Note that pebbles laying on top of the sand, prevent the formation of dunes. Where a variety of coarse material exists, ranging from pebbles to large boulders, one finds the boulders cast high up the beach or along the sides of pocket beaches, out of reach of average storms. Underneath or more towards the middle of the beach, one finds successively smaller stones. With it the slope of the beach also becomes more gradual. Some beaches are notorious for their rips.
Rips are unpredictable currents, flowing away from the beach.
They pull swimmers into deeper water and are very focused and strong. Rips arise when breaker upon breaker pushes new water onto the beach, not allowing much time for it to flow back. Normally a wave that runs up a beach will flow back underneath the next one, causing an undertow current, familiar to swimmers. Where waves arrive in rapid succession, a rip may occur, allowing the piled-up water to flow back to the sea. Because rips are narrowly focused currents, swimmers should swim out of them in a direction parallel to the beach.