Thirteen well-fixed echo-sounding traverses, repeated on three occasions during a winter and spring, have provided new data about the northern limit of a field of large sand waves that occupies much of the southern bight of the North Sea. The large sand waves were found to occur as narrow tongues up to about 11·5 km long by 3·5 km broad, which lie between low, northerly-trending ridges presumed to be of Pleistocene age. The sinuosity of the boundary as now known, coupled with the large, inherent navigational errors of earlier surveys, may well account for many of the previous indications that the boundary could have migrated for tens of kilometres. Evidence of the growth of a new sand wave is presented which, if accepted, implies an effective localized movement of the boundary of 1450 m to the north.
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