Sandstone: Sandstone, a type of sedimentary rock, looks like sand frozen in place. When quartz, feldspar, and other silica-containing minerals and rocks break into fragments between 0.1 and 2 mm (0.004 to 0.08 inches) across, scientists call the pieces sand. In deserts, on beaches, and under bodies of water, layers of sand grains build up over thousands or millions of years, until the accumulated pressure from the weight of those layers compact the sand grains into solid rock, a process called lithification. The sand grains are commonly cemented together by fine-grained quartz and calcite. Sandstone makes up 10 to 20 percent of all sedimentary rocks on Earth because its ingredients are among the most widespread minerals. They are found worldwide and form under a wide range of depositional environments and conditions. Detailed examination of the mineral grains and rock fragments in sandstones is necessary to help geologists interpret the source rock and the environment in which the sandstone was deposited. For example, coarser sand grains in the rock could indicate that blowing wind or running water removed the smallest, finest granules before lithification took place.

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.