Pressure Ridge in Sea Ice
A pressure ridge is formed where an ice sheet is broken and one piece rides over the other. The actual "pressure" is often provided by winds, storms or ocean currents that can be many miles distant acting on the edge of the ice sheet. So at the position of the pressure ridge itself, it can be eerily calm while huge forces are exerted through the ice. Such pressures are what threatened and destroyed some of the ships of the early polar explorers such as Shackleton's Endurance.
Photo; © Mike Usher - Pictures from an icebreaker cruise to Eastern Antarctica.