Upfreezing of stones in boulder clay of Central and North Poland

Abstract

The inclination and orientation of the long axes of clasts were examined in soil horizons developed from weathered till. These studies were conducted in central Poland in the tills of the Warta Stadial, and in northern Poland in the tills of three stadials of the last glaciation.

The measurement results show that the distribution of clasts differs between the upper and lower parts of the profiles. In the lower part, their arrangement is a result of glacial processes. In the upper parts, the distribution is characterized by a higher average inclination of the clasts and a greater dispersion of the main axis directions, which is an effect of repeated freezing and thawing.

A decrease in the number of clasts was observed in the central part of the frost-affected zone, and an increase in their quantity in the near-surface layer. The thermal conditions responsible for clast movements were variable. The number of freeze-thaw cycles in the periglacial environment did not exceed 50 per year in the near-surface soil layer; below a depth of 10–50 cm, it was limited to one cycle per year. The thickness of the active layer ranged from 1.5 to 2.0 meters. During the onset and final phases of the last cold period, the layer subjected to annual freezing and thawing may have been thicker. In the Holocene, the maximum number of freeze-thaw cycles in the upper part of the soil, down to 50 cm depth, was around 20 per year, and the annual frost penetration in clay soils did not exceed 1 meter.

Zones with highly advanced deformation of clast arrangement – up to 1.5 m thick, with an average inclination above 45°, and more than 70% of clasts inclined at over 30° – correspond to areas covered by ice during the Warta Stadial. The position of clasts shaped during the last cold period was only slightly altered in the Holocene.

Areas of northern Poland that were covered by the ice sheet during the earlier phases of the last glaciation show less intense clast reorientation down to depths of 70-90 cm. These changes occurred at the end of the Pleistocene and during the Holocene. In the northernmost regions of Poland, clast reorientation was observed at depths of less than 35 cm and was associated with Holocene processes.

The movements responsible for changes in clast arrangement were divided into three components: vertical upward movement, responsible for changes in the number of clasts within the profile; rotation around the long axis, affecting inclination changes; rotation around the short axis, causing increased dispersion in clast orientation.

The conducted studies show that the rate of clast rearrangement and their upward migration in till is not significant. Changes in the zone affected by annual freeze-thaw cycles become detectable with the applied method only after 17,000-19,000 years of activity.