The existing hypotheses of beach cusps development are considered. Their formation is the combined action of the beach abrasion and accumulation processes depending on the bottom slope, bottom sediments, and wave parameters. In recent decades, there are active debates about the origin of cusps. However, none of the hypotheses fully explains the mechanism of their formation. Thence the existing of some yet poorly explored hydrodynamic processes near the water edge having a significant impact on the cusps formation is supposed. A dynamic equilibrium of the beach at given seaway and the presence of rhythmic splash having sufficient force to move sediments of a given fineness are the main factors of the cusps formation. It apparently depends also on the type of wave breaking. Thus, the emergence of the cusps, their sizes and wave step are directly dependent on local morphodynamic parameters. Scalloped landforms are well represented along the shores of the Curonian and Vistula spits at the South-East Baltic Sea, and they are particularly unique in the lagoon shores. The combination of wind effected phenomena and wave action with of certain slopes of the coastal zone, leads to the development of a special type of acuteangled cusps with long “rumps” on the underwater slope. Long-term observations show that the cusps formation on the lagoon shores is quite possible and regularly re-starts under favorable conditions. At the spit shores different underwater slope parameters and seaway conditions lead to the development of the cusps significantly different from the lagoon’s ones. They are mainly the large forms with smooth contours and rounded tops.
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