months, the wedge does not split and therefore in the summer no new water is added. Casts In areas of past permafrost, ice wedges have melted and are no longer filled with ice. The wedge, which is now empty, is filled with sediment and dirt from the surrounding walls. These are called ice wedge casts and can be used to estimate the climate of hundreds of thousands of years ago. Types Ice wedges have historically been classified into two main categories: epigenetic and syngenetic. The type of an ice wedges refer to the way in which the wedge grows over time. Relatively recent field studies have revealed a new type of ice wedge: anti-syngenetic. Currently, these are the only three distinctly classified types of ice wedges observed in nature. The differences among the growth patterns of epigenetic, syngenetic and anti-syngenetic wedges then depend upon ground surface conditions, i.e. whether the ground level remains essentially unchanged or whether there is addition or loss of material. Epigenetic ‘Epigenetic’ in geological terms refers to geological features that formed after the formation of the surrounding materials. In relation to ice wedges, this means that epigenetic ice wedges are found to form in preexisting permafrost, and are not formed at the same time as the permafrost meaning they are much younger than the surrounding material. An ice wedge being epigenetic also refers to the fashion in which they grow. These types of ice wedges grow considerably wider over their lifetime, but rarely any deeper or taller. This means that epigenetic ice wedges can grow to at most 3–5 meters in width, but stay roughly the same depth/height as when they had formed. Typically, the sediments on either side of an epigenetic ice wedge, adjacent to the edges of the wedge exhibit upward bending. In an epigenetic ice wedge, the age of the ice on the periphery changes little from top to bottom. Syngenetic Syngenetic ice wedges, like the epigenetic variety, find their roots in the geological term syngenetic, which means forming at the same time as the surrounding material. This is because syngenetic ice wedges grow as the upper permafrost surface rises in response to the addition of material on the ground surface. This allows syngenetic ice wedges to grow very deep, as the surface around them rises with the accumulation of alluvium (in floodplains), peat (in tundra), and gelifluction deposits (at the bottom of