The origin of sandstone boulder aprons along the escarpments of the Stołowe Mountains: are they all rockfall-derived? A new insight into an old problem using the CONEFALL 1.0 software

A characteristic feature of sandstone-capped escarpments in the Stołowe Mountains is the occurrence of extensive boulder mantles which extend from the rock face to the footslope over distances of 300–500 m. The hypothesis implying rockfall as the process of boulder release from the rock face and subsequent transport is tested by means of Conefall 1.0 software, designed to simulate run-out zones of rockfalls. The actual extent of boulder mantles is much larger than the simulated extent, which casts doubt on the applicability of rockfall scenario. Alternative hypotheses are briefly discussed and it is concluded that a similar morphological effect can be achieved by in situ caprock disintegration and sub-caprock slope lowering and retreat

Deciphering the history of forest disturbance and its effects on landforms and soils : lessons from a pit-and-mound locality at Rogowa Kopa, Sudetes, SW Poland

The historical dimension of pit-and-mound topography has been studied at Mt Rogowa Kopa locality, Stołowe Mountains, SW Poland. This site represents one of the best developed regional examples of hummocky forest floor relief due to widespread tree uprooting and subsequent degradation of root plates. Through map analysis and dendrochronology the disturbance history was traced at least to the 1930s and most likely a strong wind episode from 1933 was the reason of forest calamity that resulted in nearly complete destruction of the original stand. However, the forest affected was a planted Norway spruce monoculture, introduced and managed at least till the beginning of 20th century, not a natural forest. The windthrow niche was then used by beech whose individuals preferentially chose mounds to grow, conserving hummocky microtopography. Changes in soil evolutionary pathways brought about by wind-driven disturbance include both homogenization (rejuvenation) and horizonation (differentiation). Evidence of soil rejuvenation includes decrease of organic carbon content and pH increase in the upper parts of soils developed on mounds in comparison with undisturbed references soils. Soil texture was relatively homogenized in pits and mounds. Dating of pit-and-mound microrelief by means of soil properties (organic carbon content, iron forms) was only partly successful. Although young age of pits and mounds is evident, the actual age inferred from soil properties was underestimated by a few tens of years. Evaluation of factors potentially controlling the propensity to widespread treethrow suggests that the type of forest is a far more important variable than local abiotic factors of bedrock geology, regolith characteristics, and slope inclination

Evidence for subsurface origin of boulder caves, roofed slots and boulder-filled canyons (Broumov Highland, Czechia)

The backslope of a sandstone cuesta in the Broumov Highland (Czechia) is cut by a complex network of canyons. Long sections of canyons have thick boulder fills which are difficult to reconcile with simple rock fall and talus development scenario. Boulder caves occur within these fills and their lowermost parts are drained by streams that evacuate fine loose sandy material produced by weathering and mechanical erosion. These boulder fills are explained as largely in situ, residual features, left after subsurface selective disintegration of rock mass, mainly joint-guided, and removal of grains by underground water. Evacuation of fines leaves voids into which overlying blocks subside but the voids may be spacious enough (largely because the rock blocks are so large) to connect into penetrable caves. Results of an analogous process may be observed along canyon walls where more densely jointed sandstone compartments develop into roofed slots. Additional evidence for an important role of subsurface processes is provided by closed depressions in the inter-canyon areas. A model of canyon evolution is offered, alternative to the existing models implying surface fluvial erosion or retreat due to spring sapping. Its applicability is limited by the thickness of a sandstone package but the origin of canyons a few tens of metres deep (up to 100 m or so) may be explained