Pack ice and grounded icebergs
 - Antarctica icebergs

This kind of ice looks smooth from a distance, but is very difficult and slow going to traverse. It is made up of last years pack ice (frozen sea-ice) that broke up, partially melted and became far from a smooth surface during the summer. Now, in the winter, the random shaped, uneven pieces have been cemented together by frozen sea water in between. The result is a very rugged surface where the quickest way to travel is by foot, vehicles or skis require so much continuous stop-start and change of direction that they actually slow you down. 

The long pole carried by the figure in the picture is known as a "bog chisel". It is a 6ft pole with a 3" wide unsharpened chisel blade on the end used to test the strength of sea-ice. If it takes three or more hefty "thwacks" to get through the ice to the sea beneath, then it is safe to walk on. If not, then it is a case of very carefully and slowly re-trace your steps to the last place where it was safe.