peer reviewed ; This article summarizes current knowledge about the viviers of the Hautes Fagnes Plateau, Belgium. They are best explained as remnants of isolated, elongate lithalsas and lithalsa plateaus. The ice that constituted them was segregation ice formed not only within the permafrost, but also above it (aggradational ice). The lithalsas formed in discontinuous permafrost. The lack of peat during growth implies an annual average temperature of -5 to -6°C in accordance with a model by Seppälä. Data are summarized from nine cuttings made through vivier ramparts. The stratigraphic evidence, including 14C and the study of volcanic dust and pollen, indicates that these periglacial mounds formed during the Younger Dryas. Forms identical with the viviers of the Hautes Fagnes have been observed in northern subarctic Québec, Canada. They result from melting of lithalsas which exist in this area, where summers are cool and permafrost is discontinuous (annual average temperature between -4 and -6°C; temperature of the warmest month between +9 and +11.5°C). Because such conditions exist in few regions, lithalsas are rare nowadays and are only known in northern Québec and Lapland. Traces of lithalsas that appeared during the Younger Dryas are known only from Ireland, Wales and the Hautes Fagnes. Similar but older forms exist in eastern England and some Holocene forms exist in Lapland. The distribution of the features that appeared during the Younger Dryas fits with the palaeoclimatical reconstruction of Isarin. This permits one to understand why these forms exist only in the British Isles and the Belgian Ardennes, but nowhere else.