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The upper surface of alluvial terraces at Mt. Sedom, Israel are covered with a biogenic crust populated by filamentous cyanobacteria. The cyanobacteria expand when wet and shrink when dry, forming when time passes cracks in the soil surface in the form of polygons. These polygons are smaller than those formed by shrinking of the mineralogical component of the substratum. Continuing to grow and expand laterally the cyanobacteria cause upfolding of the polygon margins which in time become populated with cyanophilous lichens. The lateral growth accompanied by increasing trapping of airborne dust in the cracks of thalli of cyanophilous lichens leads to the development of microridges along the former upturned margins of cracks. Increasing water storage in the depressions of the surface with augmenting roughness ameliorates the moisture regime by decreasing water runoff from the soil surface.

Cyanobacteria of the early stages of colonization occur at the drought-latent stage below the surface of the flat soil and emerge phototactically when sufficiently wetted. Their fronds are green. The microbionts on the elevated terraces representing progressive, older stages of colonization are situated above the rugged-surfaced soil and have dark thalli or fronds. The number of microbiont species, chlorophyll a, and polysaccharide content of the crust increase from the young to the old terrace. Calcite content, compaction, and linear structure increase too. All these quantitative changes lead us to regard the different stands as parts of microbiotic succession. We recommend the use of the micro-geomorphological structures as age indicators which are correlated here with the relative time sequence system of alluvial terraces in other places, even if they are not in such an obvious chronosequence. (C) 1998 Academic Press Limited.