Gneissic tors in the central European upland: Complex Late Pleistocene forms?
Résumé
Bedrock outcrops punctuating regolith-covered surfaces in the summit/upper slope and hillside positions (tors
and crags) have long been a subject of inquiry in geomorphology for their evolutionary trajectories and now also
as valuable geoheritage sites. Methodological advances in research, such as DTM-based geomorphological
analysis and terrestrial cosmogenic nuclide inventories, create new perspectives to decipher the spatial and
temporal context of tor emergence and decay. The shape, structural controls, 10Be inventories and relation to
surrounding medium-scale landforms were studied at thirteen conspicuous bedrock outcrops developed in
metamorphic and sedimentary lithologies in the Bohemian-Moravian Highland, Czechia. The shapes of studied
bedrock outcrops indicate significant litho-structural controls. The general outline is mainly governed by (sub)
vertical joints, whereas the inclination of foliation or bedding planes dictates summit and cliff morphology. Some
of the tors described in this study developed in exceptionally densely foliated and jointed gneiss, which does not
comply with the generally accepted view of tors as massive rock compartments resisting weathering. A series of
rock hardness measurements performed at three gneissic tors using Schmidt hammer show statistically significant within-tor variability of R-values, indicating different exposure times to subaerial weathering. The
complexity of landform assemblages, within which studied tors/crags are developed, varies from isolated
bedrock outcrops surrounded by smooth slopes covered with debris-rich soils to outcrops accompanied by low
rock cliffs, debris-covered steps, slope benches, boulder fields and isolated large blocks. The 10Be inventories
reveal ages mostly from the Late Pleistocene and rarely from the Holocene. Summit/upper slope tors and hillside
crags show considerable variability in maximum denudation rates ranging from 6.4 ± 0.2 to 105.6 ± 4.8 m/Ma.
Paired samples from the summits and cliffs of eight outcrops indicate three possible post-emergence evolutionary
scenarios with summit downwearing while keeping the outcrop outline, balanced cliff and summit degradation
rate, and backwearing as the dominant degradation process.