A schematic model of frost heaving processes at the example of Litzelstetten – Krähenhorn, a UNESCO World Heritage Site from the Neolithic (Lake Constance, Southern Germany)
(1) Shallow water zone with lake beds composed of glacial clay and calcareous silt. The pile heads are embedded into mobile sediments. (2) lowering water tables expose the lake floor (3) At atmospheric temperatures < 0°C the lake soils, soaked by capillary rising water and tensioned water from the adjacent soils, freeze. At the freezing front develops an ice stratum (4) with permanent freezing temperatures the upper parts of the piles are caught in the frozen soil. The ice stratum, which receives water from below and from the sides, becomes thicker and grows towards the surface. Soil and piles heave. At the lower parts of the piles emerge hollow spaces, which are filled by sediments flowing in from the sides. Some piles are teared off. (5) With increasing temperatures and thawing of the ice layer the soils lower down to their original elevation. The piles, however, encounter resistance. Pile heads remain at the elevated level and are now protruding from the surface.
(First published: M. Mainberger, Fallstudie Krähenhorn. In H. Brem, B. Eberschweiler, A. Rudigier, H. Schlichtherle, G. Schröder (Hrsg.), Erosion und Denkmalschutz am Bodensee und Zürichsee.Ein internationales Projekt im Rahmen des Interreg IV-Programms "Alpenrhein-Bodensee-Hochrhein" zur Entwicklung von Handlungsoptionen zum Schutz des Kulturgutes unter Wasser (Bregenz 2013) 241 - 246.)
Figure: Pile heads surfacing at the UNESCO World Heritag Site Horn - Hörnle I. The site was excavated in the 1980ies and covered with a protecting layer of gravel. Piles were sampled several decimetres below today´s surface. The appearance of piles is therefore a strong hint to frost heave processes.
What consequences the ongoing global warming will have in the context of frost heave processes remains uncertain. The last decade has seen considerable changes in the hydrology, the vegetation, and the fauna of Lake Constance. These include variations and amplitude of water levels and the increase of extreme weather conditions. The prehistoric sites in the litoral are subject to an ongoing monitoring program of the
Landesamt für Denkmalpflege and management plans of the Informationszentrum Pfahlbauten Baden-Württemberg.
April 2022 - Martin Mainberger
Keywords: Frost heave; Frosthebung; Pfahlfeld; Neolithic; Bronze Age; Underwater Archaeology