IS THERE A FINITE NUMBER OF SANDSTONE WEATHERING PATTERNS? EVIDENCE FROM THE HUMID TEMPERATE ZONE OF EUROPE

The rate and the periodicity of pore water evaporation from the near-surface zone of sandstone is the most important external factor, if not the only one, affecting the complexity of weathering forms on vertical cliff faces. This sole factor depends on a number of site-dependent variables (microclimate, aspect, height above cliff base, height above local base level, vegetation, etc.). The intrinsic properties of sandstone, however, have been frequently overlooked. It can be demonstrated that grain size, sorting, cementation (incl. cement mineralogy), porosity, sedimentary structures, tectonic deformation and other properties determine the geometries of the individual weathering forms and the weathering pattern as a whole.

Sandstone weathering forms on varied lithologies across Europe were documented in Fontainebleau Forest (France), Petit Suisse (Luxembourg/Germany), Pfälzer Wald (Germany), in the Bohemian Cretaceous Basin (Germany/Czech Republic) and in flysch sediments of the Carpathians. The observed recurrent weathering patterns justified the erection of eleven sandstone weathering morphofacies. Each of the morphofacies is dominated by a specific microform or a set of microforms: smooth surfaces, symmetrical cavities, accentuated sedimentary structures, spherical honeycombs, thick rock crusts, thin rock crusts with case-hardening function, stenomorphic honeycombs, polygonal cracks, siliceous karst forms, and heterolithic facies with concave forms and that with convex forms. Type localities were selected for each morphofacies and characterized as for the clast and cement composition, sedimentary structures, jointing and faulting, composition of speleothems and salt efflorescences, and pore size distribution.

Based on the information gathered, the contribution of each of the three main agents shaping the sandstone relief (physical weathering, salt weathering/case hardening, and chemical dissolution) can be quantified for each morphofacies. Application of the morphofacies concept allows 1) a prediction of weathering patterns to develop on sandstone of a given lithology, 2) determination of the effect of subtle variations in extrinsic factors on sandstone relief at an outcrop scale. This study was funded by project No. IAA300130806 of the Grant Agency AS CR.