The Changing Face of PEQ

Abstract

The spectacular relief of the sandstone inselbergs is maintained by an interplay of geological structure and weathering/erosion processes, in that vertical joints and faults provide the foci along which most weathering and erosion take place. There are three major varieties of sandstone breakdown; the first is failure of large rock masses along vertical joints, leading to rockfall. Associated with this process is the mechanical breaking of falling blocks upon impact, in some cases right down to the origjnal sand grains. Rockfall maintains vertical or steep slopes, provided basal talus is removed. The second variety is liberation of small blocks by weathering along closely-spaced joints and bedding planes. Rock prone to this style of erosion tends to be less steep and may have basal slopes buried by rubble. The third variety is grain by grain weathering by solution of cement. This process is the most fundamental, as it not only wears away the surfaces but attacks joints, leading to the processes described above. Grain by grain weathering is slowed or prevented on some surfaces by development of oxide rinds, particularly where run-off is concentrated.

These processes result in deposition of debris at slope bases, but in most cases the debris is quickly reduced to sand and removed by wind and water. Relatively clean slope bases allow slopes to retreat parallel to themselves. Where debris collects faster than it is reduced to sana, slope angles tend to decline until they reach the angle of repose of the debris.

Differences in structural expression of the various lithological units result in differing erosional characteristics. The Saleb Formation is typified by relatively gentle slopes littered with debris, due to thinly spaced joints. The Ishrin Formation, witll widely spaced vertical joints, is typified by rockfall of large masses. It is the major cliff-former. Grain by grain weathering, influenced by variations in varnish development, has produced spectacular tafoni on some Ishrin cliffs. The Disi Formation is extremely friable and although slopes are relatively gentle they are mostly rubble free. The Disi in many places forms rounded domelike shapes which may be due to exfoliation along pressure-relief joints. The Um Sahm Formation is highly fractured and similar in appearance to the Saleb.

Igneous rocks, where exposed below the sandstones, are undergoing chemical weathering, particularly along joints. The results are tors, tafoni, and large amounts of grüss.

The desert floor between inselbergs is in some places a bare bedrock surface with an integrated drainage network, and in other places a sand or playa surface. Running water seems to have been a major agent in shaping the floor, but is less important now than in the past. Transportation of sand by wind is common but there is no evidence for significant eolian aggradation or degradation at the present time.

A minimum rate of surface retreat by grain by grain weathering is 5 cm./1,000 years, based on weathering ofNabataean ruins. Overall slope retreat should be significantly greater, due to added effects of rockfall and removal of blocks by running water.