Sedimentay Rock Ventifacts

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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Imbricated clasts recording past water flow from left to right, near Harrison, Montana, USA
Courtesy of Michael C. Rygel via Wikimedia Commons

About Clastic Sediments

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.

Successive stages in the formation of grus (angular fragments) from granite due to mechanical and chemical weathering
Photograph by Marli Bryant Miller

Weathering and Sedimentary Rocks

The term weathering refers to any environmental process physical, chemical, or biological that changes rocks that are exposed at Earth's surface. Scientists define physical weathering as any kind of mechanical process that breaks down surface rocks. For example, the cycle of alternating freezing and thawing of water in rock crevasses (or cracks) breaks apart rocks, because frozen water (ice) takes up more space than liquid water. Tree and plant roots also force themselves into and between rocks to break them into smaller fragments or grains. Strong winds, especially in desert environments, may pick up sand grains, driving them into exposed rocks and eroding them and leaving marks called ventifacts (from the Latin words for wind and face). In chemical weathering, weakly acidic raindrops falling on rocks and sediments cause chemical reactions, such as dissolving halite and other salts. In biological weathering, lichens, moss, and microorganisms release acidic compounds that accelerate the breakdown of rocks and grains on which they are growing.