They fill the water just off the shore like a swarm of jellyfish — if jellyfish were round, dirty brown, and frozen solid. But how do ice boulders form? Turns out, it's kind of like the way that a piece of sea glass gets polished smooth, but in reverse. Instead of wearing away at an object, in this case the action of the waves builds an object up.
"The water temperature on the Lake Michigan is just a little bit below freezing, so you get a small piece of ice that forms in the water and as waves move back and forth it adds additional water and freezes in layers. It gets bigger and bigger, and eventually you get big balls of ice, that are pushed to the shore by the wind."