Gravity and orientated pressure as factors controlling 'honeycomb weathering' of the Cretaceous castellated sandstones (northern Bohemia, Czech Republic)


Authors: Mikuláš R

Published in: Bulletin of Geosciences, volume 76, issue 4; pages: 217 - 226;

Keywords: Honeycomb weathering, case hardening, microrelief, orientated pressure, rock arches, Cretaceous, Czech Republic,

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A thorough, 'sophisticated' geometry of honeycomb surfaces suggests that not only processes forming the pits but also some 'construction plan' has to be involved. Lateral sides of rock arches often bear honeycombs having axes parallel to the curve of the arch vault thus copying a presumed pressure vector (lines of force). Most of the pits are spindle-shaped or cellular but some are arcuate with rudimentary oblique bottoms. Cellular honeycombs are characteristically developed on vaults of spherical or parabolic rock shelters and abri. These show similar parameters as the previous group but their axes are usually close to the projection of vertical line onto the wall. Therefore, it is difficult to assess whether the vector of gravity or the vector of pressure is responsible. Surfaces of fallen blocks bear pits of double or triple shape. 1, symmetrical arcuate ones with flat bottoms; 2, symmetrical arcuate ones with oblique bottoms and oblique axes; the obliqueness, however, is presumably caused by the rock fall and not by the irregular growth; 3, 'reconstructed' honeycombs, usually larger than the two previous types; their upper parts are inclined and somewhat asymmetrical, bottoms are flat. Some blocks bear only the 'reconstructed' honeycombs. The examples show that the bottom is 're-built' more rapidly than the vault, and consequently, bottoms of arcuate honeycombs are the place of the maximum loss of the sandstone. The fact that a rock crust usually does not harden horizontal or sub-horizontal surfaces may be one of the reasons. Cross-bedded sandstones (or blocks of horizontally bedded sandstones fallen and tilted a long time ago) have flat, horizontally orientated bottoms of the pits regardless of the inclined structure of the rock. Therefore, there is a strong 'angular unconformity' between the pit bottoms and bedding of the rock. Because the honeycombs are often constituted on softer inclined beds of sandstone and occur in more-or-less regular intervals, their flat bottoms may form a regular 'staircase'.