Arcades: Products of stress-controlled and discontinuity-related weathering

Weathering patterns on sandstone cliff faces and sloping surfaces were studied from the viewpoint of gravity-induced stress combined with the presence of planar discontinuities in the rock. Critical evaluation of hundreds of photos taken in various regions around the world, numerical modelling using finite element method (including automatic stress-controlled element removal technique of erosion modelling), and physical in situ modelling of friable sandstone revealed the formation of a specific sub-group of weathering forms related to planar discontinuities. Due to their striking similarity with elements known from architecture, these pits are introduced as “arcades” or arcade-like forms. These pits and cavities vary in size but they are similar in shape, coalesced into “trainsË® along the discontinuities, and separated from each other by hourglass-shaped pillars. In their origin and geometry, these forms differ from widely known cavernous weathering forms such as honeycombs and tafoni. We have shown that arcade shape and size are controlled by the redistribution of gravity-induced stress along planar discontinuities and the resulting stress shadows. The stress shadow zones along the discontinuities are characterized by material loss owing to various weathering and erosional mechanisms. Although overlooked as yet, stress-controlled weathering along discontinuities has a strong effect on rock surface morphology, and arcades can be taken as its principal example.