Hardened fracture surfaces in quartzose sandstone and their role in origin of rock cities


Jiří Bruthans, Jan Soukup, Daniel Světlík, Jana Schweigstillová, L. Mayo

Geoscience Research Reports 46, 2013 (GRR for 2012), pages 109–115

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Rock cities in the Czech Republic are composed of numerous fracture-guided spaces (clefts) and narrow gorges. It is generally considered by geologists and geomorphologists that fractures are zones of weakness, where erosion is most efficient. Rock cities are believed to be created by downward erosion along densely fractured zones, possibly weakened by deep weathering. Interestingly the vertical fracture surfaces in Hrubá Skála sandstone in Střelec quarry is significantly firmer and has lower erodibility than the sandstone in fracture subsurface, based on tensile strength, relative erodibility and drilling resistance measurements and field observations. Based on erodibility measurements inside subsurface conduits in the quarry the hardened fracture surfaces occurred before the sandstone was exposed at ground surface. Hardening is probably of tectonic origin (deformation bands or similar feature). Hardened fracture surfaces should be considered as geomorphologicaly important type of surface hardening beside well-known rock crusts (case hardening). Based on field observations, fracture surfaces in rock cities were less erodible than underlying sandstone in time when landscape was most intensively evolving (probably during or at the end of Last Glacial). Model of evolution of rock cities is proposed based on great morphological similarity between presently evolving fracture-guided conduits in Střelec quarry and clefts in Adršpach-Teplice area. Clefts were probably developed in two steps: 1. undercutting of weak sandstone layers (undercutting horizons) by piping and frost weathering; 2. undercut vertical slabs of sandstone were mass wasted and underground spaces propagated upward forming vertically elongated cleft caves or open clefts. While first process was critical to initiate the mass wasting it is the second process, which mobilized more than 90 % of material even from cemented portions of sandstone.