ABSTRACT BOOK Sandstone Landscapes ... - Sandstones.org

TPiotr MigońDepartment of Geography, University of Wrocław, pl. Uniwersytecki 1, 50-137 Wrocław, Poland(migon@geogr.uni.wroc.pl or pmigon@yahoo.com)The role of large-scale slope failures in sandstone landscape evolutionMany sandstone landscapes around the world, especially in epiplatform tectonic settings, are characterisedby the occurrence of distinct escarpments forming cuesta faces or plateau edges. Often, top parts of theseescarpments assume the form of a vertical or steeply inclined rock slope, the height of which varies from afew to hundreds of metres. Cliff-forming sandstones are usually massive, poorly jointed, quartz-rich andwell cemented, which accounts for their high rock mass strength and the ability to cope with high tensionalstresses present within vertical, occasionally undercut rock slopes. Case hardening additionally contributesto high strength of sandstone exposed within a cliff.Lower slopes of plateau edges are often mantled by extensive chaotic talus accumulation, apparentlyderived from large-scale rock falls and slides from the upper rock slope. Amphitheatral scars, niches andalcoves within the rock slope mark the place of failure. Usually initial failure takes place at the contact ofcaprock and underlying weaker rock and then propagates upwards along developing tensional cracks. Rockslope segments affected by individual events could be as much as 200 m high and 300 m across, as in theRam sandstone area in SW Jordan. Apparent absence of sorted talus and regular debris cones indicates thatcontinuous supply of debris from the rock face plays a minor part in the rock slope evolution.The absence of talus cannot be equalised with the absence of past massive rock failures from rock slopes.Evidence from the Colorado Plateau and the Middle East demonstrates that talus disintegration at thefootslope proceeds very fast, which is primarily attributed to the presence of aggressive weatheringenvironment. Moreover, many sandstone blocks readily disintegrate on impact. There are examples ofmassive scars and niches within the cliff and very scanty talus beneath.Punctuated rock slope evolution, with long periods of relative stability interrupted by episodes of massivefailure and scarp retreat, appears to be a characteristic sandstone landscape phenomenon, unparalleled inother rock types, although it would be wrong to consider it unique for sandstone. Similarities can be soughtin the development of marine cliffs. Possible consequences for living world include repeated exposure offresh rock surfaces for colonisation by organisms, interruption of biological succession within both rockface and talus, and occasional destruction of habitats in the lower slope. Likewise, it remains an openquestion to what extent plants and animals contribute to increasing slope instability and eventually, failure.