The accretion of sediment by cyanophytes is usually associated with sediment trapping and precipitation of minerals by stromatolites in the marine intertidal and subtidal zones. Stromatolitic structures are also known to form in freshwater streams, lakes, and hot springs. All of these structures originate in aquatic environments. Current work suggests similar biogenic structures can form on dry land. Aeolian quartz sands as well as silt- and clay-size particles are trapped and held by filamentous cyanophytes which intertwine to form networks around the sand grains, and to whose sheaths the smaller particles adhere. Trapping and accretion occur whenever enough moisture is present to rehydrate dormant organisms and allow them to migrate by gliding motility to the surface of the mat. Such movements are accompanied by renewed production of polysaccharide sheath. The organo-sedimentary structures

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which result cover otherwise barren soil and rock in arid and semiarid regions of the southwestern United States. The brittle, fragile mats cover hundreds of square miles in areas undisturbed by livestock and man, and represent an accretional phenomenon in an otherwise generally erosional setting.

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