PLAY THE ANIMATION.
As you can see, the process for building a sand dune is quite simple and is continually going on. As the tide goes out it leaves a fresh batch of sand on the beach to dry out, to be caught by the wind off the sea and for the smallest, lightest and finest grains of sand to be carried off by the wind towards the dunes. On shore these new deposits build the embryo (or baby) dunes that one day will be the tall mobile dunes you have just walked through. Marram grass thrives on the new sand, with all it's nutrients, blown onto the dunes each day. The crest of the dunes is characterised by clumps of this tall and spikey grass. This grass is usefull in holding dunes in place to some degree as the long roots sink down into the dune and stabilize some of the movement in a mobile dune.