Ocean basins are filled with loose sediments — the products of erosion. Most marine sediment originates inland and is fed into oceans by rivers. Debris from cliffs and other coastal landforms provides additional sediment volume, as do skeletons, shells, teeth of marine organisms, ash from volcanoes, and even asteroids. In this interactive resource adapted from the National Park Service, learn how these different parent materials influence the color and size of the materials that compose a beach, as well as other reasons why some beaches are composed entirely of fine sand, while others are a mix of pebbles and shells.