Sedimentary Rock Sandstone

Associated Smithsonian Expert: Timothy Rose, M.S.

Geologitst Tim Rose checks out volcanic rocks in Hawaiian lava tubes.

Photographed by Richard S. Fiske, Smithsonian Institution

Tim Rose is a geologist and manager of the analytical laboratories in the Department of Mineral Sciences at the Smithsonian National Museum of Natural History. His interest in fossils and rocks began at an early age, collecting fossils at Calvert Cliffs, Maryland and rock-hounding in the pegmatite quarries of the Black Hills, South Dakota. He took his first college-level geology courses at a local community college while he was a senior in high school. After receiving his undergraduate degree in geology at the University of Delaware in 1978, he worked briefly in the oil fields of Texas and Louisiana and off the coast of New Jersey in the Baltimore Canyon. Tim joined the Smithsonian in 1980 in the fledgling department of Automatic Data Processing (now Information Technology) and after changing to a sample preparation position, he went to the University of Maryland and received an M.S. in geology in 1991. Tim is currently involved in the study of Kilauea volcano’s explosive eruptive history as well as studies of ancient Mesoamerican artifacts.

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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.

Looking down into a rock quarry in Morrison, CO. All of the rock removed is gneiss to be used in roads, buildings, and other construction projects.
Photographed by Donald E. Hurlbert, Smithsonian Institution

About Morrison Quarry

The continental crust in Colorado was constructed from several island chains that collided and merged together about 1.75 billion years ago. It was then metamorphosed at depth under heat and pressure, and magma was injected through cracks in the rock. After several million years of erosion the rocks of the Morrison Quarry became exposed at the surface. Layers of sandstone were deposited directly on top of the exposed rocks forming what is known as the "Great Unconformity," a 1.4-billion-year gap missing between one rock layer and the next. These rocks were likely uplifted during the formation of the Ancestral Rockies between 320 and 270 million years ago. The Ancestral Rockies were eventually eroded away, buried by sediments and flooded by shallow oceans. They were brought to the surface once again during the growth of the current Rocky Mountains.

Artist interpretation of the Laramide Orogeny, which led to the creation of the Rocky Mountains in the Western United States.
Image created by Karen Carr Studio, Inc., used with permission

About the Great Unconformity

Rocks record earth's history throughout time, and typically younger rocks are found above older rocks as sediments are deposited year after year. Occasionally erosion will erase rocks away and along with them a history of when they were formed. The Great Unconformity is a feature observed in Colorado where 300-million-year-old sandstone rests on top of gneiss that is about 1.7 billion years old. What happened to the 1.4 billion years in between? A record of that time was washed away, so no one can say for sure what happened in this area during that time.

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