http://en.wikipedia.org/wiki/Pressure_ridge_(ice) "A pressure ridge develops in an ice cover as a result of a stress regime established within the plane of the ice. Within sea ice expanses, pressure ridges originate from the interaction between floes"
"... By the time the Polar Pack has joined it, the ice-field presents a formidable barrier to ships. It is not simply a flat coating of ice on the surface of the ocean. Far from it. Every year on the journey south strong north-east winds drive the ice-field tight to the land; when the pressure becomes too great, huge pans of ice buckle, crack, and rear out of the sea, jamming against one another to form 'pressure ridges' like small mountain ranges. For miles the ocean is loud with the crackling and roaring of ice pans rafting on other ice pans. Ice-floes slowly turn turtle and are nipped by other floes and held in that position. It is with this constantly moving, splitting, wheeling, cracking, roaring jagged mass of ice that the sealing skippers had to contend, working their way slowly towards the seals as openings appeared, allowing ships to remain jammed for days at a time when the ice was tight, always taking their chances on being crushed and sunk." --Cassie Brown with Harold Horwood, Death on the Ice: The Great Newfoundland Sealing Disaster of 1914, Doubleday Canada, 1972.