Intenzivni rozpad karbonskeho piskovce ve skalnim previsu Certova kazatelna v Plzni

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.