Water and wind, acting over thousands or millions of years, wear down Earth's rocks and the remains of invertebrate animals, breaking them into small pieces called clasts. For example, ordinary beach sand is a collection of tiny clasts, which are composed of quartz and other minerals. Pebbles, boulders, and pieces of broken-up mollusk shells are also examples of clasts. Running water can sort small and large grains by size as the fluid and clasts flow downstream. Where clasts pile up in layers, the weight of the upper sediments presses down on the clasts below them, squeezing air and water from the spaces between the grains. The clasts are held together by cement consisting of fine-grained materials like calcite, silica, and iron oxides. Conglomerates, sandstones, and siltstones are all examples of clastic sedimentary rocks.