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Weathering features are described from an arid coastal area in northern Morocco which are indicative of invasive chasmolithic and endolithic microbial communities. Active weathering of marine terraces and karst development is associated with endolithic and epilithic algae responsible for boring, disintegration of matrix and mineral fabrics, solution and biomineralization that undermines the marine carbonate platform. Evidence of a range of biological weathering agents remains preserved, specifically calcified filaments and sporangial material. An abundance of filaments and spores representative of a consortium of algae, fungi, cyanobacteria and lichens is associated with the most denuded outcrops. The array of microbes contributes to the formation of the stromatolites to depths of 0.5 m within the limestone substrate. The preservation of stromatolites is supported by calcification of spores and filaments, with trapping and binding of carbonate held in suspension by vadose waters. The pervasive weathering on this sector of coastline has important practical implications for coastal planning and development authorities in Morocco. Copyright (C) 2003 John Wiley Sons, Ltd.