Yes chords...swamps have gasses, and various factors that effect how they freeze up...I have seen where even in the coldest winters...some swamps remain wide open...however if you pack them down...they tend to freeze up tight....if you don't pack them, the snow will insulate it on the top, and they never really freeze down.....ever notice on a lake where the sleds have a trail packed...it's tight, and solid....but just off the trail you see slush??...same principle in swamps (except swamps are usually shallower, and can have gasses that effect ice making)