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.

Regional metamorphism takes place over large areas of Earth's surface where tectonic plates crash together, pushing up mountain ranges. The metamorphic processes typically happen between temperatures of 350-650 degrees C (660-1,200 degrees F) and at depths of 5-20 km (3-12 miles). High pressures from these enormous collisions cause the rocks to recrystallize and the new mineral grains align into an arrangement like a layer cake, or a sponge when it is squeezed; that texture is called foliation. Low-grade metamorphic rocks, like slate, split easily into sheets. High-grade metamorphic rocks, like gneiss, may be foliated but do not break into sheets.

Thanks to high pressures in a particular direction, the grains in some metamorphic rocks become aligned parallel to each other, giving the rock a layered appearance. Scientists call this property foliation, from the Latin word folium, which means "leaf." In slate, which is a fine-grained metamorphic rock, the foliation occurs at a small scale due to the alignment of tiny flakes of mica and other silicate materials. Geologists say slate has good cleavage because it tends to break into parallel thin sheets. In coarse-grained metamorphic rocks such as gneiss (pronounced "nice"), the foliation may occur as alternating light- and dark-colored bands. These layers may also appear wavy or curved because the forces that changed the rock acted unevenly on the rock. Rocks that metamorphose due to high temperature rather than high pressure may not show any foliation.