I saw these snow wheels (or are they called sun wheels?) a few days back while out in the field.
They typically form on sunny afternoons when the snow starts rolling down talus slopes. This rocky slope (above) has several, best seen from a different angle (below).
Many of these aren't perfectly round, and I saw at least one that looked more like a small roller (think steamroller) - wider in the horizontal dimension than in the vertical dimension.
I didn't find much written about these, but did find some excellent photos on Flickr.