A large-scale area of pack ice contains ice of various thicknesses from zero the many meters. As the area deforms, thin ice is ridged into thicker ice, in a way that depends on the strain rate and the instantaneous thickness distribution. By equating the plastic work to the production of gravitational potential energy and the frictional dissipation in this ridging process we relate the yield curve for plastic deformation of the ice pack to the way ice thicknesses are redistributed by ridging.