Fast-formed case hardened surfaces on exposures of Hrubá Skála sandstone: microscopic, tensile strength, erodability, origin, and significance for sandstone landscape


Jana Schweigstillová, Jiří Bruthans, Lukáš Falteisek, Jan Válek, Jan Soukup

Geoscience Research Reports 46, 2013 (GRR for 2012), pages 228–234
Map sheets: Sobotka (03-34)

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On quartz sandstone landscapes in the Czech Republic, the opal cement is generally believed to be a major factor responsible for increased resistance of case hardened surfaces, which protects sandstone exposures in many areas. However there is increasing evidence showing that opal is not the necessary constituent of the case hardened surfaces and actual measurements of strength and erodibility of sandstone surfaces were missing. In Střeleč Quarry and natural sandstone exposure Apolena (Bohemian Paradise) the tensile strength and relative erodibility was measured at case hardened surfaces and on bare sandstone and sandstone microstructure and composition of binding agents was studied by various methods. Case hardened surfaces consist of a 0.5-1 mm thick zone of sandstone where most of the pores become filled by kaolinite clay reinforced by lichen hyphae. Case hardened surfaces have on average 14 times higher tensile strength than bare sandstone and are resistant to erosion by overland flow. Under optimum conditions (sun irradiation, moisture abundance) the case hardened surfaces developed in quarry within just 6 years having the same tensile strength as the case hardened surfaces at natural exposures (tensile strength 20-180 kPa). Experiments with substance that decomposes organic matter strongly indicates that organic fibers are the major factor responsible for strongly reduced erodibility of case hardened surfaces. Surprising results of the study invoked the more detail study launched in other areas with quartz sandstone exposures in Bohemian Cretaceous Basin.