Grading of pebbles by the sea

This series of pictures shows how the size of beach pebbles is 'graded' by the sea. The largest stones are nearest to the top of the beach and the smallest shingle is closest to the water's edge. As the tides come and go, stones are constantly knocked together and they are gradually ground down and eroded into smaller and smaller pebbles. The smaller they become, the more the sea is able to move them around, and finally they are ground down to form beach sand. The largest stones are only moved by the sea during violent storms and sea surges, and being heavier, they tend to remain higher up the beach. This process of grading and water erosion also takes place in rivers.