How do seashells get on the beach?

The pebbles are flint deposited in the chalk cliffs adjacent to Brighton. Through natural erosion the flints are released from the cliffs and slowly make their way through natural process to Brighton where the groynes are in place to "capture" the pebbles. You can see this in action as you can see a higher level of pebbles to the west of a groyne than on the easterly side.