Cyclic Development and Thermokarstic Degradation of Palsas in the Mid‐Alpine Zone at Leirpullan, Dovrefjell, Southern Norway

The palsas of Leirpullan are developed in deltaic sands and silts in the zone of alpine permafrost at an altitude of 1437 m (estimated mean annual air temperature −2.7°C). Five mounds that represent a chronosequence are analysed with particular reference to initiation, aggradation and degradation phases. It is inferred that geoecological factors control initial mound formation beneath areas of thin (< 20 cm) moss‐peat, which provide summer insulation for developing permafrost. Low summer temperatures and low summer precipitation appear to allow the survival of permafrost, the existence of stable mounds, and limited height growth, after the gradual successional replacement of moss‐peat by fellfield vegetation growing directly on the mineral substrate. Degradation occurs owing to thermokarstic processes, which result in the formation of rim‐ridge ramparts. ‘Exogenous’ processes associated with lateral thermal erosion by lake and river water are important at the site. Regeneration and the possible effects of climatic change are also discussed. Palsas with only a thin peat cover in the initial stage of formation but no peat cover during subsequent development and degradation are considered to be near one extreme of a morphological and genetic continuum of pure peat palsas to mineral permafrost mounds. The rim‐ridge ramparts characteristic of the degraded palsas of Leirpullan appear to provide appropriate modern analogues for relict Pleistocene forms in Wales and elsewhere in Europe formerly interpreted as pingo scars. © 1997 John Wiley & Sons, Ltd.