Mud crusts are the semi-indurated, loosely attached, upper laminae of crack-bounded mud polygons. The sizes and shapes of mud crusts are influenced by numerous factors including organisms on or in the mud during drying. Blue-green algae are among the most abundant and geologically important organisms in environments where mud crusts form. These algae commonly grow in mats consisting of tightly intertwined filaments coated by thin, gelatinous sheaths. In warm waters of high pH the algae grow rapidly and the filaments penetrate the muds to trap and bind sediment. Upon drying, the algae increase me cohesion of muds to produce crusts which maintain their integrity with rewetting. Photosynthetically produced oxygen from algal mats below mud crusts may accumulate in bubbles and cause flotation of thin, loosely attached crusts. Floating mud crusts may then be transported considerable distances by gentle wind and water currents. Upon dissipation of the supporting bubble layer the crusts sink and may then become tabular constituents of edgewide conglomerates and intraformational conglomerates and breccias. Conglomerates and breccias formed in this manner may be units of entirely conformable rock sequences.