The Remnants of Lithalsas in Western Europe and the Climate During the Younger Dryas Time

Albert Pissart
Department of Physical Geography, University de Liege

The viviers of the Hautes Fagnes Plateau (Belgium) are depressions surrounded by ramparts with a high density per unit area. Formed during the Younger Dryas, the viviers were first interpreted as remnants of pingos but are now best explained as remnants of lithalsas (mineral palsas), i.e. mounds formed by accumulation of segregation ice mainly below the permafrost base but some above the permafrost table (aggradational ice). Such cryogenic mounds are uncommon in contemporary periglacial regions. They currently exist in northern Subarctic Quebec and Lapland, at the northern limit of the discontinuous permafrost zone (mean annual air temperature -4 to -6°C), but only if summers are cool (mean July air temperature +9 to +11.5°C). Because of this narrow temperature range, remnants of lithalsas in former periglacial regions have important palaeoclimatic significance. Such remnants of Younger Dryas age have been found only in Ireland, Wales and Belgium. The palaeoclimatological reconstruction of the Younger Dryas for Western Europe that was propounded in 1997 by Isarin fits perfectly with these locations.