Large masses of ice growing in thermal contraction cracks in permafrost. In periglacial conditions, alternating freeze and thaw can lead to the formation of vertical, narrow, and deep wedges of ice in gravels. After melting, these tend to fill with sediment, forming a cast of the ice wedge seen as dark bands, easily confused with manmade features, in aerial photographs. Casts of fossil ice wedges are one of the few true indicators of former permafrost conditions. Fossil ice-wedges of this kind are seen in many sections of sand and gravel deposits in Europe. They have been used to reconstruct the extent of the periglacial zone which developed around the Devensian and Weichselian ice-sheets.