PERIGLACIAL LANDFORMS AND PROCESSES

The field of periglacial geomorphology: the landforms and processes found in cold but essentially nonglacial environments that range from humid and relatively mild (e.g. Svalbard) to extremely cold and arid (e.g. antarctic). The distinctiveness of many landforms owes much to the processes involved. These include the formation of permafrost and permafrost degradation, including the development of thermal contraction cracking, the thawing of permafrost (thermokarst), the creep of ice-rich permafrost and the formation of various forms of ground ice (e.g. wedge ice and intrusive ice. Other processes, not necessarily restricted to permafrost environments, are important because of their high magnitude or frequency. These relate to freeze-thaw processes that affect soil and bedrock, including the disintegration of exposed rock by frost weathering or other poorly understood cryogenic weathering processes and, in soils, frost heave and ice segregation (see segregation ice).

The most distinct periglacial landforms are associated with permafrost. Most widespread are ice-wedge polygons, 15–30 m in diameter, formed by the thermal contraction cracking of the frozen ground in winter. The cracks may fill with water in early summer and after a number of years ice wedges may form. Less commonly, the cracks may fill with sand. Far less widespread, but equally distinctive, are various perennial ice-cored frost mounds. The largest of these are pingos, which sometimes exceed 40 m in height. Other aggradational permafrost landforms are associated with the preferential growth of segregated ice lenses: these include palsas (formed in peat), lithalsas (palsa-like features formed in mineral soil), and peat plateaux, 1–8 m in height. Ground-ice slumps, thaw lakes and depressions (e.g. alases) and thermokarst mounds (baydjarackii) result from the degradation of ice-rich permafrost.

In relatively unconsolidated sedimentary rock, the in-situ creep of permafrost may cause nondiastrophic structures. These include up-arching beneath valley bottoms (valley bulging) and the bending, deformation and sliding of strata on slopes, leading to cambering and joint widening, or gull formation. Within the active layer, local conditions of soil moisture saturation and high pore water pressures can induce rapid shallow movements confined to the active layer, with the top of permafrost acting as a lubricated slip plane for movement, thereby controlling the depth of the failure plane. These failures, generally referred to as active-layer detachments, are frequent in the summer months on terrain underlain by ice-rich and unconsolidated shales and siltstones. In most cases, failure is initiated when the liquid limit is exceeded; this happens during years of rapid spring thaw and/or following periods of unusually heavy summer precipitation.

There are also periglacial landforms related to the cryogenic weathering of exposed bedrock. Coarse, angular rock-rubble, commonly termed blockfields in North America and Europe, and kurums in Siberia, occurs widely over large areas of the high arctic polar deserts and semi-deserts, and in the high alpine belts of some alpine zones. They surround outcrops of more resistant rock that form isolated hills or tors. In regions of extreme aridity, rectilinear, debris-veneered bedrock-controlled slopes (Richter denudation slopes) may evolve.

In upland areas, below bedrock cliffs, frost-shattered talus may accumulate as sheets and cones. Modification of talus may occur through snow-avalanche activity, debris flows, rock glacier development and pronival rampart formation.

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