotland, each winter, it's not unusual for visitors to find foam freezing on the surface.
This month, however, River Dee Trust Biologist Jamie Urquhart discovered dozens of pancakelike pieces of ice floating along the historic river.
"What we think happened is this: Foam floating about on the water started to freeze, probably at night," Dee Catchment Outreach Officer Joanna Dick said.
"Bits of frozen foam got swirled around in an eddy, and became roughly circular. Perhaps each disc grew when smaller pieces of unfrozen foam struck the disc, adhered and then froze in place," she said.
This is the first time the ice pancakes have been spotted on the River Dee. They are more commonly seen in the Antarctic or the Baltic Sea.
Below-normal temperatures for the month of December may have aided in their formation.
December temperatures in Aberdeenshire have averaged 1 degree Celsius (1.4 F) below normal. Almost every night this month, lows have dropped below freezing.
"They are a rare occurrence and conditions have to be right for them to form," Dick said.