A landscape characterized by irregular hummocky topography and interconnected linear ridges, caused by the irregular heaving and melting of ground ice under periglacial conditions (Costard and Baker (2001) Geomorph. 37, 3–4). The exact form of the depressions depends on the original distribution of ice segregations, the subsurface movement of water during warmer periods, and the presence or absence of water in the hollows, since water-filled hollows tend to perpetuate themselves. (The term ‘karst’ is used to indicate the numerous features formed by subsidence, and does not imply the presence or development of a limestone landscape.) See Kokelj and Lantz (2007) Annual Conf. Geol. Ass. Canada on thermokarst slumps and global warming, and Seppälä (1997) Geomorph. 20, 3–4 on piping in thermokarst.