Air bubbles trapped in ice as the water froze over.
I was mostly intrigued by the patterns which were created by the ice in the wheelbarrow. Unfortunately I could not get close enough for detailed photos, and the this picture gives a rough idea from a distance.
Was great to see it growing in thickness. And then the winds changed to westerns again, so the frost was send onto a low burner until the next dip.
And from this
into this: