A wave arrives but the water does not
Watch a gull sitting out beyond the SURF while waves roll underneath it. The bird goes up and down and very slightly round in a circle, and ends up more or less where it started. It is not carried to the BEACH, because the water is not going to the beach.
What travels is the energy. Each particle shoves the one in front and returns to where it was, so the shape crosses the SEA while the water stays put — the way a shake travels along a skipping rope without the rope going anywhere. Only at the end, where the seabed rises and the bottom of the WAVE drags and trips, does the water topple forwards and SPLASH up the sand.
Almost every beach is made of the same mineral
Granite is a mixture of several minerals, and once it starts weathering they do not last equally. Feldspar rots chemically into clay. Micas flake apart. Quartz does neither: it is hard, it barely reacts with anything, and it simply gets broken smaller and smaller.
So after a few million years of rivers and waves grinding the mountains down, what is left on the shoreline is the stubborn ingredient. Beaches from Cornwall to Australia are largely quartz, and a PEBBLE is a stage in the same journey rather than a different thing — given long enough, every pebble becomes sand.
The sea pulls twice a day, and drags sideways
The Moon's gravity draws the ocean into a bulge on the side of the Earth facing it. The part that surprises people is the second bulge, on the opposite side, where the water is pulled least and effectively gets left behind.
The planet spins through both of them, so most coasts get two high tides and two low tides a day rather than one. The gap between the pull of the Moon and the pull of the Sun explains the rest: when they line up you get the biggest tides of the month, and when they work at right angles you get the feeblest. Any sandcastle strategy should begin with finding out which is happening today.
Waves almost never arrive straight on. They come in at an angle, run up the sand at that angle and then drain straight back down under gravity, so every wash moves sand and shingle a small distance along the shore. Repeat it all day and the whole beach is slowly travelling sideways.
The lines of timber running down to the water are there to interrupt that. Sand piles up against one side of each groyne and thins out on the other, and the difference in level is often startling — sometimes more than a metre across a single row of planks. Without them a good many seaside towns would find their beach had moved to the next town along.
A day out in the grid
The scenery comes first: the SEA, the SUN, the BEACH and a stripy DECKCHAIR. Then the activity — dig with a BUCKET and SPADE, SWIM, SURF, PADDLE and SPLASH, or drift about on a LILO while a BOAT goes past.
The best of it is the collecting. A SHELL, a smooth PEBBLE, a CRAB or a small FISH investigated in a ROCKPOOL, a TOWEL to wrap up in afterwards, and an ICECREAM eaten at speed against the SUN. Every one of those words is short, concrete and attached to a real memory, which makes them unusually easy to spot in the grid.