Wedge-shaped, foliated ice produced in permafrost, occurring as a vertical or inclined sheet, dike or vein tapering downward, and measuring form a few mm to as much as 6 metres wide and from 1 metre to > 30 metres high. It originates by the growth of hoar frost or by the freezing of water in a narrow crack or fissure produced by the thermal contraction of permafrost.