Wang Baolai, H. M. French
Thermal contraction cracking, which is caused by the response of ground surface to the tensile stress generated by a low temperature and a rapid fall in temperature, is thought to be the mechanism by which frost fissures develop. These include active-layer ground wedges, frost-fissures of both primary and secondary infillings. Ice-wedge pseudomorphs are formed by the transformation of ice wedges when permafrost degrades. They are good indicators, therefore, of the former existence of permafrost. However, they do not have simple relationship with temperature.The following points have to be borne in mind when any paleoenvironmental reconstruction is attempted. First, soil wedge and ice wedge respond differently to the thawing of permafrost. The appearances of ice-wedge pseudomorphs vary, depending on the properties of primary infilling and the host soil types. A soil wedge may be easily preserved in both shape and size. In contrast, because of the high ice content of ice wedges and the considerable deformation on melting, structures left by the melting of ice wedges may not be recognized as ice-wedge pseudomorphs, and might be termed involutions or cryoturbations. Thus, not all ice wedges will have a recognizable pseudomorphs upon thawing. Second, the occurrence of frost fissures and their pseudomorphs is associated with a number of variables in nature. Both environmental conditions and site-specific factors should be taken into account in the paleoenvironmental reconstruction. Third, Non-cryogenic features, which resemble frost fissure pseudomorphs have been reported from regions where ice-wedge pseudomorphs were also reported. Such features have been used to reconstruct paleoenvironments. Clearly, it is necessary to reevaluate some paleoenvironmental reconstructions that have been done.
