Fine grained material was collected from an extensive North Sea mud field north of 53°30′N. Of one sample deposition and erosion velocities were measured in a series of laboratory experiments using a circular tack in which currents with different speeds could be generated. A series of consolidation periods were applied. The results are compared with tidal current velocity patterns and wave information for the North Sea obtained from published data. It is shown that the boundary between the mud field and the sandy Souther Bight is principally determined by spring tide current velocities. In the mud field net deposition of mud is possible because the increase of erosion velocity by consolidation proceeds faster than the increase of current velocity after slack tide. © 1979.