Ice wedge

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, 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. The ice wedge usually appears in a polygonal pattern known as ice wedge polygons. The cracks can also be filled with materials other than ice, especially sand, then it's called sand wedge.