"Pancake ice is a form of ice that consists of round pieces of ice with diameters ranging from a few inches to many feet, depending on the local conditions that affect ice formation. It may have a thickness of several inches. Pancake ice featers elevated rims with a nearly uniform height of a few inches. The rim is formed by piling the frazil ice/slush/etc. up the edges of pancakes when they collide, both due to random bumping into each other and because of periodic compressions at wave troughs. These rims are the first indication of the onset of the formation of the pancake ice from less consolidated forms of ice. "
Text from Wikipedia's entry on Pancake Ice.