Sedimentary Rock Clay Concretions

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.

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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.

Alternating beds of sandstone and shale, Pacifica, California, USA
Photograph by Marli Bryant Miller

Diagenesis: How Sediments Become Rocks

After a body of water deposits layers of sediments, the grains in those layers, or strata, undergo various changes in response to the pressure of the layers on top of them. Scientists use the word diagenesis from the Greek roots dia (change) and genesis (origin) to encompass all these changes. In the process called cementation, minerals grow or precipitate out of solution in the pore spaces between rock or mineral grains. Compaction is the collapse of pore spaces within a sedimentary rock due to the weight of the rock above it. Lithification is the process of hardening sediment into rock through compaction and cementation. Diagenesis takes place at temperatures of less than 200 degrees C (392 degrees F); at higher temperatures, scientists say that the rocks are undergoing metamorphism.