Structure-controlled and dissolution-facilitated? Towards a more complex understanding of the genesis and environmental controls of sandstone ruiniform relief, Stołowe Mountains tableland, SW Poland

Despite recent major advances, complex factors contributing to the development of sandstone ruiniform relief still require recognition, particularly with respect to environmental controls and the character of the weathering processes responsible for the formation of corridors. The discovery of a small cave and roofed slots in one of ruiniform terrains in the Stołowe Mountains tableland, SW Poland – with active weathering and erosional processes – allowed us to tackle the problem. Our study confirmed earlier claims that the formation of corridors might lead through the speleogenesis. We showed that the process is geologically- and environmentally-controlled and is facilitated by cement dissolution. Thick sandstone beds of subhorizontal alignment, well-developed vertical jointing, high porosity and coarse grain size account for preferential groundwater drainage and the susceptibility to erosional removal of mass. The dominance of quartz and kaolinite in mineral composition, together with the present-day temperate climate and the dense vegetation cover, result in the development of permeable and very acidic soils upon the plateau surface. Such soils support efficient rainwater percolation, produce acidic solutions and create room for fungi dominance among soil microorganisms. The development of corridors of ruiniform relief is associated with structure-controlled topographic lows upon the plateau surface, which act as sinks. There, the throughput of acidic solutions, enriched with microorganisms, is the highest and over prolonged period of time causes dissolution of clay and quartz cement. Since its content is small, the dissolution results in the reduction of rock cohesion and the susceptibility to grain-by-grain detachment. Yet, dissolution is only a facilitator, with various mechanical processes (frost weathering, wetting-weakening, dilation) earlier recognized taking part in sandstone deterioration and upward propagation of voids, until the emergence of an unroofed corridor. The remaining bedrock compartments – hardened by protective surface crust – are also subject to dissolution, but at slower pace.