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 ...
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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.