The manner in whichh a bed of weakly cohesive mud is eroded by a plain, turbulent flow of water is found experimentally to depend upon the severity of the flow over the bed. The major structures left behind on a weakly cohesive mud bed after a prolonged period of erosion also depend upon the severity of the flow. With ascending severity we obtain: (1) longitudinal rectilinear grooves, (2) longitudinal meandering grooves, (3) flute marks, (4) transverse erosional markings. The more severe flows also give rise to shear wrinkles and to erosional structures of a less well-defined nature. Of the four structures enumerated, all but longitudinally rectilinear grooves have counterparts in the fossil record. Of particular importance is evidence confirming that the flute marks are associated with separated flows and are due to processes of flow separation and reattachment.