Beach cusps are one of the strangest natural formation along the water’s edge. They are made up of a rhythmic pattern of small cusps and bays, with the cusps — the pointed horns formed by two intersecting arcs — directed towards the ocean, separated by gentler sloping bays.
The horns are made up of coarser materials and the embayment contains finer grain sediment. They are most noticeable on shorelines that have coarse material like pebbles, however, they can occur on beaches with sediment of any size. They also seem to form best when the beach is steep, and regular non-breaking waves arrive nearly at right angles to the beach.
While it’s apparent that wave action causes the formation of beach cusps, the process is not fully understood. But once cusps have been created they are a self-sustaining formation. This is because when an oncoming wave hits the horn of a beach cusp, it is split and forced into two directions. The crashing of the wave into the cusps slows its velocity, causing coarser sediment to fall out of suspension and be deposited on the horns. The waves then flow along the embayments, picking up finer sediment, and run into one another in the middle. After this collision these waves attempt to flow back out to sea where they are met by incoming waves. Therefore, once the cusp is established, coarser sediment is constantly being deposited on the horn and finer sediment is being eroded away from the embayments. This causes cusps to maintain their size, if not grow larger.
Cusps are most often a few meters long, but larger cusps have been seen to grow as large as 60 meters across.
Beach cusps have been the subject of considerable debate and research for at least the last 50 years. The question is whether the pattern is imposed on the beach by a ‘standing wave’ formed by the intersection between the waves that are approaching the shore and the so called ‘edge waves’ that develop perpendicular to the shoreline, or whether it creates itself by self organizing feedback. Most of the recent experimental work seems to support self organization, which shouldn’t be surprising since creating order by self organization is seen in a wide range of other processes from geophysics, chemistry, physics, biology, and sociology.