Winnowing process in the formation of slope-debris patterns of Socotra-Island, Yemen

Geoscience Research Reports 39, 2006 (GRR for 2005), pages 148–149

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Abstract

Spectacular forms of blocky debris on the slopes built by gabbroic rocks have been found in western part of Haggeher Mts. (Socotra Island). They occur as dark irregular patches 100 to 500 m2 in areal extent, entirely free of vegetation in contrast to the green surroundings covered with plants. At first sight, these forms resemble polygonal soils known from sub-nival areas. Here however, in tropical environment (12° from the equator), their origin requires a quite different explanation. The gabbro in the substrat of the debris is dissected by two systems of subvertical and mutually perpendicular joints. In the places of especially intensive jointing and at the junction of the two systems, the rain-water flushes fine particles of the debris into open joints and only coarse stony material unsuitable for vegetation remains on the surface. The designation "selective winnowing" can be used for this process. Low coastal plains of the island have a typical arid character with average rainfall 120 mm/y only, but this value raises ten times in the mountains making the winnowing of debris effective.