Abstrakt
Ice and sand wedges form due to repeated thermal-contraction cracking at low temperatures and filling of the cracks with ice and sand, respectively. They tend to organize into extensive polygonal networks, which are typical for polar regions with mean annual air temperature lower than -8 °C to -4 °C and at least discontinuous permafrost.
However, their relict counterparts are also present over large areas of mid-latitudes that were underlain by permafrost in the past. These ice-wedge pseudomorphs and sand-wedge casts, which can be found as wedge-like structures in vertical sections or as polygonal crop marks on aerial images, have been previously mapped in many European countries (e.g.
Belgium, France, Germany, Hungary or Poland) because they bring an important evidence about the evolution of permafrost aggradation/degradation in the past and its environmental impacts. Nonetheless, their evidence from the Czech Republic has been largely lacking or fragmentary.
This contribution therefore presents the preliminary analysis of the distribution and morphology of the Late Pleistocene ice-wedge pseudomorphs and sand-wedge casts in the Czech Republic. Mapping of ice-wedge pseudomorphs and sand-wedge casts was based on orthophotos from 2001, 2003, 2006, 2009-2010 and 2012 provided by GEODIS and TopGis and was supplemented and validated by field investigations in sandpits, quarries or technical excavations, geophysical surveying (ground penetrating radar and electrical resistivity tomography) and published geological reports.
We mapped over 700 locations where ice-wedge pseudomorphs and sand-wedge casts occur in polygonal networks or individually, which indicates that permafrost was very widespread in the Czech Republic during the Late Pleistocene. The patterns are particularly concentrated in the lowlands of the Bohemian Cretaceous Basin and the Moravian Basins and are usually embedded in sandy or gravely materials of river terraces or in silty loess covers.
Most of over 4000 polygons we have investigated in detail are pentagonal and hexagonal and their size ranges between 2 m and 32 m. Our current work aims to further extend the database based on the latest sets of orthophotos from 2014-2015 and 2016-2017 and field loggings.
We also collect samples from the ice-wedge pseudomorphs and sand-wedge casts as well as the host materials in order to determine their sedimentary properties and absolute chronology based on numerical dating. This research is supported by the Czech Science Foundation, project number 17-21612S.