Mud Ice

I went tromping through the woods today, and discovered several little fields of the strangest ice I think I've ever seen. Look - here is one of them. At first I wasn't sure it was ice, because it seemed too warm out. But they were in the shade, and I took my glove off and let one melt in my hand to make sure.



Then when I got back home, I looked for ice crystal images online and found one just like mine on Flickr.

Does anyone know how these crystals form? I figure as the mud gets colder, the water freezes and expands. What's really interesting to me is that the water molecules freeze to each other and push up out of the mud rather than forming a uniform mud-ice block. Weird. Maybe I'll try freezing some mud and see if I can grow these crystals.