A wave effectiveness parameter defined as the product of theoretical instantaneous sediment transport rate times frequency is given for wave action at the sea bed. High values of this parameter are found in waters shallower than 18 m. (10 fm.) in the southern North Sea and are held to be responsible for absence of sand waves at these depths. On the other hand, mud deposition does occur in certain shallow areas of the southern North Sea where the concentration of suspended sediment is high. Farther offshore, where suspended sediment concentration is low and wave activity moderate (as defined by the effectiveness parameter), no mud deposits are found. With increasing depth to a low wave effectiveness region, more mud is found in the bottom sediment below 30 m. (16 fm.). In the Celtic Sea the wave effectiveness parameter has the same value at 73 m. (40 fm.) as it does in the North Sea at 30 m. (16 fm.). This is close to the sand/mud boundary in the bottom sediments of both areas. Wave activity is only one factor--others being suspended sediment concentration and current velocity--influencing deposition of mud. Some ancient mudstone could have been deposited in shallow waters off exposed coasts.