The periglacial ramparted depressions of the hautes fagnes plateau: Traces of late weichselian lithalsas

Abstract

Once thought to be of human origin, ramparted circular or elongate depressions that densely cover many, but not all, summit surfaces of the Hautes Fagnes area, NE Ardenne, have been recognized since the 1950s to represent relics of periglacial mounds. We examine first the regional distribution of these periglacial ramparted depressions (PRDs), whose location seems controlled mainly by elevation and lithologic factors. Geomorphological characteristics of the PRDs and trenching data obtained from several ramparts consistently show that these landforms initially developed as lithalsas through accumulation of segregation ice in the mineral soil, whose later thaw produced the presently observed ramparted depressions. 14C dating of, and pollen data from, peat layers pre- and postdating the formation of the periglacial mounds, further supported by the presence of the 12.9-ka-old Laacher See tephra in the underlying peat, point to a period of rapid formation during the second half of the Younger Dryas. Mean annual air temperature and mean temperature of July derived for the Hautes Fagnes area (~600 m asl) during the Younger Dryas from a compilation of NW European pollen data are in excellent agreement with the temperature values observed in the western Labrador Peninsula where lithalsas are currently active, showing that PRDs are valuable paleoclimate indicators.