Beach cusps

Posted on July 25, 2013 by Alistair

 

The interaction of the Lake and a sandy beach offers fascinating physical details: Water wets the sand changing both appearance and firmness. Waves move up and down the beach sorting sand and pebbles by size. Waves shift sand along the shore sculpting inlets and the sand points. Some of these topics are illustrated and discussed on the main website on the links at Beaches.

Although uncommon, one of the loveliest details is the rhythmic pattern known as beach cusps. I have somewhat more spectacular images of them on my page, Beach Cusps, from which I lift the following explanation:

If at one spot [on the shore], the velocity of the swash [upwash of the wave] is slightly less, either because of wave interference or even a pebble or stick on the beach, then that portion of the swash will drop some of the larger sand grains. This only enhances the slow down for the next wave, which also drops some of its sand. So, a small promontory is built which slows and deflects subsequent waves. The portion of the swash which is deflected to either side of this growing cusp swings around in an arc and creates the bays. Now, those bays cause velocity differences which produce the next cusp, and so on down the beach.

Although yestermorn’s pictures are not as grand as those on the other page, the phenomenon’s rarity justifies the present attention.

A view along the shoreline shows the pattern of small cusps and bays created by the waves.

And a view from above. The waves shifted and the pattern soon vanished.

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