Relief and its memory: minute forms in sandstone surfaces

Radek Mikuláš

Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 135, 165 02 Praha 6, Czech Republic

Rock surfaces of weakly lithified sandstone rock are a specific “memory register”: it is relatively easy to form a “record” (e.g., scratch, salt erosion feature, bioerosion feature, hardened body) but its durability may be surprisingly high. Little attention has so far been paid to this aspect of the sandstone meso- and microrelief; however, the temporal analysis of the surfaces may help to date various events, e.g., fall of rock block, deforestation, and soil degradation. Especially rich record can be obtained from honeycomb surfaces. Arcuate honeycomb pits usually show gravity-controlled orientation of their flat bottoms (probably because they function as “rock dishes” in specific situations). In result, surfaces of fallen blocks often bear pits of double or triple shape. 1, symmetrical arcuate ones with oblique bottoms (i.e., the oldest, non-reconstructed generation). 2, “reconstructed” honeycombs, usually larger than the previous type; their upper parts are inclined and somewhat asymmetrical, bottoms are flat. 3, symmetrical arcuate pits with flat bottoms (i.e., the youngest, post-event generation; cf. Mikuláš 2001a).
Less frequent but well interpretable shapes of sandstone surfaces are oblique “ledges” formed through rising of soil moisture in sidewalls of sandstone pillars. Therefore, an oblique ledge may mark long-lasting unchanged position of soil cover along the walls. In the area of historic settlement of Hradsko (Kokořín Nature Reserve), several ledges can be observed on the walls of sandstone coombs. In this case, the possibility to correlate individual ledges and episodes of deforestation seems to be realistic (cf. Mikuláš 2001b).
Rock crusts, rock arches, abri, tree trunks, arcuate niches, tunnels and small precipitation forms (e.g., spherical nodules) are other features of sandstone meso-and microrelief which may bear an interpretable temporal record.

References

Mikuláš, R. 2001a. Gravity and orientated pressure as factors controlling “honeycomb weathering” of the Cretaceous castellated sandstones (northern Bohemia, Czech Republic). - Bull. Czech Geol. Survey, 76(4): 217-226.
Mikuláš R. 2001b. Poznámky ke vzniku některých prvků mikroreliéfu pískovcových skal. - Ochrana přírody, 56, 1, 19-21. Praha.

The study is a part of the Research Programme of the Institute of Geology, AS CR, Praha (No. CEZ: 23-013-912).