They call it a "wheel." See http://www.borealbirds.org/birdguide/BD0391_species.shtml. Yes, it's for feeding.

I've seen wheels with as many as 80 shovelers and as few as 4. The ones I've seen have all been circling counter clockwise. I'm pleased to hear that the wheel you saw was moving clockwise, as I had difficulty believing they would only circle in one direction.

I saw a small wheel on our Christmas Bird Count last month. Several Mallards had caught onto the game and were hanging at the edge of the wheel, facing it more or less directly, and were sharing in the stirred-up food.

Dave Jackson
Sequim
djackson at wavecable.com