The energy barrier for nucleation on the basal plane of ice has a striking manifestation when the basal orientation is present in a grain boundary groove. As the ice-water interface containing the groove advances, large planar facets grow out of the grain boundary. The facets exhibit dramatic hysteresis, and they can be up to 30 times larger than predicted by equilibrium models. Measurements of the growth direction of the facets yield insight into the nature of the nucleation process. The facets also provide a way to study the relaxation of twist along grain boundaries.