The development of vegetation in the Skaliska Basin has been reconstructed on the basis of palynological analysis and radiocarbon dating (AMS technique) of 6 sites from the late phase of the Bolling- Allerod interstadial complex to modern times. Although the area covers 90 km2, the mosaic character of habitats led to the development of different patterns of vegetation changes during the Late Glacial and Holocene. Only one site located in the eastern part of the Skaliska Basin reflected the ‘pine phase’ of Allerod, and this is the oldest data on vegetation in the Skaliska Basin. Interesting discrepancies were recorded during the Younger Dryas when patches of shrublands with Juniperus were distinct around some of the sites, while steppe with Artemisia was common in others. The beginning of the Holocene brought an expansion of birch-pine forest, but around 9600 cal. BC a cold oscillation took place which was reflected in an increase in birch in the woodlands in the western and eastern part of the Skaliska Basin. In the Preboreal chronozone elm (Ulmus) also expanded in the area but its appearance was non-synchronous. The vegetation of the Boreal chronozone was similar in the whole area and the most characteristic feature was the rapid expansion of hazel (Corylus avellana) which displaced Betula from the most of its sites. At that time a distinct redeposition of pollen material in the Parchatka river valley was detected which was probably the effect of an increase in fluvial activity of the river (humid oscillation). The following stage of vegetation development was climax woodlands with Tilia cordata, Ulmus, Quercus, Corylus avellana, and Alnus in damp places. At the beginning of the Subboreal chronozone the expansion of Quercus took place, which was subsequently replaced by Picea abies and partly Carpinus betulus. The pattern of Picea abies expansion distinctly presents two maxima which is characteristic of many sites in the north-eastern Poland. The Subatlantic chronozone is represented only by the profile from the Skaliski Forest, where, because of sandy ground, Pinus sylvestris was the dominant element. Human impact was poorly reflected through the rare occurrence of pollen grains of Cerealia type in the pollen profiles spanning the time from the Subboreal chronozone to modern times. In most profiles AMS dating produced age discrepancies, which limited the possibility of establishment of a detailed chronology. However, dates obtained from the material contaminated by mixture of glycerine, thymol and ethyl alcohol, pretreated by alcohol, showed reliable results in most cases.

Several potential Polish locations for the occurrence of fossil pingos were determined on the basis of analyses of a digital terrain model. Subsequent field reconnaissance connected with drilling into the geological structure, confirmed that one form located NW of Gdańsk, was indeed a fossil pingo. The aforementioned forms occur in a moraine plateau area related to the last ice-sheet retreat towards the Gardno phase moraine. This surface of the plateau is noticeably inclined south-north, at elevations of between 170 and 110 metres. It in fact proved possible to identify more than 80 very well-developed fossil pingos in the area investigated, with each found to consist of a central depression of average diameter 60‒80 m, as surrounded by a rampart 3–7 m high. By drilling into the central parts of the fossil pingos, we found them to be filled with organic sediments up to 6 or 7 m deep. The bottom layer of infill has carbonate and mineral-carbonate gyttjas up to 2 m thick. These are overlain by a peat layer up to 4 m thick, while these organic sediments are underlain by gley till sand. The ramparts are of sandy till frequently intercalated with silty sand. The established sequence of infilling of the central parts of the fossil pingos indicates that, in the immediate aftermath of ice-core melting, these played host to small ponds in which the accumulation of gyttja was able to take place. The gradual accumulation of lake-bottom sediments resulted in a shallowing of the ponds and the development of peat bogs. The morphological image of the above forms and initial drilling in the studied area suggest an association between their genesis and the presence of an ice-cored mound of the pingo type, experiencing subsequent degradation in the direction of the current, fossil pingo, form. Besides the classical, literal morphology of these forms, a decisive argument for acceptance of the above concept is provided by rampart lithology indicating how essential slow processes were in their accumulation. The nature and thickness of the organic infilling in the central part of a post-pingo prove equally important, suggesting an extended period of lake and peat-bog accumulation, probably lasting for the entire Holocene. The aforementioned arguments allow for the precluding of any origin linked with direct human activity (ground construction, bomb craters). The high density and close proximity and morphological similarity of the forms are likewise inimical to an identification as craters caused by above-ground meteorite explosions. Likewise, comparative analysis of the studied forms and kettle holes (usually larger irregularly-shaped larger forms of varied bottom topography) fails to indicate that the ring forms under study here have somehow arisen through the melting of buried dead ice. Analysis of deep boreholes made previously may support a geological structure of the analysed area consisting of a sand layer over 90 metres thick covered by a discontinuous till moraine several metres thick. The thick sand layer in question consists of differently-aged glaciofluvial sediments. This is a hydrogeological window connecting three main Quaternary aquifers and offering a perfect location for the ascension of groundwater. In conditions of developing discontinuous permafrost, this movement led to the creation of pingo forms in open systems on the surface. It is clear that investigation work is not currently at a stage allowing for about as to age to be made, or all details regarding evolution provided. However, the results of planned geomorphological, hydrogeological and geochronological studies should provide for both the recognition and detailed definition of the forms, thereby prompting discussion as to the evolution of permafrost during the late Weichselian transgression and recession in Central Europe.