Reconstruction of former permafrost is based on specific indicators which represent relics of features developed above or within the permafrost. Their interpretation in terms of genesis and (palaeo)climatic significance, however, is in some cases still under debate. Previously the reconstruction of the periglacial environment in Europe was limited to permafrost mapping during the coldest time of the last glaciation. Today a much more refined evolution of the palaeoclimate of the last glacial has been established, thanks to a considerable increase in registered periglacial indicators, a better understanding of the corresponding climate conditions, and more accurate and reliable dating of the periglacial phenomena.

There is no general agreement about the meaning of the word ‘palsa’. Usage and recent suggested definitions indicate that the word is chiefly used for cryogenic mounds covered by peat that were formed by an accumulation of segregation ice in the discontinuous permafrost zone. Lithalsas are similar mounds, but without any peat cover. The thickness of aggradation ice on the top of lithalsas can be considerable. Use of development and decay palsas as indicators of climatic change is difficult. The climatic conditions in which lithalsas form are much more restricted than those for palsas and, as a consequence, regions where lithalsas exist are rather rare. After melting, lithalsas leave ramparted depressions; the mass movements on the peaty slopes of palsas are less propitious to the formation of ramparts. Some of the pingo remnants described in western Europe are, more accurately, lithalsa traces.

In this paper, the authors present the results of both macroscopic and microscopic investigations on structure development created by repeated ice lensing in various loamy experiments. Experimental data are compared with observations performed on active forms in High Arctic and Alpine Mountain environments. Those observations are also compared with phenomena observed in fossil periglacial formations of Western Europe.Platy and short prismatic structure formation is bonded to the hydraulic and thermal conditions during ice segr%ation. When a long series of alternating freezing and thawing affects platy structures, the fabric evolves, also being influenced by slope and drainage conditions: cryoturbations, frostcreep, and gelifluction can appear. They are characterized by specific microfabrics which are better developed with an increasing number of cycles: this is clear in experiments where hydraulic and thermal parameters are better controlled.Vesicles are also a prominent characteristic of the surface horizon in experiments and arctic soils. The genesis of vesicles is discussed on the basis of new observations and is related to the mechanical collapse of frost-created aggregates under the mechanical work of soil air escape during soil saturation by water at thaw.

A series of palaeolandslides is described in eastern Belgium, with geophysical investigations, trench analysis and a number of 14 C dates complementing the field description. The proposed sliding mechanism stresses the importance of initial liquefaction of the upper Cretaceous Aachen sands underlying the~30-m-thick Vaals Clay Formation, in which all landslides are developed. Slope-stability analyses support the hypothesis of sudden landsliding and yield further useful information to discuss their origin. A main difficulty in determining the landslide trigger arises from insufficient absolute dates. Assuming that all landslides occurred simultaneously, we weigh the probabilities, respectively, of a climatic and a seismic trigger. Although the 150 a.d. possible date of landslide initiation falls close to one of the wettest periods of the Holocene, the spatial distribution of the deep slides and their proposed mechanism strongly suggest a seismic origin in connection with a rupture of the nearby Ostend segment of the active Hockai fault zone. However, considering that the nearby 1692, Verviers earthquake apparently caused no ground failure, it is probable that both the climatic and seismic triggers were jointly needed to provoke such deep and extended landsliding on moderate, generally stable slopes.