Sedimentary Rock Siltstone

Associated Smithsonian Expert: Leslie Hale, B.S.

Leslie Hale at the Monterey Bay Aquarium (California, USA) during a conference of museum collections managers

Photo by Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences

Leslie Hale, the Smithsonian’s rock and ore collections manager at the Smithsonian National Museum of Natural History, tells people that she is a “rock librarian.” While growing up in Bowie, Md., not far from the Smithsonian, Hale collected rocks and took a summer class on lapidary art (making jewelry out of stone). Her career choice was greatly influenced by her attendance at a magnet high school for science and mathematics and taking geology as a senior-year elective. Hale joined the Smithsonian staff shortly after finishing her bachelor’s degree at the University of Maryland in 1989. Today, she supervises two full-time staff members as well as several volunteers, interns, and contractors. She assists Smithsonian scientists and visiting researchers who want to use the museum’s extensive collection of rock and ore specimens. Hale also sends out rocks on loan to geologists at distant universities; she keeps track of the objects’ whereabouts and sends out requests for return or loan renewal. Finally, she gives tours of the Smithsonian’s geology research facilities, conducts inventories, answers questions from the public, and identifies rock specimens.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.
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Sedimentary rock formation, southwestern Utah, USA
Courtesy of Mark A. Wilson, The College of Wooster

How Sedimentary Rocks Are Formed

At or near Earth's surface, sedimentary rocks form in two ways: by the accumulation of rock grains or by the formation of a solid from minerals dissolved in water. The fragments that go into making sedimentary rocks can be as big as boulders or as small as clay particles. Over long periods of time, the upper layers of debris compress the lower layers, squeezing out excess water or air trapped between the rock fragments. Under the pressure, individual fragments eventually dissolve and stick together, or the remaining fluid within the sediment brings in other substances that act as a cement, until the sediment has turned into rock. Scientists classify many sedimentary rocks based on the size of the particles that built the rock; mudstone and sandstone, for example, originally came from fine-grained mud and sand deposits that hardened over long time periods.

Michael Wise investigates granite and pegmatite dikes that intruded into metasedimentary rocks near Mount Antero, Colorado.
Courtesy of Jennifer C. Kelly

About Sedimentary Rocks

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.