• Extensive boulder fills of canyon floors interpreted as residual materials
  • Subsidence into voids the likely process of origin of boulder fills, not rock fall
  • Boulders caves in canyons are not talus caves
  • Roofed slots provide analogy to explain larger landforms
  • Geomorphometry assists in characterizing geomorphic diversity of canyons


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.



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