Aeolian activity differentiation in the area of Poland during the period 20-8 ka BP

Abstract

Three regions, which differ in the effects of aeolian processes in the period 20-8 ka BP, have been distinguished. In the southern region accumulation of loess up to about 15-12 ka BP took place. Locally, at the margin of patches, loess became superimposed with younger sand cover. The middle and northern regions are areas of accumulation of aeolian sand deposits. This regions belong to the European "sand bełt", but the former is within the limit of the Vistulian glaciation while the latter beyond this limit. In the middle region three stages of aeolian activity have been recognized. The first stage (20-15 ka BP) is marked by intense deflation, formation of aeolian pavements, aeolisation of surface material, and incorporation of wind born grains into fluvial deposits and into sands that infill ice wedges. In the second stage (14-10 ka BP) accumulation of aeolian deposits, which gave rise to sand covers and dunes, occurred. In the Oldest Dryas widespread sandy-dusty covers and some dune hillocks were produced, in the Older Dryas accumulation of covers went on and huge dunes developed, in the Younger Dryas merely transformation of dunes occurred and small sand lenses were accumulated on the dune slopes and in the immediate neighbourhood . In the third stage, during the older Holocene the pedogenetic processes interrupted with episodes of very little accumulation on the dune slopes followed.

The breaks in aeolian activity are expressed as the fossil soils in dunes. Initial soil dates back to the Bølling, weakly developed podzolic soil or pedolith - to the Allerød, while well-developed rusty soil – to the Preboreal. From the Boreal Phase the process of podzolization began.

The northern region was exposed to wind action after retreat of the Vistulian ice sheet. At first pavements and sandy infillings of ice wedges were produced, afterwards during the Older and Younger Dryas sand covers and dunes originated with the Allerød fossil soil inside.

The northern region reveals no Oldest Dryas aeolian sands but loess of this age is present. As opposed to the middle region, there was here more abundant accumulation of sand in the Younger Dryas than in the Older Dryas period. Less frost cracks in aeolian deposits were found in the North. Appreciable differences in abrasion of aeolian grains have been observed. In the middle region wind abrasion persisted throughout the Vistulian, and was extremely intensive during the Upper Plenivistulian. This resulted in aeolo-fluvial sediments which became the source of the Late Vistulian aeolian sands. In the northern region wind abrasion began after the last ice sheet had retreated and aeolian sands originated from unchanged glacial material. Distinct aeolian abrasion of dune and cover sands in the middle region is inherited from the Plenivistulian processes and the Late Vistulian aeolian cycle introduced the slight changes, if any.

Differentiation in the effects of aeolian processes followed from the variation of environment. Over the 20-15 ka BP northern Poland was covered with the ice sheet. In middle Poland the cold desert existed. In southern Poland the tundra dominated. It was the period of intense deflation in middle Poland and of loess accumulation in the southern Polish uplands. About 15 ka BP, in the lowlands of middle Poland the discontinuous tundra vegetation appeared, which favoured the formation of sandy-dusty covers. From ca. 12.5 ka BP advancing trees gave rise to the dune building. The fixing of dunes occurred during the Allerød when the forest spread over their surfaces, and in the Preboreal Phase when the Holocene period of pedogenesis began.