What are 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.

Common Sedimentary Rocks:
Common sedimentary rocks include sandstone, limestone, and shale. These rocks often start as sediments carried in rivers and deposited in lakes and oceans. When buried, the sediments lose water and become cemented to form rock. Tuffaceous sandstones contain volcanic ash.

Clastic Sedimentary Rocks:
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. 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 than 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.

Biologic Sedimentary Rocks:
Biologic sedimentary rocks form when large numbers of living things die. Chert is a example for this type of rock, and this is one of the ways limestone can form. Limestone can also form by precipitating out of the water.

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Image shows a long gray stone lying flat in a gravel pit with a silver sign nearby
July 20, 2016

Leesburg Conglomerate

Conglomerates are sedimentary rocks that are made up of various fragments of rock interspersed with finer grained material. This particular conglomerate was deposited as a fan on the northwest side of the Culpeper Basin.

(grey-colored rock) Cherry Valley shale
March 5, 2015

Marcellus shale in central New York showing Cherry Valley limestone

Exposure of the Marcellus shale in central New York showing the Cherry Valley limestone (grey-colored rock) between the Union Springs and Oatka Creek shales of the Marcellus.

Photo of cross-beds
December 31, 2014

Large-scale cross-beds in the Paleogene Coalmont Formation

Large-scale cross-beds in the Paleogene Coalmont Formation southwest of Walden, Colorado, formed as a delta complex within a persistent lake. Sediments were chiefly eroded from the Proterozoic basement of the Park Range block to the west and transported a short distance to the Colorado Headwaters Basin.

Image: Sandstone Erosion Patterns in Zion
December 26, 2014

Sandstone Erosion Patterns in Zion

Wind and water combine to create unique patterns in the sandstone.

Image: Sandstone Detail
December 23, 2014

Sandstone Detail

Detail of the wind and water-weathered sandstone that forms the Beehive Formation in the Valley of Fire Nevada State Park

Image: Bryce Canyon Sandstone
December 17, 2014

Bryce Canyon Sandstone

Bryce Canyon is a unique sandstone formation in southern Utah. It is home to a large number of hoodoos, which are oddly shaped pillars of rock that formed due to different

Image: Sandstone Formation on the Navajo Loop Trail
December 17, 2014

Sandstone Formation on the Navajo Loop Trail

Views along the Navajo Loop Trail in Bryce Canyon National Park.

Image: Sandstone Texture
December 17, 2014

Sandstone Texture

Views along the Navajo Loop Trail in Bryce Canyon National Park.

Faulting in limestone outcrop downstream from where Greenwood Creek flows into the Colorado River.
October 31, 2012

Faulting in limestone outcrop, Greenwood Creek

Faulting in limestone outcrop downstream from where Greenwood Creek flows into the Colorado River, October 31, 2012 (photograph by Jennifer Wilson, U.S. Geological Survey).

SIR 2015–5098

Seeps along limestone outcrop downstream from where Jim John Creek flows into the Colorado River
October 31, 2012

Seeps along limestone outcrop, Jim John Creek

Seeps along limestone outcrop downstream from where Jim John Creek flows into the Colorado River, October 31, 2012 (photograph by Jennifer Wilson, U.S. Geological Survey).

SIR 2015–5098

Image: Cross-Bedded Sandstone in the Gunsight Formation
August 30, 2012

Cross-Bedded Sandstone in the Gunsight Formation

Cross-bedded sandstone of the Gunsight Formation of the Mesoproterozoic Lemhi Group. These beds are interpreted as products of alluvial sedimentation in a deltaic setting.