Why is the sand finest at the top of a beach?

Name: Codi
Price range: $

Bruny Island

Feel the sand on Neck Beach, or on any beach. High up the beach, if you feel it, it is quite fine. This is because the energy of the wave dissipates as the water comes further up the beach. The energy falls away – it drops off – so it drops the heavier sand first – and then takes the finer sand further up the beach.

When the waves come in, there is an enormous amount of energy. It loses some energy to heat as it comes in. As it loses that energy, it will start to drop the courser particles of sand and as it sweeps up the beach, it is dropping the finer particles. And that’s why you get this gradation of fine particles of sand at the top of the beach, to courser particles of sand at the waterline. At the top of the dune, the sand is very fine – because it is all wind driven.

When you get water covering an area that has normal soils on it, it will start to sort itself, and the sand particles separate out and they remain behind. You can do it with the kids at home, get a handful of soil – wash it away – the mud – tiny particles wash away first and then the particles get bigger - finally you get sand. And then you get the waves coming and wash it away clean. If you dropped a bucket of mud on the beach it won’t be there tomorrow – but the sand will. Because sand is actually quite coarse particles.

Fascinating things these beaches – they’re alive!