Chilly "crop" circles

Someone had created these two large snow spirals on the frozen Elbow River yesterday. I took the photo from a bridge going over the river. At least they left evidence of either entry or exit, otherwise I would have been totally mystified! What does puzzle me is knowing that some people are foolish enough to walk, ski or bike on any frozen expanse of water, especially as we have had so many comparatively mild days this winter!

Added on 22nd February 2007. My daughter e-mailed me the following information:

"They're made by a bike. There are a lot of stunters (not sure the correct term for them) out there who are very precise with their moves, as in competitions, they get marked on creativity, movement, perfection, etc. As when you ride your bike in a tight circle, the front and back wheel don't follow the same path, in other words, the back tire track doesn't ride over top of the front track. The rider turns in a circle, then at the centre of the swirl, they pop up on the front wheel, so only the front wheel is on the ground. Then, as they are moving forward in the straight portion, the back tire makes contact and it looks like only one track. The stunt cyclist likely new exactly what he was doing."