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.

The tiniest rock grains - less than 0.06 mm or 0.002 inch wide - may stick together to form siltstone or mudstone. The fine grain of a siltstone or mudstone is an indication that it formed from mature sediments - silts that have been weathering for a long time, have been well sorted, and may have traveled far from the rocks from which they were broken off. Generally, humans need a microscope to see the mud or clay grains that went into the formation of this sedimentary rock. Sometimes siltstone contains mica and other minerals that become aligned in a preferred direction during the formation of the rock. The resulting rock, called shale, easily breaks into pieces that look like parallel sheets.

Silt grains are rock particles even smaller than sand (less than 0.06 mm or 0.002 inch wide), so water and wind can transport them farther than sand grains. When the water in a pond appears brown and difficult to see through, it may be carrying large amounts of silt, which is slow to settle to the bottom. Quartz and feldspar, both silicates, are the most common minerals in silt. Once silt hardens into rock, scientists usually cannot see the individual grains without the aid of a microscope. Mud is a mixture of water, silt, soil, and clay (the tiniest of all rock particles). Mudstone, the sedimentary rock made of mud, often breaks into random blocks, and it may contain cracks that first appeared when the original mud was drying out.