I think those are probably flight similator contrails since the altitude doesn't seem to be high enough for contrails to form. Plus, a Cessna Skylane probably wouldn't be able to fly over them if they were contrails.
I see Walter visits one of the same sites I do. They're called Morning Glory clouds, you can find the original here, with a bit of explanation, but nobody really understands quite how they form.
I've never heard of morning glory clouds. The closest thing I've seen is the contrail feature that can be added to your microsoft flight similator. You can make them big or small and they are very realistic.