Hi martini : ) the tracks are made by the seagulls... I guess the conditions were just right... a little slushy on top and then a freeze up... must have caught a bit of snow too... now it is all gone... and covered with geese waiting to go south...
might be 10 to 20 ft across... there were 6 of them all in this end of the lake...
there isn't any spring that I know of underneath... nothing like this last winter...
no idea...