Ice wedge

Ice wedges in Sprengisandur, Iceland
Lakes in the Mackenzie delta. In the foreground, a drained lake shows large, low-centered ice-wedge polygons
Peninsula at the coast of the Arctic Ocean in the Mackenzie Delta area showing well developed ice-wedge polygons. A Caribou herd is grazing on it.
A melting pingo with surrounding ice wedge polygons near Tuktoyaktuk, Canada
Ice wedge exposed by erosion along the Beaufort Sea coast, Canada. The wedge formed by thermal contraction of the ground which opened a crack in winter. The crack filled with meltwater in the spring which then froze in the permafrost, causing the thin vertical lines of ice and sediment that form the wedge itself.

An ice wedge is a crack in the ground formed by a narrow or thin piece of ice that measures up to 3–4 meters in length at ground level and extends downwards into the ground up to several meters. During the winter months, the water in the ground freezes and expands. Once temperatures reach −17 degrees Celsius or lower,[citation needed] the ice that has already formed acts like a solid and expands to form cracks in the surface known as ice wedges. As this process continues over many years ice wedges can grow, up to the size of a swimming pool.[citation needed] Ice wedges usually appear in a polygonal pattern known as ice wedge polygons. The cracks can also be filled with materials other than ice, especially sand, and are then called sand wedges.