Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Fast disintegration of Carboniferous sandstone in rock overhang Čertova Kazatelna in Plzeň

 

Jiří Bruthans, Jana Schweigstillová, Petr Bezdička, Jan Soukup

Geoscience Research Reports 45, 2012 (GRR for 2011), pages 70–74
Map sheets: Plzeň (12-33)

Full text (PDF, 1.53 MB)

 

Abstract

Radioisotope dating indicated that walls of the studied sandstone overhangs and caves in the Czech Republic have not retreated more than 1 mm/1000 years during the Holocene. In the town of Plzeň, the sandstone overhang of Čertova Kazatelna was studied where extreme retreat rates of overhang ceiling as large as ~40 mm/year were reported bya local resident. Sandstone matrix is composed of quartz, kaolinite, illite and K-feldspar based on XRD. Surface of the rock overhang is formed by exfoliation plates several mm to a few cm thick. Unlike its dry and stable surroundings the rock overhang surface is wetted by seeping wastewater. Fallen material from rock overhang was collected on plastic foil for a period of several hours to one day during different seasons of the year. Fallen material contains 0.1-2 % gypsum in bulk and 3-11 % gypsum in fine matrix based on leaching, XRD and SEM-EDX. Maximum measured deposition of material on plastic foil (214 g/m2/day) occurred during thaw. Wetting weakening, ice wedging and possibly salt weathering are probably responsible for sandstone disintegration. Based on mass balance of sulfates in fallen material we expect long-term retreat rates of sandstone overhang surface between 3 and 12 mm/year. In sharp contrast to this extreme retreat rate, the dry surface of sandstone did not retreat by more than a few mm/97 years based on dated carving. The study demonstrated that sandstone overhangs may potentially develop within a few hundred years in case of favorable conditions.