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USGS Geology in the Parks

Sedimentary rocks

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Laminated sandstone from the Mesozoic Moenkopi Formation in Lake Mead National Recreation Area

Clastic sedimentary rock

Clastic sedimentary rocks are the group of rocks most people think of when they think of sedimentary rocks. Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock. Note the quarter for scale. This conglomerate is made up of pebbles surrounded by a matrix of finer-grained sediment. Notice the large clast in the lower right-it is a conglomerate within a conglomerate.

Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. The smallest grains are called clay, then silt, then sand. Grains larger that 2 millimeters are called pebbles. Shale is a rock made mostly of clay, siltstone is made up of silt-sized grains, sandstone is made of sand-sized clasts, and conglomerate is made of pebbles surrounded by a matrix of sand or mud.

This medium gray limestone contains many fossilized balls of algae called oncoliths (dark gray) and trace fossils left behind by burrowing animals (orange) in this approximately 550 million year old rock.

Biologic sedimentary rock

Biologic sedimentary rocks form when large numbers of living things die, pile up, and are compressed and cemented to form rock. Accumulated carbon-rich plant material may form coal. Deposits made mostly of animal shells may form limestone, coquina, or chert.

Chemical sedimentary rock

Chemical sedimentary rocks are formed by chemical precipitation. The stalactites and stalagmites you see in caves form this way, so does the rock salt that table salt comes from. This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. Eventually these minerals can be redeposited, or precipitated, when the water evaporates away or when the water becomes over- saturated with minerals.

Simple rock classification chart with links to more information on rocks.

Continue to metamorphic rocks...
Rocks and Minerals site contents


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