Abstract:

The origin of loess covers of southern Poland is connected to the Pleistocene glaciations. The particular environment of this period, characterized by cold climate, intense erosion and scarce vegetation, left various relict structures, some of them preserved in loess. Frost processes, geli- and solifluction, and thermokarst, are one of the most common processes of the periglacial period.
The largest structures associated with the periglacial environment are ice wedge pseudomorphs, often used as indicators of past permafrost extent. Their interpretation is dual, as they are marks of not only previous frost processes, but also of subsequent climate amelioration and permafrost termination.
Ice wedge pseudomorph morphology depends primarily on the type of sediment in which it is formed. Due to the potential high ground ice content, fine-grained materials like clay and silt can melt almost as long as the ice wedge itself. During melting, pressure increases significantly in the pores of the soil, often leading to dynamic filling of the post-ice wedge void. Another important factor is topography, as the position of the ice wedge on slope or below it conditions the growth of ice and its melting. Since ice wedge pseudomorphs are highly dependent on the particular site they are in, they make for a unique record of the changing climate.
Two ice wedge pseudomorphs were recently documented in Trzebnica Hills and Niemcza-Strzelin Hills (southwestern Poland). Their structure bears marks of thermokarst processes, and is likely the result of at least two stages of melting. The ice wedge pseudomorph in Trzebnica (Trzebnica Hills) is especially deformed by intense thermokarst, most likely as a result of water stagnation at the site. The second structure located in Henryków (Niemcza-Strzelin Hills) shows possible reactivation of frost processes near the end of its forming.
OSL and radiocarbon dating results point to final permafrost degradation after 15-16 ka, at the end of the Late Weichselian (MIS 2). Potential stages and processes of ice wedge pseudomorphs formation are discussed, as an attempt to interpret the complicated process of ice wedge melting and preservation.